The Characteristics And Formation Mechanism Of The Upper Fourth Sub-member Reservoir Of The Leikoupo Formation In Pengzhou Gas Field

The upper part of Pengzhou 115 well in Pengzhou area of western Sichuan has obtained industrial high-yield airflow,revealing that a reservoir is also developed in the upper part of the upper part of Lei 4,which is referred to as the upper reservoir section and has extremely high exploration and development value.According to the comprehensive reservoir anatomy of the well profile,it is revealed that there is also a set of reservoir sections in the upper part of the upper forth formation in the area,referred to as the upper reservoir section.The characteristics and formation mechanism of the upper reservoir of upper fourth Leikoupo formation are discussed by carbon and oxygen isotope anaysis,rare earth element analysis,inclusion temperature measurement,ICP-MAS analysis,thorium isotope analysis,cathodoluminescence analysis and electron probe analysis.The results of the study show that:?1?The rock types of the upper reservoir in the study area are mainly granular limestone,algae-bound limestone,microcrystalline limestone,microcrystalline dolomite,granular dolomite.Algae bond structure and grain structure are dominant,among which microcrystalline dolomite rocks the reservoir space is dominated by the pores of the paste,and intercrystalline pores and intercrystalline pores are the second.The reservoir type is mainly composed of the powder-microcrystalline cloud rock paste pore type,and micro-fine crystal cloud-dissolved pore-type reservoirs.The permeability of the upper reservoir is lower,and the correlation of pore permeability is relatively weak,showing a relatively weak positive correlation.?2?There are three stages of dolomitization in the upper reservoir section.The first stage is the same period of drought and high salinity dolomitization,the second stage is shallow burial of dolomitization in the early diagenetic stage,and the third stage is medium-deep diagenesis.The degraded calcite?¹³C and?¹⁸O values are lower than normal marine limestone,and the average composition of ⁸⁷Sr/⁸⁶Sr is higher than that of normal marine microcrystalline limestone.The deodorized calcite in the upper reservoir segment is similar to the microcrystalline limestone in terms of strontium isotope composition and rare earth element composition,indicating that the diagenetic fluid is from the interior of the carbonate rock formation.?3?There are two periods of atmospheric freshwater dissolution in the study area.The first stage is the dissolution of the same period,which is characterized by intragranular dissolution in the granular cloud rock to form intragranular dissolved pores and intragranular casting holes.;The second phase is the atmospheric water effect of the ancient epigenetic period?Indosinian I?,which is characterized by the development of irregular network-like weathering cracks,the extensive development of de-agglomerated calcite,and the extensive development of plaster pores and plaster pores.The composition of strontium isotopes and rare earth elements of degyrated calcite in the upper reservoir are similar to that of microcrystalline limestone,indicating that the carbonate formation itself controls the geochemical composition.?4?The reservoir development control factors in the upper reservoir zone of the upper area include the following aspects:closely related to dolomitization and dolomite,and the cloud-bearing facies?localized tidal flat phase?restricts the developmental distribution of the reservoir.