Characteristics and Genetic Mechanisms of Gas-Water Distribution in Deep-Buried Continuous Superimposed Tight Sandstone Gas Reservoirs in Western Sichuan Depression,Sichuan Basin

Chen Dongxia; Wang Qiaochu; Zhu Weiping; Yao Dongsheng; Xiong Liang; Zeng Jianhui; Wu Binzhou; Wang Fuwei

doi:10.3799/dqkx.2022.387

Abstract：Deep-buried superimposed tight sandstone gas reservoirs in the western Sichuan Depression are characterized by intensive heterogeneity with obvious water production.However,the mechanisms of gas-water distribution and its evolutionary processes are still unclear,which constrains the natural gas exploration and exploitation severely.Based on the data of drilling,well-logging,production and laboratory experiments,this study reveals the distribution characteristics,controlling factors and mechanisms of gas-water distribution of the tight gas reservoirs in upper Triassic Xujiahe Formation.Furthermore,this study also establishes a genetic evolution model of gas-water distribution.The research results show obvious disparity of gas-water distribution between different members of Xujiahe Formation.The gas-water distribution in the upper 4th Member of the Xujiahe Formation shows"layer-like"pattern,which is characterized by vague gas-water interface,while the regular distribution pattern(gas over water)and reverse distribution pattern(water over gas)with obvious gas-water interface only occur in part of the reservoirs.The gas-water distribution in lower 4th Member and 2nd Member of the Xujiahe Formations is characterized by"water surrounded by gas"pattern,the formation water is characterized by"island-like"distribution pattern.The hydrocarbon generating capacity of source rocks controlled the gas-water distribution at macroscopic level,while faults had dual influences of connecting and disrupting on the gas-water distribution.The reservoir heterogeneity further adjusted the water-gas distribution.The distribution of microscopic pore fluids had significant influence on water-gas distribution of the tight sandstone gas reservoirs,the three hydrodynamic fields controlled by pore fluids led to different occurrence characteristics of formation water and hence the different gas-water distribution.According to the evolution of hydrodynamic fields,hydrocarbon generating and expelling history and characteristics of tectonic evolution,the formation and evolutionary processes of formation water distribution were divided into four stages.The first stage was gas-water differentiation stage controlled by free hydrodynamic field with early-period hydrocarbon expulsion before the end of Early Jurassic.The second stage was formation water distinction stage controlled by different hydrodynamic fields with hydrocarbon expulsion peak from Early Jurassic to the end of Late Jurassic.The third stage was formation water adjustment period controlled by different hydrodynamic fields with hydrocarbon expulsion peak from Early Cretaceous to the end of Cretaceous.The fourth stage was gas-water distribution stereotype stage at present.