2021 Vol. 39, No. 6
Display Method:
2021, 39(6): 1321-1343.
doi: 10.14027/j.issn.1000-0550.2021.141
Abstract:
Dolostone-evaporite paragenesis systems （DEPS） have been widely found in Precambrian to Quaternary strata worldwide. However， despite its wide spatio-temporal distribution， the formation mechanism and controlling factors of DEPS remain poorly studied. By reference to copious literature from China and elsewhere combined with previous knowledge of DEPS in China， this study comprehensively summarizes the development characteristics， combination patterns and formation mechanism of DEPS. Firstly， its spatio-temporal distribution is clarified based on published studies： laterally， DEPS is known to be widely distributed in 51 regions globally， mainly in the northern hemisphere （mostly in Asia， followed by Europe and North America）. Comparatively， it is found less often in Africa and is only scattered in South America and Oceania. Stratigraphically， although DEPS is found in many Precambrian to Quaternary strata， it appears mostly in the Permian. The combination patterns of evaporite and dolostone in DEPS include the interbedding of dolostone and evaporite， with thick dolostone overlying thick evaporite， thick evaporite overlying thick dolostone， thick dolostone mixed with thin evaporite， and thick evaporite mixed with thin dolostone. The formation process， distribution and main controlling factors of symbiotic differences as well as the particularity and universality of the paragenetic system development are discussed. Secondly， based on macro- to microscale characterization of mineral types in DEPS， six types of evaporites and five types of dolostones have been found and their genesis is discussed. Finally， the properties， characteristics， sources and evolution paths of diagenetic fluids in DEPS are summarized. DEPS studies provide new concepts for solving the “dolostone （or dolomite？）” question， and promote the understanding of the important basic geological problems associated with DEPS. Such studies also provide theoretical guidance for the exploration of oil and gas resources in the paragenetic system.
2021, 39(6): 1344-1356.
doi: 10.14027/j.issn.1000-0550.2020.101
Abstract:
Analysis of carbonate microfacies is helpful for understanding the distribution of sedimentary facies and the evolution of paleogeography， which is of guiding significance in oil and gas exploration. Based on a large number of thin-section data and core information， seven types of microfacies （MFT1⁃MFT7） and four types of sedimentary array （MA1⁃MA4） were defined in the Ordovician M a 5 5 M a 5 5
2021, 39(6): 1357-1370.
doi: 10.14027/j.issn.1000-0550.2021.028
Abstract:
The variation of an accurate global sea level curve has always been a difficult and frontier scientific problem in sedimentary geological research. There are 2 reasons： （1） the deep-time sedimentary record is the comprehensive response to global sea level change and regional movement， but accurately eliminating the regional tectonic effect is also difficult； （2） determining paleo-water depth based on deep-time sedimentary records is still being explored. The Kalpin area of the Tarim Basin in the Late Ordovician experienced an environmental evolution from basin to basin margin to shelf， making it highly sensitive to sea level changes and an ideal area for conducting research on sea level changes based on sedimentary microfacies. The study conducted a detailed field investigation and intensive sampling analysis of the Dawangou section. Based on lithofacies sedimentology， paleontology， etc.， the two major facies of the continental shelf and the basin edge were divided， and seven microfacies are distinguished in detail： MF1， brick red /purple microbial micritic limestone； MF2， gray microbial micritic limestone； MF3， gray-green calcareous clay； MF4， gray bioclastic - lithoclastic breccia limestone； MF5， gray bioclastic silty limestone； MF6， gray bioclastic micritic limestone； and MF7， dark gray calcareous clay. Based on advanced microfacies for the sensitive zone of deep-time sedimentary records， the sea level change curve of the Upper Ordovician Kanling and Yingan Formations is constructed， and a set of methods for determining the global sea level curve can be summarized. Our sea level curve is compared with the upper Yangtze region， the southern margin of Ordos， North America， South America， Wales， and the global sea level change， and results indicate that the frequent fluctuations of sea level are similar to North America. The reconstruction plate tectonic results of the study indicate that the Caledonian tectonic movement has global corresponding characteristics during this period. Frequent plane fluctuations are mainly caused by orogenic collisions in different regions.
