Late Triassic paleogeographic inversion and its control on source rocks in the Qiangtang Basin

This paper is the result of petroleum geological survey engineering.

Objective This study elucidates the differential tectonic evolution of the Qiangtang Basin during the Late Triassic to Early Jurassic and its controlling effects on the development characteristics and spatial distribution of source rocks.

Methods This study discusses the sedimentary record of the Late Triassic tectonic transition in the Qiangtang Basin through sedimentary facies analysis and lithofacies palaeogeography reconstruction. Petroleum geochemical studies were also employed to reveal the distribution patterns of Late Triassic to Early Jurassic source rocks in the Qiangtang Basin.

Results Middle Triassic sequence exhibit shallowing-upward trends, reflecting marine regression linked to regional uplift. In contrast, Early Jurassic sequences display deepening-upward patterns driven by tectonic subsidence and transgression. We further identify a Late Triassic paleogeographic inversion event. In the northern Qiangtang Depression, marine depositional environments of the Middle Triassic transitioned into terrestrial settings during the Late Triassic. Conversely, the sourthern Qiangtang Depression shifted from Middle Triassic terrestrial domains to Late Triassic marine units.

Conclusions Our study suggests that the distribution of source rocks in the Qiangtang Basin was primarily controlled by the sedimentary change and paleogeographic inversion during the Late Triassic. Late Triassic units, likely associated with foreland basin development, formed in deep-water setting settings of the northern depression alongside coal-bearing units encircling basin margins. Meanwhile, Early Jurassic source rocks, tied to passive continental margin processes, accumulated in deep-water zones of the southern depression, including oil shale units concentrated in lagoon environments along the northern flank of the Southern Qiangtang Depression.