Dolomite Reservoir Genesis Controlled by Sedimentary Cycles: Geochemical Evidence from the Middle Permian Qixia Formation, SE Sichuan Basin

Dolostone is an important reservoir for hydrocarbons, and significant hydrocarbons have been produced in the Middle Permian Qixia Formation dolostone reservoirs in the southeastern Sichuan Basin. The origin and formation process of the dolomite reservoir in the research area are studied through thin-section, geochemical, and sedimentary cycle analyses and U-Pb geochronology. Three types of dolomites were identified, including stratiform fine-crystalline dolomite (D1), patchy fine-crystalline dolomite (D2), and saddle dolomite cement (SD). D1 and D2 exhibit a range of δ13C values from 3.39‰ to 4.21‰ and a range of δ18O values from −6.06‰ to −5.75‰, indicating a mild depletion of δ18O relative to coeval seawater while maintaining seawater-equivalent δ13C signatures. Their 87Sr/86Sr ratios and REE patterns indicate seawater-derived fluids for D1 and D2 (both test results showed a U-Pb age of ≈274 Ma) and hydrothermal origin for SD. Sedimentary cycle analysis found that the regression process in the fourth-order sequence is conducive to the formation of dolomite under the background of regression in the third-order sequence. Exposure of bioclastic shoals enabled evaporated seawater reflux, forming penecontemporaneous D1 in fluid-saturated settings. Selective dolomitization occurred in the bioturbation structure with good porosity and permeability, forming D2. In the burial stage, the hydrothermal fluid had a slight transformation on the dolomite and formed SD. This model highlights transgressive–regressive cycle controls on reservoir development, providing exploration criteria for analogous carbonate systems.