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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 production potential of the region. Methods Based on formation water geochemical analysis， various chemical parameters （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 distribution of different formation water types. Finally， the controlling factors of gas-water distribution were explored in relation to the hydrocarbon source rock， karst reservoir heterogeneity and structural features. Results The total salinity of formation water （total dissolved solids） in the 5th member of the southern Sulige gas field is relatively concentrated （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 potentially favorable exploitation zone. Type III indicates poor reservoir properties. Conclusions The distribution of formation water is controlled by three factors： hydrocarbon source rock， karst reservoir heterogeneity and local structures 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 pattern， and local structures have resulted in gas-water differentiation， with a unified gas-water interface within the same karst unit.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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