1999 Vol. 17, No. 3
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Display Method:
1999, 17(3): 331-338.
Abstract:
A fundamental problem in sequence stratigraphy is whether the sequences are global in extent and whether the sea level changes are eustatic in nature. Study on the Upper Devonian and Lower Carboniferous depositional sequences in the different facies belts on the southern margin of the Upper Yangtze Platform shows that different orders of sequences possess of distinctive distribution and correlation in space, although the sea level changes in the various orders of sequences may be primarily global in nature. The present paper regards the distribution in space, especially in different facies belts, as a criterion for defining and recognizing the various orders of sequences. The orthosequence (third-order sequence) is probably global in nature, which may be discerned in various depositional facies belts on the continental margin and can be correlated over long distances, sometimes may be worldwide. Commonly, correlation of the subsequences (fourth-order sequences with time interval of 0.5 to 1.5 Ma) is difficult in different facies belts, although some of them are probably also worldwide in distribution. A subsequence should be distinguishable and correlatable within at least one facies belt. The number of subsequences in one orthosequence varies in different facies belts.Only the number of subsequences recognized in shallow marine facies represents the correct subdivision of subsequences within one orthosequence.The higher-order sequences.including parasequences (fifth-order sequences, i. e. subsequence of Wang and Shi) and microsequences (sixth-order sequences), are regional or local in distribution. Generally their correlation are difficult even within a single facies belt.Parasepuences and microseunces may have reflected the longer and shorter Milankovitch cycles,respectively. Orthosequences and subsequences are usually recognizable in different facies belts, while parasequences and microsequences may be distinguished only in shallow marine deposits,but not in slope and basin facies deposits.The widely used meter-scale cyclothems probably include various orders of sequences and may represent parasequences (fifth-order sequences)and microsequences (sixth-sequences),even orthosequences,in different depositional facies belts.
A fundamental problem in sequence stratigraphy is whether the sequences are global in extent and whether the sea level changes are eustatic in nature. Study on the Upper Devonian and Lower Carboniferous depositional sequences in the different facies belts on the southern margin of the Upper Yangtze Platform shows that different orders of sequences possess of distinctive distribution and correlation in space, although the sea level changes in the various orders of sequences may be primarily global in nature. The present paper regards the distribution in space, especially in different facies belts, as a criterion for defining and recognizing the various orders of sequences. The orthosequence (third-order sequence) is probably global in nature, which may be discerned in various depositional facies belts on the continental margin and can be correlated over long distances, sometimes may be worldwide. Commonly, correlation of the subsequences (fourth-order sequences with time interval of 0.5 to 1.5 Ma) is difficult in different facies belts, although some of them are probably also worldwide in distribution. A subsequence should be distinguishable and correlatable within at least one facies belt. The number of subsequences in one orthosequence varies in different facies belts.Only the number of subsequences recognized in shallow marine facies represents the correct subdivision of subsequences within one orthosequence.The higher-order sequences.including parasequences (fifth-order sequences, i. e. subsequence of Wang and Shi) and microsequences (sixth-order sequences), are regional or local in distribution. Generally their correlation are difficult even within a single facies belt.Parasepuences and microseunces may have reflected the longer and shorter Milankovitch cycles,respectively. Orthosequences and subsequences are usually recognizable in different facies belts, while parasequences and microsequences may be distinguished only in shallow marine deposits,but not in slope and basin facies deposits.The widely used meter-scale cyclothems probably include various orders of sequences and may represent parasequences (fifth-order sequences)and microsequences (sixth-sequences),even orthosequences,in different depositional facies belts.
1999, 17(3): 345-354.
Abstract:
On the basis of the synthetic studies, especially the identification of the key surfaces within the Ordovician sequences(SB,FFS or TS,mfs)and the representative facies-successions in different facies zones,including platform-interior,platform-margin,and shelf-basin,exposed at the adjacent region of NW Hubei and SW Hunan on the southeastern margin of the Upper Yangtze Platform in South China,eighteen depositional sequences and the corresponding nineteen regression events are recognized. According to the recent study of the Ordovician stratigraphy,the average time duration of the sequences are estimated to be from 2Ma to 5Ma generally.It is probable that all of the sequences are of 3rd-order sequences and sea-level fluctuations.The eighteen sequences may be subdivided into three groups, including the type Ⅰand Ⅱ(Vail & Mitchum 1977;Wilgus et al.1988)and type D(the drowning sequence)(Schlager 1981;Steinhauff &Walker 1995),based on the research of benthic faunal assemblages,depositional facies and diagenesis processes of the carbonate deposits,as well as the pattern analysis of the temporal and the spatial distributions of the sequences in different facies-zones during the Ordovician.Altogether,three type Ⅰ,thirteen type Ⅱ and two type D sequences have been distinguished respectively. It has been found that most of the nineteen Ordovician regression events and the eighteen sequences mentioned above can be correlated fairly well with those developed not only in Tarim, North China,but also in Australia,North America,South America and other continents according to the data nowadays.This suggests that most of the Ordovician 3rd-order sequences and higher level cycles(the 2nd-order)are eustatic in nature,although some of them probably influenced somewhat by local and regional tectonic activity.Consequently,this reveals again that both the inner-earth factor and the orbital control of the solar system and other cosmic factors may have played an essential role in the changes of the sea-level and the deposition of the sequences. As the sequence stratigraphic framework and the physical patterns of the sequence and the systems tracts are predictable,the synthesis would be helpful to many respects of the relevant studies.The preliminary study of the Ordovician sequence stratigraphy, sea-level changes on the Yangtze Platform of China has provided a kind of synthesis in the recognition and the correlation of the 3rd-order sequences as“an integrated approach”(Wilgus et al 1988),and a link between traditional stratigraphy and the establishment of the global chronostratigraphic scale.
On the basis of the synthetic studies, especially the identification of the key surfaces within the Ordovician sequences(SB,FFS or TS,mfs)and the representative facies-successions in different facies zones,including platform-interior,platform-margin,and shelf-basin,exposed at the adjacent region of NW Hubei and SW Hunan on the southeastern margin of the Upper Yangtze Platform in South China,eighteen depositional sequences and the corresponding nineteen regression events are recognized. According to the recent study of the Ordovician stratigraphy,the average time duration of the sequences are estimated to be from 2Ma to 5Ma generally.It is probable that all of the sequences are of 3rd-order sequences and sea-level fluctuations.The eighteen sequences may be subdivided into three groups, including the type Ⅰand Ⅱ(Vail & Mitchum 1977;Wilgus et al.1988)and type D(the drowning sequence)(Schlager 1981;Steinhauff &Walker 1995),based on the research of benthic faunal assemblages,depositional facies and diagenesis processes of the carbonate deposits,as well as the pattern analysis of the temporal and the spatial distributions of the sequences in different facies-zones during the Ordovician.Altogether,three type Ⅰ,thirteen type Ⅱ and two type D sequences have been distinguished respectively. It has been found that most of the nineteen Ordovician regression events and the eighteen sequences mentioned above can be correlated fairly well with those developed not only in Tarim, North China,but also in Australia,North America,South America and other continents according to the data nowadays.This suggests that most of the Ordovician 3rd-order sequences and higher level cycles(the 2nd-order)are eustatic in nature,although some of them probably influenced somewhat by local and regional tectonic activity.Consequently,this reveals again that both the inner-earth factor and the orbital control of the solar system and other cosmic factors may have played an essential role in the changes of the sea-level and the deposition of the sequences. As the sequence stratigraphic framework and the physical patterns of the sequence and the systems tracts are predictable,the synthesis would be helpful to many respects of the relevant studies.The preliminary study of the Ordovician sequence stratigraphy, sea-level changes on the Yangtze Platform of China has provided a kind of synthesis in the recognition and the correlation of the 3rd-order sequences as“an integrated approach”(Wilgus et al 1988),and a link between traditional stratigraphy and the establishment of the global chronostratigraphic scale.
1999, 17(3): 361-366.
Abstract:
Both the Skolithos ichnofacies and the Nereites ichnofacies have been recognized in the Permian-Triassic of the eastern Kunlun orogenic belt.In the case of the strong deformation,metamorphism and position variation in an orogenic belt,it is difficult to find the body fossils and the physical depositional structures .But the identification of the environment is very important to the study of the structures and the evolution of the orogenic belt, so we must find another efficient way .Through two years of hard works on the trace fossils in eastern Kunlun orogenic belt,we identified Skolithos,Palaeophycos, Gordia,Baugouria, Thalassinoides,Rhizocorallium and so on in the lower part of Hongshuichuan Formation and merged these trace fossils into Skolithos ichnofacies.In many flgsch slices and limestones slices, we found many deep-sea trace fossils,such as Palaeodictyon,Protopalaeodictyon,Helminthoida and Chondrites, which belongs to Nereites ichnofacies . Since professor Kenneth J.Hsii made inaugyrated work in San Franciscomelange,many geologists have tried to recover the original tempoal sequences,position sequences and lithofacies sequences of the orogenic stratigraphy,But body fossils are rare in the orogenic stratigraphy,tgpical epositional structares wrer rebuilt by tectonic and metamorphosed actirities,losing their initial faces.Though two-years field work in eastern orogenic belt,we found that trace fossils can determine the sedimentary environmedt of some orogenic slices-the melange,bing significant to the recovering of lithofaties sequences of ovogenic stratigraphy. Furthermore,using trace fossils,we also can contrast stratigraphy in orogenic belt.We fourd an important feature in the lower part of Hong shuichuar Formation that the “skolithos” is very abundant.On the base of field investigation,we ascribe the stratum uhich have abundant “skolithos” and familiar lithology to the lower part of Hong shuichuar Formation. Studies of trace fossils can play a significant role in a sedimentary environmental analysis,stratigraphical correlation and identification of tectonic slices in an orogenic belt.
Both the Skolithos ichnofacies and the Nereites ichnofacies have been recognized in the Permian-Triassic of the eastern Kunlun orogenic belt.In the case of the strong deformation,metamorphism and position variation in an orogenic belt,it is difficult to find the body fossils and the physical depositional structures .But the identification of the environment is very important to the study of the structures and the evolution of the orogenic belt, so we must find another efficient way .Through two years of hard works on the trace fossils in eastern Kunlun orogenic belt,we identified Skolithos,Palaeophycos, Gordia,Baugouria, Thalassinoides,Rhizocorallium and so on in the lower part of Hongshuichuan Formation and merged these trace fossils into Skolithos ichnofacies.In many flgsch slices and limestones slices, we found many deep-sea trace fossils,such as Palaeodictyon,Protopalaeodictyon,Helminthoida and Chondrites, which belongs to Nereites ichnofacies . Since professor Kenneth J.Hsii made inaugyrated work in San Franciscomelange,many geologists have tried to recover the original tempoal sequences,position sequences and lithofacies sequences of the orogenic stratigraphy,But body fossils are rare in the orogenic stratigraphy,tgpical epositional structares wrer rebuilt by tectonic and metamorphosed actirities,losing their initial faces.Though two-years field work in eastern orogenic belt,we found that trace fossils can determine the sedimentary environmedt of some orogenic slices-the melange,bing significant to the recovering of lithofaties sequences of ovogenic stratigraphy. Furthermore,using trace fossils,we also can contrast stratigraphy in orogenic belt.We fourd an important feature in the lower part of Hong shuichuar Formation that the “skolithos” is very abundant.On the base of field investigation,we ascribe the stratum uhich have abundant “skolithos” and familiar lithology to the lower part of Hong shuichuar Formation. Studies of trace fossils can play a significant role in a sedimentary environmental analysis,stratigraphical correlation and identification of tectonic slices in an orogenic belt.
