The Formation Mechanism And The Distribution Rules Of The Reservoir Of The Leikoupo Formation In Pengzhou Gas Field

The Leikoupo formation in Pengzhou is a hot spot in the exploration of natural gas in Sichuan Basin and has good prospect for development.The formation mechanism of reservoirs and their distribution rules are discussed by drilling core observation and thin section identification,carbon and oxygen isotope analysis,orderly degree analysis,rare earth element analysis,and thorium isotope analysis.The results of the study show that:（1）The rock types of the Leikoupo Formation in the Pengzhou area are mainly microcrystalline limestone,granular limestone,algae-bound limestone,grain dolomite,and particulate dolomite And algae-bonded dolomite,among which algae-bonded rocks and crystal-grained rocks are the main reservoir rocks;the types of reservoir space in the study area include dissolved pores,intercrystalline pores,intercrystalline dissolution pores,and unfilled Tectonic fractures,in which dissolved pores are the most important reservoir space,they are characterized by anodic pores.The porosity of the upper reservoir section in the study area is relatively low,and the pore permeability is weak,while the permeability and porosity are higher than that of the upper reservoir section,and the porosity and permeability have a weak positive correlation,reflecting that the reservoir is mainly based on poor connectivity.（2）There are three stages of atmospheric water dissolution in the study area.The first stage is dissolution in the syngeneic stage.The dissolution is characterized by the dissolution of intragranular particles and mold pore;the second stage is relative to the unconformity surface in Indosinian period,the signs are mainly the following:irregular weathering cracks developed,it was network-like;infiltration sand development in the pores;localized karst or cream breccia development;A large number of dissolved pores were found in the rocks,which mainly represented paste holes and paste dissolution holes;The calcite spots were widely developed.The phase III atmospheric water dissolution was only seen in the Longshen 1 well.（3）Theδ¹³C andδ¹⁸O value of calcite spot patches in Leikoupo formation in Pengzhou area are lower than those in normal limestone,and they are difference in from fracture calcite;and the The ⁸⁷Sr/⁸⁶Sr average value of the calcite was higher than that of the normal microcrystalline limestone;In the distribution of the rare earth elements,the calcite spot patches are slightly enriched in rare earth elements compare to the normal microcrystalline limestone.（4）The reservoirs was influenced by different factors,Including the sedimentary environment,unconformity,ancient landform,etc.the sedimentary environment affects the development of the formation which contains plaster.It is the basis for reservoir development.The atmospheric water of the Indosinian period is the key to the formation of reservoirs,which makes the plaster dissolve.the ancient landform affects the hydrological system and affects the heterogeneity of reservoir.