Origins of the Upper Permian reef-dolostone and reservoir evolution in northern South China

Upper Permian reef-dolostone units in the northeastern Sichuan Basin are important units for natural gas exploration. Knowledge of dolomitization models and porosity evolution plays a key role in understanding the development of hydrocarbon systems. This study focuses on the Late Permian Panlongdong reef in the northeastern Sichuan Basin. Multiple methods, including optical (polarized and cathodoluminescent), SEM, synchrotron-radiation micro X-ray fluorescence elemental mapping, O and Sr isotopes, and in situ LA-ICPMS U–Pb dating are used to characterize petrographic, mineralogical, geochemical and geochronological nature of reef-dolostones. Our data implies that the dolomitization occurred in the early Middle Triassic (∼245 Ma) with Mg2+ supplied by the downward reflux of hypersaline seawater, accompanied by a significant increase in porosity due to selective dissolution of low-Mg calcites. During deeper burial in the late Middle – early Late Triassic, a small quantity of (micro)fractures were formed followed by the intrusion of subsurface Sr-rich brine and the enlargement of pore spaces. In the late Triassic (∼207 Ma), surface uplift occurred in South China due to continental collision between South and North China blocks, which might be responsible for the formation of a large amount of (micro)fractures and the improvement of permeability. This event was followed by Sr-depleted freshwater passing through the reef, leading to the precipitation of secondary calcite cements and decrease of reservoir porosity and permeability.