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. [ABSTRACT FROM AUTHOR]