关键词: 裂缝脉体, 古地层压力, 超压演化, 筇竹寺组, 页岩气富集, 四川盆地南部

Abstract: Natural fractures are key factors controlling shale gas enrichment and high production in Qiongzhusi Formation of Sichuan Basin. However, the development characteristics and formation stages of natural fracture systems of different origins, as well as the coupled evolution mechanisms of the fracture systems and formation pressure in Qiongzhusi Formation, remain unclear. Consequently, further investigation is required to clarify differential shale gas enrichment mechanisms. Based on multi-scale characterization of natural fractures through cores, rock thin sections, and imaging logs, combined with fluid inclusion and laser Raman spectroscopy analyses, this study systematically investigates the development patterns of natural fractures in Qiongzhusi Formation shale in Ziyang-Weiyuan area of Sichuan Basin. The formation and evolution sequences of natural fractures are established, the genetic mechanism of formation overpressure and the evolution history of formation paleo-pressure in Qiongzhusi Formation are elucidated, and the spatiotemporal coupling between fractures and formation pressures controlling differential shale gas enrichment is revealed. The results indicate that the shale reservoirs of Qiongzhusi Formation in Ziyang-Weiyuan area are dominated by tectonic fractures, mainly including tensile fractures, shear fractures, and hybrid tensile-shear fractures. Horizontally, the distribution patterns of natural fractures are primarily controlled by tectonic forces. Vertically, fracture development is controlled by the content of brittle minerals in rocks, and fractures are most developed in the middle to lower parts of the fifth layer of submember 2 of Member 1 of Qiongzhusi Formation. The natural fractures in Ziyang-Weiyuan area mainly formed during the Late Caledonian, Late Hercynian-Indosinian and Late Yanshanian-Early Himalayan periods. The fracture-filling minerals and hydrocarbon inclusions exhibit significant differences among fractures formed during different stages. The overpressure in Qiongzhusi Formation is primarily attributed to hydrocarbon generation-induced pressurization. The evolution of formation pressure can be divided into four stages, ultimately resulting in the present-day formation pressure system. During the Middle-Late Caledonian period, moderate overpressure developed throughout Ziyang-Weiyuan area. During the Late Hercynian-Indosinian period, formation pressure in Ziyang area continued to increase slowly, whereas that in Weiyuan area gradually dissipated. During the Early-Middle Yanshanian period, Qiongzhusi Formation shale entered into the peak stage of hydrocarbon generation, promoting the development of strong overpressure in Ziyang area (formation pressure coefficient of 2.00-2.92) and moderate overpressure in Weiyuan area. From the Late Yanshanian to Early Himalayan period, structural uplift of the Weiyuan area resulted in the transformation of formation pressure into a normal pressure system (formation pressure coefficient of 1.00-1.20), while the Ziyang area remained overpressured (formation pressure coefficient of 1.60-2.01). The synergistic interaction between natural fractures generated by multi-stage tectonic movements and the pressurization process associated with shale hydrocarbon generation governs the differential enrichment of shale gas in the study area. During the Early-Middle Caledonian period, natural fractures influenced the spatial variation of regional formation pressure by controlling the efficiency of lateral hydrocarbon migration and dissipation. During the Hercynian to Early-Middle Yanshanian period, the fracture system exhibited multiple functions, including hydrocarbon transport, replenishment, and localized sealing. The Ziyang area developed extremely high overpressure, whereas the Weiyuan area experienced medium-intensity overpressure under the combined effects of hydrocarbon generation-induced pressurization and tectonic compression. From the Late Yanshanian to Early Himalayan period, regional differential uplifting and reconfiguration of the tectonic framework further exacerbated the heterogeneity of hydrocarbon preservation conditions. The Weiyuan anticline underwent substantial uplift, which promoted the extensive development of through-going shear fractures and large-scale shale gas dissipation. In contrast, the Ziyang area experienced relatively weak tectonic modification, allowing the preservation of the overpressure system and providing favorable conditions for shale gas enrichment.

Key words: fracture vein, formation paleo-pressure, overpressure evolution, Qiongzhusi Formation, shale gas enrichment, southern Sichuan Basin