Characteristics and formation mechanisms of the unconformity-related paleokarst reservoirs in the Upper Sinian, Northwestern Tarim Basin, China

Abstract In the Late Sinian, the Tarim Basin experienced regional tectonic uplift and subaerial exposure. A regional unconformity was extensively distributed during the succession from the Sinian to Cambrian. As a result, a large number of secondary pores were developed in the Upper Sinian Qegebulake Formation. An integrated study based on petrographical and petrophysical analysis, geochemistry, karstology and fluid inclusions was performed to provide insight into the characteristics and formation mechanism of the Upper Sinian carbonate reservoirs. The reservoir rocks mainly include microbial dolomite, crystalline dolomite and breccias. Reservoir matrix porosity and permeability range from 0.30% to 11.06% and from 0.00156 mD to 1.49 mD respectively. Geological and geochemical characteristics suggest that three phases of dissolution were determined to be responsible for the development of the reservoirs within about 70 m beneath the unconformity. They are meteoric dissolution during subaerial exposure, and deep-seated hydrothermal dissolution and organic acid dissolution during the burial stage. The Upper Sinian dolomite underwent strong meteoric dissolution, which is supported by the appearance of the unconformity and the associated caves. The geochemical characteristics of the cave-filling calcite further indicate the diagenetic overprint from meteoric water, including the relatively negative values of δ18O (−18.46 to −8.5‰, VPDB) and wide range of δ13C (−8.11 to −0.24‰, VPDB), low ƩREE (1.77–4.23 ppm) and Yb–La and Yb–Ca variation. The occurrence of saddle dolomite as well as analytical results such as δ13C (0.67–2.52‰, VPDB) and δ18O (−13.21 to −9.81‰, VPDB) isotope signature and high homogenization temperature of fluid inclusions in quartz (87.2 to > 220 °C) and saddle dolomite (82.3–135.4 °C) suggest that the reservoirs rocks experienced interaction with hydrothermal fluids. Only bitumen occurred in the edge of secondary pores formed by dissolution of quartz and saddle dolomite, which suggests that organic acid dissolution likely occurred during the evolution of reservoirs. A geological model for the evolution of the paleokarst reservoirs is proposed through the analysis of major reservoir controlling factors. The primary pore types and reservoir petrophysical property of the Qigebulake Formation mainly depend on the sedimentation. Supergene karstification is the most important process for reservoir formation. Deep-seated hydrothermal activity has both positive and negative attribution on the formation of reservoirs. Organic acid dissolution plays an active role in the formation and preservation of reservoirs. This study provides a useful reference for hydrocarbon exploration in the deep buried Sinian carbonate reservoirs.