| The Carboniferous in the eastern Chongqing of the Sichuan Basin has experienced more than 60 years of exploration and development.Scholars generally study more structural gas reservoirs,while less research on non-structural gas reservoirs.In the recent resource evaluation,the undiscovered Carboniferous resources in eastern Sichuan are 4859×108m3,most of which are located in non-structural traps.Strengthening the study of non-structural traps in the Carboniferous in eastern Sichuan is of great significance for oil and gas exploration in the Sichuan Basin.This paper takes the Carboniferous Huanglong Formation in the eastern Chongqing in Sichuan Basin as the research object.Based on the previous studies,under the theoretical guidance of reservoir geology,sedimentology and organic geochemistry,combined with drilling,logging and thin section observation.The key points are to study the hydrocarbon source,reservoir and trap conditions of low-carbon carbonaceous natural gas accumulation,and analyze the control factors of non-structural gas reservoirs.The Huanglong Formation in eastern Chongqing can be divided from bottom to top into the first section,the second section and the third section of Huanglong Formation.The second section of Huanglong Formation is the main reservoir of Carboniferous,followed by the third srction.The main lithology is a combination of dolomite,breccia dolomite,limestone and breccia.The thickness of strata varies greatly in the region,generally 20-60m.The development of Huanglong Formation reservoir is mainly influenced by dissolution.In the study area,there are two types of fracture-type reservoirs and fracture-porosity reservoirs.Within the reservoir,different types of reservoir space are formed by dissolution.Among them,intergranular dissolved pores and intergranular pores are the most important pore types of the Huanglong Formation,which controls the development of the reservoir.The reservoir of Huanglong Formation overall porosity is between 2%.and 6%,and the permeability is concentrated between?0.110?×10-3?m2.It is a low-porosity medium-high permeability reservoir.The distribution of the reservoir is influenced by sedimentary facies and karst,which has the characteristics of great difference and strong heterogeneity on the plane.Through the comparative study of conventional composition of natural gas and carbon isotope composition of alkane,the Carboniferous natural gas comes from Lower Silurian Longmaxi Formation and Middle Permian Qixia-Maokou Formation.The source rock of Longmaxi Formation is black mudstone with high content of organic matter,and the type of organic matter is type I,which is in over-mature stage.Longmaxi high quality source rocks in eastern Chongqing area are between 90and 130m thick and have good hydrocarbon generating capacity.The source rocks of Qixia-Maokou Formation are mainly carbonate rocks.The organic carbon content is high,ranging from 0.4%to 0.7%.The organic matter type is II1,which is generally in the stage of high-over maturity.Qixia Formation-Maokou Formation source rocks are distributed steadily in eastern Chongqing with a thickness of more than 250m and a moderate preference for hydrocarbon generation.The development of gas reservoirs in Carboniferous structural low-level areas in eastern Chongqing is mainly controlled by formation thickness,sealing conditions,reservoir development scale and karstification.Gas reservoirs only develop in areas with formation thickness greater than 10m.Reservoir boundary lithology is compact,and there are abnormally high fluid pressure and pinch-out zone,which can form good lateral sealing conditions.Based on the comprehensive analysis of reservoir conditions and controlling factors of Huanglong formation in eastern Chongqing,the reservoir-forming model of structural low-level area is established.It is pointed out that natural gas in structural slope area mainly comes from Longmaxi formation and syncline area mainly comes from Qixia formation-Maokou formation. |
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