2019 Vol. 37, No. 4
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Display Method:
2019, 37(4): 661-673.
doi: 10.14027/j.issn.1000-0550.2018.178
Abstract:
The great thickness of deep-water fine-grained sedimentary rock in the K2qn Lake,Songliao Basin,offers the potential for very large-scale unconventional oil and gas exploration. Spectral analysis and wavelet analysis were used in a study of the Milankovitch cycle recorded in the K2qn1 Formation. The sedimentary paleoenvironment and its response to the Milankovitch cycle for the deep-lake facies in the Gulong Depression,Songliao Basin,were analyzed by whole-rock X-ray diffraction to determine geochemical elemental data. The formation responses to the Milankovitch cycle were established,revealing four-stage Milankovitch cycles in the K2qn1 Formation:68 meter-scale cyclic se-quences,25 level-V Milankovitch cycle sequences,6 level-IV Milankovitch cycle sequences,with one stage-Ⅲ cy-cle sequence interface. The lacustrine sedimentation in the K2qn1 Formation was mainly controlled by a short-term ec-centricity of 128 ka and a slope of 48 ka. The sedimentation environment indicates that the climate changed from dry to wet,with increasing water depth,reductive enhancement,and decreasing salinity. The combination of the six envi-ronments indicates the relationship between the thick shale lithology and the sedimentary environment. The layered gray mudstone deposited in a warm, humid climate is conducive to the development of oil shale formation.
The great thickness of deep-water fine-grained sedimentary rock in the K2qn Lake,Songliao Basin,offers the potential for very large-scale unconventional oil and gas exploration. Spectral analysis and wavelet analysis were used in a study of the Milankovitch cycle recorded in the K2qn1 Formation. The sedimentary paleoenvironment and its response to the Milankovitch cycle for the deep-lake facies in the Gulong Depression,Songliao Basin,were analyzed by whole-rock X-ray diffraction to determine geochemical elemental data. The formation responses to the Milankovitch cycle were established,revealing four-stage Milankovitch cycles in the K2qn1 Formation:68 meter-scale cyclic se-quences,25 level-V Milankovitch cycle sequences,6 level-IV Milankovitch cycle sequences,with one stage-Ⅲ cy-cle sequence interface. The lacustrine sedimentation in the K2qn1 Formation was mainly controlled by a short-term ec-centricity of 128 ka and a slope of 48 ka. The sedimentation environment indicates that the climate changed from dry to wet,with increasing water depth,reductive enhancement,and decreasing salinity. The combination of the six envi-ronments indicates the relationship between the thick shale lithology and the sedimentary environment. The layered gray mudstone deposited in a warm, humid climate is conducive to the development of oil shale formation.
2019, 37(4): 674-689.
doi: 10.14027/j.issn.1000-0550.2018.171
Abstract:
The North Qaidam Orogenic Belt experienced a complicated ocean-land transformation process in the PanAfrican-Qilian period,and developed a trench-arc-basin system in the convergent margin in the Cambrian-Early Or-dovician. Consequently,the NWW-SEE trending Tanjianshan island arc belt and the South Oulongbuluq back-arc ba-sin formed along the North Qaidam Orogenic Belt,and developed a set of carbonate-clastic-volcanic sedimentary for-mations. During this time,a basin-scale coupling system was formed between sedimentary evolution of the southern margin of the Oulongbuluq microcontinent and convergence forces between it and the Qaidam block,triggering a se-ries of geological events. They are linked by a series of genetic mechanisms,which have a common macroeconomic background. In the Early Paleozoic era a tectonic shift from passive to active continental margin occurred at the south-ern margin of the Oulongbuluq microcontinent,as a consequence of northward subduction by the North Qaidam Ocean. This induced multi-period volcanic eruption events,and formed numerous sets of volcanic rocks,pyroclastic rocks and intercalated metamorphic clasolite in the Tanjianshan island arc belt. Meanwhile,the southern margin of Oulongbuluq microcontinent changed from a shallow water carbonate platform to deep water slope,and regularly de-veloped concentrated event deposits such as carbonate gravity flow deposits (submarine fan,turbidite,etc.) and slumping deposits. In addition,a mass of island-arc material and uplifted continental crust basement caused by subse-quent collision between the Tanjianshan island-arc belt and the Qaidam block provided important provenance for sub-sequent sandy debris flow and clastic turbidite deposition in the South Oulongbuluq back-arc basin. The isotopic chro-nology data shows that the development time of these event deposits is consistent with the relatively most active period of tectonism at the northern margin of the Qaidam Basin area and early in the Paleozoic. Therefore,these stable-distri-bution volcanics and gravity flow deposits with concentrated and regular distribution formed in the aftermath of sub-duction and arc-continent collision, and there is a coupling relationship between them.
The North Qaidam Orogenic Belt experienced a complicated ocean-land transformation process in the PanAfrican-Qilian period,and developed a trench-arc-basin system in the convergent margin in the Cambrian-Early Or-dovician. Consequently,the NWW-SEE trending Tanjianshan island arc belt and the South Oulongbuluq back-arc ba-sin formed along the North Qaidam Orogenic Belt,and developed a set of carbonate-clastic-volcanic sedimentary for-mations. During this time,a basin-scale coupling system was formed between sedimentary evolution of the southern margin of the Oulongbuluq microcontinent and convergence forces between it and the Qaidam block,triggering a se-ries of geological events. They are linked by a series of genetic mechanisms,which have a common macroeconomic background. In the Early Paleozoic era a tectonic shift from passive to active continental margin occurred at the south-ern margin of the Oulongbuluq microcontinent,as a consequence of northward subduction by the North Qaidam Ocean. This induced multi-period volcanic eruption events,and formed numerous sets of volcanic rocks,pyroclastic rocks and intercalated metamorphic clasolite in the Tanjianshan island arc belt. Meanwhile,the southern margin of Oulongbuluq microcontinent changed from a shallow water carbonate platform to deep water slope,and regularly de-veloped concentrated event deposits such as carbonate gravity flow deposits (submarine fan,turbidite,etc.) and slumping deposits. In addition,a mass of island-arc material and uplifted continental crust basement caused by subse-quent collision between the Tanjianshan island-arc belt and the Qaidam block provided important provenance for sub-sequent sandy debris flow and clastic turbidite deposition in the South Oulongbuluq back-arc basin. The isotopic chro-nology data shows that the development time of these event deposits is consistent with the relatively most active period of tectonism at the northern margin of the Qaidam Basin area and early in the Paleozoic. Therefore,these stable-distri-bution volcanics and gravity flow deposits with concentrated and regular distribution formed in the aftermath of sub-duction and arc-continent collision, and there is a coupling relationship between them.