2021, 39(6): 1371-1386.
doi: 10.14027/j.issn.1000-0550.2021.059
Abstract:
Ooids developed all over the world in the Lower Triassic， and giant ooids appeared. The ooid shoal at the margin of the Feixianguan Formation platform in northeastern Sichuan comprises strongly heterogeneous and complex ooid types. The relationship between ooid type， grain size parameter evolution and hydrodynamic forces were studied for deposits in the Longtancun， Laixi and Yudongzi regions using field measurement， microscopic identification and grain-size analysis. Seven primary divisions and nine subdivisions of ooids were found， and fractured ooids may be classed separately. In addition to skewness， other parameters （e.g.， average particle size， standard deviation， kurtosis and graphic curve shape of particle size data） are strongly correlated. The classifications suggest that the ooid shoal at Longtan village indicates a change from an active marginal zone to a stable ooid sand flat as hydrodynamic conditions gradually decreased. At Laixi， a deposition cycle of active marginal zone-stable ooid sand flat to active marginal zone is evident， with decreasing and then increasing hydrodynamic force. The hydrodynamic force decreases with upward at Yudongzi. However， there is poor correlation between the horizon containing giant and the overall trend of changes in ooid grain type and size， possibly because the abnormally large particle size in the giant ooids is affected by a combination of microbial action and the rapid increase of seawater carbonate saturation rather than rapid change in hydrodynamic force. Therefore， when judging the hydrodynamic conditions of sedimentary environments containing giant ooids using grain-size parameters， it is not possible to draw a conclusion from grain size alone， but it is necessary to analyze the correlations between ooid grain-size parameters.
2021, 39(6): 1387-1405.
doi: 10.14027/j.issn.1000-0550.2021.066
Abstract:
Sedimentary manganese carbonates， especially the black shale type， are generally considered to have been deposited in a deepwater， restricted and reductive sedimentary environment. Deposits of manganese carbonate are well developed in the black shale near the top of the Ediacaran Doushantuo Formation in the Chengkou area of the northern Yangtze Craton. The black shale host rock and cerium anomaly of manganese carbonates is thought to be vital evidence of the sedimentary environment of manganese carbonates， but there is a lack of systematic sedimentological evidence， with the result that the sedimentary environment of the manganese carbonates in the Chengkou area remains controversial. In this study， the sedimentary textures and structures of the manganese carbonates in this area are described in detail， based on field and core observations and thin section microscopy. This work included a detailed sedimentological study of the Chengkou manganese deposit， and a model of the sedimentary environment was constructed. It was found that the Chengkou manganese carbonate deposit is mainly layered， and contains well-developed stromatolites， oncolites， thrombolites and dendrolites. The frequent bonding growth of the oncolites indicates that the Chengkou manganese deposits are typical microbialites with several kinds of microstructure： clotted， spherical， filamentous， tubular， fibrous and radial. The sedimentary records from drill cores show alternate deposits of rhodochrosite and collophanite. In the rhodochrosite， all types of microbialite are deposited cyclically. Normal grain bedding and intraclast layers indicative of stronger hydrodynamic force are rarely recorded. The collophanites are characterized by intraclast depositions in a strong hydrodynamic environment and often overlie the rhodochrosites， evidence of the cyclic sedimentary nature of the series of manganese-bearing rock to phosphorus-bearing rock and indicates gradually shallowing sedimentary water at the end of the Doushantuo Formation in the Chengkou area. When considered together with the sedimentary characteristics of the Doushantuo Formation in adjacent areas， Member I of the formation in the Chengkou area evidently developed a barrier-free coastal sedimentary system without a dolomite cap. Member II was a carbonate ramp sedimentary system with mixed siliciclastic sediments. The manganese carbonate microbialites mainly developed in the barrier and bonding reef sedimentary environment in the lower part of the inner carbonate ramp. It is notable that the development of barriers and bonding reefs at the end of the Doushantuo Formation was an important dynamic factor in the evolution of the sedimentary basin in the study area， promoting the evolving regional sedimentary system from carbonate ramp to carbonate platform.
