Characteristic And Evolution Model Of Reservoirs Of Thin Interbeded Sandstones Of The Upper Es4 In Lijin Sag

"This paper focus on the case study of thin interbeded sandstones of the Upper Es4 in Lijin sag.Based on core observation,common thin section and cast thin section identification,scanning electron microscopy observation,fluorescent and cathodoluminescence observation,as well as analysis of data of X-ray diffraction,mercury injection,core properties and grain-size testing,the characteristic and diagenesis evolution of reservoirs were studied.Moreover,quantitative research on porosity evolution of reservoirs have been done also.It turns out that most of the thin interbeded sandstones in study area are arkose and lithic arkose in which there are lots of cement.In general,the physical properties of reservoirs in study area is bad.The porosity of thin intebeded sandstones are not more than 15%,while the permeability are not more than 50mD.Secondary pores,especially acid secondary pores formed by the dissolution of carbonates,account for most of the pore space.The pore structure of interbeded sandstones can be classified into three types.Compicated diagenesis had happened in thin interbeded sandstones in the study area.There are five active fluid episodes of diagenetic environment.According chronological,they are alkaline,acidic,alkaline,acidic and alkaline.The diagenetic evolution sequence of interbeded sandstones in the study area is compaction,cementation of pyrite/carbonate micrite envelope/gypsum-dissolution of feldspar/carbonate micrite envelope-cementation of carbonate,dissolution of quartz-dissolution of feldspar/lithic/carbonate,quartz overgrowth-dissolution quartz,cementation of carbonate/pyrite.The main clay minerals in thin interbeded sandstones in the study area are illite,kaolinite,chlorite and ilite smectite mixed layer,the formation and distribution of which have been influenced by diagenesis largely.Kaolinite formed in acid diagenetic environment during early diagenetic-middle diagenetic A1 stage(1600-3200m),most of which is the alteration of feldspar.Illite was generated during A2 period of middle diagenetic stage,because of alteration of kaolinite and reaction between K-feldspar and kaolinite.In addition,quartz overgrowth always accompanying with the formation of illite.Chlorite mainly formed from late period of A2 period of middle diagenetic stage(>3500m).The exiting state of chlorite is pompon and flaky.According to quantitative research on porosity evolution of reservoirs,the average original porosity of beach bar is 41%,while that of storm beach bar is 31.35%.Because of the formation of carbonate micrite envelope during syndiagenetic,the impact of compaction was weaken.Therefore at that time,the average porosity of the former was 33.2%,while the average porosity of the latter was 27.1%.When it came to B period of early diagenetic stage,under the influence of compaction and cementation,the average porosity of beach bar and storm beach bar were changed to 19.4%and 15.2%.In A1 period of middle diagenetic stage,because of dissolution happening in acid diagenetic environment,the number of secondary pores increased.The average porosity of beach bar and storm beach bar increased to 21.8%and 17.5%.During A2 period of middle diagenetic stage,affected by alkaline fluid,most of secondary pores were destroyed,then the average porosity of storm beach bar decreased to 7.7%.