1999, 17(3): 374-382.
Abstract:
Having patch reefs,mud mounds and biostromes,the reef types are various in Silurian strata on the north & south margin of Sichuan basin.Owing to the simple regional tectonic movement,the diagenesis, process of the reef carbonate was “progressive diagenesis”which show that the process was from syngenesis,short time fresh water influence get have strongly influence on the reef limestones.Among them,the cementation,corroasion,silicification and dolomitization are the most important.During the burial diagenesis period,the pore spaces in the reefs were filled by calcite cements with multi-generations.The reservoir capacity of the reefs decreased because of the weak epidiagensis influence.The organisms skeletons have the strong influence and reflection to the diagenesis.The original pores and fractures of the reef limestones are mainly skeleton pores and body cavity with some fenestrae in the mudstones and thrombolites.Some intergranular pores and inter-spar crystal pores are in the grainstone.The secondary pores and fractures are epidiagenetically dissoved and corroded cavities and fractures well as structural fractures.According to the clay mineral thermal evolution and inclusions analysis.the period of primary migration of hydrocarbon closed to the burial diagenetic period of the reefs,which was about in the Early Triassic.There were at least two times hydrocarbon migration traces in the reefs,primary migration happened in Early Triassic and secondary migration was about the period of Himalayan orogeny.Because of the strong cementation,the pore spaces in the reef reservoirs derased.
Having patch reefs,mud mounds and biostromes,the reef types are various in Silurian strata on the north & south margin of Sichuan basin.Owing to the simple regional tectonic movement,the diagenesis, process of the reef carbonate was “progressive diagenesis”which show that the process was from syngenesis,short time fresh water influence get have strongly influence on the reef limestones.Among them,the cementation,corroasion,silicification and dolomitization are the most important.During the burial diagenesis period,the pore spaces in the reefs were filled by calcite cements with multi-generations.The reservoir capacity of the reefs decreased because of the weak epidiagensis influence.The organisms skeletons have the strong influence and reflection to the diagenesis.The original pores and fractures of the reef limestones are mainly skeleton pores and body cavity with some fenestrae in the mudstones and thrombolites.Some intergranular pores and inter-spar crystal pores are in the grainstone.The secondary pores and fractures are epidiagenetically dissoved and corroded cavities and fractures well as structural fractures.According to the clay mineral thermal evolution and inclusions analysis.the period of primary migration of hydrocarbon closed to the burial diagenetic period of the reefs,which was about in the Early Triassic.There were at least two times hydrocarbon migration traces in the reefs,primary migration happened in Early Triassic and secondary migration was about the period of Himalayan orogeny.Because of the strong cementation,the pore spaces in the reef reservoirs derased.
1999, 17(3): 390-396.
Abstract:
Based on the theory of sequence stratigraphy, the sequence stratigraphic characteristics of Ordovician weathering residuals in central Ordos Basin were studied, and the high resolution sequence stratigraphic framework for the Mawu-l submember was established, in which 4 parasequences, i.e., Mawu1-1, Mawu1-2, Mawu1-3 and Mawu1-4 were identified. Generally, the parasequence sets of Mawu-l submember formed at the descent stage of sea-level, when the frequent change of sea-level resulted in multiple marines flooding surfaces and transgressive surfaces, and the deposits mainly consist of abnormal salinity possessing gypsum flat rocks. A parasequence may include one or more weathering-diagenesis cycles resulted from the change of sea-level. The top erosion surface of a parasequence corresponds to the maximum descent stage of sea-level. The relationship pattern between sea-level variation and weathing-diagenesis (karstification) was set up in this paper. The processes and controlling factors for inter-layer karstification and the weathering-diagenesis (karstification) at the later stage were presented from the genesis point of view. It was revealed that in carbonate high-resolution sequence stratigraphic framework, the sea level varying periods of different hierarchies controlled the sedimentary stratigraphy and weathering-diagenesis (karstification) of different hierarchies. Thus the multi-stage and multi-cycle characteristics of carbonate weathering residuals were determined. Therefore, the establishment of weathering-diagenesis pattern for parasequeces of Mawu1 submember is useful for further understanding of distribution pattern of weathering residuals, and for the effective prediction of weathering-crust reservoirs. The relationship between parasequences and reservoir characteristics were also analyzed. It was declared that reservoir characteristics of prapasequences 1 to 4 are integrated reflection of sedimentation and diagenesis. The influences of parasequence boundaries on the weathering-crust reservoir of Mawu-1 were also emphasized. These boundaries not only caused component and structure variations of rocks, but also provided materials for inter-layer karstification, since inter-layer karstification generally occurred at the top of parasequences, which exposed when the sea-level descended, and thereafter karstification took place. The vertical distribution of vugs caused by inter-layer karstification has obvious multi-cycles as primitive gypsum dotted sedimentation, while the horizontal distribution is controlled by sedimentary facies. In the study area, the accumulation capacity of parasequences 1 and 2 was influenced by karstific erosion at the later stage. Vugs in parasequence 3 are main hydrocarbon bearing spaces in the central gas field. Vugs in parasequence 4 are not main hydrocarbon bearing spaces for the limitation of karstification. Therefore, studies of sequence boundaries of different hierarchies are useful for the prediction of vertical and horizontal distributions of oil-gas reservoirs.
Based on the theory of sequence stratigraphy, the sequence stratigraphic characteristics of Ordovician weathering residuals in central Ordos Basin were studied, and the high resolution sequence stratigraphic framework for the Mawu-l submember was established, in which 4 parasequences, i.e., Mawu1-1, Mawu1-2, Mawu1-3 and Mawu1-4 were identified. Generally, the parasequence sets of Mawu-l submember formed at the descent stage of sea-level, when the frequent change of sea-level resulted in multiple marines flooding surfaces and transgressive surfaces, and the deposits mainly consist of abnormal salinity possessing gypsum flat rocks. A parasequence may include one or more weathering-diagenesis cycles resulted from the change of sea-level. The top erosion surface of a parasequence corresponds to the maximum descent stage of sea-level. The relationship pattern between sea-level variation and weathing-diagenesis (karstification) was set up in this paper. The processes and controlling factors for inter-layer karstification and the weathering-diagenesis (karstification) at the later stage were presented from the genesis point of view. It was revealed that in carbonate high-resolution sequence stratigraphic framework, the sea level varying periods of different hierarchies controlled the sedimentary stratigraphy and weathering-diagenesis (karstification) of different hierarchies. Thus the multi-stage and multi-cycle characteristics of carbonate weathering residuals were determined. Therefore, the establishment of weathering-diagenesis pattern for parasequeces of Mawu1 submember is useful for further understanding of distribution pattern of weathering residuals, and for the effective prediction of weathering-crust reservoirs. The relationship between parasequences and reservoir characteristics were also analyzed. It was declared that reservoir characteristics of prapasequences 1 to 4 are integrated reflection of sedimentation and diagenesis. The influences of parasequence boundaries on the weathering-crust reservoir of Mawu-1 were also emphasized. These boundaries not only caused component and structure variations of rocks, but also provided materials for inter-layer karstification, since inter-layer karstification generally occurred at the top of parasequences, which exposed when the sea-level descended, and thereafter karstification took place. The vertical distribution of vugs caused by inter-layer karstification has obvious multi-cycles as primitive gypsum dotted sedimentation, while the horizontal distribution is controlled by sedimentary facies. In the study area, the accumulation capacity of parasequences 1 and 2 was influenced by karstific erosion at the later stage. Vugs in parasequence 3 are main hydrocarbon bearing spaces in the central gas field. Vugs in parasequence 4 are not main hydrocarbon bearing spaces for the limitation of karstification. Therefore, studies of sequence boundaries of different hierarchies are useful for the prediction of vertical and horizontal distributions of oil-gas reservoirs.
1999, 17(3): 403-408.
Abstract:
Tidal flat,delta and lake deposition of Permo-Carboniferous are well-developed in the Ordos basin with an evolution sequence changed gradually from marine to continental facies upwards.In Carboniferous,the transgression was caused by North China Sea and Qilian sea in which the former was of the largest range. The Lower part of Benxi Fm.is tidal flat facies with very large distribution range but the Sandbody in it is relatively small.The Upper part of Benxi Fm.is the mixed deposition of turbidity and fresh water in epicontinental sea,in which formed some small bars and the delta front only distributes in the northeast of the basin.Most of elongated sandbodies from well S201 of the north to well Q1 of the south extend along NNE.Transgression range of the early period of Taiyuan Fm.in late Carboniferous is larger than that of Benxi Fm. At that time,under the influence of weak tectonic movement,subdued sea-floor,weak hydrodynamism and less sediment supply,the study area mainly developed tidal flat and delta deposits.During late period of Taiyuan Fm.,the basin mainly developed tidal flat except the northeastern part of the basin where the submerged distrbutary channel of delta front prevailed .In Permian,Qinling trough began to contract again,and all north China Platform uplifted resylting in the rapid regression of sea water from east and west part of the Drdos basin.The great change of the palaeotopography and palaeoclimate led to the much wider deposition range than that of the Carboniferous. Delta plain was widespreed in the basin in the early period of Shanxi Fm.of Lower Permian.In depositional area,water became shallow and wide and the thickness of the deposits became thin.As a result of the palaeotopography,the shape of delta composed of several stacked sandbodies showed digitated,implying the obvious differential subsidence and sedimentary types between the north and south of the study area.In the north,Delta plain was well-developed,and the delta frout prevailed in the south where the submerged distributary channel from Jingbian to Ansai is the largest sandbody which is also the major reservior in the study area.In the period of Shihezi Fm.,the climate changed from warm humid to dry and hot resulting in the rapid decrease of the plant,and a set of continental clastic deposits were formed.Along with the continuous rising of the prevenance area,the paleocurrent direction was always to the south.Delta plain composed of distributary channel and flood plain was formed in the north Shihezi Fm.and digitate delta front developed in the south where riversf flowed into the lake.Widespread lacustrine mudstone formed in the Upper Shihezi Fm.and Shiqianfeng Fm. of Upper Permian became the best regional caprocks in the study area. In Permo-Carboniferou,the distributary channel sanbodies of deltaplain and submerged distributary channel sandbodies of delta front are the best reservoirs,and the tidal flat sandbody and fingered sandbody of delta front are also favorable are for exploration.