2019, 37(4): 690-701.
doi: 10.14027/j.issn.1000-0550.2018.172
Abstract:
Numerous burrows occur in the third member of the Ordovician Majiagou Formation in western Henan Province. A study of the filling characteristics of burrows and the evolution of burrow-mediated pores was conducted on burrow-fills in the research area,based on the observation of outcrops together with polarizing optical microscope, cathodoluminescence microscope and scanning electron microscope. The results showed that macro-scale selective dissolution occurred in four stages. (1) Burrows are almost all either fully filled or semi-filled,mostly with calcite or dolomite;occasional burrows are unfilled. (2) Burrow-mediated pores occur commonly,consisting of dissolution pores and micro-cracks,mainly around the burrow boundary. (3) Burrow-mediated pores were controlled by burial dolomitization and dissolution in the early Hercynian cycle and Indo-Chinese epoch. Most of the dissolution pores were formed in the Hercynian period. (4) Burrow-mediated selective dissolution depends on the degree of crystalliza-tion of the dolomite in the burrow-fills. Micro-dissolution pore formation is influenced by the presence of fluids.
Numerous burrows occur in the third member of the Ordovician Majiagou Formation in western Henan Province. A study of the filling characteristics of burrows and the evolution of burrow-mediated pores was conducted on burrow-fills in the research area,based on the observation of outcrops together with polarizing optical microscope, cathodoluminescence microscope and scanning electron microscope. The results showed that macro-scale selective dissolution occurred in four stages. (1) Burrows are almost all either fully filled or semi-filled,mostly with calcite or dolomite;occasional burrows are unfilled. (2) Burrow-mediated pores occur commonly,consisting of dissolution pores and micro-cracks,mainly around the burrow boundary. (3) Burrow-mediated pores were controlled by burial dolomitization and dissolution in the early Hercynian cycle and Indo-Chinese epoch. Most of the dissolution pores were formed in the Hercynian period. (4) Burrow-mediated selective dissolution depends on the degree of crystalliza-tion of the dolomite in the burrow-fills. Micro-dissolution pore formation is influenced by the presence of fluids.
2019, 37(4): 702-712.
doi: 10.14027/j.issn.1000-0550.2018.181
Abstract:
Iron oolites of the Middle Devonian Yangmaba Formation in the Longmenshan area are an important form of iron deposit in China's Ningxiang-type oolitic hematite deposit. To recover their sedimentary environment and ge-netic mechanism,the sedimentary characteristics of iron oolites were studied in detail by thin section identification and scanning electron microscope analysis and detailed measurement of the Ganxi Shigouli section. The study shows that the eutrophic iron is concentrated in layers and lenses of bioclastic limestone,oolitic bioclastic limestone and bioclastic oolitic limestone. The surrounding rock is silty mudstone and mudstone. Iron oolites are important stores and carriers of iron and occur in four types,depending on the difference in mineral composition:microlites,chlorite oolites,ferritic oolites and iron oolites. Iron oolites are deposited in a lagoon at the edge of the reef. The distribution of oolites enabled the genetic distribution pattern of oolitic iron at the reef edge to be established. In turn,from the outer reef shoal to the inner shoal,to the lagoon,normal or ferritic oolites,chlorite oolites,chlorite thin-skin oolites and iron were developed.
Iron oolites of the Middle Devonian Yangmaba Formation in the Longmenshan area are an important form of iron deposit in China's Ningxiang-type oolitic hematite deposit. To recover their sedimentary environment and ge-netic mechanism,the sedimentary characteristics of iron oolites were studied in detail by thin section identification and scanning electron microscope analysis and detailed measurement of the Ganxi Shigouli section. The study shows that the eutrophic iron is concentrated in layers and lenses of bioclastic limestone,oolitic bioclastic limestone and bioclastic oolitic limestone. The surrounding rock is silty mudstone and mudstone. Iron oolites are important stores and carriers of iron and occur in four types,depending on the difference in mineral composition:microlites,chlorite oolites,ferritic oolites and iron oolites. Iron oolites are deposited in a lagoon at the edge of the reef. The distribution of oolites enabled the genetic distribution pattern of oolitic iron at the reef edge to be established. In turn,from the outer reef shoal to the inner shoal,to the lagoon,normal or ferritic oolites,chlorite oolites,chlorite thin-skin oolites and iron were developed.
2019, 37(4): 713-722.
doi: 10.14027/j.issn.1000-0550.2018.165
Abstract:
The Cenozoic India-Eurasia collision and uplift of the Tibetan Plateau remain hot topics. The Yanyuan Ba-sin in southwest Sichuan province is located in the Sichuan-Yunnan block,which is between the Red River and Xianshuihe-Xiaojiang fault zones. It is a tectonic escape basin that formed during the continental collision,and its evolution process is valuable in revealing the history of the continental collision and uplift of the Tibetan Plateau. In this paper,detailed grain size measurements were carried out on the Pliocene-Early Pleistocene sediments in the Yanyuan Basin,and the grain size data were processed using wavelet transform technology. The results indicate that wavelet transform data can clearly show potential cycles of grainsize variations and,therefore,can provide an effec-tive way to divide the sedimentary stratum interface quantitatively. According to the wavelet transform grain-size anal-ysis,the sedimentary stratum in the Yanyuan Basin can be divided into two level-1,six level-2,and six level-3 muta-tions,which reflect the tectonic and sedimentary environment changes on different time scales. Further analysis of the grain-size distributions within the representative samples for different periods prove that this sequence is mainly com-posed of sediments from lake and river facies. Combined with the grainsize level-1 mutation of the wavelet transform data,the formation and evolution of the Yanyuan Basin can be divided into three stages:5.4-4.7 Ma is the formation phase of the basin,the sedimentary type is mainly a shallow lake subfacies,and the wavelet transform curve shows a high amplitude shift feature;4.6-3.6 Ma is the expansion phase of the basin,the sedimentary types are mainly deep and half-deep lake subfacies,and there is a low amplitude shift feature in the wavelet transform grainsize curve; 3.6-1.8 Ma corresponds to the basin contraction and filling phase,and the features of sedimentary facies and wavelet transform curves are similar to those of the basin formation phase. The formation of the Yanyuan Basin in the Early Pliocene (5.4-4.7 Ma) is related to an India-Eurasia collisional event,while the sedimentary environment change that occurred following 3.6 Ma corresponds to an important uplift event of the Tibetan Plateau.
