Accumulation Characteristics And Analysis Of Accumulation Conditions Of Tight Oil Reservoir In Chang 7 Member Of Yanchang Formation In Qingcheng Area,Ordos Basin

The Chang 7 Member of the Ordos Basin,which is belonging to deep lacustrine-semideep lacustrine deposits,develops major source rocks and tight reservoirs,with the wide distribution of tight oil reservoirs in this horizon.In this paper,the tight oil reservoir in the Chang 7 Member in Qingcheng Area was taken as a research object,by employing logs,well logging,drilling and test,and geochemical analytical data,effective source rock,tight sandstone reservoir,source-reservoir combination,and genetic type of crudes have been systematically studied to clarify the enrichment conditions and major controls of tight oil reservoirs and to establish an accumulation model,thereby providing theoretical guidance for the exploration of tight oil reservoirs.It is indicated in the study that there are two types of effective source rock,black shale and dark-colored mudstone in the Chang 7 Member in Qingcheng Area.Among them,the black shale,which is characterized by great thickness,wide distribution,high organic matter abundance,good organic matter type,and moderate thermal maturity,is the major source rock in the study area,followed by dark-colored mudstone.The rock types of tight reservoirs are dominated by feldspathic lithic sandstone and lithic arkose sandstone,with poor physical properties;the pore types are mainly characterized by dissolved pore of feldspar and intergranular pore.The pore structure types include the type of small pore and fine throat and the one with micro-pore and micro-throat,showing poor connectivity.It belongs to a typical tight sandstone reservoir,in which the source-reservoir combination can be divided into four types.Types I and II source-reservoir combination are substantially developed,showing best oil-bearing properties.Types III and IV sourcereservoir combination are secondary to them,with poor oil-bearing properties.Macroscopically,effective source rock,sandbody distribution and source-reservoir combination are the major controls for the enrichment of tight oil reservoirs in the Chang 7 Member in the study area.Detailedly,effective source rocks provide abundant material basis for tight oil reservoirs.The thickness and distribution of sandbodies,which are important places for the accumulation of oils,significantly control the distribution and scale of tight oil reservoirs.The source-reservoir configuration also plays a pivotal role in controlling the accumulation of tight oil reservoirs.