Lithofacies Characteristics And Sweet Spot Prediction Of Mixed Rocks Of E₃² In Ganchaigou Area,Yingxiongling Structural Belt

The upper member of lower Ganchaigou formation in Ganchaigou area of Qaidam Basin is a set of tight lacustrine fine-grained mixed sedimentary rock reservoirs with good exploration potential.However,the reservoir lithology is complex and heterogeneous,which seriously restricts the fine study of reservoir.Therefore,this paper makes full use of well logging,core,rock thin sections,cast thin sections,scanning electron microscopy and abundant analytical and laboratory data to study lithofacies characteristics,organic matter characteristics and reservoir characteristics in the study area.Using artificial intelligence method to identify lithofacies;according to a variety of geochemical indexes,to determine the organic matter characteristics;using conventional mercury injection,nuclear magnetic resonance and other methods to determine the pore structure characteristics;using brittle mineral component method to determine the brittle characteristics of reservoir.Finally,the distribution characteristics of the lithofacies in the study area are determined,the comprehensive grading evaluation criteria of the reservoir is established,and the"sweet spot"in the study area is predicted.The research results show that:（1）eight lithofacies types are developed in the upper member of lower Ganchaigou formation in the study area,including massive limy dolomite lithofacies,lamellar argillaceous dolomite lithofacies,sandstone lithofacies,calcareous shale lithofacies and mudstone lithofacies.The abundance of organic matter in the study area is low,and the types of organic matter are type I and type II₁,which belong to the low mature stage.（2）There are five types of reservoir space:intercrystalline pore,intercrystalline dissolved pore,intergranular pore,micropore and micro-fracture.The pore structure can be divided into four types,such as intergranular pore-flake throat and intercrystalline dissolved pore-flake throat.Among them,intercrystalline pore-flake throat and intercrystalline dissolved pore-flake throat are mostly developed in massive limy dolomite lithofacies,lamellar argillaceous dolomite lithofacies and calcareous shale lithofacies,and have the best physical properties,which are the most widely developed and favorable reservoir space in the study area.The oil content of massive limy dolomite lithofacies and lamellar argillaceous dolomite lithofacies is the best,and the brittleness of massive limy dolomite lithofacies and sandstone lithofacies is the best.（3）The evaluation standard of reservoir comprehensive classification is established and the reservoirs are classified into three levels.Further,the lithofacies,porosity and brittleness are taken as the key parameters for the prediction of"sweet spot",and the"sweet spot"is divided into two levels:class I and Class II.（4）An intelligent lithofacies identification method based on neural network is established,and the characteristics of lithofacies spatial distribution are determined by combining logging and seismic.Taking the II-6 sand group as an example,the spatial distribution characteristics of lithofacies are determined.On the whole,the Argillaceous shale lithofacies and mudstone lithofacies deposits are set as the background,and the lithology changes quickly in the longitudinal direction.On the plane,the massive limy dolomite lithofacies and lamellar argillaceous dolomite lithofacies are mainly distributed in the northwest of the study area and around well Chai 905 area.The calcareous shale lithofacies are usually distributed around the lamellar argillaceous amellar argillaceous dolomite lithofacies.In well areas such as Chai 903,sandstone lithofacies deposits are intermittently developed and distributed in the shape of a beach,and sandy shale lithofacies are usually distributed around the sandstone lithofacies.（5）Based on the distribution characteristics of lithofacies and the inversion results of key parameters of"sweet spot"evaluation,the prediction of"sweet spot"is realized.Taking II-6-1 and II-6-2 small layers as examples,Class I"sweet spot"is mainly distributed in the vicinity of well area of Chai 10,Chai 3-2,Chai 9 and Chai 905.Class II"sweet spot"is generally distributed around Class I"sweet spot",with good contiguity.