Geochemical evidence of paleo-depositional environment of Triassic Adula Formation source rocks of eastern Qiangtang Basin

Abstract

The mudstone within the Upper Triassic Adula Formation constitutes a crucial hydrocarbon source rock reservoir in the Qiangtang Basin, yet there is considerable debate regarding its paleo-depositional environment. Through systematic studies, including petrology, organic geochemistry, and elemental geochemistry analyses of the Adula Formation at the Eertuolongba section in the Quemocuo area of the eastern Qiangtang Basin, this study investigated the paleo-depositional environment of the Adula Formation source rocks and its impact on hydrocarbonsource rock development. The total organic carbon (TOC) content of the Adula Formation mudstone ranges from 0.27% to 3.46%, with an average of 1.60%, indicating overall favorable source rocks with locally deve-loped high-quality source rocks. The shelf facies mudstone in the lower section of the Adula Formation was formed during a phase of sea-level rise, characterized by deeper, oxygen-poor water, a semi-arid to semi-humid paleoclimate, moderate chemical weathering, minimal terrigenous input, and higher salinity. The deposition of the upper section occurred during a phase of basin contraction and demise, suggesting a transition from shelf facies to delta facies. This period was marked by relatively shallower, oxygen-rich water and a shift from arid to semi-arid to semi-humid paleoclimate, with increased terrigenous input and a saline to semi-saline water environment influenced by freshwater input. The source rocks of the Adula Formation are felsic volcanic rocks, primarily from a continental island arc tectonic setting, likely sourced from the island arc source domain in the Jinsha River suture zone during the Early to Middle Triassic. In the lower section of the Adula Formation, the TOC content of shelf facies mudstone shows a robust positive correlation with redox condition indicators, suggesting that source rock development was mainly controlled by oxygen-poor water conditions during sea-level rise period. In contrast, no significant correlation was observed between TOC content and paleo-environmental parameters in the upper shelf delta facies mudstones. Mudstones with TOC content greater than 2% were likely deposited during periods of high terrigenous input and relatively humid paleoclimate, indicating that source rock development was influenced by multiple factors such as paleoclimate and terrigenous input.

Keywords

• paleo-depositional environment
• source area tectonic background
• source rock
• adula formation
• qiangtang basin