The Cenozoic India-Eurasia collision and uplift of the Tibetan Plateau remain hot topics. The Yanyuan Ba-sin in southwest Sichuan province is located in the Sichuan-Yunnan block,which is between the Red River and Xianshuihe-Xiaojiang fault zones. It is a tectonic escape basin that formed during the continental collision,and its evolution process is valuable in revealing the history of the continental collision and uplift of the Tibetan Plateau. In this paper,detailed grain size measurements were carried out on the Pliocene-Early Pleistocene sediments in the Yanyuan Basin,and the grain size data were processed using wavelet transform technology. The results indicate that wavelet transform data can clearly show potential cycles of grainsize variations and,therefore,can provide an effec-tive way to divide the sedimentary stratum interface quantitatively. According to the wavelet transform grain-size anal-ysis,the sedimentary stratum in the Yanyuan Basin can be divided into two level-1,six level-2,and six level-3 muta-tions,which reflect the tectonic and sedimentary environment changes on different time scales. Further analysis of the grain-size distributions within the representative samples for different periods prove that this sequence is mainly com-posed of sediments from lake and river facies. Combined with the grainsize level-1 mutation of the wavelet transform data,the formation and evolution of the Yanyuan Basin can be divided into three stages:5.4-4.7 Ma is the formation phase of the basin,the sedimentary type is mainly a shallow lake subfacies,and the wavelet transform curve shows a high amplitude shift feature;4.6-3.6 Ma is the expansion phase of the basin,the sedimentary types are mainly deep and half-deep lake subfacies,and there is a low amplitude shift feature in the wavelet transform grainsize curve; 3.6-1.8 Ma corresponds to the basin contraction and filling phase,and the features of sedimentary facies and wavelet transform curves are similar to those of the basin formation phase. The formation of the Yanyuan Basin in the Early Pliocene (5.4-4.7 Ma) is related to an India-Eurasia collisional event,while the sedimentary environment change that occurred following 3.6 Ma corresponds to an important uplift event of the Tibetan Plateau.
2019, 37(4): 723-734.
doi: 10.14027/j.issn.1000-0550.2018.166
Abstract:
Three distinct lithostratigraphic units A,B,and C have been recognized according to field work and thin section observations. The crudely stratified Unit A deposited in subtidal environments is characterized by monotonous wackstone intercalated with bioclastic wackstone-packstone. The well-bedded Unit B consists largely of bivalve float-stones in a matrix of bioclastic packstone deposited in subtidal conditions. Unit C,disconformably overlying Unit B, is composed of complex lithofacies associations that deposit in variable environments from subtidal to supratidal set-tings,sometimes in lagoon environments. According to the biota range chart and stable carbon isotope data from the Mt. Faito section,which can be used to analyze the biosedimentary diversity,different climatic and sea water condi-tions can be inferred as well. Unit A is characterized by a "Chlorozoan" association,which indicates warm and oligo-trophic conditions;Unit B is dominated by "Chlorozoan" and "Foramol" associations that indicate relatively warm and mesotrophic conditions;and Unit C is a combination of "Microbial" and "Foramol" associations,hence mesotro-phic and temperate thermal conditions. Therefore,local early Aptian biosedimentary shifts and paleoenvironmental turnovers exerted by the increasing temperature,CO2,and nutrient contents have been interpreted on a global scale by analyzing the biosedimentary evolvement and correlating the C-isotope curve with those of other early Cretaceous carbonate platforms and basins worldwide.
Three distinct lithostratigraphic units A,B,and C have been recognized according to field work and thin section observations. The crudely stratified Unit A deposited in subtidal environments is characterized by monotonous wackstone intercalated with bioclastic wackstone-packstone. The well-bedded Unit B consists largely of bivalve float-stones in a matrix of bioclastic packstone deposited in subtidal conditions. Unit C,disconformably overlying Unit B, is composed of complex lithofacies associations that deposit in variable environments from subtidal to supratidal set-tings,sometimes in lagoon environments. According to the biota range chart and stable carbon isotope data from the Mt. Faito section,which can be used to analyze the biosedimentary diversity,different climatic and sea water condi-tions can be inferred as well. Unit A is characterized by a "Chlorozoan" association,which indicates warm and oligo-trophic conditions;Unit B is dominated by "Chlorozoan" and "Foramol" associations that indicate relatively warm and mesotrophic conditions;and Unit C is a combination of "Microbial" and "Foramol" associations,hence mesotro-phic and temperate thermal conditions. Therefore,local early Aptian biosedimentary shifts and paleoenvironmental turnovers exerted by the increasing temperature,CO2,and nutrient contents have been interpreted on a global scale by analyzing the biosedimentary evolvement and correlating the C-isotope curve with those of other early Cretaceous carbonate platforms and basins worldwide.
2019, 37(4): 735-748.
doi: 10.14027/j.issn.1000-0550.2018.183
Abstract:
This is the first report of Upper Cretaceous eolian deposits in the Honghuatao Formation at the eastern mar-gin of the Hengyang Basin in central southern Hunan,providing a new understanding of the paleogeographic environ-ment of this area in the Upper Cretaceous. The Honghuatao Formation comprises a set of medium to fine-grained ma-genta feldspar quartz sandstone and lithic quartz sandstone. Large plates,wedge-shaped staggered bedding,bi-direc-tional staggered bedding,parallel bedding and ventifacts are common,all indicating an eolian desert environment. Optical microscope examination shows that suspended components such as mud and mica are absent,and dish-shaped pits,crescent-shaped craters,and upturned (flat) jointed particles are visible under the electron microscope. Comprehensive analysis confirmed that the sandstone is an eolian deposit,which has important paleoenvironmental significance. The discovery indicates that the climate was dry and hot;desertification and saline lakes were obvious, and food shortage might have led to the migration of dinosaurs in the Hengyang Basin. The dip characteristics of the foreset laminae of sand dunes has enabled the characteristics of surface wind belts in Hunan province to be restored, which provides a new basis for supporting the connection between the Hengyang Basin and its surrounding red basins. The discovery provides valuable material for the study of eolian deposition in Hunan province.
