Geochemical Characteristics And Geological Significance Of Organic-rich Shale In Chongqing Area

In this paper,two sets of shale of the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation in Xiushan,Youyang and Chengkou areas among Chongqing are used as research objects,and two sets of shale formation are analyzed by using organic and inorganic geochemical parameters.Based on the organic and inorganic geochemical parameters,the sedimentary environment of two sets of shale formation was analyzed.The main controlling factors of the formation and preservation of two sets of shale organic matter were discussed.The reservoir formation conditions of two sets of shale gas were compared and analyzed.The study suggests that the Lower Cambrian Niutitang Formation shale deposits in the low-latitude shallow-semi-deep water-shelter environment in Chongqing area,and its provenance environment is dominated by active continental margins.The climate is characterized by dryness and heat,and the water environment mainly closed-semi-closed,the depth of water changes greatly from Chengkou to Youyang and Xiushan,the salinity is high,and it is accompanied by the participation of seabed hydrothermal activity.The formation of the shale organic matter is affected by the redox conditions.Its paleo-salinity and paleoclimate are not conducive to the formation of organic matter;The Lower Silurian Longmaxi shale deposits in the shallow-water shelf environment of the middle and low latitudes.The source structure environment is dominated by continental island arcs,and the tectonic activities are more intense so that volcanic activity in the sediment source area is relatively intense.The climate of the sedimentary period is characterized by moist and warm climate,the water closure condition is good,the weak oxidation-reduction environment is dominant,the water depth changes little,the salinity is relatively low due to the melting of global glaciers.Its good water closure condition and warm and humid environment provide favorable conditions for the formation of organic matter.The depth and salinity of water have little effect on the formation of organic matter.The shale gas accumulation conditions of the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation in Chongqing area are compared and analyzed.The organic matter maturity of the Lower Cambrian Niutitang Formation shale is higher than that of the Lower Silurian Longmaxi Formation,and the thermal evolution is also higher;The rock and mineral characteristics of the two sets of shale are similar,and the brittle mineral content is higher,which is conducive to the later fracturing transformation.The porosity and permeability of the Longmaxi Formation are higher than those of the Niutitang Formation,and the pore structure is also superior to the Niutitang Formation.Studies have shown that the reservoir conditions of the Lower Silurian Longmaxi Formation are better than those of the Lower Cambrian Niutitang Formation.The paper also compares and analyzes the main controlling factors of shale gas accumulation in the Lower Silurian Longmaxi Formation in study area to the Jiaoshiba area in Fuling.The sedimentary environment,rock characteristics and organic matter formation and storage conditions of the two sets of shale are similar,but the late tectonic transformation in the study area is stronger than that in the Jiaoshiba area.This is the reason why the Longmaxi Formation shale failed to find industrial shale gas reservoir.