Abstract:

The exploration and development of shale gas in Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in Nanchuan and its adjacent blocks have yielded fruitful results. However, it is crucial to pay closer attention to the comprehensive use of fluid characteristics for analyzing the differences in shale gas preservation conditions in each block. Research findings reveal the following key points: ① With the prolongation of recovery time, the mineralization degree of the produced water gradually increases, exhibiting notable differences from fracturing fluid. This suggests the presence of the presence of movable CaCl₂-rich formation water in shale layer, characterized by a mineralization degree exceeding 50 g/L; ② Enriched and high-yield wells exhibit low water production, low mineralization and rich in NaHCO₃, which are indicative of condensate water; ③ Under different preservation conditions, the deuterium oxygen isotopes of the produced water vary with time. The high pressure stable block in the basin gradually deviates from the atmospheric precipitation line, while the normal（low） pressure complex block outside the basin remains close to the atmospheric precipitation line; ④ From the inside to the outside of the basin and from deep to shallow, the homogenization temperature of fluid inclusions in shale fracture filled calcite veins gradually decreases（from 240 ℃ to 90 ℃）. Simultaneously, the metamorphism coefficient of the inclusions also gradually increases, reflecting the degree of differential damage of shale gas preservation conditions.

Key words: shale gas, preservation condition, hydrochemical characteristics, isotope, Nanchuan