Effective Source Rock And Its Control To Crude Oil In The Chang 7 Member Of The Ordos Basin

The Ordos Basin is an important petroliferous basin in China and tight reservoir in the Chang 7 Member has become a pivotal source of increased reserves and production of oil and gas resources.In this paper,based on data such as log,well logs and core observation and combined with the analytical results from organic geochemistry,organic petrology,GC-MS characteristics of saturated hydrocarbon,and field emission scanning electron microscopy,the characteristics of source rocks were analyzed.It is indicated in the results that the source rocks in the Chang 7 Member are mainly composed of black shale and dark-colored mudstone.The black shale is superior to the dark mudstone in terms of organic matter abundance and type,both reaching the standard of high-quality source rocks.Additionally,the Ro values of both types of source rock are distributed in 0.6% ? 1.08%,which indicate the thermal stage of hydrocarbon generation.According to the relationship between total organic carbon and pyrolysis residual hydrocarbon,and chloroform asphalt “A”,the TOC cutoff of effective source rock(TOC >1.5%)is determined.It is considered that the black shale and partial dark-colored shale in the Chang 7 Member are effective source rocks.The black shale is mainly developed in the third submember of the Chang 7 Member and the dark-colored mudstone is distributed in distributed in each submember of the Chang 7 Member.There are mainly three types of crude oil in the Chang 7 Member,including Type A,Type B,and Type C.Amongst them,Type A crude oil mainly comes from black shale,and Type C crude oil mainly comes from dark-colored mudstone,however,Type B crude oil is the mixture of the former two.The major hydrocarbon generation period for the two source rocks in the target layer was in the Early Cretaceous,which is consistent with the hydrocarbon generation period determined from fluid inclusions,showing evident controls to the accumulation of oil and gas.The distribution and type of effective source rocks in the Chang 7 Member correspond to that of crude oil,and the thickness and organic matter abundance of effective source rocks are positively correlated with the production of crude oil.The Class I source-reservoir combination in which the black shale is best developed exhibits the highest oil and gas production,indicating that the source rocks of different lithologies in the Chang 7 Member have good controlling effects on different types of crude oil distributed among them.This study clarified the cutoff of effective source rocks,finely characterized the effective source rocks of each small layer,and analyzed the controls of hydrocarbon generation period,type,distribution,quality,and source-reservoir combination type to the generation,type,distribution,and production of oil and gas,thereby providing important theoretical basis and guidance for the exploration and development of tight oil and gas in the Chang 7 Member of the Ordos Basin.