Tidal flat,delta and lake deposition of Permo-Carboniferous are well-developed in the Ordos basin with an evolution sequence changed gradually from marine to continental facies upwards.In Carboniferous,the transgression was caused by North China Sea and Qilian sea in which the former was of the largest range. The Lower part of Benxi Fm.is tidal flat facies with very large distribution range but the Sandbody in it is relatively small.The Upper part of Benxi Fm.is the mixed deposition of turbidity and fresh water in epicontinental sea,in which formed some small bars and the delta front only distributes in the northeast of the basin.Most of elongated sandbodies from well S201 of the north to well Q1 of the south extend along NNE.Transgression range of the early period of Taiyuan Fm.in late Carboniferous is larger than that of Benxi Fm. At that time,under the influence of weak tectonic movement,subdued sea-floor,weak hydrodynamism and less sediment supply,the study area mainly developed tidal flat and delta deposits.During late period of Taiyuan Fm.,the basin mainly developed tidal flat except the northeastern part of the basin where the submerged distrbutary channel of delta front prevailed .In Permian,Qinling trough began to contract again,and all north China Platform uplifted resylting in the rapid regression of sea water from east and west part of the Drdos basin.The great change of the palaeotopography and palaeoclimate led to the much wider deposition range than that of the Carboniferous. Delta plain was widespreed in the basin in the early period of Shanxi Fm.of Lower Permian.In depositional area,water became shallow and wide and the thickness of the deposits became thin.As a result of the palaeotopography,the shape of delta composed of several stacked sandbodies showed digitated,implying the obvious differential subsidence and sedimentary types between the north and south of the study area.In the north,Delta plain was well-developed,and the delta frout prevailed in the south where the submerged distributary channel from Jingbian to Ansai is the largest sandbody which is also the major reservior in the study area.In the period of Shihezi Fm.,the climate changed from warm humid to dry and hot resulting in the rapid decrease of the plant,and a set of continental clastic deposits were formed.Along with the continuous rising of the prevenance area,the paleocurrent direction was always to the south.Delta plain composed of distributary channel and flood plain was formed in the north Shihezi Fm.and digitate delta front developed in the south where riversf flowed into the lake.Widespread lacustrine mudstone formed in the Upper Shihezi Fm.and Shiqianfeng Fm. of Upper Permian became the best regional caprocks in the study area. In Permo-Carboniferou,the distributary channel sanbodies of deltaplain and submerged distributary channel sandbodies of delta front are the best reservoirs,and the tidal flat sandbody and fingered sandbody of delta front are also favorable are for exploration.
1999, 17(3): 414-421.
Abstract:
The Sinian—Middle Devonian System in the Tarim Basin can be divided into two I order sequences (megasequences), six II order sequences (supersequences) and seventeen III order sequences (sequences). The geotectonic evolution is characterized by the splitting and closing cycle of the Paleo-Xinjiang cratonic plate from the Sinian era to Middle Devonian period. The evolution of Tarim Basin underwent the craton marginal rift basin stage (Z-O) and the retro arc foreland basin stage (S-D2). The Megasequence I was composed of sediments of the craton marginal rift basin stage, of which the SupersequenceⅠA was the filling product of the continental rift basin filling during the Sinian era, the SupersequenceⅠB and the SupersequenceⅠC were the fills of the passive continental marginal depressional basin from the Cambrian era to the early Ordovician period, and the SupersequenceⅠD was the fills of the retro arc tensional basin in the middle-late Ordovician period. The MegasequenceⅡ standed for the sedimentation of the retro arc foreland basin stage, of which the SupersequenceⅡA in the East Tarim was the fills of retro arc foreland basin in the Silurian era and the SupersequenceⅡB was the filling product of the peripheral foreland basin in the early and middle Devonian period. It was proved by the study of the sequence stratigraphy of the Sinian～Middle Devonian system in the Tarim Basin that the tectonism controlled the formation of most of the 3-orders sequences. The eustasy finally played an important role in the formation of most of the sequences during the stable passive continental marginal depressional basin and intra-craton depressional basin evolution stage.
The Sinian—Middle Devonian System in the Tarim Basin can be divided into two I order sequences (megasequences), six II order sequences (supersequences) and seventeen III order sequences (sequences). The geotectonic evolution is characterized by the splitting and closing cycle of the Paleo-Xinjiang cratonic plate from the Sinian era to Middle Devonian period. The evolution of Tarim Basin underwent the craton marginal rift basin stage (Z-O) and the retro arc foreland basin stage (S-D2). The Megasequence I was composed of sediments of the craton marginal rift basin stage, of which the SupersequenceⅠA was the filling product of the continental rift basin filling during the Sinian era, the SupersequenceⅠB and the SupersequenceⅠC were the fills of the passive continental marginal depressional basin from the Cambrian era to the early Ordovician period, and the SupersequenceⅠD was the fills of the retro arc tensional basin in the middle-late Ordovician period. The MegasequenceⅡ standed for the sedimentation of the retro arc foreland basin stage, of which the SupersequenceⅡA in the East Tarim was the fills of retro arc foreland basin in the Silurian era and the SupersequenceⅡB was the filling product of the peripheral foreland basin in the early and middle Devonian period. It was proved by the study of the sequence stratigraphy of the Sinian～Middle Devonian system in the Tarim Basin that the tectonism controlled the formation of most of the 3-orders sequences. The eustasy finally played an important role in the formation of most of the sequences during the stable passive continental marginal depressional basin and intra-craton depressional basin evolution stage.
1999, 17(3): 430-434.
Abstract:
Kuche Depression,situated in the northern part of Tarim Basin, is a "dustpan”shaped depression with steep slope in north side and gentle slope in the south side in Mesozoic Era.There is a set of hugely thick,alluvial—lake classic deposition system in it.Three types of deltas and they are recognized:fan-deltas,braided-river deltas and meandering-river deltas and they are very abundant in the lake margin.Their characters are distinct and distinctions are clear:(1)fan-deltas,formed where an alluvial fan is deposited directly into lake from an adjacent highland, are alternatively deposited by gravity flows sediments,formed by gusty、momentary and catastrophic event,and tractive current sediments,formed between catastrophic events,gravity current sediments are dominant in fan-deltas.Delta plain subfacies are similar to alluvial fan facies,and channel bars take the form of lentiform and their thickness is small and various.(2)braided-river deltas,formed where a braided fluvial system progrades into a lake,are deposited by river tractive current sediments,controlled by torrential flood in general,and are seasonal sediments,of which the delta plain subfacies are similar to braided river facies,channel bars are abundant,overall,sand bodies take the form of layered bodies,but internally,they are composed of many finer-upward lenticular sand bodies that pile up each other,cross-beddings are abundant,and especially,exceedingly rich lateral accretion cross-beddings are the distinct character of braided-river deltas.The rock types of this kind delta are mostly grain-supported sandstones and conglomerates.(3)meandering-river deltas,formed where a meandering fluvial system progrades into a lake,are deposited by river tractive current sediments that are correspondingly continuous river production,of which the delta plain subfacies are similar to meandering river facies,and channel bars take the form of layered bodies,with abundant cross seddings.Certainly,there is close inherent relationship in three types of deltas,not only coexist at the same term,but also transform along with geologic development.
Kuche Depression,situated in the northern part of Tarim Basin, is a "dustpan”shaped depression with steep slope in north side and gentle slope in the south side in Mesozoic Era.There is a set of hugely thick,alluvial—lake classic deposition system in it.Three types of deltas and they are recognized:fan-deltas,braided-river deltas and meandering-river deltas and they are very abundant in the lake margin.Their characters are distinct and distinctions are clear:(1)fan-deltas,formed where an alluvial fan is deposited directly into lake from an adjacent highland, are alternatively deposited by gravity flows sediments,formed by gusty、momentary and catastrophic event,and tractive current sediments,formed between catastrophic events,gravity current sediments are dominant in fan-deltas.Delta plain subfacies are similar to alluvial fan facies,and channel bars take the form of lentiform and their thickness is small and various.(2)braided-river deltas,formed where a braided fluvial system progrades into a lake,are deposited by river tractive current sediments,controlled by torrential flood in general,and are seasonal sediments,of which the delta plain subfacies are similar to braided river facies,channel bars are abundant,overall,sand bodies take the form of layered bodies,but internally,they are composed of many finer-upward lenticular sand bodies that pile up each other,cross-beddings are abundant,and especially,exceedingly rich lateral accretion cross-beddings are the distinct character of braided-river deltas.The rock types of this kind delta are mostly grain-supported sandstones and conglomerates.(3)meandering-river deltas,formed where a meandering fluvial system progrades into a lake,are deposited by river tractive current sediments that are correspondingly continuous river production,of which the delta plain subfacies are similar to meandering river facies,and channel bars take the form of layered bodies,with abundant cross seddings.Certainly,there is close inherent relationship in three types of deltas,not only coexist at the same term,but also transform along with geologic development.
1999, 17(3): 443-448.
Abstract:
he braided river delta plain and front are well developed in Sanjianfang Formation reservoir,in which the main microfacies include braided river distributary channel,channel bar,underwater distributary channel and zones between distributary channels.The sedimentary microfacies in Sanjianfang reservoir and its effect on waterflooding result are studied through determination of sedimentary,combination of dynamic and static information as well as observation of production test.Based on the analysis of lithological,electrical and corresponding physical characters,the relationship between lithology and electricity is firstly set up,then according to the depositional cycle developped by sedimentation of each distributary river from formation to development and abandonment,the layer correlation is made by each well in matrix profiles.On the basis of what mentioned above,the top and bottom are as well as distribution range of every unit, i.e.,sandstone group,sandstone layer and substratum are determined.Finally,Sanjianfang Formation is subdivided into five sandstone groups,eighteen sandstone layers and twenty-four substrata. Controlled by the depositional microfacies,the movement of water—oil and the variation of production are primarily affected by the horizontal distribution of sandbodies.The relatively good physical properties,connection and high water-intake capacity results in high velocity of injecting water in distributary channels and quick increase of water in production wells.For this reason the high-yield wells produces no water at the beginning and much water at last. In vertical profiles,controlled by the sedimentary cycles,the highly produced layers are mainly the third and the forth substrata located in the middle-low part of every sandstone group.On the whole,affected by the plane and vertical heterogeneity,the injected water tends to breakthough in one direction horizontally and one layer vertically which further increases contradivtion between intralayer and interlayer during waterflooding.
he braided river delta plain and front are well developed in Sanjianfang Formation reservoir,in which the main microfacies include braided river distributary channel,channel bar,underwater distributary channel and zones between distributary channels.The sedimentary microfacies in Sanjianfang reservoir and its effect on waterflooding result are studied through determination of sedimentary,combination of dynamic and static information as well as observation of production test.Based on the analysis of lithological,electrical and corresponding physical characters,the relationship between lithology and electricity is firstly set up,then according to the depositional cycle developped by sedimentation of each distributary river from formation to development and abandonment,the layer correlation is made by each well in matrix profiles.On the basis of what mentioned above,the top and bottom are as well as distribution range of every unit, i.e.,sandstone group,sandstone layer and substratum are determined.Finally,Sanjianfang Formation is subdivided into five sandstone groups,eighteen sandstone layers and twenty-four substrata. Controlled by the depositional microfacies,the movement of water—oil and the variation of production are primarily affected by the horizontal distribution of sandbodies.The relatively good physical properties,connection and high water-intake capacity results in high velocity of injecting water in distributary channels and quick increase of water in production wells.For this reason the high-yield wells produces no water at the beginning and much water at last. In vertical profiles,controlled by the sedimentary cycles,the highly produced layers are mainly the third and the forth substrata located in the middle-low part of every sandstone group.On the whole,affected by the plane and vertical heterogeneity,the injected water tends to breakthough in one direction horizontally and one layer vertically which further increases contradivtion between intralayer and interlayer during waterflooding.
