Formation Mechanism and Implications of Tectonic Breccia-Related Dolostone Reservoirs in the Ediacaran Dengying Formation, Central Sichuan Basin

Deep and ultra-deep carbonate reservoirs in the Ediacaran Dengying Formation of the central Sichuan Basin are important targets for natural gas exploration. Although previous studies have documented depositional and diagenetic controls on Dengying reservoir development, the formation mechanisms and reservoir effects of tectonic breccia remain insufficiently constrained. This study integrates core and thin-section observations, FMI logs, petrophysical measurements, fluid-inclusion microthermometry, carbon and oxygen isotopes, and U–Pb dating of saddle dolomite cement to clarify how tectonic breccia formed and how it modified the dolostone reservoir quality. The results indicate that tight laminated dolomicrite was fragmented within a strike-slip fault-damage-zone system during Late Caledonian weak compression and localized transtension, forming 1–5 cm angular to sub-angular clasts and an inter-breccia fracture-pore framework. The U–Pb dating (ca. 440 ± 17 Ma) roughly ties saddle dolomite cementation to the brecciation driven by Late Caledonian localized stretching. Brecciation created the early fracture-pore system, whereas saddle dolomite cementation produced the strongest early pore-loss effect and later fillings further reduced the residual pore space. Later calcite cementation, bitumen–sulfide filling, and open fractures are interpreted as reservoir-modification stages inferred from cross-cutting relationships, the burial history, and regional geochronological constraints rather than direct dating. These data indicate that effective fracture-pore reservoirs were preserved locally, where the early breccia framework remained partly open or was later reconnected by open fractures. This study clarifies how fault-related brecciation, staged cementation and filling, and late fracture preservation jointly controlled the preservation of the deeply buried carbonate reservoir quality.