272-6 Natural fracture characteristics and vein-forming fluid evolution of shales of the Wufeng-Longmaxi Formation in the complex tectonic zone of the southeastern Sichuan Basin, China

Session: Faults, Fractures, and Geomechanics for the Energy Transition (Posters)

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Abstract:

Natural fractures, generated by tectonic reworking and faulting, are widely and variably distributed in shale reservoirs, playing a crucial role in fluid dynamics at micro- to nano-scales. These fractures, particularly when filled with calcite veins, serve as valuable records of paleo-pressure conditions, revealing the historical evolution of fluid systems, hydrocarbon generation, and the dynamic changes in fracture architecture. Understanding the interplay between fractures and fluid dynamics is essential for interpreting shale gas generation, migration, and preservation processes.

This study focuses on calcite veins filling fractures in the shale of the Paleozoic Wufeng-Longmaxi Formation in the southeastern Sichuan Basin. Petrographic observations, geochemical analyses, fluid inclusion studies, and laser Raman spectroscopy, combined with burial history modeling, were used to reconstruct the pressure evolution and fluid dynamics of shale reservoirs. Three types of calcite veins were observed within the shale fractures: bedding-parallel veins, broken veins, and high-angle veins. The results show that bedding-parallel veins, formed between ~180 and 165 Ma, grew in a low-temperature (148.1–168.3°C) and closed environment due to overpressures from hydrocarbon generation. These veins record the pressure increase in the shale reservoir during early burial. In contrast, broken and high-angle veins, formed between ~140 and 95 Ma, grew in a high-temperature (163.5–206.4°C) and open environment associated with tectonic uplift and horizontal compression. These veins reflect the pressure release in the shale reservoir during the later stages of tectonic deformation.

The shale reservoir in the southeastern Sichuan Basin's complex tectonic zone transitioned from overpressure to normal pressure (hydrostatic) during the Yanshan period, with pressure decreasing from an overpressured state (~106 MPa) in the early Yanshanian (Late Jurassic) to a normal pressure state (~61 MPa) in the late Yanshanian. Folding, tectonic uplift, and erosion along the southeastern margin of the Sichuan Basin caused shale gas to migrate and dissipate along décollement surfaces and fractures, leading to a rapid decline in reservoir pressure.

This study reveals the key role of natural fractures in fluid migration and reservoir pressure evolution in complex structural areas, while also clarifying the importance of micro- to nano-scale natural fractures in shale gas preservation. Moreover, it enhances the understanding of shale gas pressure evolution and preservation in similar complex tectonic settings worldwide.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8435

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