Diagenetic evolution and effects on reservoir development of the Dengying and Longwangmiao formations, Central Sichuan Basin, Southwestern China

The deeply buried Lower Cambrian Longwangmiao Formation and Upper Ediacaran Dengying Formation from the Sichuan Basin, China, have a total natural gas reserve up to 3 × 10¹² m³. The complex diagenetic evolution and their impacts on the present-day reservoir quality have not been systematically elucidated, hampering the current exploration. Crucially, the integration and comparation diagenetic study on these two formations, which may be able to shed new lights on reservoir formation mechanism, are yet to be systemically evaluated. By compiling geochemistry data, including carbonate U-Pb ages and petrophysics data, coupled with new petrology, trace elements, and strontium isotope data, of various types of diagenetic carbonates, this study aims to decipher the potential links between diagenesis and reservoir development of both formations. Intriguingly, similar diagenetic sequence, which contains five distinctive dolomite phases, is established in both formations. The matrix dolomite (D1) and early dolomite cement (D2) were likely formed by reflux dolomitization, as inferred by their nearly syn-depositional U-Pb ages and elevated δ¹⁸O caused by seawater evaporation. The subsequent moderate burial dolomite cement (D3) was most plausibly the product of burial compaction as indicated by its lighter δ¹⁸O and slightly younger U-Pb ages compared with D1 and D2. Whereas deep burial dolomite cements (D4 and D5) yield markedly depleted δ¹⁸O, elevated ⁸⁷Sr/⁸⁶Sr, along with much younger U-Pb ages and higher precipitation temperatures, suggesting that they were likely linked to hydrothermal fluids. Despite the wide occurrence of meteoric and organic acids leaching and thermochemical sulfate reduction, they may have only played a subsidiary role on these reservoirs development. Instead, superior reservoir quality is tightly linked to tectonics as inferred by higher reservoir quality closely related to the well-developed fractures and faults filled with abundant hydrothermal minerals. Notably, good reservoirs in both formations are mainly attributed to high permeability caused by tectonics. Hence, this new contribution emphasizes the crucial role of tectonics on spatially explicit reservoir prediction of deep to ultra-deep (up to > 8000 m) carbonates in the Sichuan Basin, as well as other sedimentary basin analogues in China.