岩溶储层地层水地球化学特征及气水分布规律OA

[Objective]The 5th member of the Ordovician Majiagou Formation in the southern Sulige gas field primarily consists of karst reservoirs,with proven reserves exceeding 100 billion cubic meters.However,during development,severe water production and complex gas-water relationships have been observed,and the distribution patterns and controlling factors of gas and water remain unclear,significantly constraining the release of the gas pro-duction potential of the region.[Methods]Based on formation water geochemical analysis,various chemical parame-ters(i.e.,Na+/Cl-,desulfurization,Mg2+/Ca2+,Na+/Ca2+and Na++Cl-/Mg2+coefficients)were analyzed to determine the formation environment of the water.The distribution patterns of gas and water were then studied by identifying the water-bearing intervals in individual wells,and by cross-well gas reservoir profile comparisons and the planar distri-bution of different formation water types.Finally,the controlling factors of gas-water distribution were explored in re-lation to the hydrocarbon source rock,karst reservoir heterogeneity and structural features.[Results]The total salini-ty of formation water(total dissolved solids)in the 5th member of the southern Sulige gas field is relatively concen-trated(mean 83 598.7 mg/L),with slightly acidic pH.The water type is predominantly CaCl2,and exhibits low chloride-sodium coefficients,low desulfurization coefficients,low magnesium-calcium coefficients,low sodium-calcium coefficients and high metamorphism coefficients,all indicating good formation sealing favorable for gas reservoir preservation.These parameters show similar characteristics over the area,controlled by the karst reservoirs.The formation water is classified into three types,based on genetic differences,spatial distribution and reservoir properties:Type I(structural low-position water),Type II(isolated lenticular water)and Type III(low-permeability trapped water).Type I is mainly developed in karst areas,controlled by karst residual hill units,with a unified gas-water interface,and is the primary region for high-efficiency wells.Type II is sporadically distributed and influenced by the planar distribution of karst depressions and troughs,with moderate reservoir properties,representing a poten-tially favorable exploitation zone.Type III indicates poor reservoir properties.[Conclusions]The distribution of for-mation water is controlled by three factors:hydrocarbon source rock,karst reservoir heterogeneity and local struc-tures within individual karst units.Insufficient hydrocarbon supply from the source rock is the fundamental reason for the development of formation water.The heterogeneity of the karst reservoir dictates the gas-water distribution pat-tern,and local structures have resulted in gas-water differentiation,with a unified gas-water interface within the same karst unit.