This is the first report of Upper Cretaceous eolian deposits in the Honghuatao Formation at the eastern mar-gin of the Hengyang Basin in central southern Hunan,providing a new understanding of the paleogeographic environ-ment of this area in the Upper Cretaceous. The Honghuatao Formation comprises a set of medium to fine-grained ma-genta feldspar quartz sandstone and lithic quartz sandstone. Large plates,wedge-shaped staggered bedding,bi-direc-tional staggered bedding,parallel bedding and ventifacts are common,all indicating an eolian desert environment. Optical microscope examination shows that suspended components such as mud and mica are absent,and dish-shaped pits,crescent-shaped craters,and upturned (flat) jointed particles are visible under the electron microscope. Comprehensive analysis confirmed that the sandstone is an eolian deposit,which has important paleoenvironmental significance. The discovery indicates that the climate was dry and hot;desertification and saline lakes were obvious, and food shortage might have led to the migration of dinosaurs in the Hengyang Basin. The dip characteristics of the foreset laminae of sand dunes has enabled the characteristics of surface wind belts in Hunan province to be restored, which provides a new basis for supporting the connection between the Hengyang Basin and its surrounding red basins. The discovery provides valuable material for the study of eolian deposition in Hunan province.
2019, 37(4): 749-757.
doi: 10.14027/j.issn.1000-0550.2018.170
Abstract:
Abundant marine trace fossils are preserved in Devonian strata in the Longmenshan area. Geological investigation of the Shigouli outcrop in the Jiangyou area found trace fossils preserved in tempestite from the upper Yangmaba Formation,and are systematically described. A total of five ichnogenera were identified:Chondrites, Changchengia,Rhizocorallium,Skolithos and Zoophycos. Of these,Chondrites-Zoophycos ichnofacies reflect a shallow shelf environment affected by storm action. The order in which the trace fossils appeared was identified from the cross-cutting relationship between the traces and their positions:Chondrites,Zoophycos,Rhizocorallium, Changchengia and Skolithos. The Chondrites and Zoophycos ichnofossils in the tempestite had the distinctive features of opportunistic species,and are important indicators of the sedimentary environment. Numerous thick branched Chondrite burrows reflect the oxygen content of the water at that time. This not only controlled the burrow diameter,it may also imply a link between burrow diameter and the severity of the living environment. The Zoophycos in the tempestite revealed that its trace maker was living in shallow shelf deposits in the Devonian.
Abundant marine trace fossils are preserved in Devonian strata in the Longmenshan area. Geological investigation of the Shigouli outcrop in the Jiangyou area found trace fossils preserved in tempestite from the upper Yangmaba Formation,and are systematically described. A total of five ichnogenera were identified:Chondrites, Changchengia,Rhizocorallium,Skolithos and Zoophycos. Of these,Chondrites-Zoophycos ichnofacies reflect a shallow shelf environment affected by storm action. The order in which the trace fossils appeared was identified from the cross-cutting relationship between the traces and their positions:Chondrites,Zoophycos,Rhizocorallium, Changchengia and Skolithos. The Chondrites and Zoophycos ichnofossils in the tempestite had the distinctive features of opportunistic species,and are important indicators of the sedimentary environment. Numerous thick branched Chondrite burrows reflect the oxygen content of the water at that time. This not only controlled the burrow diameter,it may also imply a link between burrow diameter and the severity of the living environment. The Zoophycos in the tempestite revealed that its trace maker was living in shallow shelf deposits in the Devonian.
2019, 37(4): 758-767.
doi: 10.14027/j.issn.1000-0550.2018.167
Abstract:
A set of deep-water gravity flow deposits was developed in the Bayingebi Formation in the Guaizihu Depres-sion. Although the buried depth is more than 3 400 m, the reservoir exhibits good physical properties. In order to illus-trate the characteristics of this high-quality reservoir, petrological features, reservoir properties, and pore structure characteristic were analyzed based on the integrated application of thin section observations, scanning electron micros-copy (SEM), mercury penetration, and petrophysical properties, combined with the burial history features. We then discuss the formation mechanism. The results show that the sand bodies are mainly composed of feldspathic litharen-ite with low compositional maturity, low porosity, and extra-low permeability (the average porosity is 11.4%; the aver-age permeability is 6.093×10-3 μm2). The types of reservoir space are dominated by residual intergranular pores, fol-lowed by dissolved intergranular and intragranular pores, and a small amount of microfractures. The throat of the reser-voir is small, and the pore/throat ratio is relatively large. This reservoir is mainly characterized by medium-large pores and a medium-thin throat. The primary textural and mineralogical compositions of the sediments were the fundamental conditions for the formation of the high-quality reservoir in the Bayingebi Formation. Good initial physical properties developed due to strong hydrodynamic conditions, coarse-grained sediments, medium-sorting, and low matrix content. In addition, high hard particle contents gives the reservoir a stronger ability to resist compaction, and some feldspar and rock debris provide material basis for later dissolution. Early rapid burial, the later stage of tectonic uplift, and the hydrocarbon generation of organic matter played an important role in the formation of overpressure and dissolution. The combination of abnormal high pressure, dissolution by organic acid, and hydrocarbon filling were the key factors in forming the high-quality reservoir in the deep-burial Bayingebi Formation.
A set of deep-water gravity flow deposits was developed in the Bayingebi Formation in the Guaizihu Depres-sion. Although the buried depth is more than 3 400 m, the reservoir exhibits good physical properties. In order to illus-trate the characteristics of this high-quality reservoir, petrological features, reservoir properties, and pore structure characteristic were analyzed based on the integrated application of thin section observations, scanning electron micros-copy (SEM), mercury penetration, and petrophysical properties, combined with the burial history features. We then discuss the formation mechanism. The results show that the sand bodies are mainly composed of feldspathic litharen-ite with low compositional maturity, low porosity, and extra-low permeability (the average porosity is 11.4%; the aver-age permeability is 6.093×10-3 μm2). The types of reservoir space are dominated by residual intergranular pores, fol-lowed by dissolved intergranular and intragranular pores, and a small amount of microfractures. The throat of the reser-voir is small, and the pore/throat ratio is relatively large. This reservoir is mainly characterized by medium-large pores and a medium-thin throat. The primary textural and mineralogical compositions of the sediments were the fundamental conditions for the formation of the high-quality reservoir in the Bayingebi Formation. Good initial physical properties developed due to strong hydrodynamic conditions, coarse-grained sediments, medium-sorting, and low matrix content. In addition, high hard particle contents gives the reservoir a stronger ability to resist compaction, and some feldspar and rock debris provide material basis for later dissolution. Early rapid burial, the later stage of tectonic uplift, and the hydrocarbon generation of organic matter played an important role in the formation of overpressure and dissolution. The combination of abnormal high pressure, dissolution by organic acid, and hydrocarbon filling were the key factors in forming the high-quality reservoir in the deep-burial Bayingebi Formation.