2021, 39(6): 1406-1424.
doi: 10.14027/j.issn.1000-0550.2021.036
Abstract:
The Luoping biota has been preserved in the middle of the second member of the Guanling Formation of the Middle Triassic， and is typical of the biological recovery and spreading that took place after the mass extinction at the end of the Permian. The paleoenvironment of the second member of Guanling Formation in the Jiangbian village section， Luoping county， Yunnan province， was analyzed for microfacies and sedimentary geochemical compositions. The diagnosis of the sedimentary facies and a few paleoenvironmental proxies indicated that these strata underwent four sedimentary stages. Stage I mainly comprise restricted platform and open platform facies characterized by low MoEF， V/Cr and medium UEF， indicating an oxic-suboxic environment. Stage II deposits are mainly tidal flats with restricted platform facies， as indicated by relatively low MoEF， UEF and V/Cr typical of an oxic environment. Stage III mainly consists of intraplatform basin facies characterized by significantly high MoEF， UEF， and V/Cr， suggesting an anoxic environment. Stage IV comprises mainly tidal flat deposits with shoal and open platform facies characterized by low MoEF， V/Cr and medium UEF， indicating an oxic-suboxic environment. The Luoping biota was mainly preserved in Stage III， implying that anoxia played an important role in the extraordinarily well-preserved Middle Triassic marine reptiles and other marine fossils.
2021, 39(6): 1425-1439.
doi: 10.14027/j.issn.1000-0550.2021.124
Abstract:
A travertine is a special chemical or biochemical deposition. They are widely distributed in the interior of the continent and contain important geological information and are special reservoirs of oil and gas. The factors that affect the deposition of travertines are complex and diverse. Based on the investigation of the formation process and classification of travertines， this article discusses the controlling factors of travertine formation from five aspects： climate environment， hydrogeological conditions， physical and chemical conditions of water bodies， biological activities， and structural activities. Then， the travertine in the Tabei area of Xinjiang province is compared with foreign travertine deposits in terms of depositional environment， model， and deposition rate. The travertine deposits show good seasonal stratification under a warm and humid environment in north Tarim. The travertine in Wudaopan area was deposited into a large amount of terrigenous debris and greatly affected by biological activities. In Liuhuanggou， travertines are a product of hydrothermal upwellings resulting from fault activity. By comparing and summarizing the findings， the travertine that filled in the fractured caves in Tabei has well-developed internal pores， good connectivity， and good oil content display. Therefore， the travertine has a certain storage capacity.
2021, 39(6): 1440-1450.
doi: 10.14027/j.issn.1000-0550.2021.119
Abstract:
Substantial discoveries and breakthroughs have been made in the search for natural gas in the middle Sichuan paleo-uplift and upper Ediacaran strata from the end of the Neoproterozoic （Sinian Dengying Formation） in the peripheral slope area of the Sichuan Basin. This detailed study of late Ediacaran strata and sedimentation in the non-inherited structural slope area of the Sichuan Basin is of significant benefit to Dengying Formation exploration. Comprehensive interpretation of geological and geochemical data and geophysical logging for the region， taking previous research reports into account， enabled fourth-order sequence division and correlation of SQ4. The sedimentary evolution of the system tract is also discussed. The findings show： （1） SQ4 is divided into five fourth-order sequences. Influenced by their sedimentary paleogeomorphology， SQ4-1， SQ4-2 and SQ4-3 developed in the slope area of north Sichuan； SQ4-3， SQ4-4 and SQ4-5 developed in the high region of the paleo-uplift. （2） In the Dengying Formation， SQ4 comprises a low system tract and a transgressive system tract from bottom upwards. The low system tract evolved from multi-stage core agglomerate beach sedimentation. The transgressive system tract indicates evolution of inter-hill core sand debris beach dome cap sedimentation. In the Gaoshiti Moxi area of the middle Sichuan uplift， SQ4 is the result of the evolution of a sedimentary transgressive system tract and a highstand system tract from bottom upwards. The transgressive system tract and the north Sichuan slope area are isochronous deposits， and the highstand system tract represents the sedimentary evolution of inter-hill sand debris beach and conglomerate stone beach near Qiuping. （3） Conditions of the low system tract of Dengying Formation SQ4 may have formed lithological hydrocarbon traps. The SQ4-1 and SQ4-2 low system tracts in the north Sichuan slope region cover large areas， and favor lithological trap exploration.
