Research On Effectiveness Of Oil Charging In The Tight Sandstone Reservoir Of The Chang 7 Member,Ordos Basin

Tight sandstone oil is the most important and practical part of unconventional oil and gas exploration in China,and also the most replacement resource of petroleum.However,the effectiveness of micro-nano pore throat system to oil charging and accumulation is a key question for the tight oil accumulation,which is different from the conventional hydrocarbon accumulation.Taking the Chang 7 tight sandstone reservoir of Yanchang formation,the Ordos Basin as example,the reservoir basic characteristics and diagenetic evolution,pore throat system heterogeneity and oil charging mechanism were comprehensively studied,to clarify the effectiveness of oil charging in the tight sandstone reservoir.Result shows that the Chang 7 tight sandstone reservoir in the study area is mainly composed of lithic arkose and feldspathic litharenite with silt-fine grain size and high plastic component content.The physical properties are obviously controlled by the mineral composition,particle size and sorting.The oiliness of the reservoir is obviously affected by the physical properties,pore structure and charging force.Reservoir has experienced strong compaction and carbonate cementation,and feldspar dissolution has been widely developed,but the dissolution strength varies greatly.Compaction is the dominant factor for reservoir densification.Siliceous cementation and clay mineral filling are the key factors for reservoir densification;late iron-containing carbonate cementation further strengthens the degree of reservoir densification and ultimately lead to the heterogeneity of physical properties and pore throat distribution.At the moment of large-scale hydrocarbon charging in the Chang 7 Member,the reservoir has been densified.Reservoir space types are mainly dissolved pores,residual intergranular pores,and clay mineral intergranular pores.Pore throat distribution presents strong heterogeneity,and micro-nano pore throat system are developed,and pore throat connectivity is poor.Tight sandstones have a wide range of pore throat distribution,and the pore throats of different sizes have obvious differences in the contribution to physical properties.Movable fluid saturation is mainly controlled by physical properties and pore structure.That is,the better the physical properties,the larger the pore throat radius,and the larger the movable fluid volume.The heterogeneity of pore throat distribution is obviously affected by geological factors,such as mineral composition,particle size and sorting.That is,clastic minerals(quartz+feldspar)are favorable geological factors for the pore throat distribution tending to the direction of large pores,while clay minerals and carbonate minerals are its unfavorable geological factors.Particle size and sorting are important parameters that affect the compaction of reservoir.Coarse particle size and good sorting are conducive to the retention of intergranular pores,which often causes the pore throat distribution to tend to the direction of large pores.The"characteristic pore throat radius"defined by the physical properties and the median pore throat radius,reflects the three characteristics of rock storage space,seepage capacity and pore throat size.It can effectively divide the Chang 7 tight sandstone reservoir into four types.The physical simulation experiment of oil charging in natural cores shows that the oil charging process of tight sandstone reservoir is mainly affected by physical properties,pore structure and charging force.There is a power function relationship between pore throat radius threshold and the charging force,which indicates the coupling of pore throat size and charging force is the key to determining oil charging and enrichment.In addition,the relationship models between charging force and effective accumulation space are also established in the Chang 7 tight sandstone reservoir of different type,which can be used to estimate the oil oiliness of reservoir.The effective pore throat radius threshold of oil charging in the Chang 7 tight sandstone reservoir is about 0.1?m.The different tight sandstone reservoirs present difference in the effectiveness of oil charging:Types I and II₁ reservoirs are effective reservoirs for tight sandstone oil accumulation,which is the most realistic object of tight sandstone oil exploration and development,while Types II₂ and III reservoirs are ineffective tight reservoirs for oil accumulation.