Main Control Factors And Models Of Biogas Reservoir Formation In Shallow Crude Oil Degradation In Gaoyou Sag,Subei Basin

The focus of exploration in the North Jiangsu Basin is on oil reservoirs,and gas reservoir research is relatively weak.Along with the continuous discovery of oil reservoirs,several natural gas reservoirs have also been discovered.However,because the work of natural gas is only in its infancy,and the degree of exploration is low,the type,accumulation pattern and exploration potential of natural gas have yet to be clarified.In recent years,heavy oil reservoirs and associated gas reservoirs have been discovered in the shallow layers(<1500m)of Gaoyou dimple.However,from the perspective of gas reservoir size,the gas-bearing area is small,the reserves are relatively limited,the distribution is scattered on the plane,and the exploration is difficult.Therefore,it is necessary to clarify the source of such gas reservoirs,the main controlling factors of accumulation,and clarify their accumulation patterns.This paper first combines the reservoir data and geochemical analysis data to analyze that this type of natural gas is crude oil-degrading biogas,confirms its associated relationship with heavy oil reservoirs,and clarifies the conditions for the formation of heavy oil reservoirs.Subsequently,the oil-gas transmission system was used as the main line to make an in-depth study on the formation conditions and main controlling factors of the gas reservoir,and pointed out the hydrocarbon accumulation characteristics,and established a shallow crude oil degradation biogas accumulation model in Gaoyou dimple.The next step of exploration and deployment in the sag provides a basis.Analysis of crude oil physical properties,family composition,chromatograms and other organic geochemical data shows that the shallow heavy oil in Gaoyou dimple is a secondary heavy oil.The main reason for thickening is biodegradation,and the degree of thickening is mainly controlled by temperature and groundwater The influence of type,salinity,etc.,of which salinity is more obvious;analysis of natural gas composition and carbon isotope analysis shows that shallow natural gas reservoirs are formed by the degradation of anaerobic microorganisms and have the characteristics of secondary biogas For example,high methane content,high nitrogen content in nonhydrocarbon gas,heavy carbon isotope composition of carbon isotope of carbon dioxide and carbon isotope composition of methane,and lighter carbon isotope composition of methane.At the same time,it was found that the shallow gas in the Gaoyou dimple has an associated relationship with the crude oil biodegraded heavy oil reservoir,and is genetically related.By comparing and analyzing Huang Jue,Zhouzhuang Gas Reservoir and Weizhuang Gas Reservoir in the slope belt,the following conclusions are drawn: the main controlling factors of biogas accumulation are the lateral migration ability of the faults around the heavy oil reservoir and the sealing of caprock Biomass accumulation model is mainly divided into two types: in-source(gas cap gas)and out-source(gas layer gas).The source refers to the heavy oil reservoir,and the in-source degraded gas reservoir basically belongs to the same trap as the heavy oil reservoir.There is basically no obvious migration,which is mainly developed in the formation of Funing Formation in the slope belt;the out-source degraded gas reservoir is mainly developed in the formation of Dainan Formation,Sanduo Formation and Yancheng Formation in the fault terrace belt.