2021, 39(6): 1451-1465.
doi: 10.14027/j.issn.1000-0550.2021.078
Abstract:
Recent exploration for hydrocarbons in the deep carbonate rocks of the Shunbei area of the Tarim Basin has been fruitful. Drilling and seismic data indicate that global sea-level change during the Lower to Middle Ordovician divided the carbonate rocks into a second-order sequence containing eight third-order sequences. Three sequence development models are proposed⁃transgressive， highstand and lowstand⁃and their influencing factors are analyzed. Cores and rock slices of Penglaiba strata below the sequence boundary clearly show dissolution pores， indicating lowstand and highstand karsts in the reservoir. Analysis of the actual geological conditions concludes that the fault/karst theory of the Tahe area is not applicable to the Shunbei area. Karstification below the sequence boundary is the main factor in the formation of the Shunbei reservoirs.
2021, 39(6): 1466-1477.
doi: 10.14027/j.issn.1000-0550.2021.072
Abstract:
The reconstruction of litho-paleogeography has an important role in understanding the evolution of sedimentary environments and the spatial distribution of sedimentary formations， and for hydrocarbon exploration. The unique location and evolution of structures caused complicated tectonic⁃sedimentary differentiation of the Permian in the Sichuan Basin. Previous studies have reported differences in their understanding of the original geographical pattern of the Permian in the Sichuan Basin. This has limited the prediction of favorable exploration facies belts in the Chihsia and Maokou Formations to a certain extent. In this study， the stratigraphy and sequence boundaries were interpreted from combined seismic and well log data and field data surrounding the Sichuan Basin. On this basis， the Chihsia Formation was divided into two third-order sequences， SQ1 and SQ2， and the Maokou Formation into three third-order sequences， SQ3， SQ4 and SQ5. Five paleogeographic maps were created based on the sedimentary facies evidence and characteristics of these 3rd-order sequences. These reveal that the tectonic features transitioned from the northeast of the Chihsia stage to the northwest of the Maokou stage. Tectonic-sedimentary differentiation features transition from the uplift and depression of the quasi-stable background in the Chihsia stage to platform/trough differentiation of the unstable background in the Maokou stage. The favorable exploration facies belt in the Chihsia Formation in the quasi-stable period was mainly due to the relatively flat landform， with slight uplift under water. The dolomite shoal facies is evident in a “belt and ring” structure. The unstable Maokou Formation was influenced by the regional extensional structural background and syndepositional normal fault activity. Favorable exploration facies belts occur in “trough and peninsula” structures. The platform margin between the Chihsia and Maokou periods marks the origin of shallow-water geomorphology. The favorable exploration areas are mainly in the backshore cloud shoal facies belt inside the margin. This differs from reef-sedimentary zones， where favorable exploration facies belts occur at the edge of the platform of a high-energy reef beach along the shore.
2021, 39(6): 1478-1490.
doi: 10.14027/j.issn.1000-0550.2021.105
Abstract:
Studies of the sedimentary facies and sequence stratigraphy have indicated that the main depositional environments of the Lower Triassic Jialingjiang Formation in the eastern Sichuan Basin were a restricted platforms and evaporite platforms. Two third-order sequences （SQ1， SQ2） and five fourth-order sequences （SSQ1⁃SSQ5） were recognized. Sediments in the Jialingjiang strata evolved from a restricted platform to an evaporite platform， then to restricted platform again， and finally to an evaporite platform. The main hydrocarbon reservoirs and producing layers appear in the SSQ1-SSQ2 sequence. Sequences SSQ1⁃SSQ5 are further subdivided into transgressive systems tracts （TST） and highstand systems tracts （HST）. Taking these sequences as mapping units， lithofacies paleogeographic maps were prepared for each of the fourth-order tracts. Evidence from the study of the distribution of favorable reservoirs indicates that the restricted platform-to-platform shoal in the SSQ1 and SSQ2 sequences are key areas for future exploration.