1999, 17(3): 454-457.
Abstract:
It's important to exactly determine the lower limit of petrophysical parameters for reservoir in calculation of reserves.Laboratory test is commonly used to get the lower limit values.However,the test is complicated and takes a long time.As for intensively anistropic reservoir,a large amount of data should be obtained which means much more time and costs.Therefore an easy,fast and economic method is necessary to be provided instead of traditional test.Distribution function is one of the most important mathematic characteristics of geological body.Distribution curve is often used to analyse the distribution regularity of the studied object.The distribution function curve method presented in this paper is a new statistic method used to determine the lower limit of petrophysical parameters for reservoir.Based on making full use of data from well logging interpretation available in oil-gas field,the disribution curves for effective and non-effective gas-bearing reservoirs are drawn in the same coordinate system respectively.Theoretically,if there exists a point of intersection for the two curves,the corresponding frequency must be zero.Actually,there is a superimposed region because of the mixed effects for the effective and non-effective gas-bearing reservoir.Under the condition of no seperation of mixed effects,the value corresponding to the intersection point for the two curves is taken as the lower limit.This method is applied to the determination of lower limit for the gas-bearing resevoir of Zhongbu Gas Field in Ordos Basin.The results coincides well with the values obtained by methods of accumulative production capacity,experience statistics and laboratory test,which demonstrates that this method is successful and valuable to be extended.
It's important to exactly determine the lower limit of petrophysical parameters for reservoir in calculation of reserves.Laboratory test is commonly used to get the lower limit values.However,the test is complicated and takes a long time.As for intensively anistropic reservoir,a large amount of data should be obtained which means much more time and costs.Therefore an easy,fast and economic method is necessary to be provided instead of traditional test.Distribution function is one of the most important mathematic characteristics of geological body.Distribution curve is often used to analyse the distribution regularity of the studied object.The distribution function curve method presented in this paper is a new statistic method used to determine the lower limit of petrophysical parameters for reservoir.Based on making full use of data from well logging interpretation available in oil-gas field,the disribution curves for effective and non-effective gas-bearing reservoirs are drawn in the same coordinate system respectively.Theoretically,if there exists a point of intersection for the two curves,the corresponding frequency must be zero.Actually,there is a superimposed region because of the mixed effects for the effective and non-effective gas-bearing reservoir.Under the condition of no seperation of mixed effects,the value corresponding to the intersection point for the two curves is taken as the lower limit.This method is applied to the determination of lower limit for the gas-bearing resevoir of Zhongbu Gas Field in Ordos Basin.The results coincides well with the values obtained by methods of accumulative production capacity,experience statistics and laboratory test,which demonstrates that this method is successful and valuable to be extended.
1999, 17(3): 464-472.
Abstract:
Tow sediment wave fields are developed in a depth zone of about 1 000-1 100m in the north-eastern Rockall Trough.Their wavelengths vary from 1 to 2 km and waveheights from 18 to 20 m,covering area of 350km2 and 20km2 respectively.Sedment waves migrate upslope and include generally an upslope climbing,asymmetry upslope and sinusoidal shape in stratigraphic section.The conclusions proposed by former scholars that sediment waves of large area were formed by the overflow moving from the Norwegian Ses and the Lee-Wave modle has caused the upslope migration of the sediment waves of small area have many problems.Considering the paleoclimate and paleocirculation,internal waves coul be generated in study area while every unit of the sediment waves was being formed.The formation of internal waves is related to contour currents closely.Resonnable hydrolic mechanism of sediment waves formation can be obtained accord-ing to the theory of internal wave.Generated under different paleoclimatic and paleocirculative conditions,the combining of two internal waves propagating toward the same direction,unique internal wave and internal stand wave which inter-sected sea floor successively would form a general basal climbing,intermidiate asymmetric and upper sinusoidal shape in stratigraphic section.
Tow sediment wave fields are developed in a depth zone of about 1 000-1 100m in the north-eastern Rockall Trough.Their wavelengths vary from 1 to 2 km and waveheights from 18 to 20 m,covering area of 350km2 and 20km2 respectively.Sedment waves migrate upslope and include generally an upslope climbing,asymmetry upslope and sinusoidal shape in stratigraphic section.The conclusions proposed by former scholars that sediment waves of large area were formed by the overflow moving from the Norwegian Ses and the Lee-Wave modle has caused the upslope migration of the sediment waves of small area have many problems.Considering the paleoclimate and paleocirculation,internal waves coul be generated in study area while every unit of the sediment waves was being formed.The formation of internal waves is related to contour currents closely.Resonnable hydrolic mechanism of sediment waves formation can be obtained accord-ing to the theory of internal wave.Generated under different paleoclimatic and paleocirculative conditions,the combining of two internal waves propagating toward the same direction,unique internal wave and internal stand wave which inter-sected sea floor successively would form a general basal climbing,intermidiate asymmetric and upper sinusoidal shape in stratigraphic section.
1999, 17(3): 482-485.
Abstract:
Core analysis of Ordovician carbonate rocks from a well in Tarim basin in Northwest China showed that the carbonate source rock consisted of three main components,namely pure carbonate matrix,argillaceous carbonate band and stylolite seam.From organic carbon analysis,it can be seen that the organic carbon contents in argillaceous carbonate band and stylolite seam were very high,with the average of 1.14% and 1.92% respectively and that in pure carbonate matrix was very low,with the mean value of 0.098%.Whereas,the distribution of S1／TOC value from Rock Eval analysis was completely different,with highest value in pure carbonate samples and much lower values in argillaceous carbonate band and stylolite seam samples.But the three kinds of samples from the same depth in the well did not show much differences in S2／TOC and Tmax values,indicating that the kerogen contained in these three kind samples from the same depth had the same thermal maturity and the same hydrocarbon potential.The significant differences in S1／TOC values among the three kinds samples were resulted from different efficiency of hydrocarbon expulsion in different kinds of carbonate components. The lack of mechanical compaction and the low hydrocarbon generation amount due to the low organic carbon content resulted in the lack of expulsion force in pure carbonate source rock,so the expulsion efficiency was very low and most generated hydrocarbons still remained in the source rock,which accounted for the high S1/TOC value in pure carbonate samples.Whereas,the powerful driving force for hydrocarbon expulsion in argillaceous carbonate band and stylolite seam,resulted from the intensive mechanical compaction owing to their high mud content and the high hydrocarbon generation amount due to the high organic carbon content,made the most generated hydrocarbons migrate out of argillaceous band and stylolite seam and caused the low values of S1/TOC. Stylolite seam and argillaceous band were the components of high organic carbon content in carbonate source rocks.The high organic carbon content in stylolite seam was the result of the accumulation of organic matter in the seam by pressure solution,and the high organic carbon content in argillaceous band was mostly from original sedimentation.In the thickness of 650m Ordovician carbonate rocks in the above well in Tarim basin,the total thickness of stylolite was 20m and that of argillaceous carbonate band was 130m. They were the most important contributors to the Ordovician carbonate source rock in Tarim basin.
Core analysis of Ordovician carbonate rocks from a well in Tarim basin in Northwest China showed that the carbonate source rock consisted of three main components,namely pure carbonate matrix,argillaceous carbonate band and stylolite seam.From organic carbon analysis,it can be seen that the organic carbon contents in argillaceous carbonate band and stylolite seam were very high,with the average of 1.14% and 1.92% respectively and that in pure carbonate matrix was very low,with the mean value of 0.098%.Whereas,the distribution of S1／TOC value from Rock Eval analysis was completely different,with highest value in pure carbonate samples and much lower values in argillaceous carbonate band and stylolite seam samples.But the three kinds of samples from the same depth in the well did not show much differences in S2／TOC and Tmax values,indicating that the kerogen contained in these three kind samples from the same depth had the same thermal maturity and the same hydrocarbon potential.The significant differences in S1／TOC values among the three kinds samples were resulted from different efficiency of hydrocarbon expulsion in different kinds of carbonate components. The lack of mechanical compaction and the low hydrocarbon generation amount due to the low organic carbon content resulted in the lack of expulsion force in pure carbonate source rock,so the expulsion efficiency was very low and most generated hydrocarbons still remained in the source rock,which accounted for the high S1/TOC value in pure carbonate samples.Whereas,the powerful driving force for hydrocarbon expulsion in argillaceous carbonate band and stylolite seam,resulted from the intensive mechanical compaction owing to their high mud content and the high hydrocarbon generation amount due to the high organic carbon content,made the most generated hydrocarbons migrate out of argillaceous band and stylolite seam and caused the low values of S1/TOC. Stylolite seam and argillaceous band were the components of high organic carbon content in carbonate source rocks.The high organic carbon content in stylolite seam was the result of the accumulation of organic matter in the seam by pressure solution,and the high organic carbon content in argillaceous band was mostly from original sedimentation.In the thickness of 650m Ordovician carbonate rocks in the above well in Tarim basin,the total thickness of stylolite was 20m and that of argillaceous carbonate band was 130m. They were the most important contributors to the Ordovician carbonate source rock in Tarim basin.
1999, 17(3): 493-498.
Abstract:
Natural gas hydrate as a potential natural fuel resource, which occur worldwide in the polar regions, mostly associated with onshore and offshore permafrost, and in sediment of out continental and insular margins will provide human enormous energy in the coming 21 century. Three aspects of gas hydrates are important: their fossil fuel resource potential, their role as a submarine geologic hazard and their effects on global climate change. Gas hydrates have become the focus of intense international interest over the past few years because of their abundance and their potential role as a fuel, which could mitigate global climate change. Natural gas hydrate future face to challenges.
Natural gas hydrate as a potential natural fuel resource, which occur worldwide in the polar regions, mostly associated with onshore and offshore permafrost, and in sediment of out continental and insular margins will provide human enormous energy in the coming 21 century. Three aspects of gas hydrates are important: their fossil fuel resource potential, their role as a submarine geologic hazard and their effects on global climate change. Gas hydrates have become the focus of intense international interest over the past few years because of their abundance and their potential role as a fuel, which could mitigate global climate change. Natural gas hydrate future face to challenges.
1999, 17(3): 339-344.