2019, 37(4): 768-784.
doi: 10.14027/j.issn.1000-0550.2018.154
Abstract:
The Ordos Basin is one of the largest petroliferous basins in China. Exploration has shown that the Shan2 to He8 members in the SE part of the basin has considerable exploration potential. Previous studies have focused mainly on the sand body distribution within a single member,but have not discussed the overall sedimentary evolu-tion for a number of members. To infer the transition from meandering river delta to braided river delta and analyze the main controlling factors of the evolutionary process,and to reconstruct the true evolution,in this study facies log-ging from core samples was integrated with analytical data to recognize the delta sedimentary facies. Changes of the base-level elevation enabled the sediment supply to be recognized,and to reconstruct the paleo-geomorphology and paleoclimate in the study area,and the evolution of a meandering river delta to a braided river delta. The elevation of the base-level surface sloped downwards between Shan2 and He8,enhancing channel sedimentation and increasing the supply of sediment. It was then inferred that the gradation of paleo-thickness due to compaction gradually in-creased the ancient slope. Analysis of lithofacies together with geochemical characterization indicated that the ancient air temperature gradually decreased. A comprehensive analysis of these four factors has provided a theoretical basis for the study of the evolution of meandering river deltas to braided river deltas. Such reconstruction of the sedimentary evolution process is significant for hydrocarbon exploration. In addition,the study provides a theoretical basis for the spatiotemporal evolution of various types of delta.
The Ordos Basin is one of the largest petroliferous basins in China. Exploration has shown that the Shan2 to He8 members in the SE part of the basin has considerable exploration potential. Previous studies have focused mainly on the sand body distribution within a single member,but have not discussed the overall sedimentary evolu-tion for a number of members. To infer the transition from meandering river delta to braided river delta and analyze the main controlling factors of the evolutionary process,and to reconstruct the true evolution,in this study facies log-ging from core samples was integrated with analytical data to recognize the delta sedimentary facies. Changes of the base-level elevation enabled the sediment supply to be recognized,and to reconstruct the paleo-geomorphology and paleoclimate in the study area,and the evolution of a meandering river delta to a braided river delta. The elevation of the base-level surface sloped downwards between Shan2 and He8,enhancing channel sedimentation and increasing the supply of sediment. It was then inferred that the gradation of paleo-thickness due to compaction gradually in-creased the ancient slope. Analysis of lithofacies together with geochemical characterization indicated that the ancient air temperature gradually decreased. A comprehensive analysis of these four factors has provided a theoretical basis for the study of the evolution of meandering river deltas to braided river deltas. Such reconstruction of the sedimentary evolution process is significant for hydrocarbon exploration. In addition,the study provides a theoretical basis for the spatiotemporal evolution of various types of delta.
2019, 37(4): 785-797.
doi: 10.14027/j.issn.1000-0550.2018.177
Abstract:
The rock fabric,mineral composition,main trace-element geochemistry and carbon and oxygen isotopic characteristics of lacustrine dolomite in the Lower Ganchaigou Formation of the Qaidam Basin were studied,and the origin of the dolomite was analyzed. The sedimentary environment of dolomite in the study area was shallow water and a relatively closed lacustrine environment. The dolomite is predominantly micritic,and occurs in layers and blocks. It is seen as algal,gypsiferous,and grain dolomite. The degree of ordering is generally low. The ratio of Mg2+ to Ca2+ is low. The Mn,Fe,Na and Sr contents are all high in the major elements,indicating that the water body was highly sa-line and that it was relatively closed. Light rare earth elements among the trace elements are relatively enriched,with negative europium anomalies,reflecting a low-temperature diagenetic environment and shallow burial. Oxygen iso-topes have a low negative value,indicative of salinized water overall. Carbon isotopes are low-positive or negative, mainly due to atmospheric water and biological effects. The study suggests that the dolomite was formed penecontem-poraneously in a relatively closed saline water environment. The Mg2+ in the dolomitic fluid is mainly derived from leaching by atmospheric water and deep brine,with a small amount from algal decomposition. The Mg2+ was enriched by evaporation and salt mineral precipitation,and the enriched fluid was dolomitized by seepage-reflux infiltration of calcite.
The rock fabric,mineral composition,main trace-element geochemistry and carbon and oxygen isotopic characteristics of lacustrine dolomite in the Lower Ganchaigou Formation of the Qaidam Basin were studied,and the origin of the dolomite was analyzed. The sedimentary environment of dolomite in the study area was shallow water and a relatively closed lacustrine environment. The dolomite is predominantly micritic,and occurs in layers and blocks. It is seen as algal,gypsiferous,and grain dolomite. The degree of ordering is generally low. The ratio of Mg2+ to Ca2+ is low. The Mn,Fe,Na and Sr contents are all high in the major elements,indicating that the water body was highly sa-line and that it was relatively closed. Light rare earth elements among the trace elements are relatively enriched,with negative europium anomalies,reflecting a low-temperature diagenetic environment and shallow burial. Oxygen iso-topes have a low negative value,indicative of salinized water overall. Carbon isotopes are low-positive or negative, mainly due to atmospheric water and biological effects. The study suggests that the dolomite was formed penecontem-poraneously in a relatively closed saline water environment. The Mg2+ in the dolomitic fluid is mainly derived from leaching by atmospheric water and deep brine,with a small amount from algal decomposition. The Mg2+ was enriched by evaporation and salt mineral precipitation,and the enriched fluid was dolomitized by seepage-reflux infiltration of calcite.
2019, 37(4): 798-811.
doi: 10.14027/j.issn.1000-0550.2018.187
Abstract:
Two gravity piston core samples were taken for investigation from a typical Late Pleistocene-Holocene slope-confined canyon in the Baiyun Sag,Pearl River Mouth Basin. Sediment analyses from the core located at the canyon head revealed the presence of five sand layers containing particles of quartz,shell fragments,and abundant foraminifera. Sedimentary features (e.g., inverted ages and eroded base) showed that the sand in each layer was trans-ported by gravity currents,but other features also revealed the action of traction currents,evident from (1) cumula-tive frequency diagrams and C-M cross-plots,which showed that the coarse particles were transported by rolling; (2) well-sorted sand particles;and (3) top or basal lithological contacts and bidirectional cross-bedding within some in-tervals. These sedimentary features,considered together,indicate that the sediments were deposited at the canyon head by gravity current,which itself was possibly modified by active bottom currents in the northern South China Sea. The sediment waves recognized in the canyon,together with in-situ mooring observations,have raised the postulation that the bottom current was caused by the interplay of complex canyon morphology and internal waves and tides, which would have also led to the observed cyclic variation of cross-bedding thickness and the exceptional abundance of planktonic foraminifera. These conclusions increase the knowledge of possible mechanisms of sand sedimentation within the canyon,and have important implications for the prediction of deepwater reservoirs for both hydrocarbons and natural gas hydrate.
