Reservoir Evaluation Of The Triassic Leikoupo Formation,in The PZ Area Of The Western Sichuan Depression

Marine natural gas in the west of Sichuan has great potential for exploration and construction.The Middle Triassic LeiKoupo formation,which has not been well studied,is the most realistic potential growth and production target.The in-deep study of the marine oil and gas reservoir formation conditions,trap evaluation,reservoir prediction,oil and gas analysis is restricted due to the properties of this area.LeiKoupo formation is old and has thick stratigraphic thickness formed by corrosion.The reservoir heterogeneity is high.It is difficult to calculate the reservoir parameters quantitatively and the characteristics of the fluid distribution deeply.The seismic resolution and the signal to noise ratio of the marine stratigraphic system are also poor.There is an urgent need for careful interpretation of the fine reservoir parameters in the study area,establishing of accurate fluid identification standards and comprehensive evaluation of the reservoir classification which are very important for the subsequent reservoir and reserve evaluation,geological modeling,development plan constitution.In order to achieve comprehensive evaluation of the gas reservoir in LeiKoupo formation the techniques and methods usually used in carbonate reservoir research have been applied from the point-line–surface to study the reservoir characteristics and provenances,gas reservoir characteristics,reservoir logging evaluation,and reservoir prediction based on the combinations of macroscopic and microscopic,static and dynamic,geology and geophysics.Based on the understanding of the regional sedimentary background,the main sedimentary microfacies are identified from the lithology and electrical properties based on the field profile,the drilling coring and logging well of LeiKoupo formation.The seismic reflection characteristics analysis has been conducted to study the seismic signs of different sedimentary facies and to recognize the different types of sedimentary microfacies.The results demonstrate that the section 4 of the PZ area in the western Sichuan is medium-weak sedimentary carbonate platform sediments.The sub-3 of section 4 is localized tidal-flat depositional environment.The dolomite-lime flat and algae clouds flat pond provide a sedimentary and diagenetic basis for the extensive development of the reservoir while quasi-synchronic-shallow burial dolomitization provides favorable conditions for high-quality reservoir development.Dolomite is distributed on the plane,stable and continuously in the vertical direction.The subsection can be divided into gray clouds,gray algae cloud pond,algae beach,a gray cloud pond,algae cloud,cloud flat.Favorable micro-phase Yun Ping and Zaoyun Ping which tidal-flat facies.Based on the analysis of the cores and flakes of 4 wells,the study area is mainly composed of algae-bound dolomite and grain dolomite,followed by gray dolomite and limestone.The lithologic identification neural network model has been established by using the BP nonlinear neural network identification method.The lithology of the single well is explained by this model and agree well with the experiment results.The porosity of the whole diameter is 1.03⁶.47%while the average porosity is 3.69%.The vertical permeability is 0.0043¹.89×10^-3?m² with an average at 0.25×10^-3?m².Z1,Z2 and Z3 are the best among the 8 wells studied.The"karst type"reservoir with"strong peaks+relative low impedance"seismic response is found in the study area based on high resolution processing,wave impedance inversion and porosity inversion study.The reservoir in the Long Men piedmont tectonic belt is widely distributed in the northeastern part of the study area.The predicted reservoir thickness is 30⁴5 m.The reservoir is thick with good physical properties and high abundance of gas.The effective thickness of reservoir petrophysical lower limit has been confirmed by analogy method.In theory,the effective porosity of the lower limit should be less than 2%when the gas flow is good.It's an effective way to combine the depth of the double side of the absolute value of resistivity,normal probability distribution method,longitudinal wave time difference and longitudinal wave velocity ratio method and other methods to evaluate the reservoir properties and achieve reservoir fluid identification.The evaluation standard of reservoir is established based on the“four property relationship”results.Based on the wave impedance vs effective thickness by inversion,the porosity,permeability,gas saturation and pore throat structure were selected as the comprehensive evaluation parameters of gas reservoir.The study area is divided into favorable reservoir area and favorable reservoir area,which is mainly distributed in the relative high site of Jinma-Yazi River structure and local high point area of Shiyang Chang.The characteristics of the upper reservoir segment distribution show that there is no significant change in the lateral direction.However,the quality is poor which mainly composed of type?followed by type?and type?reservoir.The lower reservoir section is the main production layer,which can be divided into the upper and lower reservoir subsection.The main reservoirs in the Z3 well and Z2well are distributed in the upper part of the lower reservoir,indicating that the favorable reservoirs of the Jinma and Yazi River are mainly distributed in the upper part of the lower reservoir.The main reservoirs in the Z1 well are mainly distributed in the lower part of the lower reservoir.