Genesis of Upper Permian Changxing Formation dolomites, south of Kaijiang-Liangping Trough, SW China: evidence from petrology, geochemistry, and fluid inclusions

Dolomitization has a profound influence on the quality and distribution of the Changxing Formation reef-shoal gas fields in the south of Upper Permian Kaijiang-Liangping Trough. These gas fields consist of two types of matrix replacement dolomite (D1 and D2) and some saddle dolomite (SD). A comprehensive petrographic and geochemical examination indicates that D1 dolomite is formed by the replacement of matrix limestone by seepage-reflux of penesaline seawater during the syndiagenetic-early diagenetic stage. This interpretation is supported by δ18O values slightly higher than those of dolomites precipitated from coeval seawater and REE patterns similar to those of matrix limestones, reinforced by 87Sr/86Sr ratios that greatly overlap those of matrix limestones and coeval well-preserved fossils. Dolomitization by penesaline seawater rather than by hypersaline evaporated seawater is also supported by the absence of massive evaporites. D2 dolomite, with C-O-Sr isotopes and Fe-Mn concentrations comparable to those of D1, formed through the hydrothermal alteration of D1 at temperatures between 163.2 and 189.3 °C during Early Triassic. SD precipitated directly from hydrothermal fluids at temperatures between 152.4 and 197.8 °C. The hydrothermal origins of D2 and SD are also supported by more negative δ18O values than those of dolomite formed from coeval seawater, higher 87Sr/86Sr ratios, and positive Eu anomalies.