Two gravity piston core samples were taken for investigation from a typical Late Pleistocene-Holocene slope-confined canyon in the Baiyun Sag,Pearl River Mouth Basin. Sediment analyses from the core located at the canyon head revealed the presence of five sand layers containing particles of quartz,shell fragments,and abundant foraminifera. Sedimentary features (e.g., inverted ages and eroded base) showed that the sand in each layer was trans-ported by gravity currents,but other features also revealed the action of traction currents,evident from (1) cumula-tive frequency diagrams and C-M cross-plots,which showed that the coarse particles were transported by rolling; (2) well-sorted sand particles;and (3) top or basal lithological contacts and bidirectional cross-bedding within some in-tervals. These sedimentary features,considered together,indicate that the sediments were deposited at the canyon head by gravity current,which itself was possibly modified by active bottom currents in the northern South China Sea. The sediment waves recognized in the canyon,together with in-situ mooring observations,have raised the postulation that the bottom current was caused by the interplay of complex canyon morphology and internal waves and tides, which would have also led to the observed cyclic variation of cross-bedding thickness and the exceptional abundance of planktonic foraminifera. These conclusions increase the knowledge of possible mechanisms of sand sedimentation within the canyon,and have important implications for the prediction of deepwater reservoirs for both hydrocarbons and natural gas hydrate.
2019, 37(4): 812-824.
doi: 10.14027/j.issn.1000-0550.2018.174
Abstract:
In the Paleogene,a set of mixed sediments in a salinified-lake basin was developed to the west of the Yingxiong Ridge,controlled by regional tectonic movement and the local climate. The rock series have particular sed-imentary facies successions,which are very important in the study of high-frequency sequence division and paleogeo-graphic evolution of the salinified-lake basin. Upper member strata of the Lower Ganchaigou Formation consist of dark,fine-grained mixed sediments with complex mineral composition,mainly composed of terrigenous clasts,car-bonate minerals and evaporite minerals,which respectively correspond to the pre-salinization stage,the initial salini-zation stage and the final salinization stage of the lake water. In the study area,the strata are divided into seven fourth -order sequences (Psq1-Psq7). Except for Psq1 and Psq2,they occur in three parts:clastic rocks,carbonate rocks and evaporite. Theoretically,deepwater salt forms by the stratification of heavy brine as it sinks. As a result,salt rock in deep depression areas is thicker than in low uplift areas. Therefore,taking into consideration the fourth-order se-quence,the paleogeomorphology of the upper member of the Lower Ganchaigou Formation in the Yingxi area was able to be restored in accordance with the relative thicknesses of the salt rock strata. Psq1and Psq2 developed in the early stages of lake evolution,when the area covered by the salt-water lake was very limited. During the development of Psq3 to Psq5,the degree of salinization gradually increased,and the scope of salt-water lake continually expand-ed,with thick evaporite being formed in the depression area. Then,during the deposition of Psq6,the topography of the study area tended to be flatter due to filling and leveling of the previous stages,and the range of height differences was considerably reduced. Finally, during the deposition of Psq7, the western area of the lake in the study area gradu-ally became shallower and the sedimentary center began to migrate eastward.
In the Paleogene,a set of mixed sediments in a salinified-lake basin was developed to the west of the Yingxiong Ridge,controlled by regional tectonic movement and the local climate. The rock series have particular sed-imentary facies successions,which are very important in the study of high-frequency sequence division and paleogeo-graphic evolution of the salinified-lake basin. Upper member strata of the Lower Ganchaigou Formation consist of dark,fine-grained mixed sediments with complex mineral composition,mainly composed of terrigenous clasts,car-bonate minerals and evaporite minerals,which respectively correspond to the pre-salinization stage,the initial salini-zation stage and the final salinization stage of the lake water. In the study area,the strata are divided into seven fourth -order sequences (Psq1-Psq7). Except for Psq1 and Psq2,they occur in three parts:clastic rocks,carbonate rocks and evaporite. Theoretically,deepwater salt forms by the stratification of heavy brine as it sinks. As a result,salt rock in deep depression areas is thicker than in low uplift areas. Therefore,taking into consideration the fourth-order se-quence,the paleogeomorphology of the upper member of the Lower Ganchaigou Formation in the Yingxi area was able to be restored in accordance with the relative thicknesses of the salt rock strata. Psq1and Psq2 developed in the early stages of lake evolution,when the area covered by the salt-water lake was very limited. During the development of Psq3 to Psq5,the degree of salinization gradually increased,and the scope of salt-water lake continually expand-ed,with thick evaporite being formed in the depression area. Then,during the deposition of Psq6,the topography of the study area tended to be flatter due to filling and leveling of the previous stages,and the range of height differences was considerably reduced. Finally, during the deposition of Psq7, the western area of the lake in the study area gradu-ally became shallower and the sedimentary center began to migrate eastward.
2019, 37(4): 825-833.
doi: 10.14027/j.issn.1000-0550.2018.175
Abstract:
In order to dissect the characteristics of underwater distributary channel and discuss its significance for the formation of lithological traps,the sedimentary microfacies characteristics of an underwater distributary channel were analyzed by multi-parameter analysis. It is proposed that underwater distributary channel microfacies sandbodies which develop in different paleogeomorphological units are of great significance to the formation of lithological traps because of their different physical properties. The results show that two paleogeomorphological units are present in an ancient low rise and an ancient trench in the second section of the Sangonghe Formation in the study area:a non-rotat-ing distributary channel was developed in the ancient trench,and a rotating distributary channel developed on the sides of the low rise. The physical properties of the normal cyclic underwater distributary channel sandbody gradually deteriorated from bottom to top,and lateral blockages formed at the sides of the reverse cyclic distributary channel reservoir,which was beneficial to the formation of lithological traps. On this basis,a lithological trap model was es-tablished and applied for the positioning of a pre-exploration well in the oilfield, and achieved good results.
