Characteristic And Controlling Effect On Natural Gas Accumulating Of Xujiahe Formation Source Rock In Northeast Sichuan Basin

The geological,geochemical and hydrocarbon generation characteristics of the Xujiahe Formation and its controlling effect on natural gas accumulation were studied based on logging,geological or geochemical data,comparing the characteristics and differences of source rocks in each interval to discuss the the influence on natural gas enrichment.Through core observation and field survey,combined with logging data,the development intervals and its horizontal and vertical distribution characteristics of source rocks are defined,and the source-reservoir combination types are classified based on the thickness of source rocks and the configuration relationship between them.The T₃x²,T₃x³ and T₃x⁵ are determined as the main source rocks which contain three types of source rocks:dark mudstone,carbonaceous mudstone and coal,in which the dark mudstone is the main source rock while carboniferous mudstone and coal are locally developed.The thickness of source rocks decreases northward on the plane,and there are three types of combination between source rocks and reservoirs vertically:reservoir inyerlaid in source,source-reservoir interbeded and source inyerlaid in reservoir.Based on geochemical testing data,the organic carbon content of different types of source rocks was recovered by logging evaluation method,and the geochemical characteristics of source rocks were identified.The results show that the organic matter abundance of source rocks is medium-good type,the kerogen types of which are mainly type II₂-III,and it has reached the stage of high maturity to over maturity.The main source rocks are the upper T₃x²,T₃x³in Yuanba area,the T₃x⁵ in Bazhong area,the T₃x³ and lower T₃x⁵ in Tongnanba area.The thermal evolution history of source rocks in Xujiahe Formation was restored by basin simulation software,and the hydrocarbon generation intensity of different lithologic source rocks in each interval was calculated by organic carbon method.It is determined that source rocks of Xujiahe Formation in the study area began to generate a great deal of hydrocarbon amostly in the Late Jurassic-Early Cretaceous,and the differences in hydrocarbon generation characteristics between different areas are small.The calculation results of hydrocarbon generation intensity show that the dark mudstone had large hydrocarbon generation intensity and wide hydrocarbon generation range,while the coal source rocks had large hydrocarbon generation intensity and narrow hydrocarbon generation range.The west Yuanba area,the eastern part of Tongnanba and Bazhong area are the main hydrocarbon generation area of Xujiahe formation.Based on the data of components and carbon isotopes of natural gas,the genetic types and sources of natural gas in the study area are determined,and then the enrichment characteristics of source gas and the influence of source rocks of Xujiahe Formation are clarified.The natural gas of the Xujiahe Formation in Chuandongbei area is dominated by coal-type gas,which distributes in Yuanba area of T₃x²,T₃x³ and the whole area of T₃x⁴,while the marine-origin gas which came from P₃w is mixed of T₃x² in Yuanba area and T₃x²,T₃x⁴ in Tongnanba area.The influence of source rocks on natural gas enrichment range is mainly controlled by hydrocarbon generation centers,and the series are related to the different source-reservoir combinations.The main manifestations are the high yield of natural gas enrichment near hydrocarbon generation centers,the source-reservoir interbeded favorable for natural gas reservoir formation.