The Paleo-environment Of Source Rock Formation And Geological Evaluation Of Shale Oil In The Songliao Basin

Shale gas has gradually become one of hotspots of global unconvention resources exploration and development,but study on shale oil is still infancy.The shale oil is the same source and reservoir in-situ oil accumulation,that of formation mechanism,evolution,enrichment and paleo-environmental conditions in the research field.The mudstones or shales of Qingshankou(K2qn)and Nenjiang(K2n)Formation(samples from two wells and four profiles)are reviewed as object in this study;By developing organic geochemistry,thermal simulation experiment in closed system,semiquantitative mineral compositions and gas adsorption and so on,the aim is to understanding paleo-environmental condition,the process of hydrocarbon formation and evolution,physical characteristics of mudstone/shale,and the geological condition and exploration target of shale oil.The major conclusions are as follows:(1)The abudant C30 marine steranes were characterized by periodic variation in source rock of Songliao Basin,implying multiple transgression events during the sedimentary period: The high abundant C30 marine steranes in the lower parts of K2n1 and K2n2 indicated large scale of transgression,leading to intermittently anoxic and stratified events;There was a rapidly and strongly transgression in the lower part of K2qn2,but stratified events depend on action of source input,microbial action and climate in this period;It showed relatively stable semi-deep and deep lake during the sedimentary of K2qn1.There was relatively high water salinity in the lower of K2n2 and the water in the lower of K2n2 was characterized by freshwater,brackish water and salt water environment on the basis of transgression,and salinity in the lower of K2qn2 fell in between the above two.(2)The hydrocarbon generation and evolution of two sets of source rocks were performed by gold tube closed system,acquiring shale oil and gas accumulation and evolution stages: When Easy%Ro was under 0.68%,it was assigned to invalid or inefficient oil generation stage;when Easy%Ro was in the range of 0.68~1.18%,it implyed main oil generation,and volatile hydrocarbon was abundant in 1.18~1.76%;when Easy%Ro ranged of 1.18~2.25%,it represented to wetness or condensate;when Easy%Ro exceed 2.25%,methane gas was as result of pyrolysis products.Therefore,K2 qn and K2 n source rocks have entered into different evolutions of later and earlier to main oil generation,respectively,and those in a suitable range of hydrocarbon expulsion were exploiting potential of shale oil in Songliao Basin;For volatile hydrocarbon in the Songliao basin,at the range of early and late generation stage,K2 qn source rocks had a certain development potential in suited geological condition,but K2 n source rocks were in earlier generation stage and no development value.The carbon isotopes of residual kerogens amd asphaltenes,derived thermal two keogens,showed gradually positive and a good correlation between kerogens amd asphaltenes with increasing maturity in the oil windows.Therefore,shale oil evolution and enrichment could be indentified by the correlation of carbon isotopes of kerogens amd asphaltenes on the basis of shale oil in situ reservoir.(3)The mudstones of the K2qn1,lower of the K2n1 and K2n2 were best source rocks(type I/II),lower of the K2qn2 in the second place(type I/II),and the upper of K2n1 was poor(type III)in this study.The mudstones of the K2qn1,with high maturity and hydrocarbon yields,were the main target of the development and exploration of unconventional resources.(4)There were differences between the K2 qn and K2 n in clay mineral composition: The mudstones/shales of the K2 qn contained a high content of illite and well-organized illite-montmorillonite mixed-layer,and occupied mainly position by abundant montmorillonite in the K2 n,but total clay mineral percents were about 40~50% similar distribution for two sets of source rocks.The contents of brittle mineral and quartz were relatively high and enough industrial fracturing.(5)The difference between residual oil(S1)and adsorption oil(SP)could be an important method to estimate whether or not movable shale oil in mudstones/shales.S1-SP,oil saturation and production index show commonly movable shale oil resource and sedimentary uninterrupted mudstones/shales in the K2 qn when burial depths exceed 2,065 m for well H14 possessing of a certain industrial value.These suggested that there was shale oil potential in the K2 qn shale of Songliao Basin.(6)The result of gas adsorption for mudstones/shale of the Songliao basin and that of kerogens showed orders of the pore structure parameters as following:Surface area: K2 n mudstone/shale>kerogens>K2qn mudstone/shale;Pore volume: kerogens>K2n mudstone/shale>K2qn mudstone/shale;Average pore diameter: kerogens>K2qn mudstone/shale>K2n mudstones/shale.(7)Those organic matter(TOC and group components)in different pore sizes were mainly controlled by expulsion efficiency of hydrocarbon in the K2qn1 mudstones/shales,specific performance: there were no correlation,positive correlation and negative correlation between microporous,mesoporous and macroporous and TOC or residual hydrocarbon(saturate and aromatic),respectively.The expulsion thereby was reflected on releasing process of residual oil in mesoporous.Most of mudstones/shales of the Songliao Basin are present at or before oil peak with the generation of hydrocarbons and further inhibit of pore structure development;Illite-montmorillonite made a contribution to the development of the specific surface area of the mudstones/shales in the Songliao Basin.