In order to dissect the characteristics of underwater distributary channel and discuss its significance for the formation of lithological traps,the sedimentary microfacies characteristics of an underwater distributary channel were analyzed by multi-parameter analysis. It is proposed that underwater distributary channel microfacies sandbodies which develop in different paleogeomorphological units are of great significance to the formation of lithological traps because of their different physical properties. The results show that two paleogeomorphological units are present in an ancient low rise and an ancient trench in the second section of the Sangonghe Formation in the study area:a non-rotat-ing distributary channel was developed in the ancient trench,and a rotating distributary channel developed on the sides of the low rise. The physical properties of the normal cyclic underwater distributary channel sandbody gradually deteriorated from bottom to top,and lateral blockages formed at the sides of the reverse cyclic distributary channel reservoir,which was beneficial to the formation of lithological traps. On this basis,a lithological trap model was es-tablished and applied for the positioning of a pre-exploration well in the oilfield, and achieved good results.
2019, 37(4): 834-846.
doi: 10.14027/j.issn.1000-0550.2018.188
Abstract:
The structural setting of the Sichuan Basin in the latter part of the Early Cambrian epoch appears as a sin-gle depression between two uplifts. The climate was hot and dry,favoring the development of gypsum-salt rocks, which characteristics were analyzed in a combination of field profile,drilling section,thin section and regional geo-logical survey. The present study of lithofacies paleogeography of gypsum-salt rocks compares the diverse hydrody-namic characteristics,sedimentary structure,single-layer thickness,development of areal scale and location,and fi-nally establishes a sedimentary model of Early Cambrian gypsum-salt rocks in the Sichuan Basin,with the following results. (1) The gypsum-salt rocks mainly comprise gypsum-bearing dolomite and gypsum rocks from 0.74 m to 419.9 m thick. The sedimentary centers are located in the Chongqing-Hejiang and Lichuan-Pengshui areas. The overall dis-tribution characteristics are "mountain type". (2) The lithological association of gypsum-salt rocks falls into three cat-egories:association of dolomite or mudstone (sand) rocks with gypsum-salt rocks,developed in the upper tidal zone;rhythmically interbedded associations of gypsum-salt rocks and dolomite in relatively shallow,low-lying areas in the upper tidal zone;and the association of gypsum-salt rocks with dolomite or shale in deeper depressions with re-stricted circulation in the upper-to inter-tidal zone. (3) In multiple-lagoons (one main lagoon and two subsidiary la-goons),the gypsum-salt rocks were formed mainly in static lagoons in southeastern Sichuan,with two subsidiary la-goons in disturbed lagoons in eastern Sichuan and in mixed tidal depressions in southwestern Sichuan.
The structural setting of the Sichuan Basin in the latter part of the Early Cambrian epoch appears as a sin-gle depression between two uplifts. The climate was hot and dry,favoring the development of gypsum-salt rocks, which characteristics were analyzed in a combination of field profile,drilling section,thin section and regional geo-logical survey. The present study of lithofacies paleogeography of gypsum-salt rocks compares the diverse hydrody-namic characteristics,sedimentary structure,single-layer thickness,development of areal scale and location,and fi-nally establishes a sedimentary model of Early Cambrian gypsum-salt rocks in the Sichuan Basin,with the following results. (1) The gypsum-salt rocks mainly comprise gypsum-bearing dolomite and gypsum rocks from 0.74 m to 419.9 m thick. The sedimentary centers are located in the Chongqing-Hejiang and Lichuan-Pengshui areas. The overall dis-tribution characteristics are "mountain type". (2) The lithological association of gypsum-salt rocks falls into three cat-egories:association of dolomite or mudstone (sand) rocks with gypsum-salt rocks,developed in the upper tidal zone;rhythmically interbedded associations of gypsum-salt rocks and dolomite in relatively shallow,low-lying areas in the upper tidal zone;and the association of gypsum-salt rocks with dolomite or shale in deeper depressions with re-stricted circulation in the upper-to inter-tidal zone. (3) In multiple-lagoons (one main lagoon and two subsidiary la-goons),the gypsum-salt rocks were formed mainly in static lagoons in southeastern Sichuan,with two subsidiary la-goons in disturbed lagoons in eastern Sichuan and in mixed tidal depressions in southwestern Sichuan.
2019, 37(4): 847-857.
doi: 10.14027/j.issn.1000-0550.2018.164
Abstract:
The Upper Ordovician Wufeng-Lower Silurian Longmaxi organic-rich shale in the Sichuan Basin and its peripheral regions,whose thickness is 20-60 m,is the main focus for the exploration and development of shale gas in China. According to our research in recent years,the temporal and spatial distribution of the organic-rich shale varies greatly. Based on the observation of key sections,core drilling,lithosphere biostratigraphy,geochemistry and miner-al analysis,the spatial-temporal distribution characteristics of the organic-rich shale in the Sichuan Basin and its sur-rounding areas were clarified. Three types of organic-rich shale depositional evolutionary models were established, which are described as follows:(1) The "two sags sandwiched between three uplifts" model in the Changning-Weiyu-an area:the organic-rich shale deposits at WF2-LM5 in Changning and at LM1-LM8 in Weiyuan. The thickness of the organic-rich shale in the uplift is much thinner than that in the sags. (2) The "stage-progressive distribution pat-tern" model in the Chongqing and Qianjiang area:The closer the area is to the interior of the basin,the later the sedi-mentation of the organic-rich shale tends to end. The Jiaoshiba area is the depositional center of the shale,while the thickness of other areas is relatively thin. (3) The "stable uplifting" model in northeastern Sichuan:The organic-rich shale in Well Wuxi 2 in the center of the basin has a large thickness and a long depositional duration (WF2-LM9). The thickness of the shale in the marginal area of the basin is significantly thinner,and the region presents an overall upward trend. With the passage of time,the organic shale depositional area becomes smaller and smaller. In general, the deposition of these shales is mainly influenced by the Guangxi movement. The organic-rich shale sedimentary cen-ters tend to move northward gradually,and the shale only deposits in the Weiyuan and Wuxi areas after the Aeronian stage in the Sichuan Basin. The research results have an important guiding significance for the micro-layer division and fine reservoirs contrast of the Wufeng-Longmaxi shale in this area.
