Comparison Study On Oil And Gas Accumulation In Shale

As important types of unconventional oil and gas, oil shale and shale gas attract more and more attentions nowadays. The paper tries to research oil shale and shale gas comprehensively. Taking shale rich in organic matter as subject, the northern Bogda Mountain, Xinjiang and Upper Yangtze as study areas, then making systematically comparable analysis between oil shale and shale gas and finally predict the distribution of oil shale and shale gas in China.Oil shale is fine sedimentary rock of high ash content and solid combustible organic matter, shale gas is natural gas accumulation in shale in the forms of free gas and adsorption gas. The source and reservoir is the same one respectively for oil shale and shale gas. Both two require hydrostatic reduction sedimentary environment and high organic content which also decrease the importance of preservation conditions and structural traps. As to accumulation mechanism, geological characteristic, evolution feature, controlling factor and distribution law, there exist tremendous differences between oil shale and shale gas. The core difference lies in hydrocarbon generation characteristics. For example, oil shale requires genetic potential kerogen, basically no humic organic matter and the organic remains in immature stage which only generates bio-methane and little immature-low mature oil, while shale gas can be generated from all types and every evolution stage of kerogen and shale gas can also be biogenetic gas, thermogenic gas or the mixture of both.Sedimentation determines the composition and type of shale, while tectonic movement determines the distribution and evolution stage of shale. Oil shale or biogenetic shale gas can be formed when subsidence or tectonic uplift occurs with theⅠandⅡtype kerogen at the immature stage; at the mature stage, fractured reservoir or thermogenic shale gas can be formed; at the over-mature stage, cracked shale gas can be formed. BecauseⅢtype kerogen doesn’t have genetic potential, oil shale or fractured reservoir can’t be formed during the kerogen evolution. Shale gas and oil shale are important types of shale hydrocarbon system, they could constitute both ends of the time and space in the hydrocarbon accumulation mechanism spectrum. Oil shale in northwest China is mainly the aphytal sedimentation of Permian. The tectonic uplift after Jurassic make the oil shale has the characteristics of shallow depth or outcrop on the surface, great thickness, stable distribution, low degree of thermal evolution, high oil yield, etc. Gas bearing shale in southern China is mainly Palaeozoic marine deposits. It has the characteristics of strong later tectonic movements, thick individual layer, wide distribution, high degree of thermal evolution, high abundance of organic matter, etc. The shale in southern China has a large potential of shale gas resources.The geological age for oil shale formation in China concentrates in Mesozoic and Cenozoic and mainly distributes in Songliao Basin, Ordos Basin and Junggar Basin, while shale gas distributes widely from Palaeozoic to Cenozoic and Palaeozoic in South, Jurassic in Northwest, Carboniferous-Permian in North China, Triassic in Ordos Basin, Cretaceous in Songliao Basin and Paleogene in eastern rift basins are all favorable for shale gas accumulation.