2021, 39(6): 1491-1505.
doi: 10.14027/j.issn.1000-0550.2021.098
Abstract:
The ancient marine strata in large basins in western China （the Ordos， Sichuan and Tarim Basins） are complicated by multi-stage structural transformation， which makes it difficult to accurately identify and analyze sedimentary formations and identify oil and gas exploration targets. This paper presents a comprehensive analysis of the sedimentary tectonic background and infill sequences of the marine carbonate rocks in these basins， and establishes the geological structural model of typical sedimentary formations. It is found that the marine carbonates in these basins were deposited during the pre-Mesozoic discrete period of the China Block； the Ordos Basin is quasi-stable， and there was almost no clastic rock cushion in most areas before the deposition of the ancient carbonate rocks. The sub-stable Sichuan Basin and active Tarim Basin each have a clastic base. Ancient underlying rifts developed in the basement and at the margins of the three basins， containing ochsenkopf （Niutou）-type formations. During the pre-Indosinian， a number of different-scale paleo-uplifts were extensively formed， around which “para-anticlinal” formations were deposited. During tectonically stable intervals， an epicontinental sea covered the marine interior craton， and dolomite sheet formations developed in the wide tidal platform. Few of these have been studied until now. The strongly concealed “Niutou”， “para-anticlinal” and “dolomite sheet” formations at great burial depths and subject to tectonic/sedimentary differentiation， are all important targets for oil and gas exploration. Therefore， how to accurately extract information about these structures that were formed and re-formed by multiple cycles of tectonic movement， and identify the fine structures of the three basins and their hydrocarbon accumulation， are challenges for the oil and gas exploration of deep marine strata， and further study is very necessary.
2021, 39(6): 1506-1516.
doi: 10.14027/j.issn.1000-0550.2021.074
Abstract:
The origin of the dolomite in the Qixia Formation in western Sichuan and its relationship to the Emeishan basalt eruption is controversial. The dolomites were divided into four types on the basis of field section and core observations， petrological studies， whole rock REE， C， O and Sr isotope and mineral micro-area C and O isotope analysis： residual granular dolomite， crystalline dolomite， saddle dolomite and vein-filling dolomite. The residual granular dolomite is thick and massive， mainly with flat planar and subhedral crystals， with dark cathodoluminescence （CD）. Its REE distribution pattern is similar to micritic limestone， with negative Ce and Eu anomalies and slightly negative O isotopic values. C and Sr isotopic values are mostly contemporaneous with seawater， and micro-area C and O isotopic analysis showed that the C isotopic values of dolomite with a foggy center and bright edge are contemporaneous with seawater. The bright edge of the O isotope is more negative than the foggy center， which indicates that this kind of dolomite was initially formed by shallow seawater burial， then reworked by subsequent thermal fluid superimposition. The dolomite grains are dense and massive， with dark CD. The REE distribution pattern is also similar to that of micritic limestone， but with negative Ce and slightly positive Eu anomalies. The C and Sr isotopes are the same as in seawater of the same period； the O isotope is slightly more negative than the seawater. The C and O isotopes of dolomite grain micro-areas of different size show similar properties， indicating that it was mainly distributed in the same period， formed by shallow burial in seawater， but recrystallized by later thermal fluid， forming anisotropic dolomite. The curved-surface allomorphic coarse-grained saddle dolomite occurs as fracture and dissolution cavity infill. Under the microscope， it has wavy extinctions and bright red color. Its C isotope is similar to that of Permian seawater， its O isotope is distinctly negative and Eu is positive， indicating that saddle dolomite is hydrothermal in origin. Because it has passed through silicate strata， its Sr isotope is obviously high. The petrological and geochemical characteristics of this dolomite indicate a late burial origin. In summary， the dolomite in the Qixia Formation in western Sichuan is mainly of shallow burial origin， then superimposed and re-formed by hydrothermal solution during the Emeishan basalt eruption. Only saddle dolomite occurring as fracture and cavity infill is of hydrothermal origin.
