Reservoir Characteristics And Main Controlling Factors Of Chang 8 In Jiyuan Area Of Changqing Oilfields

Using core,well logging,oil testing data,based on the stratigraphic division and correlation and sedimentary microfacies,this paper focuses on Chang 8 reservoir characteristics,regularities of distribution and the main control factors of the physical property of reservoir physical property.Then criteria for reservoir classification and evaluation is built and favorable reservoir development zone is predicted.Researches showes that shallow water delta front sub facies develope during sedimentary period of Chang 8,including underwater diversion channel,underwater natural levee,branch Bay.The study find that reservoir rock types of Chang 8 in Jiyuan area is mainly feldspathic lithic sandstone and lithic feldspathic sandstone,with medium maturity structure and general composition maturity.The reservoir of Chang 8 in Jiyuan area experiences compaction,pressure solution,cementation,metasomatism and dissolution and so on,totally in the middle diagenetic A stage.The typical diagenetic sequence of the study area is as follows: compaction-micritic calcite cement-qartz,feldspar increase-clorite film-dissolution of feldspar and debris,formation of kaolinite(Oil Breakthrough)-autogeny-quartz,autogeny-feldspar-iron calcite cement-dissolution of feldspar,debris,iron calcite.Diagenetic process has undergone alkaline and acidic diagenetic environment,and these two environments appeared alternately.Reservoir space is primarily consist of intergranular porosity and intragranular dissolution pores.The average reservoir porosity is 8.84%,and the average permeability is 0.65×10-3?m2.The lower limit of effective reservoir porosity is 5.62%,and The petrophysical lower limit values of an effective reservoir is 0.10×10-3um2.The physical property of the reservoir in the Chang 8 reservoir group is better than the physical properties of the upper part.Deposition had an fundamental effect on the reservoir's physical properties,while diagenesis plays a decisive role in the late stage of the reservoir.The reservoir's physical properties are the best in Underwater Diversion Channel,with its porosity 8%~18% and permeability 0.3 um2~14 um2.While Shunt Bay's physical properties are the worst,with its porosity less than 6% and permeability less than 0.1×10-3um2.The initial porosity of the original sediments in the study area is between 40.28% and 41.55%.The primary porosity of the reservoir is reduced by 30.53% by compaction,which had the most influence on the physical properties of the reservoir.The porosity is further reduced by 4.85% by cementation and metasomatism.Dissolution in the later stage produces some secondary pores,contributing in an increase of porosity by 3.04%,which has a limited improvement of the reservoir.Finally,based on the reservoir evaluation standard,favorable reservoir development area is predicted in order to provide certain reference significance for oil and gas exploration in the region.