Research On Heterogeneity And Reservoir Characteristics Of Chang 8 Reservoir Of Yanchang Formation In Shuimogou Area,Fuxian County,Northern Shannxi Province

Shuimogou area is a new oil-producting region in Ordos Basin,whose main oil-bearing layer is Chang 8 formation of Upper Triassic.At present,the well area has entered the stage of water injection development,but there are some problems such as rapid decline in production,low productivity in single wells,and poor water injection.Previous geological studies can’t meet the requirements of refined development of this oil field.Therefore,the study of sedimentary microfacies,sand-bodies superimposed patterns and distribution characteristics,reservoir heterogeneity,“four-property”relationship and reservoir characteristics in Shuimogou area has guiding significance for the adjustment of development plan.Guided by relevant theories of reservoir sedimentology and reservoir geology,based on logging data of 155 wells in Shuimogou area,combined with core observation,testing and production data,this paper researches the sedimentary microfacies types,sand body superimposition and distribution characteristic,“four-property”relationship,reservoir heterogeneity,reservoir characteristics and main controlling factors of Chang 8 oil-bearing formation.The Chang 8 oil-bearing formation in Shuimogou region develops the delta front subfacies sediment.The distributary channel sand bodies are the main framework sand bodies,which spread in continuous sheets along the NE-SW direction.Vertically,the sand stacking style of Chang 8 oil-bearing formation in Shuimogou region can be divided into separate,overlapping,tangential and substitutional type.In plane,it can be divided into isolated,butt-joint,lateral and substitutional type.The connectivity of sand body varies greatly among small layers,among which the Chang 8²-1-1 layer is the best.The fluid distribution has been influenced by thickness and stacking pattern of the sand body.Vertically,reservoirs are mainly distributed in multi-layer superimposed and thick sands.In plane,it is mainly distributed in lateral and substitutional sand body.The lower limit of reservoir lithology of Chang 8 oil-bearing formation in Shuimogou region is packsand,the lower limit of oil content is oil trace,the porosity is 8%,the permeability is0.12×10^-3μm²,and the lower electrical limit is RILD≥16Ω·m,Δt≥222μs/m,So≥20%.The rock types are mainly arkose and debris-arkosic sandstone,with intergranular pores,intragranular dissolved pores and residual intergranular pores,and the main cements are illite,chlorite,calcite,etc.The reservoir’s pore throat type is mainly samll pore-micro throat,with the relatively higher displacement pressure and median pressure,which belongs to the low-porosity and ultra-low permeability type.The heterogeneity of Chang 8 reservoir is strong,which is dominated by positive rhythm with well-developed interlayer.Reservoir heterogeneity has some influence on reservoir distribution.The reservoir sandstone distribution area has better reservoir physical property,poorer development of interlayer,weaker heterogeneity,and richer oil and gas accumulation.The thickness of sand body on the flank of distributary channel is thinner,the physical property is poorer,the interlayer is well developed,the heterogeneity is stronger,and the enrichment of oil and gas is poor.The oil content of the Chang 8 oil-bearing formation in Shuimogou region varies greatly.In plane,oil mainly concentrates in the middle and southwest of the study area.while in vertical,it mainly concentrates in the Chang 8^2-1 reservoir.The Chang 8 oil-bearing formation belongs to lithologic reservoir,the underwater distributary channel sand body controls reservoir distribution,nose uplift structure controls reservoir accumulation,and lateral shielding of tight sandstone prevents oil and gas escaping.