Characterization Of Shale Reservoir Multiscale Heterogeneity And Its Relationship With Hydrocarbon

The Lower Paleozoic Wufeng-Longmaxi Formation shale reservoir of the Sichuan Basin is one of the most realistic formations of shale gas exploration and development in China.The large-scale commercial development has been realized in the Fuling shale gas field.The analysis of characteristics of shale reservoir heterogeneity is an important part of evaluating shale gas reservoirs time and space distribution in the sedimentary basin.The discussion of the relationship between shale reservoir heterogeneity and hydrocarbon content is the basis for the prediction of shale gas reservoir geological dessert.Based on the analysis of the tectonic setting,geochemical characteristics,petrological characteristics,and gas-bearing characteristics of shale gas wells,the shale gas reservoir macro-heterogeneity characteristics,micro-heterogeneity characteristics,and gas-bearing characteristics are discussed.The relationship between shale reservoir heterogeneity and hydrocarbon content is confirmed,and the control factors of shale gas difference enrichment and the accumulation models are discussed.The Wufeng-Longmaxi Formation shale reservoir has significant difference in macro-heterogeneity characteristics(sedimentary facies and lithofacies)and micro-heterogeneity characteristics(pores as the main parameter).Three third-order sequences,SQ0,SQ1,and SQ2,were developed from the bottom to the top of the Wufeng-Longmaxi Formation.The SQ0 developed a transgressive systems tract in the Wufeng Formation,and has a stability and continuity distribution,which deposit in deep shelf.The SQ1 is the lower strata of the Longmaxi Formation.At the early of SQ1,deep shelf was developed.While at the late of SQ1,the northwest was mainly shallow shelf,and the southeast was mainly coast.The SQ2 is the upper strata of the Longmaxi Formation.At the early of SQ2,the sedimentary facies were shallow shelf and tidal flat from northwest to southeast.While at the late of SQ2,the study area was given priority to coastal facies.The Wufeng-Longmaxi Formation Shale is mainly developed 9 types of lithofacies.The organic-rich siliceous shale is mainly developed in the lower section of the formation,and has a stable and good connectivity distribution,whose thickness is gradually thinning from northwest to southeast.At the middle and upper section of the formation,the lithofacies changes fast,the various types of lithofacies are appear,and the lateral continuity become bad,which showing a strong spatial heterogeneity.The organic pore in the Wufeng-Longmaxi Formation Shale generally shows elliptical,gneissic or irregular,etc.The pore size of organic pore is generally between 5 nm and 200 nm.The secondary dissolution pore generally shows elliptical or irregular,and the pore size is generally less than 400 nm.The secondary intergranular pore generally shows slit or wedge-shape,and pore size is generally between 0.5?m and 2 ?m.The micropore,mesopore,and macropore are developed well in the Wufeng-Longmaxi Formation Shale.The mesopore and macropore account for about 76% of the total pore volume,while the micropore account for about 77% of the total specific surface area.The surface porosity of the Wufeng-Longmaxi Formation Shale is mainly provided by the pores smaller than 400 nm.The pores in the organic matter are most developed,whose surface porosity is between 17% and 21%.The multi-scale heterogeneity of the Wufeng-Longmaxi Formation shale reservoir has an obvious controlling function on hydrocarbon content.The adsorbed gas storage capacity of the Wufeng-Longmaxi Formation is mainly controlled by the micropore,while the control function of mesopore is weak.The free gas storage capacity of the Wufeng-Longmaxi Formation is mainly controlled mesopore and macropore.The organic matter has an obvious controlling effect on pore development,which controls the adsorption and desorption capacity of shale gas.The lithofacies combination of organic-rich siliceous shale and organic-rich clay shale is favorable for shale gas generation and preservation.The deep shelf generally has high hydrocarbon content.The good shale gas reservoirs of the Wufeng-Longmaxi Formation are mainly concentrated in the inside of the Sichuan Basin.The shale gas content of the outside Sichuan Basin is generally lower than that of the inside of the Sichuan Basin.The size of the gas reservoir is controled by the residual strata,while its sealing ability is the key for the preservation of shale gas reservoir.The shale gas reservoir is a process of dynamic balance,and the most favorable accumulation models of shale gas reservoir is the type of extra source supply + reservoir sealed(ESS-RS).The shale gas reservoirs of the Sichuan Basin are mainly developed in the inside of the basin,which has the characteristics of positive tectonics,moderate stratum buried,good stratum continuity,good stratum closed boundary,late uplifting time,small uplift amplitude,and extra gas supply.