Oil And Gas Accumulation Zones Characteristics And Distribution Evaluation Of Leikoupo Formation,Sichuan Basin

As a result of the Indosinian movement,the Leikoupo Formation suffered erosion of different degree in different areas in Sichuan Basin.Leikoupo Formation in the Lu zhou ancient uplift has been eroded away and eroded in the Kaijiang ancient uplift to T₂l₂¹ formation.At present,in the Leikoupo Formation along the northern slope of the Luzhou ancient uplift in the Sichuan Basin,Moxi T₂l¹ gas reservoir,Wolonghe T₂l¹ gas reservoir and several gas-bearing structures were discovered.Zhongba T₂l³ gas reservoir,Longgang T₂l⁴ gas reservoir,Yuanba T₁l⁴ gas reservoir,Pengzhou-xinchang T₂l⁴ gas reservoir were discovered in the south rim of Tianjingshan ancient uplift.In this paper,through the observation of rock thin section,major and trace element analysis,carbon and oxygen isotope analysis,cathodoluminescence analysis and X-ray diffraction analysis of the Leikoupo Formation reservoir,combined with the current exploration and research status of the Leikoupo Formation,the characteristics of sources,reservoir and cap were studied.Because the distribution of natural gas is of a certain regularity and regionality,the concept of combining oil and gas accumulation zones is used to divide and study the natural gas accumulation zone of Leikoupo Formation.Based on the analysis of carbon and oxygen isotopes and major and trace elements,it is suggested that the sedimentary environment of Leikoupo Formation is higher salinity,warmer,and more enclosed environment.Sedimentary facies are fringe?barrier?carbonate platform sediments,which can be divided into restricted-evaporation platform subfacies,platform marginal shoal subfacies and tidal flat subfacies,and in the restricted-evaporation platform subfacies they can be further classified as lagoons,the lagoon edge and platform inner shoal.From the field profile and core observations,it was found that Leikoupo Formation is mainly a set of dolomite,limestone and gypsum salt rock deposits.The reservoir types are shoal reservoirs,paleokarst reservoirs,and crystalline carbonate reservoirs.The reservoir space is mainly composed of dissolved pores,holes,and fractures,and Leikoupo reservoir belongs to medium-low pore and medium-low permeability reservoir.In the analysis of reservoir origin,it was found that the formation of dolomite reservoirs in Leikoupo Formation was controlled mainly by sedimentary microfacies,tectonics,dolomitization,and dissolution.Sedimentary facies control the spatial distribution of reservoirs.Penecontemporaneous dissolution and tectonic movement are the key to the formation of pores and fractures.Dolomitization plays an important role in the preservation of early pores,and supergene karstification and burial karstification can modify reservoirs.The conventional logging data and core analysis data combined with the integrated method in machine learning are used to divide and predict the carbonate lithology of the reservoir.The random forest algorithm and gradient tree boosting have good results in prediction.Logging features,logging elements,and seismic reflection features were used to identify karst reservoirs at the top of Leikoupo Formation.It was found that karst reservoirs have high gamma characteristics in conventional logging curves,have high manganese,iron,and aluminum contents in logging elements feature,and have low-wave impedance,weak reflection,and disorderly reflections in seismic reflection.According to the definition of oil and gas accumulation zone,Leikoupo Formation is identified as two natural gas accumulation zones:the natural gas accumulation zone in the north slope of the Luzhou ancient uplift and the natural gas accumulation zone on the south rim of the Tianjingshan ancient uplift.Combining the existing exploration examples of hydrocarbon reservoirs,the characteristics of natural gas accumulation zones are analyzed.It is suggested that the accumulation model of the north slope of Luzhou ancient uplift natural gas accumulation zone is controlled by dual source hydrocarbon supply?Permian source rocks,Xujiahe Formation source rocks?vertical and lateral transportations?unconformity,faults,and pore-fracture?,and the shoal and karst reservoirs.The south rim of the Tianjingshan ancient uplift gas accumulation zone is controlled by multi-source hydrocarbon supply(Permian source rocks,Xujiahe Formation source rocks and the Leikoupo Formation source rocks,vertical and lateral transportations?unconformity,faults,and fracture?,and the shoal and karst reservoirs.This paper uses the analogy method as a theoretical basis,combines the probabilistic statistical principles,and uses the geological analogy-probability method to calculate the probabilistic distribution of oil and gas geological resources in the natural gas accumulation zone of the Leikoupo Formation.The natural gas geological resources of the north slope of Luzhou ancient uplift and the south rim of Tianjingshan ancient uplift are 10500?10⁸m³ and 13700?10⁸m³,respectively.