Dolomite origin and its implication for porosity development of the carbonate gas reservoirs in the Upper Permian Changxing Formation of the eastern Sichuan Basin, Southwest China

Abstract High-quality carbonate gas reservoirs of the Upper Permian Changxing Formation (P 2 ch) in the eastern Sichuan Basin are mainly dolostones. Subsurface core samples from the Longhui-Tieshan area were studied to determine the origins of dolomitizing fluids and interpret porosity evolution during dolomitization. Petrographically, two dolomite types were identified, including micritic to fine-sized dolomites (type 1) and fine-to coarse-sized sucrosic dolomites (type 2). The type 1 dolomites, characterized by micritic crystals ( 500 μm) and high Mg/Ca order degrees (mean value of 0.787). Low Fe and Mn concentrations (163 ppm and 69 ppm, respectively), high homogenization temperatures (60 °C–200 °C), depleted δ 18 O values (mean value of −5.14‰), and Sr isotope compositions (mean value of 0.707643), collectively suggest that the dolomitizing fluid of type 2 dolomites was likely derived from buried, imprisoned brine water closely associated with Early Triassic seawater but unrelated to meteoric water. During the first stage of porosity evolution (i.e., stage A), carbonate porosity was slightly decreased by dolomitizing fluids preferentially replacing micritic components and CO 3 2− ions involvement. During the second stage (i.e., stage B), carbonate porosity was slightly increased because molecular replacement occurred under an environment with insufficient CO 3 2− ions. During the advanced stage (i.e., stage C), dolomitization was intensively weakened because of CO 3 2− ions depletion, yet the acidic dissolution and structural movements produced considerable pores and fractures, forming high-quality reservoirs. Type 1 dolomites are volumetrically minor and have no reservoir potential, effective reservoirs are closely associated with type 2 dolomites. A burial dolomitization model of type 2 dolomites is proposed based on the previous analyses. Dolomitizing fluids moved downward through open faults and unconformities into P 2 ch carbonates. Potentially high-quality reservoirs likely occur at locations close to dolomitizing fluid conduits.