Study Of The Organic Matter Shales Lithofacies And Reservoir Characteristics Of The Upper Permian Dalong Formation In Western Hubei

The North American“Shale Gas Revolution”promoted global energy transformation and triggered breakthroughs and innovations in petroleum geology theory and technology.The key issue of shale gas exploration and development is the accurate understanding of reservoirs.The large-scale exploration of shale gas has been the focus and hotspot of oil and gas geology research in China for decade.However,there are still many problems in shale reservoir research.The organic-rich shale has extremely complex mineral composition and diagenetic evolution process,which leads to its strong reservoir heterogeneity.The unclear understanding of the evolution of pore structure and the control factors constrained the understanding of the shale gas enrichment law and the yield evaluation of the Dalong Formation.In addition,compared with conventional reservoirs,the organic-rich shale has a smaller pore structure?mainly nano-scale pores?,the wider range of pore sizes,which challenged to traditional reservoir research methods.Therefore,how to reasonably characterize the organic-rich shale reservoir structure has become the main problem in the current pore structure research.In this study,the organic-rich shale of the Dalong Formation in western Hubei Province was taken as the research object.And the lithofacies,reservoir characteristics and pore structure of the organic shale of the Dalong Formation in western Hubei province were studied based on the sediment coring,thin section observation,scanning electron microscope?SEM?and sedimentary geochemistry analysis.The model of organic matter enrichment of the Dalong Formation in western Hubei was established,and the factors controlling the quality development of the shale reservoirs in the Dalong Formation were identified.Through the above comprehensive analysis and research,the following main conclusions and understandings are drawn:SEM,X-ray diffraction analysis and silicon isotope analysis were used to identify the terrigenous clastic in quartz of the Dalong Formation,as well as the biogenetic and clay mineral transformation of the Dalong Formation shale,and the clay minerals of the Dalong Formation shale are almost entirely composed of illite.According to the shale mineral composition and organic matter content and structure,a shale facies division method based on"organic matter+mineral composition"was established.Five types of lithofacies were identified in the study area,including organic-rich siliceous shale,organic-medium siliceous shale,organic-rich calcium-siliceous mixed shale,calcareous shale and argillaceous shale.The Enshi-Xuanen area mainly developed organic-rich siliceous shale facies,and the Jianshi area mainly developed organic-rich calcium-silicon mixed shale.Geochemical elements?i.e.,U,V,and Mo?and pyrite framboid data indicate that an anoxic and sulfurized environment predominated during the early deposition of the Dalong Formation.U_EF-Mo_EF covariation map shows that the study area was an open sedimentary basin in the Late Permian.The values of Cu/Al in the Dalong Formation were 17.59×10^-4,indicating that Western Hubei had extremely high primary productivity in the late Permian.The positive correlation between Cu,Ni,and V elements indicates that the hypoxia in the late Permian seawater was controlled by higher primary productivity.The input organic matter of the land source or ocean surface is decomposed by microorganisms,and accompanied by a huge amount of oxygen consumption,the bottom water body is deprived of oxygen,which is conducive to the preservation of organic matter.Driven by climate change,the Western Hubei in Late Permian experienced different chemical weathering processes.The trend of CIA values in the Dalong Formation indicates that the Late Permian climate had experienced a significant reduction process,followed by a rapid increase in temperature at the end of the Late Permian.The organic enrichment of the Dalong was closely related to the paleoclimate change.The rapid warming enhanced the terrestrial chemical weathering process,resulting in a sharp increase in the supply of marine nutrients to the ocean.Excessive supply of terrestrial nutrients makes the ocean more hypoxic and acidified,resulting in a decrease in primary productivity in South China,which ultimately led to the end of the organic enrichment process.Diagenesis types such as compaction,cementation,dissolution,metasomatism,clay mineral transformation and organic hydrocarbon generation were identified in the Dalong Formation shale through SEM observation and scanning electron microscopy.Research has shown that compaction was the main reason for the decrease of porosity.Although siliceous cementation blocked pore throat,it was beneficial to development of the reservoir fracture.Organic hydrocarbon generation promoted the development of mineral dissolution pores,whereas it was not conducive to the formation of authigenic minerals.The diagenetic signs such as illite crystallinity and vitrinite reflectance indicated that diagenetic stage of the Dalong Formation shale belongs to the intermediate diagenetic B-stage B to the late diagenetic stage.High-resolution scanning electron microscopy analysis showed that the black shale of the Dalong Formation developed intergranular pores,intragranular pores,organic pores and inorganic fissures.Among them,secondary organic pores are the most important pore type of organic-rich shale.The secondary organic matter pores are irregularly elliptical-circular and honeycomb structure partially.The pore size of organic matter in the shale of the Dalong Formation was obviously small,and it was not developed in some samples.This is because the formation uplifted too early and the abnormal high pressure lost too early.The pore of organic matter shrinked and deformed under the overburden rock pressure,and even disappeared.Scanning electron microscopy?SEM?showed that there was a close relationship between the lithofacies and the development of pore characteristics.Low pressure CO₂ adsorption,low pressure N₂ adsorption,high pressure MIP,combined with NLDFT model were adopted to conduct full-aperture splicing of shale pore structure,thereby quantitatively analyzing the pore structure and pore size distribution characteristics of different lithofacies in the Dalong Formation.The results show that micropores,mesopores and macropores contributed 59.1%,18.3%and 22.7%respectively to the total pore volume,while micropores and mesopores accounted for 30.7%and 67.9%of the total specific surface area,respectively,which is the main space for gas adsorption.There are some differences in pore development in different shale.The proportion of micropores in siliceous shale is the highest,while that in argillaceous shale is the highest.Organic carbon content was positively correlated with microporous pore structure parameters,and clay minerals were positively correlated with mesoporous pore structure parameters,indicating that both organic matter and clay minerals developed nanopores.And organic matter mainly provides micropores,while clay minerals played a positive role in the development of mesopores.The reservoir quality development of the Dalong Formation in the study area is controlled by a variety of factors.Favorable sedimentary environment was the key to the high organic carbon content of the Dalong Formation shale.The extremely high primary productivity in western Hubei controlled the enrichment of organic matter and formed the“productivity preservation model”of the organic matter of the Dalong Formation shale in the late Permian,However,the tectonic movement determined the burial history of shale.The Dalong Formation shale began to deepen from the Late Permian and experienced rapid uplift and strong tectonic deformation during the late Cretaceous.The early rapid burial was conducive to the rapid hydrocarbon generation of the organic matter of the Dalong Formation shale,and depth reached maximum in Late Cretaceous.The experiment shows that the average vitrinite reflectance was2.16%,which indicates that the Dalong Formation shale reached a high maturity to the over-maturation stage,which was extremely beneficial to the development of organic pores.The rapid rise of the stratum is not conducive to the preservation of abnormal high pressure in the reservoir.The early pressure loss of the Dalong Formation shale during the rapid uplift of the stratum made the organic pores shrinked and deformed under the pressure of the overlying rock,which was not conducive to the preservation of organic pores.Organic pores are the most important type of pores in shale reservoirs.Organic hydrocarbons are beneficial to the development of organic pores and acid dissolution,but compaction and cementation have a filling and plugging effect on pores.The Dalong Formation shale cement was mainly composed of brittle mineral quartz and calcite,which was beneficial to the development of fracture joint under pressure.The lithofacies affected the primary pore morphology and distribution of shale,and the differences in different facies diagenesis constrained the pore structure of the reservoir.According to the above comprehensive analysis,the main controlling factors of the Dalong Formation shale reservoir development were higher organic carbon content and special tectonic effects.