2021, 39(6): 1517-1531.
doi: 10.14027/j.issn.1000-0550.2021.065
Abstract:
Based on field investigation and geochemical analysis， it is believed that the dolomites of the Longwangmiao Formation in the Tuhechang and Nanshanping sections， southeast Sichuan Basin， differ from other dolomites in the area， being mainly of reflux seepage origin. Tuhechang dolomite is of burial origin， and the dolomite in Nanshanping is of mixed-water origin. The Tuhechang dolomites occur in the lower part of the Longwangmiao Formation， and did not have the geological conditions suitable for forming reflux seepage dolomites. The temperature of inclusions of the Tuhechang dolomites shows a formation temperature higher than 105 °C. Its Mn element and Fe element content are relatively high （average 792.5×10–6 and 3 428.8×10–6 respectively）， with good correlation. The average δ18O content is –8.18‰， which is significantly lower than for dolomites in other sections of the study area. These all reflect a burial environment in the Tuhechang section. The main part of the Nanshanping section lies in a slope facies zone， and did not have the geological conditions for the formation of reflux seepage dolomite. Nanshanping dolomite contains an obviously low value for Na element， and δ18O is also obviously negative， both indicating that it was formed in a low-salinity fluid environment at a relatively low temperature. In addition， Nanshanping dolomite shows obvious U element enrichment， indicating that its formation was related to atmospheric fresh water， and it is therefore a mixed-water dolomite. These two dolomite types have relatively good physical properties， which is highly significant for reservoir development in the southeastern Sichuan Basin.
2021, 39(6): 1532-1545.
doi: 10.14027/j.issn.1000-0550.2021.093
Abstract:
The beach facies dolomite in the Qixia Formation is well developed in the Moxi-Gaoshiti area， central Sichuan Basin. At present， the mechanism of differential dolomitization in beach facies is still unclear， which hinders the prediction of high-quality dolomite reservoirs. This study combined an examination of the petrology and stratigraphy， grain type and tectonic background to determine the main controlling factors and genetic models of different types of dolomitization by analyzing the trace elements， carbon， oxygen and strontium isotope characteristics of different types of dolomites. The results show that， in the study area， the Qixia Formation dolomites are predominantly fine crystals， followed by medium crystals and coarse crystals. Most of the crystal forms are semi-automorphic to automorphic. The dolomite has an obvious residual grain structure， indicating that the original lithology was granular limestone. The cathodoluminescence of the fine-grained， and fine-to-medium crystalline dolomite is generally dark red to red in color. The distribution pattern of rare earth elements is similar to that of limestone of the same period. The 87Sr/86Sr ratios of the dolomites lie within the range of Permian seawater， indicating that the diagenetic fluid of the dolomite has the same homology as the limestone deposited in seawater. The δ13C value of dolomite （3.73‰⁃4.19‰） is similar to that of limestone （3.61‰⁃4.93‰） in the same period， indicating that the dolomite and limestone have the same carbon source. The Sr content decreases significantly from limestone to dolomite and Mn content increases， together indicating that the limestone was metasomatized to form dolomite after a particular mode of diagenesis： in this case， by replacement of porous granular limestone with Mg2+-rich fluid in the buried strata. The cathodoluminescence of medium-to-coarse crystalline dolomite is red， with obvious zonal characteristics， and has high Mn content， low Sr/Ba ratio and positive Eu anomaly. The 87Sr/86Sr ratios are higher than for seawater in the same period. The δ18O value is between -8.06‰ and -8.52‰. The higher homogenization temperature of the inclusions and the negative δ18O value together indicate that the buried dolomitization process was also affected by high local temperatures. This type of dolomite is formed from a continuous and sufficient supply of dolomitic fluid during burial. Overall， buried dolomitization was the main cause of the dolomite in this area. Mg2+-rich fluid in the formation migrated under the dual influences of pressure and thermal convection， which promoted the movement of dolomitizing fluid. However， in some areas， the presence of saddle-shaped dolomite indicates that it was subsequently subjected to various degrees of hydrothermal transformation in the later period.
