Fluid activity characteristics of shallow shale veins in the Wufeng–Longmaxi Formation in the Shixi syncline in northern Guizhou and their significance for shale gas preservation

Abstract

Preservation conditions are key to enriching shallow shale gas. Therefore, the veins of two typical wells in the Shixi syncline in northern Guizhou as research objects. Based on core observation, vein thin-section observation, cathodoluminescence, calcite in situ U-Pb dating, fluid inclusion microthermometry, Raman spectrum shift, single well basin simulation, and C-O isotope geochemical analysis, the researchers clarified the characteristics of the veins and the differences in paleofluid activity, as well as their significance for shale gas preservation. The results showed that: 1) a small number of high-angle fractures had developed mainly in the Shixi syncline in northern Guizhou. The width of viens is small and filled primarily with early calcite-quartz and late calcite. The inclusions were mainly methane and brine. The proportion of brine inclusions in the SD1 well was greater than that in the SX1 well. The SD1 well has experienced more intense second-stage uplift and denudation, and the consequent gas loss is serious. 2) The results of the C-O isotope analysis showed that most of the vein-forming fluids were derived from marine carbonate rocks. The O₃b had obvious negative deviations of δ¹³C and δ¹⁸O, which were modified by exogenous or deep hydrothermal fluid. The C-O isotope difference between the vein bodies and the surrounding rock of the SX1 well was small, and the preservation conditions were better. 3) The difference in gas content in the Shixi syncline depends on the degree of damage to gas reservoir preservation conditions caused by burial depth and other factors. Shixi syncline is a wide and gentle syncline with central retention enrichment mode. Therefore, clarifying the development of shale veins, the characteristics of paleofluid activity, and their significance for shale gas preservation can lay a foundation for studying the enrichment mechanism of shallow shale gas and thus guide further exploration and development.