Abstract:
There are well exposed carbonate and terrigenous mixtures in the Lower-Middle Devonian in the northwestern Yangtse Craton,China.On the bases of measuring sections,investigating boundary surfaces,and correlating decade sections,hybrid depositional sequences are recognized,and mixings in systems tracts and tectonic significance are analyzed in the paper. Eight hybrid depositional sequences can be distinguished from the top Pragian to the early Givetian in the study area.They are nominated S4,S5,S6,S7,S8,S9,S10,S11 separately which can be classified into two hybrid depositional sequence sets SS2 (S4-S7),and SS3 (S8-S11).Sedimentation in TSTs,Little SMTs and HSTs of depositional sequences in SS2,is characterized by in-situ mixing and punctuated mixing.And source mixings could be only completed in the early TSTs of S7 and S8,but the facies mixing is predominated in early TSTs of most depositional sequences in SS3 and in the HST of S9.It is proposed that mixing of carbonate and terrigenous sediments can be done in anyone of LST/SMT,TST,and HST.This is slightly different from the propositions,among which someone suggested that a mixing could have been only undertaken in HST or in LST. The changes of systems tracts thickness,and relationships between mixing and tectonic are also discussed in the paper.Each of the depositional sequences in SS2 is characterized by the pattern of TST≥HST+SMT except for the S7,and vice versa,any of depositional sequences in SS3 has a texture of HST≥TST+SMT/LST.A relatively active tectonic subsidence was transferred to a stable tectonism at the SMT of S7 in SS2,and a pattern of relatively active to stable tectonism in each of S8 S9,S10,S11 is induced in SS3.That is to say,the patterns of transgression to regression (T-R) could be a way of continuous and slow transgression to relatively fast regression in SS2,and be the tendency of relatively fast transgression to slow regression in each depositional sequence of SS3. We have three questions on the intervals between large-scale boundary surfaces.①If an extensively tectonic activity has ever done at the gap between the pure terrigenous sediments (Pingyipu Formation) and the mixed carbonate and siliciclastic sediments.②If an exposing event existed that is displayed by ooidal hematite between the SS2 and SS3.③If a sea-level falling event with a tectonic uplift took place at the transition of the hybrids to carbonate.
There are well exposed carbonate and terrigenous mixtures in the Lower-Middle Devonian in the northwestern Yangtse Craton,China.On the bases of measuring sections,investigating boundary surfaces,and correlating decade sections,hybrid depositional sequences are recognized,and mixings in systems tracts and tectonic significance are analyzed in the paper. Eight hybrid depositional sequences can be distinguished from the top Pragian to the early Givetian in the study area.They are nominated S4,S5,S6,S7,S8,S9,S10,S11 separately which can be classified into two hybrid depositional sequence sets SS2 (S4-S7),and SS3 (S8-S11).Sedimentation in TSTs,Little SMTs and HSTs of depositional sequences in SS2,is characterized by in-situ mixing and punctuated mixing.And source mixings could be only completed in the early TSTs of S7 and S8,but the facies mixing is predominated in early TSTs of most depositional sequences in SS3 and in the HST of S9.It is proposed that mixing of carbonate and terrigenous sediments can be done in anyone of LST/SMT,TST,and HST.This is slightly different from the propositions,among which someone suggested that a mixing could have been only undertaken in HST or in LST. The changes of systems tracts thickness,and relationships between mixing and tectonic are also discussed in the paper.Each of the depositional sequences in SS2 is characterized by the pattern of TST≥HST+SMT except for the S7,and vice versa,any of depositional sequences in SS3 has a texture of HST≥TST+SMT/LST.A relatively active tectonic subsidence was transferred to a stable tectonism at the SMT of S7 in SS2,and a pattern of relatively active to stable tectonism in each of S8 S9,S10,S11 is induced in SS3.That is to say,the patterns of transgression to regression (T-R) could be a way of continuous and slow transgression to relatively fast regression in SS2,and be the tendency of relatively fast transgression to slow regression in each depositional sequence of SS3. We have three questions on the intervals between large-scale boundary surfaces.①If an extensively tectonic activity has ever done at the gap between the pure terrigenous sediments (Pingyipu Formation) and the mixed carbonate and siliciclastic sediments.②If an exposing event existed that is displayed by ooidal hematite between the SS2 and SS3.③If a sea-level falling event with a tectonic uplift took place at the transition of the hybrids to carbonate.
1999, 17(3): 355-360.
Abstract:
The integration between Lower Yangtze plate and South China plate took place at the end of Silurian.The integration brought about early to meso-Devonian stratigraphic hiatus.The lower Devonian Wutong Formation forms a connecting link between the preceding and the following sedimentation which recorded the tectonic evolution in this region.Our work on the sedimentary and geochemical characteristics of the Wutong Formation may plays an important role in studying the tectonic system transformation in this region. In this paper, the typical sedimentary characteristics of Wutong Formation, such as storm gravel bank, flushing cross-stratum, beach ridge, tricrest bed ripple, plant and trace fossil taphofacies characteristics, etc., has been recorded and summarized in detail. All these sedimentary characteristics show that the Wutong Formation is a set of littoral facies sedimentary formation and this sedimentary environment is far away from that of the underbedding lower Paleozoic. The main chemical composition and trace elements of mudstone have been quantitatively analyzed.Its main chemical compositions are characterized of high SiO2 and K2O/Na2O but low MgO. The great difference in chemical composition between Wutong Formation and the underlying lower Paleozoic formations suggest the tectonic system change at the beginning of early Devonian in Lower Yangtze region, and this conclusion is consistent with the result from the sedimentary characteristics and trace elements. In trace elements, Zr, Hf compositions distribute far away from that of upper Silurian in Zr-Hf graphs and shows to be proximal environment. B, Ga, Ba, Sr, Rb etc. and their relative ratio are characterized of littoral facies and paralic sedimentation. Summing up the sedimentary and geochemical characteristics and regional geology, the authors put forwards that the Wutong Formation is a set of littoral facies sedimentary formation that occurs in back-arc foreland basin, and this tectonic background continued to the end of early Carboniferous.
The integration between Lower Yangtze plate and South China plate took place at the end of Silurian.The integration brought about early to meso-Devonian stratigraphic hiatus.The lower Devonian Wutong Formation forms a connecting link between the preceding and the following sedimentation which recorded the tectonic evolution in this region.Our work on the sedimentary and geochemical characteristics of the Wutong Formation may plays an important role in studying the tectonic system transformation in this region. In this paper, the typical sedimentary characteristics of Wutong Formation, such as storm gravel bank, flushing cross-stratum, beach ridge, tricrest bed ripple, plant and trace fossil taphofacies characteristics, etc., has been recorded and summarized in detail. All these sedimentary characteristics show that the Wutong Formation is a set of littoral facies sedimentary formation and this sedimentary environment is far away from that of the underbedding lower Paleozoic. The main chemical composition and trace elements of mudstone have been quantitatively analyzed.Its main chemical compositions are characterized of high SiO2 and K2O/Na2O but low MgO. The great difference in chemical composition between Wutong Formation and the underlying lower Paleozoic formations suggest the tectonic system change at the beginning of early Devonian in Lower Yangtze region, and this conclusion is consistent with the result from the sedimentary characteristics and trace elements. In trace elements, Zr, Hf compositions distribute far away from that of upper Silurian in Zr-Hf graphs and shows to be proximal environment. B, Ga, Ba, Sr, Rb etc. and their relative ratio are characterized of littoral facies and paralic sedimentation. Summing up the sedimentary and geochemical characteristics and regional geology, the authors put forwards that the Wutong Formation is a set of littoral facies sedimentary formation that occurs in back-arc foreland basin, and this tectonic background continued to the end of early Carboniferous.
1999, 17(3): 367-373.
Abstract:
According to the relation between relative sea level changes rates and reef growth rates,Permian reef in southern China can be devided into three types:offlapping reef,aggradational reef and progradational reef.When relative sea level rising rate is higher than reef growth rate,offlapping reef configuration was pingroduced.When relative sea level rising rate is equivalent to reef growth rate,aggradational reef configuration was formed.There are three types of progradational reef configuration:1) During relative sea level rising rate is lower than reef growth rate;2) Stillstand of relative sea level;3) relative sea level fall.Permian reef in southern China distributed mainly over Nanpanjiang area and eastern Sichuan to western Hubei area.There are the Maokou Age progradational reef and the Changxing Age aggradational reef in Nanpanjiang area.There are both offlapping reef and progradational reef of the Changxing Age in eastern Sichuan to western Hubei area. There are typical models of diagenetic and oil-gas pool in different types of reef. Firstly, relative sea level changes control on reef heterogeneous reservoir rocks and pore rich accumulation zone. Distribution of the time and space of petroleum in reef were controlled by reef heterogeneity. It is very important to recognize the regular patterns of the time and space of reef pore rich accumulation zone for exploration of the reef oil and gas pool. Secondly,relative sea level changes control on mechanism and configurations of the elements of the reef oil and gas bearing system.Analysis of the reef oil and gas-bearing system could help us preliminarily evaluate the potential of reef oil and gas-bearing system before drilling.Thirdly,different types of the reef complexes have their own contributing factors and regular patterns of the time and space.Through analysis of the contributing factors and regular patterns of the time and space,we might predict buried reef more precisely.
According to the relation between relative sea level changes rates and reef growth rates,Permian reef in southern China can be devided into three types:offlapping reef,aggradational reef and progradational reef.When relative sea level rising rate is higher than reef growth rate,offlapping reef configuration was pingroduced.When relative sea level rising rate is equivalent to reef growth rate,aggradational reef configuration was formed.There are three types of progradational reef configuration:1) During relative sea level rising rate is lower than reef growth rate;2) Stillstand of relative sea level;3) relative sea level fall.Permian reef in southern China distributed mainly over Nanpanjiang area and eastern Sichuan to western Hubei area.There are the Maokou Age progradational reef and the Changxing Age aggradational reef in Nanpanjiang area.There are both offlapping reef and progradational reef of the Changxing Age in eastern Sichuan to western Hubei area. There are typical models of diagenetic and oil-gas pool in different types of reef. Firstly, relative sea level changes control on reef heterogeneous reservoir rocks and pore rich accumulation zone. Distribution of the time and space of petroleum in reef were controlled by reef heterogeneity. It is very important to recognize the regular patterns of the time and space of reef pore rich accumulation zone for exploration of the reef oil and gas pool. Secondly,relative sea level changes control on mechanism and configurations of the elements of the reef oil and gas bearing system.Analysis of the reef oil and gas-bearing system could help us preliminarily evaluate the potential of reef oil and gas-bearing system before drilling.Thirdly,different types of the reef complexes have their own contributing factors and regular patterns of the time and space.Through analysis of the contributing factors and regular patterns of the time and space,we might predict buried reef more precisely.
1999, 17(3): 383-389.
