Chronology and geofluids characteristics of calcite cement in the Upper Triassic Xujiahe Formation tight sandstone reservoir, Wstern Sichuan Basin, SW China

Abstract

Geological fluid activities significantly influence diagenesis and hydrocarbon accumulations in reservoir rocks. However, it remains challenging to precisely determine the timing of these activities. This study aims to investigate the chronology and geofluids characteristics of calcite cement that leads to reservoir tightness in the Upper Triassic Xujiahe Formation in the Western Sichuan Foreland Basin, which is a critical exploration target for hydrocarbon resources. We examine calcite cement as a marker of fluid activities, employing petrological analyses, U-Pb dating, and in-situ carbon-oxygen isotope analysis. These methods are integrated with simulations of regional burial and thermal histories to assess pore evolution and hydrocarbon accumulation mechanisms in these tight sandstone reservoirs. Results reveal two main calcite cementation stages within the second member of the Xujiahe Formation (Xu-2 Member): early cements from the Middle Jurassic around 175 ± 11 Ma and late cements from the Late Jurassic between 161 and 156 Ma. Initially, basin-derived alkaline fluids and near-surface atmospheric freshwater supplied the calcium and carbon for early cements during ongoing basin subsidence. Calcium and carbon for the late calcite cements came from the dissolution of carbonate rock fragments by organic acids. The heavier δ¹³C is primarily associated with in-situ generation and consumption of carbon within the system, while the δ¹⁸O_fluid, which is close to modern seawater values, is mainly related to active water-rock reactions induced by processes such as the dissolution of carbonate rock fragments by organic acids. The closed diagenetic system due to calcite cementation in the Late Jurassic led to reservoir tightening. Therefore, basin-scale chronological constraints on calcite cement are of great significance for quantitatively studying sandstone reservoir densification and exploring the mechanism and control factors of hydrocarbon accumulation.