Alkaline Lake Diagenesis And Its Influence On Oil And Gas Of The Permian Fengcheng Formation In The Mahu Depression,Junggar Basin,China

The Mahu Depression in Junggar Basin is one of the oil-rich Depressions in the basin,which has become the research hotspot in recent years.The Fengcheng Formation in Mahu Depression is the key horizon of oil and gas exploration.The strata of the Fengcheng Formation is relatively special,which is alkaline lake environment and influenced by volcanic activities,furthermore,the Fengcheng Formation experienced different diagenesis and formed different rock minerals than in normal lake environment.Therefore,it has important and special research value.This study is based on core slices,using rock and mineral identification,scanning electron microscopy(SEM),electron probe microanalysis(EPMA),cathodoluminescence(CL)and laser raman(LR)testing methods to explore the petrological and mineralogical characteristics of the Fengcheng Formation in the Mahu Depression of Junggar Basin,and analyze the type of alkaline diagenesis and diagenetic conditions experienced during the Fengcheng Formation to establish an alkaline sedimentary evolution-diagenesis model.The Fengcheng Formation experienced alkaline diagenesis,the strata developed a peperite comprised of alkaline mafic-intermediate volcanic rocks,carbonate rocks,clastic rocks,mudstone and rich reedmergnerite rocks,and three major diagenetic mineral assemblages have been identified: Carbonate mineral assemblages(calcite + ferreous dolomite),reedmergnerite and carbonate mineral assemblages(reedmergnerite + calcite +ferreous dolomite),reedmergnerite and alkaline mineral assemblages(reedmergnerite +shortite + trona).These three mineral assemblages can be divided into two types of alkaline diagenesis.The first type of mineral assemblages is related to low-temperature alkaline diagenesis,and the last two types of mineral assemblages are related to medium-high temperature alkaline diagenesis influenced by hydrothermal fluid.Low-temperature alkaline diagenesis is characterized by the generation of ferreous dolomite belts,quartz dissolution,agglutination of the laumontite as well as the cementation of chlorite,the medium-high temperature alkaline diagenesis is represented by the formation of reedmergnerite.Quartz dissolution and the cementation of chlorite have dual effects on the physical properties of the reservoir.The pores created by the dissolution of laurentite cementation are conducive to the accumulation of oil and gas.The appearance of alkaline minerals such as reedmergnerite contributes to the generation and transformation of oil and gas,and the larger pores generated by the dissolution of alkaline minerals can develop into good oil and gas storage spaces.The occurrence of alkaline diagenesis is affected by conditions such as provenance,climate,and water-rock reaction,the hydrolysis of alkaline mafic-intermediate volcanic rocks and hydrothermal fluids provide the material conditions for the formation of the rocks,and the prolonged arid climate facilitates the formation of the special alkaline environment of the“alkaline lakes”,where water-rock reactions play an important role in the formation of the three major mineral assemblages.Combining the sedimentary and diagenetic factors of the provenance,climate,water-rock reaction and the diagenetic environment,a sedimentary-diagenetic model for the alkaline environment of the Fengcheng Formation has been established.The model is divided into four stages: before deposition of the Fengcheng Formation,when the Fengcheng Formation are deposited,when the Fengcheng Formation strata begin to be buried into a low-temperature alkaline diagenetic stage,and a medium-high temperature alkaline diagenetic stage with increasing burial depth and hydrothermal influence.