2021, 39(6): 1546-1564.
doi: 10.14027/j.issn.1000-0550.2021.112
Abstract:
Magnesium （Mg） is a major rock-forming element and biological nutrient element， and is thus an important link between continents， oceans and the Earth’s interior. Marine carbonates are a major reservoir of Mg and an important component of the global Mg cycle. Using Mg isotopes to trace the deposition-diagenesis process of marine carbonate formation is the prerequisite for reconstructing the deep-time seawater Mg isotopic composition （δ26MgSW） and quantifying the Earth’s historical marine Mg cycle. Studies using this approach have made considerable progress in the past two decades； for example： （1） investigation of Mg isotope fractionation during the formation of different carbonate minerals has been enhanced； （2） numerical models of Mg isotopic behavior have been established， providing constraints on the dolomitization process； and （3） the use of Mg isotopes to trace the early diagenetic fluid system has been explored. These have provided a theoretical base for using marine carbonate data to reconstruct δ26MgSW. The deposition-diagenesis process and its effects on Mg isotopic composition should be considered when selecting information from marine carbonate archives to reconstruct the marine Mg cycle. Critical evaluation combined with geochemical models are necessary when choosing an archive for reconstructing deep-time δ26MgSW.
2021, 39(6): 1565-1579.
doi: 10.14027/j.issn.1000-0550.2021.073
Abstract:
The top of the Majiagou Formation in the Lower Paleozoic of the Ordos Basin experienced long-term weathering and denudation and formed a weathered crust karst reservoir dominated by gypsodolomite. In this study， the pore type and filling behavior of the gypsum dolomite were examined， and the karstification properties of the gypsodolomite were analyzed. The difference between the karstification characteristics of gypsodolomite and traditional carbonate karstification was studied， and a karstification model of the gypsum dolomite development area is discussed. Due to their strong hydrophilicity and solubility higher than either dolomite and calcite， anhydrite nodules and gypsum crystals readily absorb water， then dissolve to form selective pores in the fabric. This leads to the formation of honeycomb dissolution pores in the gypsodolomite layers. Due to the high solubility and mechanical instability of anhydrite， the weathered crust reservoir space led to interbedded gypsodolomite， dolomite and calcite dolomite in the study area， consisting mainly of dissolution pores in the anhydrite nodules and crystals together with associated expansion microcracks. The pore sizes are self-limiting. The lithology of gypsodolomite and sedimentary facies-anhydrite in a tidal flat of the upper tidal zone were the essential materials and environmental conditions for the formation of the reservoirs， and thus led directly to the strata-controlled distribution of gypsodolomite gas reservoirs. The surface gypsodolomite was firstly leached by fresh water to form honeycombed pores. Due to the unique physical and chemical properties of gypsodolomite， the karst water in the weathering crust was mainly permeated by diffusion.Karstification in the gypsodolomite development area was due to the dissolution of gypsodolomite by diffusion permeation， which is the fundamental reason why the karstification properties and model differ from those for carbonate rocks.
2021, 39(6): 1580-1592.
