Reconstructing hydrocarbon accumulation and burial history of Permian reservoirs in northwest Sichuan Basin, China: Integrated fluid inclusion thermometry and U-Pb dating

Abstract

The western Sichuan basin is a typical foreland basin, and great discovery of oil and gas exploration has been done here, but the history of hydrocarbon accumulation and burial history evolution is still uncertain. Fluid inclusion thermometry and isotopic dating are employed in this study to reconstruct the burial history and hydrocarbon accumulation of the Western Sichuan Foreland Basin. Furthermore, research was conducted research on the diagenesis, reservoir-forming stages and differences of the Qixia Formation reservoirs at different tectonic locations in the northwest of Sichuan. The results show that in the Shuangyushi area (concealed front-edge belt), the upper reservoirs of the Qixia Formation are mainly composed of sparry dolomitic limestone, with a diagenesis mineral sequence of: (1) the early microcrystalline calcite, (2) recrystallized calcite, (3) quartz, albite, pore bitumen in the surrounding rock, and (4) dolomite and calcite in fractures. In the Longmen Mountain piedmont area (thrust nappe belt), the outcrop rock of the Qixia Formation is mainly dolomitic limestone, with a diagenesis mineral sequence: (1) microcrystalline calcite, (2) recrystallized calcite, (3) quartz and bitumen, and (4) calcite filled in both pores and fractures. In the Shuangyushi area, the reservoirs of the Qixia Formation underwent crude oil charging from the Middle Triassic to Middle Jurassic, subsequent natural gas charging in the Late Cretaceous, and final natural gas reservoir destruction and adjustment at Late Cretaceous (81.2 ± 8.4 Ma). While, the outcrop of the Qixia Formation in the Longmen Mountain piedmont area recorded oil charging from the Lower Cambrian source rocks at Late Triassic (217.1 ± 0.9 Ma) and the destruction of oil reservoirs during the Miocene to Pleistocene (i.e., Himalayan orogeny). By comparing the petrological characteristics, fluid inclusion development and reservoir-forming periods of the two areas, it is considered that these disparities arise from the areas’ distinct tectonic positions within the foreland basin, leading to divergent responses to the Indosinian and Himalayan orogeny, as well as the resulting hydrocarbon accumulation processes.