Evolution of multistage dolomitization fluids in the Upper Ediacaran Qigebrak Formation, northern Tarim Basin, China: Effects on reservoir formation

The deep-buried dolomites of the Upper Ediacaran Qigebrak Formation in the Tarim Basin possess substantial hydrocarbon potential. Although diagenetic processes, particularly dolomitization, are recognized as critical for reservoir evolution in this formation, their specific impacts on reservoir quality remain poorly understood. This study utilizes newly acquired petrographic, stable isotope, and radiogenic isotope data, integrated with carbonate U–Pb ages and temperature data, to elucidate the relationship between dolomitization processes and reservoir evolution in the Qigebrak Formation. Seven distinct dolomite phases were identified: dolomicrite (D1), very fine crystalline dolomite (D2), fine crystalline dolomite (D3), fibrous dolomite cement (D4), bladed dolomite cement (D5), fine to medium crystalline dolomite cement (D6), and coarse crystalline saddle dolomite cement (D7). D1 and D2 exhibit δ¹³C and ⁸⁷Sr/⁸⁶Sr ratios consistent with coeval Ediacaran seawater, indicating an initial syngenetic dolomitization event in restricted lagoon/tidal flat environments. This event formed via near-surface dolomitization driven by the reflux of slightly evaporative seawater. D4 and D5 also precipitated in near-surface settings under seawater dolomitization conditions, but their depleted δ¹³C and δ¹⁸O values with elevated ⁸⁷Sr/⁸⁶Sr ratios suggest the involvement of meteoric water in the precipitation process. In contrast, most D3 and D6 were formed through burial dolomitization at elevated temperatures. D7 originated from hydrothermal dolomitization at 135–150 ℃, characterized by progressively depleted δ¹⁸O ratios with increasing burial depth and the mixing of ⁸⁷Sr-enriched hydrothermal fluids. Notably, early syngenetic dolomitization preserved primary pores in the Qigebrak Formation despite long-term burial, whereas later burial and hydrothermal dolomitization primarily adjusted the pre-existing pore systems. This study enhances our understanding of multistage dolomitization processes in the Qigebrak Formation and provides insights for future exploration of Precambrian successions.