Abstract:
In the East Yunnan—West Sichuan area,dolostones are well developed and can be divided into two types:massive dolostones and patchy dolostones. Massive dolostones are light gray and gray,and mainly consist of fine or medium dolomites.The δ13C is +0.8‰～+3.5‰(PDB) and averages +2.7‰;δ18O is -6.8‰～-9.1‰(PDB) and averages -7.8‰.Sr is 25×10-6～52×10-6 and averages 39×10-6;Na is 52×10-6～74×10-6 and averages 64×10-6.Patchy dolostones are gray and consist of dolomite patches and calcite patches,with dolomite patches over 50%.The dolomite patches are several centimeters to over 20 cm in size and consist of fine to medium dolomite crystals.The dolomite crystals are euhedral or subhedral and are commonly 60%～90%.The δ13C is +3.1‰～+4.0‰ and averages +3.6‰;δ18O is -6.4‰～-8.1‰ and averages -7.8‰.Sr is 46×10-6～68×10-6 and averages 55×10-6;Na is 60×10-6～89×10-6 and averages 74×10-6. Various data show that massive dolostones and patchy dolostones are the same in origin and different only in dolomitization degree.These dolostones were formed under burial conditions,and the dolomitizing water was fresh meteoric water leaching basalts.The Mg2+ for dolomitization came from weathering and dissolution of basalts.Very thick basalts formed high mountains.which formed great relief of landform and large water head and made the meteoric water leaching basalts flow deep into underground.Such water resulted in large scale dolomitization of limestones.Thus,the dolomitization mechanism in this area may be called "dolomitization through leaching of basalts”.
In the East Yunnan—West Sichuan area,dolostones are well developed and can be divided into two types:massive dolostones and patchy dolostones. Massive dolostones are light gray and gray,and mainly consist of fine or medium dolomites.The δ13C is +0.8‰～+3.5‰(PDB) and averages +2.7‰;δ18O is -6.8‰～-9.1‰(PDB) and averages -7.8‰.Sr is 25×10-6～52×10-6 and averages 39×10-6;Na is 52×10-6～74×10-6 and averages 64×10-6.Patchy dolostones are gray and consist of dolomite patches and calcite patches,with dolomite patches over 50%.The dolomite patches are several centimeters to over 20 cm in size and consist of fine to medium dolomite crystals.The dolomite crystals are euhedral or subhedral and are commonly 60%～90%.The δ13C is +3.1‰～+4.0‰ and averages +3.6‰;δ18O is -6.4‰～-8.1‰ and averages -7.8‰.Sr is 46×10-6～68×10-6 and averages 55×10-6;Na is 60×10-6～89×10-6 and averages 74×10-6. Various data show that massive dolostones and patchy dolostones are the same in origin and different only in dolomitization degree.These dolostones were formed under burial conditions,and the dolomitizing water was fresh meteoric water leaching basalts.The Mg2+ for dolomitization came from weathering and dissolution of basalts.Very thick basalts formed high mountains.which formed great relief of landform and large water head and made the meteoric water leaching basalts flow deep into underground.Such water resulted in large scale dolomitization of limestones.Thus,the dolomitization mechanism in this area may be called "dolomitization through leaching of basalts”.
1999, 17(3): 397-402.
Abstract:
The tectonic setting of the study area was aulacogen in Mesoproterzoic to Early Paleozoic, and impactogen in Late Paleozoic.The lithostrtiagaphic units contain Jingyuan Formation,Yanghugou Formation and Taiyuan Formation during Late Carboniferous in Hulusitai section of Ordos basin.Based on the study of high reselution sequence cycle and depositional systems,sixteen T-R high-reslution sequence cycles occur from Namurian C stage to Stephanian stage in Late Carboniferous strata.There developped barrier-lagoon systems (Jingyuan Fm.and Yanghugou Fm.),barrier-lagoon with carbonate platform composite systems (JingyuanFm.and Yanghugou Fm.)and tidal delta systems(Taiyan Fm.).They average about 1.9 Ma and range from 1.1～3.3 Ma in length,reflecting forth-order relative sea level change cycles,and belong to High-resolution Sequence Cycles.Each T-R sequence is composed of transgressive system tract,early highstand system tract,later highstand system tract,with a few record of forced-regression system tract.In general,lowstand ersional sequence bondary and transgressive surface are merged,with no or few record of the lowstand system tract.Transsion came from Qilian Sea during early-middle stage of Late Carboniferous.Qilian Sea and Huabei Sea were merged in Late Taiyuan stage.But they are not coincident with forth-order sea level fluctuation.It is charcterizied of forth-order sea-kevel change by the slow rising firstly and then dast rising with interenvening period of fall,and then fast falling with a few forced-regression.The third-order sea level change reflects that transgression began in Namurian C stage and reached maxium transgrssion in Wesphalian stage and occoured regression in the end of Carboniferous.So,the relative sea level change from the record of the stratigraphy sequences are the composite sea level change with small-order cycles superimposed with high-order cycles subsidence and sediment influx.Finally,sequence stratigarphic model of paleocoastal plain environment has been proposed.
The tectonic setting of the study area was aulacogen in Mesoproterzoic to Early Paleozoic, and impactogen in Late Paleozoic.The lithostrtiagaphic units contain Jingyuan Formation,Yanghugou Formation and Taiyuan Formation during Late Carboniferous in Hulusitai section of Ordos basin.Based on the study of high reselution sequence cycle and depositional systems,sixteen T-R high-reslution sequence cycles occur from Namurian C stage to Stephanian stage in Late Carboniferous strata.There developped barrier-lagoon systems (Jingyuan Fm.and Yanghugou Fm.),barrier-lagoon with carbonate platform composite systems (JingyuanFm.and Yanghugou Fm.)and tidal delta systems(Taiyan Fm.).They average about 1.9 Ma and range from 1.1～3.3 Ma in length,reflecting forth-order relative sea level change cycles,and belong to High-resolution Sequence Cycles.Each T-R sequence is composed of transgressive system tract,early highstand system tract,later highstand system tract,with a few record of forced-regression system tract.In general,lowstand ersional sequence bondary and transgressive surface are merged,with no or few record of the lowstand system tract.Transsion came from Qilian Sea during early-middle stage of Late Carboniferous.Qilian Sea and Huabei Sea were merged in Late Taiyuan stage.But they are not coincident with forth-order sea level fluctuation.It is charcterizied of forth-order sea-kevel change by the slow rising firstly and then dast rising with interenvening period of fall,and then fast falling with a few forced-regression.The third-order sea level change reflects that transgression began in Namurian C stage and reached maxium transgrssion in Wesphalian stage and occoured regression in the end of Carboniferous.So,the relative sea level change from the record of the stratigraphy sequences are the composite sea level change with small-order cycles superimposed with high-order cycles subsidence and sediment influx.Finally,sequence stratigarphic model of paleocoastal plain environment has been proposed.
1999, 17(3): 409-413.
Abstract:
The relationship between clastic composition and tectonic setting has provided a useful method to recognize the link of sedimentology and tectonic type.Based on the systematic analysis of regional sedimentary-tectonic character and clastic composition to determine the basin tectonic type of the Mesozoic in the Ordos Basin,it could be concluded as follows: First,there was a sinistral sheared stress field in mechanical property of the North China craton during the Mesozoic.There formed two major tectonic elements under the deformation of the stress field.One is the large uplifted zone in the east,and the other is the Ordos Basin in the west.Simultaneously,the west edge thrust belt of the basin was formed.The main structure of the Mesozoic in the Ordos Basin was presented as“deep depression in the west-slope and forebulge in the east”.Also,there were a large amount of conglomerate-clastic rocks of the Upper Triassic, the Upper Jurassic,and the Lower Cretaceous in the deep depression.These sedimentary tectonic charactes showed that the Mesozoic of the Ordos Basin is most likely to be a quasi-foreland basin. Meanwhile,the subsidence center of the Mesozoic in the Ordos Basin could not coincide with the sedimentary center,These two centers were not only changable but also located at different positions.From the late Triassic to the Early Cretaceous,the subsidence center of the west depression migrated along the thrust orientation,and the sedimentary center in the middle part of the basin circled anti-clockwisely.Obviously,It was the result of the west edge thrust belt of the basin and the sinistral sheared stress deformation of the North China craton during the Mesozoic. Futhermore,most of the Mesozoic samples fall in the Recycled Orogen Provenances in the QFL and QFLT triangular diagram delinested by Dickinson(1979),It is suggested that the clastic resources of the fills in the Mesozoic be like that of the foreland basin.However,the most chemical component ratios of the Mesozoic samples are not of the typical foreland basin character,but similar to the quasi-foreland basin.For example,Fe2O3+MgO=4%～6%,TiO2=0.3%～0.5%，K2O/Na2O=0.8%～1.7%,Fe2O3/K2O=1.1%～1.9% and SiO2/Al2O3=5.5%～8.9%. Therefore,the Mesozoic Ordos Basin was not a typiocal foreland basin,but a quasi-foreland one .It was the most result of the west edge therust belt of the basin during the Meszozic, especially during the late Triassic,the late Jurassic and the Early Cretaceous.
The relationship between clastic composition and tectonic setting has provided a useful method to recognize the link of sedimentology and tectonic type.Based on the systematic analysis of regional sedimentary-tectonic character and clastic composition to determine the basin tectonic type of the Mesozoic in the Ordos Basin,it could be concluded as follows: First,there was a sinistral sheared stress field in mechanical property of the North China craton during the Mesozoic.There formed two major tectonic elements under the deformation of the stress field.One is the large uplifted zone in the east,and the other is the Ordos Basin in the west.Simultaneously,the west edge thrust belt of the basin was formed.The main structure of the Mesozoic in the Ordos Basin was presented as“deep depression in the west-slope and forebulge in the east”.Also,there were a large amount of conglomerate-clastic rocks of the Upper Triassic, the Upper Jurassic,and the Lower Cretaceous in the deep depression.These sedimentary tectonic charactes showed that the Mesozoic of the Ordos Basin is most likely to be a quasi-foreland basin. Meanwhile,the subsidence center of the Mesozoic in the Ordos Basin could not coincide with the sedimentary center,These two centers were not only changable but also located at different positions.From the late Triassic to the Early Cretaceous,the subsidence center of the west depression migrated along the thrust orientation,and the sedimentary center in the middle part of the basin circled anti-clockwisely.Obviously,It was the result of the west edge thrust belt of the basin and the sinistral sheared stress deformation of the North China craton during the Mesozoic. Futhermore,most of the Mesozoic samples fall in the Recycled Orogen Provenances in the QFL and QFLT triangular diagram delinested by Dickinson(1979),It is suggested that the clastic resources of the fills in the Mesozoic be like that of the foreland basin.However,the most chemical component ratios of the Mesozoic samples are not of the typical foreland basin character,but similar to the quasi-foreland basin.For example,Fe2O3+MgO=4%～6%,TiO2=0.3%～0.5%，K2O/Na2O=0.8%～1.7%,Fe2O3/K2O=1.1%～1.9% and SiO2/Al2O3=5.5%～8.9%. Therefore,the Mesozoic Ordos Basin was not a typiocal foreland basin,but a quasi-foreland one .It was the most result of the west edge therust belt of the basin during the Meszozic, especially during the late Triassic,the late Jurassic and the Early Cretaceous.
1999, 17(3): 422-429.
