Geological And Geochemical Characteristics Of Tight Oil-source Rocks In Central And Western China

With the reduction of conventional oil and gas resources,unconventional oil and gas?tight oil and gas,shale oil and gas,etc.?show an increasingly important position.North America has achieved great success in tight oil exploration and development.Inspired by the success of tight oil in North America,China's tight oil exploration and development has also made considerable progress,but overall it is still in its infancy.Many unconventional petroleum geology problems for the exploration and development of tight oil in China's terrestrial sedimentary system need to know in depth.Tight oil refers to a reservoir that is trapped in or in close proximity to a high-quality source rock formation and that has not accumulated through large-scale,long-distance migration,forming a large area that co-occupies or is in close proximity to the source rock system.Continuously distributed oil and gas resources.Source rocks play a dominant role in the formation of tight oils.Based on systematic research on the research progress of tight oil at home and abroad,this paper focuses on the main tight oil exploration areas of the Ordos Basin,Qaidam Basin and Santanghu Basin in the central and western of China,then compare and analyze geological and geochemical characteristics of each tight oil exploration area.Focusing on the relationship between source rock characteristics and tight oil formation,and exploring the relationship between source rock and tight reservoirs in order to deeply understand the main Geology and Geochemistry control factors of tight oil accumulation.?1?According to the comparison of tectonic features of Ordos Basin,Qaidam Basin and Santanghu Basin,high quality hydrocarbon source rock and tight reservoir are developed in the depression and slope area which is the main distribution area of tight oil resources.The geological background of the multi-bulging,multi-depressions multi-sedimentary center,multi-source and multi-stage tectonic activities of the lake basin determines the geological conditions of the formation and distribution of the dense oil of continental facies.The size of hydrocarbon reservoir is determined by the size of hydrocarbon,and the slope of the slope area determines the distribution area.?2?High quality,widely distributed source rocks generally determine the formation and distribution of tight oils.The source rock of Chang7 in the Ordos Basin is mainly lake mudstone and shale,with a area of 1×10⁴km²,and the type of organic is?-?,the TOC is mainly between 0%and 20%,which is high quality effective hydrocarbon source rocks.The abundance of organic matter of high quality hydrocarbon source rocks in Qaidam Basin is 0.5³.5%in Jurassic(J₁₊₂),0.4¹.2%in the E₃²,0.4¹.2%in the N₁,and more than 2%in some area.The main type of organic matter is?-?type,parts of organic material is?type in the middle and lower Jurassic,belongs to the good hydrocarbon source rocks.the organic matter type of hydrocarbon source rocks is?-?type in Lucaogou of Santanghu Basin.TOC is generally greater than 1%,which is a set of high-quality hydrocarbon source rocks.High-quality source rocks lay the foundation of crude oil formation,and its distribution area also defines the distribution of tight oils.?3?Widely distributed tight reservoirs provide favorable reservoir space for tight oil accumulation.The above tight oil exploration area mainly develop compact reservoirs of tight sandstones,carbonate rocks,and mixed rocks.The tight reservoirs are spatially and continuously distributed.Micron-sized pores and nano-sized pores develop in large areas in tight reservoirs.,and the pores constitute the main reservoir space of the reservoir.The reservoir of the Chang7 in the Ordos Basin is mainly fine silt and siltst,and the porosity is between 4.8¹2.6%which the favorable exploration area is30000⁵0000km².In Qaidam basin,tight oil reservoir occupies area of 5×10⁴ km²,the main rock type are feldspar sandstone and lithic feldspar sandstone reservoir,In the×iaganchaigou formation?E₃²?,the average porosity is 14.45%,in Shangganchaigou formation?N₁?,the average porosity is 8.2%,In the middle and lower Jurassic(J₁₊₂),the average porosity is of 11.8%.In Lucaogou formation of Santanghu Basin,prospective tight oil-bearing area of about 1000 km²,reservoir lithology is mainly cloud mass siltstone,Sandy dolomite and micritic dolomite.The porosity is between 4¹6%.The tight reservoir has the characteristics of wide area of distribution,the porosity of basically greater than 4%and the excellent reservoir capacity.?4?Reasonable source and reservoir allocation relationship is the key to tight oil accumulation High-quality hydrocarbon source rocks and dense reservoirs are spatially stacked on top of each other,causes extensive contact between source rocks and reservoirs which Provides reasonable source-reservoir space conditions for tight oil accumulation.For example,in the ordos basin,The sandstone reservoirs are symbiotic with the oil shale and the black mudstone,and reservoirs have a large contact area with source rocks.In the qaidam basin,the favorable sedimentary sand bodies are interbeded with the dark mudstone of the lake,forming multiple source reservoirs.in the dessert segment of Santanghu Basin,dessert segment are interbeded with the non-dessert segment.Reservoir is encapsulated by source rack.With the good source relations of configuration,the contact area of source rock and reservoir significantly increase is enhanced,and extremely the distance of petroleum migration is shortened,which contribute to the hydrocarbon source rocks take better hydrocarbon generation and expulsion and accumulate oil and gas by the reservoir effectively.?5?The power of pro×imal accumulation is mainly the differential pressure from the high quality source rocks.The pressure difference of hydrocarbon potential source is generally 10¹5Mpa,and the generated crude oil is transported to the tight reservoir distance and filled with continuous filling.So,the high-quality source rocks with thermal evolution in the late oil generation stage is more conducive to tight oil accumulation.At this stage,not only hydrocarbon generation pressurization may be optimal,but also the hydrocarbon properties produced are relatively good.