doi: 10.14027/j.issn.1000-0550.2021.063
Abstract:
Microporous micritic limestone forms important hydrocarbon reservoirs in the Middle East， and is clearly different from the three major reservoir types in China， namely， marine paleo-karst reservoirs， reef-shoal reservoirs and stratified dolostone reservoirs. The matrix texture is of two kinds： tight mosaic textures， and chalky textures. The micrites occur in three crystal forms： micro-rhombic， rounded and anhedral. Rounded micrite crystals form the best reservoirs， followed by micro-rhombic micrite reservoirs and anhedral tight reservoirs. Micropores （< 10 μm diameter） are the main pores in the micrites， in addition to spongy matrix dissolution pores， mold holes and dissolution channels. The formation of micropores is mainly restricted by the original mineral components and the diagenetic conditions. Low-magnesium calcites are the essential precursor minerals. Relative mineralogical stability may preserve primitive structures that include intergranular micropores. The formation of pores in micritic limestones is the result of two stages of early atmospheric freshwater leaching during shallow burial and organic acid dissolution during buried diagenesis. In the shallow freshwater burial case， calcite overgrowth （the Ostwald ripening process） leads to early cementation that prevents compaction while partially retaining the original structure and intergranular micropores， and increases permeability by eliminating small crystals. Freshwater selective leaching also forms widely developed mold holes. In deeper burial diagenesis， the dissolution of organic acid fluid forms spongy matrix dissolution pores and dissolution channels， which also leads to the formation of rounded micrite crystals.
2021, 39(6): 1593-1608.
doi: 10.14027/j.issn.1000-0550.2021.096
Abstract:
In this study， the margin platform in the central eastern region to the west of the Kaijiang-Liangping Trough was divided into two areas： （1） the main central Longgang area， and （2） the eastern Longgang area， to determine the properties， genesis， patterns and differences of the reservoir. The main Longgang area is a steep-slope platform margin type. The reef⁃shoal complex growth close to the outer edge of the platform margin has a ridgeline shape， with multiple stages apparent in the vertical direction. The eastern part of the Longgang area is a gently sloping platform margin； the reef⁃shoal complex is the product of broad horizontal and vertical development. The former type of reservoir space contains constitutively selective intragrain dissolution pores， intergrain dissolution pores and non-constitutively selective dissolution fractures and structural fractures， producing low porosity and low permeability. The latter type consists mainly of intercrystal pores， intercrystal dissolved pores， intergrain dissolved pores and structural dissolution joints， resulting in a fracture-type reservoir possessing extremely low porosity and low permeability. Development of the reef⁃shoal complex reservoir was completely controlled by atmospheric freshwater dissolution， reflux infiltration and dolomitization， together with early diagenetic near-surface karstification， burial dissolution and buried dolomitization. The water body in the eastern Longgang area was relatively deep， resulting in weak atmospheric freshwater dissolution in the syngenetic stage， and weak near-surface dissolution in the early diagenetic stage. The high-quality reservoir is influenced by fractures and buried dissolution.
2021, 39(6): 1609-1621.
doi: 10.14027/j.issn.1000-0550.2021.121
Abstract:
Oil and gas exploration of the Majiagou Formation in the Daniudi area， Ordos Basin， continues to make breakthroughs. Practice has shown that the whole Fifth member of the Majiagou Formation （Ma5） contains gas， but its multiplicity of reservoir types and complex genetic backgrounds have not been conducive to classification prediction. To systematically summarize the reservoir genetic types in this study， the reservoir space types and main influences on reservoir development were investigated by core observation， thin section identification， cathodoluminescence thin section observation， whole-rock X-ray diffraction analysis and carbon and oxygen isotope analysis. These demonstrate that， in the study area， the Ma5 reservoirs contain intercrystal pores， intercrystal dissolution pores， microcracks， residual intergrain pores and intragrain dissolution pores. The reservoir developed in a confined evaporative platform as a result of seepage-reflux dolomitization. Epigenetic karstification had no obvious influence on the reservoir in the Ma55 submember. Genetic analysis in the study area suggests four types of reservoir： gypsum-dissolved， weakly re-formed fractured dolomite， powder-crystal dolomite and syngenetic dissolution granular reservoirs， with gypsum-dissolved reservoirs and powder-crystal dolomite reservoirs being the main types. Gypsum-dissolved reservoirs are related to the salinity of the sedimentary environment and karstification during the supergene period， and contain mainly gypsum mold pores. Powder-crystal dolomite reservoirs are the result of seepage-reflux dolomitization during the period of sea-level decline， and the reservoir space consists of intercrystal pores. This summary is significant as a guideline for subsequent high-quality reservoir predictions in the Ma5 member in the Ordos Basin.
