The Formation Of The Reservoir In The Upper Triassic Chang 7 Oil Layer Of Jiyuan Area,Ordos Basin

Shallow-water delta sedimentary rocks are very important for oil and gas accumulation,particularly for unconventional hydrocarbon resource.The energetically flourish of the exploration and exploitation of unconventional petroleum request a full understanding of the principle of reservoir's formation and the controls on reservoir quality.This paper employed numerical simulation,sedimentary geomorphology,core lithology,laboratory petrography techniques,stable isotopes and other collected petrophysic data,and comparison analysis of modern and ancient sedimentary record to analyze the formation of the reservoir in the Upper Triassic Chang 7 oil layer of Jiyuan area,Ordos Basin.Based on the results of numerical simulation and the Poyang lake morden sedimentary analysis,the deposition processes of the Chang 7 oil layer was deduced.In the interface of river(terminal distributary channel)and lake,inertial and friction perform dominantly,scouring occurred pervasively in the previous settled sediments.The variation of the effluent energy in the river mouth(e.g.caused by seasoning and rainfall)lead to frequent conversion of deposition and erosion,and unstable architecture of sedimentary elements.Consequently,the reservoir is a complex that consist of potentially scoured radial mouth bar(splay),distributary channel deposition(shallow-upward facies succession)and the lobes coalesced,generally showing as a protuberance lakeward.The pore-throat structure of the reservoir is complicate.Only 40% of porosity is likely useful for mercury injection and the throats take a reasonable proportion.The pore-throat structure controls on the porosity and permeability of the reservoir as a combination function.This function was established by an integrated parameter(?),which include total pore space,maximum and average throat radius and pore/throat volume ratio.The original composition of the sediment impacts on the types and strength of the diagenesis in the burial processes significantly.Different composition suggests different sedimentary hydro-dynamic.Carbonate cements,including calcite,ferroan calcite and dolomite,and kaolinite are the main authigenic minerals occurred in the Chang 7 oil layer sandstone.The authigenesis of carbonate cements cover the whole period of diagenesis.Organic acid,accompanied by the hydrocarbon generation of mudstone,enormously affects the chemical environment of pore fluid,which lead to pervasively dissolution of feldspar,resulting precipitation of kaolinite.Generally,compaction dominates porosity-loss of the reservoir;carbonate cements impact on the reservoir quality in various degree.The micropores in the matrix and secondary micropores,such as the intercrystalline pores of the kaolinite,consist of the major pore space of the reservoir.Low content of carbonate cement and relative high content of clay minerals(authigenic clay minerals and clay matrix)lead to relatively high porosity and permeability.The relative high content of matrix suggests relatively low hydro-dynamic of the sedimentary environment,inferring insignificant scouring.