The Origin Of Complex Tight Reservoir Of The Lower Cretaceous,Erlian Basin,China

There is a set of tight reservoir with fine grains and complex lithologies-a mixed lithology of terrigenous clastic,carbonate,and volcaniclastic rocks-in the Lower Cretaceous,Erlian basin.This reservoir in several sags has found oil and gas,however,it has not obtained large break-through for its not-unified lithologic classification,uncertainties of lithological characteristics,reservoir space and origin.In this study,we take several major sags of Erlian Basin as an example.Based on observation of cores,thin sections and SEM,the petrology,mineralogy and diagenesis of the complex tight reservoir are studied.Moreover,through unconventional and conventional reservoir physical properties tests and analysis of stable carbon and oxygen isotopes and trace/rare earth elements,the origin of the complex tight reservoir and its pore throat structure are determined.In combination with drilling and logging data,observation of cores and thin sections,and X-ray analysis,the complex tight reservoir has various lithology types with poor lithologic continuity and various mineral types.Based on the the lithology classification schemes of peperite and terrigenous clastic rock,the lithology of the complex tight reservoir are divided into three categories and sixteen similar categories.Different categories of lithogies have different horizontal and vertical distribution.Tuff is interbeded in thick mudstones and distributed in semi-deep lake and deep lake subfacies.Tuffite and complex lithological mudstones are interbeded in siltstones and thick mudstones,and mainly distributed in shallow lake subfacies.Complex lithological sandstones are usually interbeded with siltstones and sandstones,and distributed in the distal fans.Through application of conventional physical properties tests such as high pressure mercury and unconventional tests such as constant speed mercury injection and nuclear magnetic resonance(NMR),the complex tight reservoir is characterized by various reservoir space types with micro-and nano-pore structures including intercrystal pore,intergranular pore,intragranular pore and micro fractures.Different categories of lithogies have different types and distribution of reservoir space.Tuff mainly develops intercrystal pore and intergranular pore.Tuffite and complex lithological mudstones mainly develop intercrystal pore and micro fractures.Dolomite and complex lithological sandstones mainly develop intergranular and intragranular pores.Based on analyses of the petrology,mineralogy,diagenetic minerals and cutting relationship between minerals,this complex tight reservoir has medium diagenetic strength within the middle diagenetic stage A1 to A2.Different categories of lithogies have different diagenetic types and strength.Fine-grained rocks including tuff,tuffite and complex lithological mudstones have experienced strong diagenesis within the middle diagenetic stage A2.Tuff mainly experienced devitrification,and tuffite and complex lithological mudstones experienced dolomitization and calcitization.Coarser-grained rocks including complex lithological sandstones have experienced relatively weak diagenesis within the middle diagenetic stage A1.It is dominated by calcitization such as late stage calcite and ferro dolomite filling pores or replacement of detrital grains.Based on observation of cores,thin sections and SEM,macroscopic occurrence and mineralogy of authigenic dolomite,and analyses of trace elements and stable C/O isotopes,three categories of dolomitization are established in this study.The first is penecontemporaneous dolomitization,mainly occurring in dolomite.The second is associated with methanogenesis and mainly occurred in dolomitic tuffite and dolomitic mudstone.Its Mg2+ is mianly derived from hydrolysis and alteration of volcanic materials.The last is burial dolomitization mainly occurring in dolomitic tuffite and dolomitic sandstone.Its Mg2+ is mianly derived from dissolution of feldspar and clay mineral transformation.