The Origin And Characteristics Of Special Rocks Reservior Of The Aershan Formation, Erennaoer Sag

There is a set of tight reservoir and special rocks with fine grains and enriched oil&gas,whose lithologies is a mixed lithology of terrigenous clastic,carbonate,and volcaniclastic rocks in the Aershan Formation,Cretaceous,Erennaoer sag,Erlian Basin.Although this reservoir has oil and gas shows in several sags,it has not obtained large break-through for its not-unified lithology classification,distribution and origin.Based on the aboved problems,the research of.the special rocks of petrology characteristics,physical characteristics,reservoir space,diagenesis,pore structure and genesis mechanism are investigated.Combined with drilling and logging data,observation of cores and thin sections,cathodoluminescence,SEM,and X-ray analysis,based on the the lithology classification schemes of peperite and terrigenous clastic rock,the lithology of the studied special rocks are divided into three categories and thirteen similar categories.Different categories of lithogies have different horizontal and vertical distribution.Tuffite,dolomite and special lithological mudstones?calcareous mudstone,dolomitic mudstone,tuffaceous shale?are mainly distributed in semi-deep lake and shallow lake subfacies.Special lithological sandstones?calcareous sandtone?and siltstones?calcareous siltstone,dolomitic siltstone,tuffaceous siltstone?are usually distributed in the fan delta front and front delta subfaciesThrough application of high pressure mercury and constant speed mercury injection,nuclear magnetic resonance?NMR?,the permeability of calcareous sandstone is best,calcareous sandstone and dolomitic siltstone is better.The special rocks develops various reservoir space types including intercrystal pore,intergranular pore,intragranular pore and micro fractures.Special lithological sandstones and siltstones mainly develop intergranular and intragranular pores.Tuffite and dolomite are mainly develop intercrystal pore.Special lithological mudstones have substrate pores.And cracks in the region of the various lithology is also developed.Based on analyses of the thin sections,cathodoluminescence and SEM,the diagenetic types and diagenetic evolution characteristics are initial studied.Peperite and special rock mudstone,whose diagenesis is strong,are mainly in the middle diagenetic stage A₂.The diagenesis of peperite and the special rock mudstone are mainly carbonate cementation,especially dolomite,followed by calcareous and siliceous cementation.However,the diagenesis of the special rock sandstone with relatively coarse grain size is relatively weak,are mainly in the middle of the diagenetic stage A₁ and developed siliceous cementation.Based on features of petrology,diagenetic evolution and geochemistry?isotopes and trace elements?,the causal model of special rocks,including tuffaceous rocks and three categories dolomitic rocks are established in this study.The tuffaceous rocks are formed by the combined action of volcanic ash and water.The first one is caused by penecontemporaneous dolomitization,mainly occurred in dolomite and layered dolomitic mudstone.The second one is associated with methanogenesis and occurred in star-like dolomitic tuffite and dolomitic mudstone.The last is mainly caused by burial dolomitization which is often occurred in Special lithological sandstones and massive tuffite.On the basis of previous research,the environment of favorable reservoir development is preliminary predicted.The most favorable reservoirs are mainly located in the front fan delta and fan delta front which are developed dolomitedsiltstone and calcareous siltstone.The more favorable reservoirs are mainly developed in the dolomited siltstone,calcareous siltstone and calcareous sandstones,which are also located in the front fan delta and fan delta front.