Early Dolomitization Mechanism of the Upper Ediacaran Qigebrak Formation, Northwestern Tarim Basin: Evidence from Petrography, Rare Earth Elements, and Clumped Isotope

The deeply buried Upper Ediacaran Qigebrak Formation dolostones in the Tarim Basin are promising future hydrocarbon exploration targets in China. However, the origin of these pervasive matrix dolomites is not well understood, which hampers further hydrocarbon exploration. In this study, petrographic, isotopic (C, O, and Sr), rare earth element (REE), and clumped isotope analyses were performed to unravel the mechanisms of early dolomitization. Petrographic investigations indicate that the Qigebrak Formation carbonates were completely replaced by three distinct types of dolomites: (1) dolomicrite (MD-1), (2) fabric-preserving dolomite (MD-2), and (3) fabric-destructive dolomite (MD-3). Despite different crystal textures, these three dolomite types have a narrow range of δ13C and 87Sr/86Sr values similar to those of coeval seawater. Furthermore, their seawater-normalized REE compositions display a seawater-like REE pattern with positive Ce anomalies. These findings suggest that the dolomitization fluids were seawater derived. From the clumped isotope temperature (TΔ47 ≈ 60 °C) and the δ18O water values of the dolomitization fluids, it can be inferred that the main mechanism for the formation of matrix dolomites was seepage-reflux dolomitization by mesosaline to penesaline seawater in the evaporative environment. MD-1 and MD-2 precipitated from mesosaline to penesaline seawater in slightly evaporated settings. MD-3 was likely formed via recrystallization of MD-1 and/or MD-2 at a greater depth. This study provides an insight into early dolomitization processes related to mesosaline to penesaline seawater, which may make the origins of dolomite reservoirs with similar geological backgrounds better understood.