Abstract:
The formation and evolution of the sequences in continental down-faulted basin are mainly controlled by eposodic tectonism.The strength of boundary fault activity and the velocity of basement depression are the main reason that lead to the formation of different sequences.Principal marks to identify sequence types are the sequence boundary characters.The mechanisms of formation of sequence boundary and sequence surface are the uplift of whole lacustrine basin,the cease of the movement of boundary fault and the depression of basement.Based on these studies,four types of sequences have been identified from the lower Cretaceous strata in the northern Wuerxun sag,i.e.,simply faulted sequence,quickly faulting-depressed sequence,syngenetic faulting-depressed sequence and simply depressed sequence.The development of simply faulted sequence and simply depressed sequence are mainly controlled by sag boundary fault and basement depression respectively,whereas the formation of quickly faulting-depressed sequence and syngenetic faulting-depressed sequence are dependent on both boundary fault and basement depression.Due to different tectonic background,these sequences are different in scale and composition. Different kinds of sequences have different system tracts and petroleum potential,and the most favorable sequence types are the quickly faulting-depressed sequence and syngenetic faulting-depressed sequence,for they both contain best source,reservoir and seal rocks.
The formation and evolution of the sequences in continental down-faulted basin are mainly controlled by eposodic tectonism.The strength of boundary fault activity and the velocity of basement depression are the main reason that lead to the formation of different sequences.Principal marks to identify sequence types are the sequence boundary characters.The mechanisms of formation of sequence boundary and sequence surface are the uplift of whole lacustrine basin,the cease of the movement of boundary fault and the depression of basement.Based on these studies,four types of sequences have been identified from the lower Cretaceous strata in the northern Wuerxun sag,i.e.,simply faulted sequence,quickly faulting-depressed sequence,syngenetic faulting-depressed sequence and simply depressed sequence.The development of simply faulted sequence and simply depressed sequence are mainly controlled by sag boundary fault and basement depression respectively,whereas the formation of quickly faulting-depressed sequence and syngenetic faulting-depressed sequence are dependent on both boundary fault and basement depression.Due to different tectonic background,these sequences are different in scale and composition. Different kinds of sequences have different system tracts and petroleum potential,and the most favorable sequence types are the quickly faulting-depressed sequence and syngenetic faulting-depressed sequence,for they both contain best source,reservoir and seal rocks.
1999, 17(3): 435-442.
Abstract:
The Turpan Basin is located in the eastern part of the Tian Shan and has a complex polycyclic sedimentary and tectonic history from the Late Permian to the Late Tertiary.Main stratigraphic boundaries follow in general tectonically induced sedimentary cycles,which are bounded by unconformities.More than 10 000 metres of clastic sediments were accumulated during the evolution of the basin.The sedimentary facies are abundant and the sedimentary environments changed with time.The oldest parts of the true basin fill are of Late Permian age.They and mostly nonmarine deposits and overlay unconformably Upper Carboniferous volcanic rocks and consist of lacustrine and fluvial dark mudstones,siltstone,sandstone and conglomerate.The Triassic strata consist of red coarse clasts in the lower part and dark,fine clastic sediments with coal and coal streaks in the upper part, indicating alluvial and lacustrine-fluvial environments with climate change from arid to humid during the Triassic.The Jurassic sediments form the most widely distributed and thickest part of the Turpan Basin,as well as the most important productive sequences of oil,gas and coal,The Lower and Middle Jurassic sediments consist of grey and green fine clastic coal-bearing rocks which were deposited in a lacustrine-swamp environment.The Upper Jurassic consists of variegated coarse clasts rocks,reflecting piedmontfluvial environment with arid climate.In the Cretaceous,the sedimentary range was more limited and the sedimentary environment belongs to a lacustrine system.The Tertiary strata are distributed through the entire basin and are the youngest of the basin fill investigated in this study.Conglomerate and sandstone are dominant in the lower part and deposited in a braided fluvial/alluvial environments,while mudstone and silty mudstone are dominant in the upper part and are a lacustrine deposition.Local interlaid gypsum beds and salt beds with red mudstone indicate a deposition in an arid climate and a local salt-water lake environment. Paleocurrent analysis reveals a complex pattern of sediment fluxes into the basin.Along with the tectonic variation,the source rock regions of the basin varied with time. From Permian to Jurassic,the Jueluotage Shan was the dominant source rock region for the Turpan Basin.The locally uplifted areas of the Bogda Shan(e.g.at Taoshuyuan)were only minor providers of source rocks .The paleocurrent directions were from south to north.The Qitai paleouplift was an important provenance area for the northern region of the basin,which leading to the transportation of sediments from north to south .Since the Latest Jurassic,the Bogda Shan gradually folded and uplifted,building another source rock region for the basin.The paleocurrent directions were from north to south.In the Cretaceous and Tertiary,the Bogda Shan was an important source rock region.This region was continually strongly uplifted in the Cenozoic age and the dominant source rock region for the basin with transport directions from north to south.In the Hami Sag,the Haerlike Shan was a continuously eroding region and an ever important provenance area,along with the Jueluotage Shan.The paleocurrent directions were always from north to south.
The Turpan Basin is located in the eastern part of the Tian Shan and has a complex polycyclic sedimentary and tectonic history from the Late Permian to the Late Tertiary.Main stratigraphic boundaries follow in general tectonically induced sedimentary cycles,which are bounded by unconformities.More than 10 000 metres of clastic sediments were accumulated during the evolution of the basin.The sedimentary facies are abundant and the sedimentary environments changed with time.The oldest parts of the true basin fill are of Late Permian age.They and mostly nonmarine deposits and overlay unconformably Upper Carboniferous volcanic rocks and consist of lacustrine and fluvial dark mudstones,siltstone,sandstone and conglomerate.The Triassic strata consist of red coarse clasts in the lower part and dark,fine clastic sediments with coal and coal streaks in the upper part, indicating alluvial and lacustrine-fluvial environments with climate change from arid to humid during the Triassic.The Jurassic sediments form the most widely distributed and thickest part of the Turpan Basin,as well as the most important productive sequences of oil,gas and coal,The Lower and Middle Jurassic sediments consist of grey and green fine clastic coal-bearing rocks which were deposited in a lacustrine-swamp environment.The Upper Jurassic consists of variegated coarse clasts rocks,reflecting piedmontfluvial environment with arid climate.In the Cretaceous,the sedimentary range was more limited and the sedimentary environment belongs to a lacustrine system.The Tertiary strata are distributed through the entire basin and are the youngest of the basin fill investigated in this study.Conglomerate and sandstone are dominant in the lower part and deposited in a braided fluvial/alluvial environments,while mudstone and silty mudstone are dominant in the upper part and are a lacustrine deposition.Local interlaid gypsum beds and salt beds with red mudstone indicate a deposition in an arid climate and a local salt-water lake environment. Paleocurrent analysis reveals a complex pattern of sediment fluxes into the basin.Along with the tectonic variation,the source rock regions of the basin varied with time. From Permian to Jurassic,the Jueluotage Shan was the dominant source rock region for the Turpan Basin.The locally uplifted areas of the Bogda Shan(e.g.at Taoshuyuan)were only minor providers of source rocks .The paleocurrent directions were from south to north.The Qitai paleouplift was an important provenance area for the northern region of the basin,which leading to the transportation of sediments from north to south .Since the Latest Jurassic,the Bogda Shan gradually folded and uplifted,building another source rock region for the basin.The paleocurrent directions were from north to south.In the Cretaceous and Tertiary,the Bogda Shan was an important source rock region.This region was continually strongly uplifted in the Cenozoic age and the dominant source rock region for the basin with transport directions from north to south.In the Hami Sag,the Haerlike Shan was a continuously eroding region and an ever important provenance area,along with the Jueluotage Shan.The paleocurrent directions were always from north to south.
1999, 17(3): 449-453.
Abstract:
The No.2 member of Shahejie Formation(Paleogene) is one of the major target beds in Dongpu depression. An investigation based on seismic, well-logging and core data indicates that the member consists of alluvial fan sediments in the southern west sag. The sediments are mostly brown-grey brown finer lithic arkose and coarse siltstone, interbedded with brown mudstone,with medium-low compositional and textural maturation.Some sedimentary structures, such as graded bedding, small-scale cross bedding, ripple lamination and horizontal bedding caused by density and traction flow deposition,and such as mud crack,burrow,swash mark and curty bedding caused by exposure and others, are seen in cores,and scatters are distributed along a line parallel to C=M line in CM plot.The features show that the sediments are deposited under the influence of density and traction flow.The study area is combined into a depositional unit with the West Slope because of a decrease of the boundary faults action in the depositional age of the member, resulting in finer-grained front middle-fan deposition in the study area and coarse-grained proximal fan in the West Slope of Dongpu depression. This is why the alluvial fan sediments of Es2 member have finer grain-size in the study area. In seismic profiles and well-logging curves, three sedimentary cycles are recognized in the member, showing an fining-upward trend. The cycles are correlated between wells in the area, indicating the control of tectonics and weather.
The No.2 member of Shahejie Formation(Paleogene) is one of the major target beds in Dongpu depression. An investigation based on seismic, well-logging and core data indicates that the member consists of alluvial fan sediments in the southern west sag. The sediments are mostly brown-grey brown finer lithic arkose and coarse siltstone, interbedded with brown mudstone,with medium-low compositional and textural maturation.Some sedimentary structures, such as graded bedding, small-scale cross bedding, ripple lamination and horizontal bedding caused by density and traction flow deposition,and such as mud crack,burrow,swash mark and curty bedding caused by exposure and others, are seen in cores,and scatters are distributed along a line parallel to C=M line in CM plot.The features show that the sediments are deposited under the influence of density and traction flow.The study area is combined into a depositional unit with the West Slope because of a decrease of the boundary faults action in the depositional age of the member, resulting in finer-grained front middle-fan deposition in the study area and coarse-grained proximal fan in the West Slope of Dongpu depression. This is why the alluvial fan sediments of Es2 member have finer grain-size in the study area. In seismic profiles and well-logging curves, three sedimentary cycles are recognized in the member, showing an fining-upward trend. The cycles are correlated between wells in the area, indicating the control of tectonics and weather.
1999, 17(3): 458-463.
Abstract:
Element geochemical and heavy mineral methods are applied to study provenance of Holocene sediments in Subei coastal plain. Based on core analyses, that sedimentary strata developed in Subei coastal plain during Holocene mainly include littoral facies, offshore-neritic facies and tidal flat facies from the lower part to the upper part. The authors firstly resarched elemental and heavy mineral compositions of the Changjaing and the Huanghe sediemnts, and study shows there are obvious differences between both sediments. Some elements and heavy minerals can be applied to trace the Changjiang and the Huanghe sediments and study the diffusions and mixture of both rivers in coastal zones. And then, elemental and detrital mineral compositions of Holocene sediments from four drilling cores in Subei coastal plain are determined in detail. Analyses reveal that the Huanghe River is the dominant provenance and supplied more matters for formation of Holocene sediments than the Changjiang River. During early Holocene, influence of the Changjiang matter exceeded that of the Huanghe matter. After the maximal transgression in Holocene, the Huanghe River arrived the South Yellow Sea and carried abundant sediment, and the Changjiang River supplied limited matter by a narrow distributary and its influence was small. During the last two thousand years, sediments of this area were mainly from the Huanghe River. Study also shows that elemental geochemical method is more useful than heavy mineral method in provenance discrimination because the former can effectively avoid the influence of hydraulic sorting.
