Differential enrichment process of shale gas in Cambrian Qiongzhusi Formation in middle section of Deyang-Anyue rift trough, Sichuan Basin: evidence from fracture veins and fluid inclusions

In recent years, shale gas exploration in the Cambrian Qiongzhusi Formation within the Deyang-Anyue rift trough of the Sichuan Basin has achieved significant breakthroughs, establishing it as a crucial succession area for unconventional hydrocarbon exploration in the basin. Focusing on the Qiongzhusi Formation shale in the middle section of the rift trough, a series of analytical techniques were comprehensively employed, including core observations, thin-section analysis, cathodoluminescence (CL) imaging, laser in-situ U-Pb isotope dating of carbonate rock, fluid inclusion petrography, laser Raman spectroscopy, homogenization temperature testing, and basin numerical simulations. This study systematically investigated the fracture development characteristics, mineral-filling sequence, and the types, components, and thermobaric parameters of fluid inclusions, revealing the differential shale gas enrichment mechanisms in the study area. The results showed that bedding-parallel fractures, low- to high-angle tensile fractures, and high-angle shear fractures were developed. Specifically, well Z201 in the central rift trough exhibited densely developed high-angle shear fractures, while well WY1H on the western slope of the rift margin showed relatively weak fracture development. In contrast, well W201 in the high structural position of the Weiyuan area is characterized by high-angle shear fractures accompanied by hydrothermal mineral fillings. The fractures were predominantly filled with calcite, with local occurrence of quartz, barite, dolomite, and pyrite. Hydrocarbon inclusions in the vein minerals were diverse in type, with trapping temperatures decreasing in the order of methane > high-saturation hydrocarbon > bitumen. The trapping pressure for methane indicated that the Qiongzhusi Formation once widely experienced widespread overpressure to strong overpressure conditions. Combined with burial and thermal evolution histories, the results suggested that hydrocarbon accumulation in the study area experienced three stages: a shale oil generation stage from the Late Permian to Early Triassic, a shale gas enrichment stage from the Middle Jurassic to Early Cretaceous, and a shale gas adjustment stage from the Late Cretaceous to the present. Late-stage tectonic adjustments showed clear spatial differences. The central rift trough experienced minimal adjustments, preserving intact strong overpressure and thus serving as the optimal exploration target. The western slope of the rift trough margin remained overpressured with good gas-bearing properties. In contrast, the high structural position on the western rift trough margin experienced intense uplift and denudation during the Himalayan period, which reduced the pressure coefficient to 1.0 and significantly decreasing its gas-bearing capacities. Therefore, careful evaluation of preservation conditions is critical for exploration.