Geological Conditions Of Shale Gas Accumulation In Black Rock Series Of Lower Paleozoic In ZhenBa-XiXiang Area Of Western Daba Mountain

The Sichuan Basin is an important area for shale gas exploration and development in China,and the shale gas development in the Lower Paleozoic Longmaxi Formation and Niutitang Formation has made great breakthroughs.The research area is located in the northeast margin of Sichuan Basin,which is far from shale gas geological research and exploration and development in Sichuan Basin.Therefore,it is of great practical significance for the exploration and development of shale gas in Zhenba-Xixiang area of western Daba Mountain.Based on the theoretical guidance of sedimentology,element geochemistry,oil and gas geochemistry,reservoir geology and other disciplines,this paper analyzes the petrological,sedimentary facies,reservoir and organic geochemical characteristics of Lower Paleozoic Longmaxi Formation and Niutitang Formation shale in Zhenba-Xixiang area of Daba Shanxi Section,and defines the shale gas accumulation conditions in the study area.On this basis,the favorable area of shale gas in the study area is predicted.Field outcrop and core observation analysis show that both Longmaxi Formation and Niuitang Formation in the study area are shallow shelf sedimentary systems,and the sedimentary facies distribution lithology is relatively stable,the longitudinal bottom-up grain size gradually coarses,and the deep shelf subfacies deposits gradually transition to shallow shelf subfacies deposits,with the latter being the dominant.The black shale rich in organic matter is mainly preserved in the deepwater shelf subphase.Elemental geochemistry and analysis of abundant strawberry pyrite observed by SEM indicate that the Longmaxi Formation and Niuitang Formation both reflect strong hypoxic reduction conditions,which are conducive to the enrichment of organic matter in shale.The results of whole rock X-ray diffraction analysis show that the composition minerals of Longmaxi Formation and Niuitang Formation in the study area include brittle minerals(quartz,feldspar,carbonate minerals and a small amount of pyrite)and clay minerals.The brittle minerals of Longmaxi Formation account for 47%～84%of the total minerals,with an average content of 67%,and the clay minerals account for 16%～53% of the total minerals,with an average content of 33.1%.The brittle minerals account for 60%～77% of the total minerals in Niutitang Formation with an average content of 68%,and the clay minerals account for 23%～40% of the total minerals with an average content of 32.4%.The clay minerals in Longmaxi Formation and Niutitang Formation mainly include illite and chlorite,and contain no other clay minerals or little content.Polarized light microscopy and scanning electron microscopy showed that the microscopic pore types of Longmaxi Formation included intergranular pores,corrosion pores,intergranular pores,cast film pores and micro-cracks,and organic pores were rare.The micropore types of Niutitang Formation consist of intergranular pores,corrosion pores,organic pores,intergranular pores and micro-fractures,while the macrocracks mainly develop structural fractures and interlayer foliation fractures.Organic geochemical analysis shows that the Longmaxi Formation is a good source rock with an abundance of organic matter ranging from 0.06% to 4.24%,with an average of 1.93%.The kerogen type is mainly type I,with a small amount of type II kerogen.The Niuitang Formation belongs to the good-excellent source rock class,with organic matter abundance ranging from 0.18% to 12.3% with an average of2.85%.The kerogen type is type I.The organic matter maturity of the two source rocks is relatively high,and both have reached the stage of high-over-maturity evolution.Based on the analysis of shale gas accumulation conditions of Lower Paleozoic Longmaxi Formation and Niutitang Formation in the study area,and according to the classification criteria of shale gas favorable areas,it is predicted that the Longmaxi Formation and Niutitang Formation are favorable areas of shale gas,and the shale gas accumulation potential and scale of the Niutitang Formation in the west of the study area are better and larger than that of the Longmaxi Formation.