The Upper Ordovician Wufeng-Lower Silurian Longmaxi organic-rich shale in the Sichuan Basin and its peripheral regions,whose thickness is 20-60 m,is the main focus for the exploration and development of shale gas in China. According to our research in recent years,the temporal and spatial distribution of the organic-rich shale varies greatly. Based on the observation of key sections,core drilling,lithosphere biostratigraphy,geochemistry and miner-al analysis,the spatial-temporal distribution characteristics of the organic-rich shale in the Sichuan Basin and its sur-rounding areas were clarified. Three types of organic-rich shale depositional evolutionary models were established, which are described as follows:(1) The "two sags sandwiched between three uplifts" model in the Changning-Weiyu-an area:the organic-rich shale deposits at WF2-LM5 in Changning and at LM1-LM8 in Weiyuan. The thickness of the organic-rich shale in the uplift is much thinner than that in the sags. (2) The "stage-progressive distribution pat-tern" model in the Chongqing and Qianjiang area:The closer the area is to the interior of the basin,the later the sedi-mentation of the organic-rich shale tends to end. The Jiaoshiba area is the depositional center of the shale,while the thickness of other areas is relatively thin. (3) The "stable uplifting" model in northeastern Sichuan:The organic-rich shale in Well Wuxi 2 in the center of the basin has a large thickness and a long depositional duration (WF2-LM9). The thickness of the shale in the marginal area of the basin is significantly thinner,and the region presents an overall upward trend. With the passage of time,the organic shale depositional area becomes smaller and smaller. In general, the deposition of these shales is mainly influenced by the Guangxi movement. The organic-rich shale sedimentary cen-ters tend to move northward gradually,and the shale only deposits in the Weiyuan and Wuxi areas after the Aeronian stage in the Sichuan Basin. The research results have an important guiding significance for the micro-layer division and fine reservoirs contrast of the Wufeng-Longmaxi shale in this area.
2019, 37(4): 858-867.
doi: 10.14027/j.issn.1000-0550.2018.179
Abstract:
On the basis of core logging,well logging and other data sources,this study examined the mode of high-resolution sequence development and how the base level cycles evolved at all levels in the Beixi area of the Beier De-pression in the Hailar Basin. The results showed that five types of sequence boundary developed in the Nantun Forma-tion within the study area. These are divided into three long-term base-level cycles and 12 medium-term base-level cy-cles. Stratigraphic thickness is controlled by changes in paleogeomorphology and datum surface. The ancient land-scape has had a great influence on thickness variation,forming the ‘south-thick gradual transition to north-thick’ stratum model. As an example,the effect of datum elevation on phase sequence,phase domain and source reservoir distribution was analyzed in LSC1. It is considered that during LSC1 deposition,the change of base level comprised an early slow-rising stage,a fast-rising stage,a further slow-rising stage,a slow decline stage,and finally a rapid de-cline stage. A fan delta,a lake bed fan and a lake depositional system were developed in the study area. The sandbod-ies in MSC1,MSC2 and MSC5 are well developed and have beneficial physical properties. The geometrical shape is predominantly multi-layered. Thick-layer distributary channel deposits constitute the main reservoir sandbodies. MSC3 represents the longest time period of the largest lake,forming high-quality hydrocarbon source rocks. The source reservoir is close to the bottom, and a superior reservoir formation has resulted.
On the basis of core logging,well logging and other data sources,this study examined the mode of high-resolution sequence development and how the base level cycles evolved at all levels in the Beixi area of the Beier De-pression in the Hailar Basin. The results showed that five types of sequence boundary developed in the Nantun Forma-tion within the study area. These are divided into three long-term base-level cycles and 12 medium-term base-level cy-cles. Stratigraphic thickness is controlled by changes in paleogeomorphology and datum surface. The ancient land-scape has had a great influence on thickness variation,forming the ‘south-thick gradual transition to north-thick’ stratum model. As an example,the effect of datum elevation on phase sequence,phase domain and source reservoir distribution was analyzed in LSC1. It is considered that during LSC1 deposition,the change of base level comprised an early slow-rising stage,a fast-rising stage,a further slow-rising stage,a slow decline stage,and finally a rapid de-cline stage. A fan delta,a lake bed fan and a lake depositional system were developed in the study area. The sandbod-ies in MSC1,MSC2 and MSC5 are well developed and have beneficial physical properties. The geometrical shape is predominantly multi-layered. Thick-layer distributary channel deposits constitute the main reservoir sandbodies. MSC3 represents the longest time period of the largest lake,forming high-quality hydrocarbon source rocks. The source reservoir is close to the bottom, and a superior reservoir formation has resulted.
Main Factors Controlling Carbonate Reservoir Formation: Case study of Tazhong block III, Tarim Basin
2019, 37(4): 868-877.
doi: 10.14027/j.issn.1000-0550.2018.176
Abstract:
Carbonate rock is the major target horizon for Paleogene oil/gas exploration in the Tazhong area of the Tarim Basin. Study of the major factors controlling reservoir formation plays an important role in carbonate explora-tion. The complexity of reservoir forming in Tazhong block Ⅲ has restricted exploration,so analysis of carbonate res-ervoir deposition and later re-formation was carried out in terms of tectonic evolution to confirm that the high-energy facies controlled by faulted uplifting was the material basis for karstification. High-energy shoals and inner platform gentle shoals are widely developed in Tazhong block Ⅲ. The feather-shaped fractured zone and echelon faults in the NE strike-slip fault system are favorable areas for reservoir development. The paleostructures in the crucial period of hydrocarbon accumulation controlled the migration pathway for Paleogene oil/gas migration from north to south in Ta-zhong block Ⅲ,where a nose-like structure is most favorable for oil/gas migration and accumulation. Based on the above work,a comprehensive evaluation of reservoir formation was carried out based on regional seal distribution, reservoir prediction and existing drilling records, to direct future exploration.
Carbonate rock is the major target horizon for Paleogene oil/gas exploration in the Tazhong area of the Tarim Basin. Study of the major factors controlling reservoir formation plays an important role in carbonate explora-tion. The complexity of reservoir forming in Tazhong block Ⅲ has restricted exploration,so analysis of carbonate res-ervoir deposition and later re-formation was carried out in terms of tectonic evolution to confirm that the high-energy facies controlled by faulted uplifting was the material basis for karstification. High-energy shoals and inner platform gentle shoals are widely developed in Tazhong block Ⅲ. The feather-shaped fractured zone and echelon faults in the NE strike-slip fault system are favorable areas for reservoir development. The paleostructures in the crucial period of hydrocarbon accumulation controlled the migration pathway for Paleogene oil/gas migration from north to south in Ta-zhong block Ⅲ,where a nose-like structure is most favorable for oil/gas migration and accumulation. Based on the above work,a comprehensive evaluation of reservoir formation was carried out based on regional seal distribution, reservoir prediction and existing drilling records, to direct future exploration.
