Permian karst porous limestone reservoir in the syncline area of southern Sichuan Basin:Origin and new field of natural gas exploration
With the continuous advancement of oil and gas exploration,the exploration direction gradually expands from structural highs to syncline areas.According to the traditional understanding,the reservoirs in syncline areas are generally tight,so how to find high-quality reservoirs is one of the key factors to achieve breakthroughs in oil and gas exploration in syncline areas.Taking the limestone of Middle Permian Maokou Formation in the syncline area of southern Sichuan Basin as an example,this paper comprehensively analyzes the drilling cores,thin sections,well logging and geochemical data.Based on this,the characteristics and genesis of porous limestone reservoirs in syncline areas are researched.In addition,a reservoir development model is established,and the new direction of natural gas exploration is pointed out.And the following research results are obtained.First,the karst porous carbonate reservoir developed in the Maokou Formation in the syncline area of southern Sichuan Basin is lithologically dominated by micritic-sparry bioclastic limestone and sparry bioclastic limestone,and its reservoir space is mainly acted by the pores formed through fabric selective dissolution,such as biological coelomopores,biological moldic pores and intragranular dissolved pores.Second,the average porosity of the Maokou Formation reservoir in the study area is 4.07%,and the average permeability is 0.273 mD.There is a significant positive correlation between porosity and permeability,so it is a typical porous reservoir.Third,the Maokou Formation reservoir is in a low content of Al,Sc,Zr and total rare earth(ΣREE)as well as the content of Mn and Fe.Its carbon isotope values are within the distribution range of those of calcite precipitated in the Middle Permian seawater,and the oxygen isotope shows a certain negative bias.Fourth,the formation of the karst porous reservoir in the Maokou Formation is related to the penecontemporaneous karst,which is controlled by the sedimentary microgeomorphology and the high-frequency sequence,and the dissolution fluid is mainly meteoric water.In conclusion,karst porous reservoirs are generally developed in the Sichuan Basin,and this kind of reservoir is far away from unconformity and modified by penecontemporaneous karstification controlled by sedimentary palaeogeomorphology and facies belt.Therefore,the exploration target should be changed to"favorable sedimentary facies belts controlling reservoirs in a large scale",and syncline area will be an important new field for future natural gas exploration in the Sichuan Basin.
Porous reservoirKarst reservoirPenecontemporaneous karstMiddle Permian Maokou FormationSouthern Sichuan BasinSyncline area
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