The Characteristics And Analysis Of Exploration Potential Of Tight Oil Reservoir Of Yanchang Formation, Ordos Basin

Tight oil is an unconventional oil resources,which play a very important position in the global energy structure.Crude oil physical property of tight oil is good,light oil layers could be found in tight oil exploration.Tight oil produce an extremely low perm-eability layer of shale,siltstone,sandstone or carbonate,tight oil reservoir and organic-rich source rocks are closely linked.Ordos Basin is located in North China block west,it tectonic setting is stable.This basin is one of China's major oil and gas basins and important and typical zone of tight oil development in China.The low permeability and ultra-low permeability reserves of proven reserves accounted for91.4%in Changqing petroleum exploration area.Based on years of technical research,Changqing oilfield has been a huge success in the field of tight oil exploration and development.it has successfully realized more than 0.3mD ultra-low permeability reservoirs(Ansai,Xifeng,Jiyuan,Huaqing oilfields)development.The tight oil of ground air permeability less than 0.3mD is the research object today.Yanchang Formation tight oil mainly develop in the semi-deep lake and deep lake facies,the minister Chang 7 reservoir oil-bearing sandstone is the most typical.The tight oil has a wide distribution,hydrocarbon source rock condition is superior,tight sandstone reservoir,complex pore structure,physical property is poor,high oil saturation,oil property is good,the characteristics of reservoir pressure coefficient is low.Triassic Chang7 lacustrine hydrocarbon source rock is the main hydrocarbon source rocks with high quality in Ordos Basin.The Chang-7 hydrocarbon source rock was considered to consist of the mudstones and shale,whereas the mudstones and oil shale of the Chang-7 subsection were considered to be the best source rocks in the Ordos Basin.The stage for the deposition of the Chang-7 subsection underwent maximum lake flooding.Therefore,the Chang-7 subsection deposited in the semi-deep and deep lacustrine environments with thicknesses in the range of 60–110 m.Large-scale and abundant organic high-grade hydrocarbon sources developed in the middle of the lake basin.Chang 7 rich organic shale and black mudstone rock developed rich organic matter laminated structure,organic matter abundance is high,the types of organic matter and residual hydrocarbon content are good,with sapropel type–hybrid,the maturity of the mud and shale is high.The Chang7 lacustrine high-quality hydrocarbon source rocks are mainly distributed in the region of jiyuan-huachi-yijun,it is about 16 m thick,the most up to 60 m,covers an area of 3.3×10~4km~2.Dark mudstone are mainly distributed in the region of west side of jiyuan,wuqi and luochuan area,average thickness of 17 m,the most thickness of 124 m,covers an area of 5.4×10~4km~2.Chang7 high-quality hydrocarbon source rocks is the high production rate of hydrocarbon and hydrocarbon expulsion efficiency,the TOC abundance of the Chang7 hydrocarbon source rocks is relatively high,TOC in the Chang7 shale accounts for 13.81%on average and that in the Triassic mudstones for 5.0%.The hydrocarbon expulsion rate of shale is mainly distributed from 70%to 90%,with77.3%on average,the Chang 7 black mudstone hydrocarbon expulsion rate are mainly distributed from 20%to 80%,average 42.7%.Chang7 shale hydrocarbon generation total hydrocarbon generation quantity 1012.2×10~8t,black mudstone hydrocarbon generation quantity 216.4×10~8t,Chang7 hydrocarbon source rocks with a total amount of hydrocarbon generation of 1228.6×10~8t.Triassic Chang 7 sedimentary facies is the half deep lake-deep lake facies of inland depression basin,its sediment is fine grained.Chang 7 sedimentary period,the shape of lake basin bottom is an asymmetry depressed-type lacustrine basin with gentle slope in the east and steep slope in the west,which control the thick layer of sand body distribution.The sedimentary subfacies is meandering river delta in the northeast of Basin,the sands are delta front underwater distributary channel sand.The sedimentary subfacies is braided river delta in the Southwest of Basin,the sands is combination of gravity flow deposits in central lake basin.The reservoir is large-scale fine grained sandstone.Underwater distributary channel sand and sandy debris flow is the main reservoir.Tight sandstone reservoir distribution is stability,wide area and large thickness.Chang 7 dense sandstone reservoir distribution area is approximately11500 km~2,single sand thickness is approximately 5~18 m,cumulative thickness is approximately 18~55 m,reservoir thickness is great.The rock types of Chang 7 tight oil reservoirs are mainly Cuttings feldspathic sandstone,the sediment granularity is mainly fine and the interstitial content is relatively high.The reservoir is particularly tight,with the permeability less than0.3mD,it is the total penetration of 92.5%.The porosity is generally from 6%to 13%,it is the total porosity of 87.5%.The pore type is generally a dissolved pore or intergranular pores-dissolved pores,with fine small throats,the pore-section area ratio is generally less than 2.5%.Pore filling reduces the pore throat connectivity,pore throat radius is small,narrow range.The micron pore is dense oil reservoir main reservoir space,the micron pore type is residual intergranular pore and feldspar dissolved pore-based.The micron pores were bundles,banded and spherical pore shape,the micron pore connectivity is good.The nanoscale pore throat network is complexity.The nanometer pore types are mainly intergranular pore,feldspar dissolved pore and organic pore.The reservoirs of micron pore is development,large pores quantity and good connectivity.The number of large pores of nanoscale reservoir is relatively less and poor connectivity.The micron throat development degree,connectivity better than the nanoscale throat.Tight oil occurs in the presence of"isolated pores",connecting large pores and microcracks.The ranged of main volume is 100-500?m~3 under Micrometer-sized droplets.The volume of the oil droplets is 0-10~7nm~3.The abnormal pressure of Chang 7 hydrocarbon source rocks is the main driving force in tight oil accumulation.Base on thermodynamic modeling and fluid inclusions test restore ancient pressure of oil pool accumulation,which is 18~26MPa.The pressure differential of source rocks and tight sandstone is generally 12~15MPa,excess pressure is highest 25MP.The limit of pore throat of Chang 7 tight oil is 75nm.The results of the study of burial history and thermal history that Late Jurassic and Early Cretaceous is the main charging period of Chang 7 hydrocarbon.The reservoir densification of Triassic Chang7 tight oil is simultaneous mass accumulation of tight oil.The carbonate cementation is an important factor of the Chang 7 reservoir densification.Studies have shown that the densification time of Chang 7 reservoir is108Ma.The lake sandstone reservoir of the large area of distribution,oil shale and black mudstone are neighbor.The tight oil can be formed under abnormally high pressure.By the analysis of geological conditions,deepen research tight oil forming mechanism and main controlling factors.The results that thick source rocks,large and stable distribution reservoir sand bodies,good source storage configuration and effective filling conducive to the formation of large-scale Tight oil reservoir distribution.Based on the research of tight oil resources evaluation method,combined with case analysis of basin comparison,optimization and demonstrate the volume method,EUR analogy method and resource abundance analogy method of three kinds of resources evaluation for Erdos basin Yanchang Formation tight oil method.According to industry specifications resource evaluation,we developed the plan of tight oil resource evaluation of Ordos Basin and divided the evaluation unit,set up scale area,screening evaluation methods.We build the key parameter selection criteria,determined by the impact parameter Factors selected to the research and critical parameters and carried out resource evaluation.Volume method,EUR analogy method and resource abundance analogy method calculation of Yanchang Formation of tight oil resources volume was 31.16 tons,25.49 tons and 32.97 tons,determined the tight oil resources of 30 tons,and has formed a set of suitable for tight oil resources Erdos method of basin evaluation system.