Element geochemical and heavy mineral methods are applied to study provenance of Holocene sediments in Subei coastal plain. Based on core analyses, that sedimentary strata developed in Subei coastal plain during Holocene mainly include littoral facies, offshore-neritic facies and tidal flat facies from the lower part to the upper part. The authors firstly resarched elemental and heavy mineral compositions of the Changjaing and the Huanghe sediemnts, and study shows there are obvious differences between both sediments. Some elements and heavy minerals can be applied to trace the Changjiang and the Huanghe sediments and study the diffusions and mixture of both rivers in coastal zones. And then, elemental and detrital mineral compositions of Holocene sediments from four drilling cores in Subei coastal plain are determined in detail. Analyses reveal that the Huanghe River is the dominant provenance and supplied more matters for formation of Holocene sediments than the Changjiang River. During early Holocene, influence of the Changjiang matter exceeded that of the Huanghe matter. After the maximal transgression in Holocene, the Huanghe River arrived the South Yellow Sea and carried abundant sediment, and the Changjiang River supplied limited matter by a narrow distributary and its influence was small. During the last two thousand years, sediments of this area were mainly from the Huanghe River. Study also shows that elemental geochemical method is more useful than heavy mineral method in provenance discrimination because the former can effectively avoid the influence of hydraulic sorting.
1999, 17(3): 478-481.
Abstract:
Occurrences of organic matter in Paleogene argillaceous rock of Jiyang depression are primarily studied by means of fluorescent microscopy. The occurrences of organic matter in Paleogene argillaceous rock are classified into three types which are named as “enrichment type、dispersion type and part-enrichment type”, respectively. “Enrichment type” means that the organic matters are distributed plentifully along bedding plane in the rock, which deposited mainly in moderate depth—deep lake facies and is rich in organic matter; “Dispersion type” means that the organic matter are scattered randomly in the rock that deposited mostly in fluvial-delta facies or shore -shallow lake facies or turbidity current deposist, which is various in abundance of organic matter; (c) “part enrichment type”is intermediate one with the property of both “ enrichment type" and “dispersion type”. Combined with part quantitative analysis data about seaing ability of argillaceous rocks, three different composition properties are concluded as follows : (1) The sealing ability of the argillaceous rock was worse when organic matters were rich and occurred with “enrichment type” in the argillaceous rock， (2) The argillaceous rock had a certain sealing ability when organic matters were rich and occurred with “dispersion type”in the argillaceous rock， (3) The sealing ability of the argillaceous rock was better when organic matters were poor and occurred with “dispersion type” in the argillaceous rock. Thus,the effect caused by occurrence of organic matter no the sealing ability of argillaceous rock should not be ignored. It can be one of the important basis for us to evaluate sealing ability of argillaceous rock over oil-gas reservoir.
Occurrences of organic matter in Paleogene argillaceous rock of Jiyang depression are primarily studied by means of fluorescent microscopy. The occurrences of organic matter in Paleogene argillaceous rock are classified into three types which are named as “enrichment type、dispersion type and part-enrichment type”, respectively. “Enrichment type” means that the organic matters are distributed plentifully along bedding plane in the rock, which deposited mainly in moderate depth—deep lake facies and is rich in organic matter; “Dispersion type” means that the organic matter are scattered randomly in the rock that deposited mostly in fluvial-delta facies or shore -shallow lake facies or turbidity current deposist, which is various in abundance of organic matter; (c) “part enrichment type”is intermediate one with the property of both “ enrichment type" and “dispersion type”. Combined with part quantitative analysis data about seaing ability of argillaceous rocks, three different composition properties are concluded as follows : (1) The sealing ability of the argillaceous rock was worse when organic matters were rich and occurred with “enrichment type” in the argillaceous rock， (2) The argillaceous rock had a certain sealing ability when organic matters were rich and occurred with “dispersion type”in the argillaceous rock， (3) The sealing ability of the argillaceous rock was better when organic matters were poor and occurred with “dispersion type” in the argillaceous rock. Thus,the effect caused by occurrence of organic matter no the sealing ability of argillaceous rock should not be ignored. It can be one of the important basis for us to evaluate sealing ability of argillaceous rock over oil-gas reservoir.
1999, 17(3): 486-492.
Abstract:
Tarim Basin, with an area of 560,000 square kilometers and a sedimentary sequence of 14,000 meters in thickness,although nearly more than ten medim-sized oil&gas fields discovered but without a giant one in ten years exploration,is still a perspective area for potential petroleum exploration due to its attractive petroleum geological settings. Because of its abundance in gas-condensate,a number of studies focused on the light hydrocarbons in oil&gas have been presented.For better understanding the genetic relationship of light hydrocarbons among rock-oil-gas, this paper deals with the composition characteristics of the light hydrocarbons acidolysised from the Palaeozoic sedimentary rock and their comparison with those of oil and gas. The experimented core samples,with their depth ranged from 4 000m-5 877m and geological time dated to Cambrian,Ordovician and Carboniferous repectively,are located on Lunnan,Tazhong etc.six oil-gas fields or structures. The core samples firstly were smashed under the lower temperature and of which in 60-100 mesh sized were collected to be used in extracting light hydrocarbons.After the acidolysis device,along with 40g sample and 50ml distilled water in it, was evacuated to vacuum,it was heated to 70-80℃ by water bathing,and then dripping phosphoric acid(85%) to react with carbonate minerals to release the light hydrocarbons reserved and CO2 produced,the later was neutralized using KOH(saturated) finally in gas-washing flask and the accumulated light hydrocarbons was analyzed by HP5880 GC equipped with HP-PONA column and data processing system. The data calculated from the acidolysised light hydrocarbons(Table 1)display a complete different distribution compared with the marine oil-gas nearby.(1) Heptane values ranging from 12.0%-24.6% and the average is 19.0%.Isoheptane values ranging from 1.7 to 2.91 and the average is 2.15.Both are much lower than those of the marine oil-gas nearby,for example,both averages of Lunnan structure are 35.8% and 2.9 respectively.(2)Lower content of light aromatic componds,with the average of 0.95% in C6-7calculating unite,compared with that of 4.96% in oil and gas-condensate.(3)Different distribution pattern in alkane and cycloalkane,namly ∑nC6-7＜∑iC6-7and ∑CPs＞∑CHs,contrary to the distribution patterns in oil and gas-condensate.(4)Lighterδ13C1‰ values,averaged -54.8‰,as well as lower (C1+ C2)% values( ＜50%),compared with natural gas.Meanwhile one similarity on Mongo K Index is recognized,namly in Tazhong and Yaha oil - gas and rock all possess higher Mongo K values compared with other oil&gas bearing structuresBased on the distributive characteristics of acidolysised light HCs,conclusions obtained as follows:(1)The lower maturity acidolysised light HCs obtained in the experiment is the earlier HCs which was sourced mainly from marine bacteria and algae and preserved in the crystal lattice of carbonate mineral.the reservior rematuration is not effective even under the depth of 4 500m-5 900 during such a long geological period mainly because of the increasingly higher pressure and the lower earth thermal gradient as well as the lower geothermal flux in recent and late geological in Tarim Basin.(2) The similar higher Mongo K values coexisted in rock-oil-gas indicat that there is some relationship between the Ordovician rock and the oil-gas in Tazhong and Yaha oil field.(3)Because of the maturity deviation reflected by light HCs between the rock and oil&gas deposites nearby,it demonstrats that the higher matured marine oil&gas is mainly sourced from the deeper Plaeozoic sources perhaps in the sedimentary center of the Basin by undergoing a long distance of migration,to which the Palaeozoic rock near the reservior contribute little at least in low molecule HCs and also no exchang of light HCs between them perhaps due to the relative closing property of the crystal lattice.
Tarim Basin, with an area of 560,000 square kilometers and a sedimentary sequence of 14,000 meters in thickness,although nearly more than ten medim-sized oil&gas fields discovered but without a giant one in ten years exploration,is still a perspective area for potential petroleum exploration due to its attractive petroleum geological settings. Because of its abundance in gas-condensate,a number of studies focused on the light hydrocarbons in oil&gas have been presented.For better understanding the genetic relationship of light hydrocarbons among rock-oil-gas, this paper deals with the composition characteristics of the light hydrocarbons acidolysised from the Palaeozoic sedimentary rock and their comparison with those of oil and gas. The experimented core samples,with their depth ranged from 4 000m-5 877m and geological time dated to Cambrian,Ordovician and Carboniferous repectively,are located on Lunnan,Tazhong etc.six oil-gas fields or structures. The core samples firstly were smashed under the lower temperature and of which in 60-100 mesh sized were collected to be used in extracting light hydrocarbons.After the acidolysis device,along with 40g sample and 50ml distilled water in it, was evacuated to vacuum,it was heated to 70-80℃ by water bathing,and then dripping phosphoric acid(85%) to react with carbonate minerals to release the light hydrocarbons reserved and CO2 produced,the later was neutralized using KOH(saturated) finally in gas-washing flask and the accumulated light hydrocarbons was analyzed by HP5880 GC equipped with HP-PONA column and data processing system. The data calculated from the acidolysised light hydrocarbons(Table 1)display a complete different distribution compared with the marine oil-gas nearby.(1) Heptane values ranging from 12.0%-24.6% and the average is 19.0%.Isoheptane values ranging from 1.7 to 2.91 and the average is 2.15.Both are much lower than those of the marine oil-gas nearby,for example,both averages of Lunnan structure are 35.8% and 2.9 respectively.(2)Lower content of light aromatic componds,with the average of 0.95% in C6-7calculating unite,compared with that of 4.96% in oil and gas-condensate.(3)Different distribution pattern in alkane and cycloalkane,namly ∑nC6-7＜∑iC6-7and ∑CPs＞∑CHs,contrary to the distribution patterns in oil and gas-condensate.(4)Lighterδ13C1‰ values,averaged -54.8‰,as well as lower (C1+ C2)% values( ＜50%),compared with natural gas.Meanwhile one similarity on Mongo K Index is recognized,namly in Tazhong and Yaha oil - gas and rock all possess higher Mongo K values compared with other oil&gas bearing structuresBased on the distributive characteristics of acidolysised light HCs,conclusions obtained as follows:(1)The lower maturity acidolysised light HCs obtained in the experiment is the earlier HCs which was sourced mainly from marine bacteria and algae and preserved in the crystal lattice of carbonate mineral.the reservior rematuration is not effective even under the depth of 4 500m-5 900 during such a long geological period mainly because of the increasingly higher pressure and the lower earth thermal gradient as well as the lower geothermal flux in recent and late geological in Tarim Basin.(2) The similar higher Mongo K values coexisted in rock-oil-gas indicat that there is some relationship between the Ordovician rock and the oil-gas in Tazhong and Yaha oil field.(3)Because of the maturity deviation reflected by light HCs between the rock and oil&gas deposites nearby,it demonstrats that the higher matured marine oil&gas is mainly sourced from the deeper Plaeozoic sources perhaps in the sedimentary center of the Basin by undergoing a long distance of migration,to which the Palaeozoic rock near the reservior contribute little at least in low molecule HCs and also no exchang of light HCs between them perhaps due to the relative closing property of the crystal lattice.
