Tight Reservoir Characteristics And Sweet Spot Prediction Of Chang 8 Reservoir In Ganquan Oilfield

Aiming at the problems of unclear understanding of the enrichment law of Chang 8 tight reservoir in Ganquan Oilfield of Xiasiwan area,in-depth study of tight oil’s’sweet spot’,and lack of evaluation standard of’sweet spot’in Chang 8 tight reservoir,based on logging,core and analysis test data,the reservoir characteristics of Chang 8 tight reservoir in the study area,the main controlling factors and enrichment law of tight oil enrichment,the establishment of geological standards for tight oil’sweet spot’evaluation and the identification and prediction of tight oil’sweet spot’were systematically studied by means of fine description of reservoir characteristics,logging interpretation of reservoir physical properties,fine identification and classification evaluation of’sweet spot’.The results show that:（1）The tight sandstone reservoir of Triassic Chang 8 member in Ganquan oilfield is delta front subfacies underwater distributary channel sand body.The sandstone of Chang 8 reservoir is mainly lithic feldspar sandstone and feldspar lithic sandstone.The clastic components are characterized by high feldspar and high quartz.The clastic particles are mainly subangular.The reservoir sandstone has the characteristics of fine grain size,poor sorting and poor roundness.The physical properties of Chang 8 reservoir in the study area are poor.The surface porosity is 0.7%～19.1%,with an average of 11.1%and a median of 11.7%.The permeability is 0.01×10^-3μm²～4.8×10^-3μm²,with an average of 0.252×10^-3μm²and a median of 0.229×10^-3μm².The overburden porosity is between 0.27%and 14.37%,with an average of 7.78%and a median of 7.34%.The permeability is between 0.0013×10^-3μm²～1.2783×10^-3μm²,the average value is 0.1259×10^-3μm²,and the median value is0.0049×10^-3μm².Therefore,Chang 8 reservoir belongs to low-ultra-low porosity and ultra-low permeability reservoir,that is,tight reservoir.According to sedimentary microfacies,sand body structure,effective thickness of sand body,physical properties,pore type,pore structure and other parameters,the Chang 8 sandstone reservoir in the study area is subdivided into four categories:I,II,III and IV,among which the II and III reservoirs in the study area are more developed.（2）The Chang 8 reservoir in Ganquan oilfield is low porosity,low permeability-tight sandstone,no edge/bottom water,elastic-dissolved gas drive lithologic reservoir.The enrichment of tight oil in Chang 8 member is mainly controlled by five factors:high quality source rock,high relative physical property,micro structure,reservoir heterogeneity and source reservoir configuration.High-quality source rock is an important basis for tight oil accumulation.The high value area of relative physical properties under the background of dense physical properties is a favorable condition for enrichment;the development of micro-structure provides good trap conditions for Chang 8 tight oil accumulation.Heterogeneity is one of the important factors affecting hydrocarbon accumulation.Excellent source storage configuration is an indispensable condition.（3）According to the two main controlling factors of source rock and reservoir quality,it can be divided into four types of source-reservoir configuration relationships:’excellent source-excellent reservoir’,’excellent source-good reservoir’,’excellent source-medium reservoir’and’excellent source-poor reservoir’.Among them,Chang 8 reservoir mainly develops’excellent source-good reservoir’and’excellent source-medium reservoir’combination types.The better the source-reservoir configuration,the more conducive to the enrichment of tight oil.（4）Based on the analysis of the enrichment regularity of Chang 8 tight oil,the sweet spot index evaluation was carried out through the macroscopic development characteristics of the reservoir,the quality of the source rock,the quality of the reservoir,the oil content,the heterogeneity and the fracturing of the reservoir.The parameters such as lithology,porosity,permeability,oil saturation,source rock characteristics,heterogeneity and brittleness factor were optimized,and the sweet spot identification standard of Chang 8 tight oil was established.The Chang 8 reservoir in the study area is divided into four types of’desserts’:I,II,III,and IV.Among them,type I dessert is thick sand-optimal permeability dessert,type II dessert is medium-thick sand-medium permeability dessert,type III dessert is medium-thin sand-low permeability dessert,and type IV dessert is thin sand-differential permeability dessert.（5）On the basis of single well’s sweet spot identification and classification evaluation,the vertical and horizontal distribution characteristics of sweet spots are described.Chang 8₂oil layer of Chang 8 oil layer of Yanchang Formation in Ganquan Oilfield mainly develops type I and II desserts,and a small amount of type III and IV desserts.III and IV desserts are mainly developed in Chang 8₁oil layer.Among them,type I and type II desserts are mostly developed in the middle of the main channel and the middle of the branch channel of the underwater distributary channel;type III and IV desserts are mostly developed in the middle and side edge of the branch channel of the underwater distributary channel.