Reflux dolomitization and magma-related thermal alteration in the lower Cambrian carbonate successions, Tarim Basin, northwestern China

Abstract

Petroleum exploration in the Tarim Basin currently targets the lower Cambrian dolostone reservoir. Accurate prediction of massive dolostone distribution requires precise constraints on their genesis. While thermal alteration is widespread, its impact on reservoir quality remains inadequately studied. This study integrates petrological, mineralogical, and geochemical analyses to understand dolostone genesis and thermal alteration effects. The δ18O values (3.3‰ to 5.6‰ VSMOW), derived from Δ47, identify evaporative seawater as the primary dolomitizing fluid. Combined 87Sr/86Sr and δ34S data constrain the dolomitization timing from the late early Cambrian (stage 4) to the middle Cambrian (Guzhangian). Platform evolution, from a carbonate ramp to a rimmed shelf, coupled with the warming climate, drove brine reflux dolomitization. Thermal alteration induced recrystallization of early dolostones and precipitation of hydrothermal sanidine, quartz, and dolomite cements, characterized by high 87Sr/86Sr ratios and positive Eu anomalies. Variations in dolomite crystal size resulted from differential thermal heating by an underlying magmatic intrusion. Multimineral veins proximal to intrusions attest to magma-hydrothermal mineralization. Replacive quartz, quartz cements, and the latest dolomite cements, exhibiting 87Sr/86Sr values higher than 0.712 and significant positive Eu anomalies, record the upwelling hydrothermal activity. The upward decrease of recrystallization intensity and precipitation temperatures of hydrothermal minerals indicates that both processes originated from the underlying magmatic sill. This study establishes a genetic model for dolomitization in the lower Cambrian reservoirs and clarifies the impact of magmatic-hydrothermal processes on deep-burial dolostones.