Seismic Description Of Dolomite Reservoir Of Leikoupo Formation In Jinma-Yazihe Longmenshan Piedmont,Western Sichuan Basin

With the further exploration of marine facies in western Sichuan,high-yield industrial gas flow was taken from Middle Triassic Leikoupo Formation in many focused exploration wells,which indicates the bright prospecting future of Leikoupo Formation.Currently,the three drilled wells are all good indicators.Although drilling results show the total thickness of dolomite reservoirs is large,the single layer is thick and the heterogeneity is strong without obvious seismic response.Due to the large depth of the strata in Leikoupo Formation in western Sichuan,the resolution of seismic data is low and the signal-to-noise is poor.Furthermore,the formation was influenced by multi-stage tectonic movements and the induced fractures are complicated,which has increased the difficulty of fine description of reservoir to a certain extent.According to the previous research and seismic characterizing issues of Leikoupo Formation,we summarized the characteristics and the main controlling factors of the target reservoir.We believe sedimentary facies and fractures control the developing magnitude of good dolomite reservoir.Due to different geological characteristics of reservoir are guided by different factors,the corresponding seismic responses are also different.Therefore,based on the geological knowledge and seismic interpretation,this paper presents the description of target reservoir against the main controlling factors by reasonable seismic method and attribute analysis.The seismic facies analysis is mainly carried out by the waveform classification method to determine the favorable band and show the plane distribution of the sedimentary facies.Impression and residual thick method help us rebuild the palaeo-geomorphology and search for favorable developed karst reservoirs.According to the seismic modeling,we obtained the seismic response characteristics and built the identification mark of dolomite reservoirs,while the amplitude attribute analysis is based on this modeling results.The inverted reservoir parameters including wave impedance and porosity are used to identify the better physical properties developed area.After describing the fracture distribution by multi-attribute analysis,we finally get the fracture developed area.Based on the controlling factors of sedimentary facies and fractures,combing with paleo-geomorphology rebuilding,multi-attribute analysis and inversion results,the multi-seismic method can be used to characterize the target reservoir.The following knowledge are obtained:?1?The favorable seismic facies of the upper and lower reservoirs are determined by waveform classification.The seismic facies of upper reservoir varied from strong to weak with M₁ complex waveform,and small peaks are found between M₁ and L₁ layer.So the upper reservoir is probably developed with subtidal lime and dolomite flat of microfacies.The seismic facies of lower reservoir varied with strong peaks?second phase?and trough,so it is probably intertidal dolomite-lime flat and algae clouds flat of microfacies developing area.?2?According to the results of seismic modeling analysis,different seismic response characteristics of target reservoirs are summarized,which shows great connection between the amplitude energy of the reflected wave at the bottom of the reservoir and the physical properties of the karst reservoir.The stronger the amplitude of second phase,the greater the thickness and better the physical properties of reservoir.?3?Paleo-geomorphology rebuilding helps to determine the dissolution developed zone.The study area is an ancient geo-slope unit tilted to the northwest with short-time exposure and weak dissolution,while the dissolution of Yazihe is stronger.Attribute analysis reflects the plane distribution characteristics.The submember of the fourth Leikoupo formation is widely distributed with dolomite reservoirs?lower?,and the favorable area mainly develops in Yazihe structural band,where the reservoir is scaled.According to the wave impedance and porosity inversion,we can distinguish good reservoir.The physical properties of target upper reservoir is poor while the lower is good.Pore enriched area is developed in the vicinity of YaS1,and the physical properties of PZ1 in Jinma structural band is moderate,while the large area near YS1 is relatively poor.Multi-attribute analysis can be helpful to determine the fracture developing zone.The fractured form is dominated by crush network,and the Jinma tectonic belt in PZ1 is developed with fractures,and the fractures of the Bailu tectonic belt are also developed.Finally,combining the distribution characteristics of the sedimentary facies,we are able to predict the favorable high quality reservoir.?4?Different reservoirs with different geological characteristics are controlled by different factors and also response seismic differently.Base these controlling factors,choosing appropriate seismic characterization method helps to improve the reliability.