Discovery of Carboniferous-Permian source rocks in the Qiangtang Basin, North Tibetan Plateau and its geological significance

The Qiangtang Basin in the North Tibetan Plateau is the largest marine petroliferous basin in the Chinese mainland that has yet to achieve a breakthrough in oil and gas exploration. It is a key target for searching for new petroleum resources. The exploration progress has been stagnant, primarily due to inadequate basic geological studies and unresolved key scientific issues, such as unclear hydrocarbon genesis and sources, which hinder systematic evaluation of the basin’s resource potential. As a Late Paleozoic-Mesozoic superimposed basin, it preserves a complete sedimentary sequence dominated by Mesozoic strata, which are traditionally regarded as the main hydrocarbon source, supported by bitumen and oil seeps. In contrast, Late Paleozoic strata outcrop sporadically with limited research. Previous petroleum resource assessments of the basin predominantly focused on Mesozoic sedimentary strata, with limited attention on the petroleum potential of Paleozoic strata.This study focused on the Carboniferous-Permian strata in the Riwanchaka-Rejuechaka area of the North Qiangtang Basin on the basis of 1:50000 geological mapping. Late Paleozoic successions are dominated by stable shallow marine carbonate platform deposits, including: the Lower Carboniferous Riwanchaka, Upper Carboniferous Walongshan, Middle Permian Xueyuanhe and Upper Permian Rejuechaka Formations. They are mainly composed of limestone and fine clastic rocks, accompanied by warm-water fossils and local reef limestone. Here, a series of source and reservoir rocks were firstly identified, and distinct petroliferous odors were detected in dense micritic limestone, bioclastic limestone and reef limestone from the Riwanchaka and Walongshan Formations. The Xueyuanhe and Rejuechaka Formations showed poor hydrocarbon shows due to late-stage fracturing and dissolution.The source rocks, including argillaceous and carbonate lithologies, have variable thickness and organic geochemical properties. 68 carbonate and 78 argillaceous samples were analyzed for total organic carbon (TOC). The results show that the TOC values of argillaceous rocks are 0.08%–19.40% (average=2.80%) for the Rejuechaka Formation, 0.29%–16.60% (average=1.12%) for the Xueyuanhe Formation, 0.06%–2.54% (average=0.49%) for the Riwanchaka Formation, and 0.10%–1.56% (average=0.22%) for the Walongshan Formation. Most argillaceous rocks were poor to moderate source rocks, with a small number of good source rocks (TOC>2%). Carbonate rocks generally exhibit low TOC values (0.12%–0.26%) and are predominantly non-source rocks with individual poor ones.The discovery of Late Paleozoic source rocks in the Qiangtang Basin not only reveals the multi-source nature of the Qiangtang Basin but also significantly expands exploration targets to deeper stratigraphic intervals. These Late Paleozoic source rocks feature in-situ source-reservoir assemblages with appreciable hydrocarbon generation potential. Together with the overlying Mesozoic reservoirs, they formed a “lower generation, upper accumulation” configuration, in which unconformities, faults, and connected sand bodies serve as migration pathways, and thick regional seals provide favorable preservation conditions. Furthermore, their hydrocarbon potential could be comparable to coeval source rocks in the Tethyan petroliferous basins (e.g., the Caspian, Tarim and Sichuan Basins), though high-quality source rocks account for a relatively small proportion. Studies have shown that Late Paleozoic source rocks mainly occurred in the central Qiangtang area, but more high-quality ones could be distributed beneath the deeply buried Mesozoic strata in the Qiangtang Basin, where deformation is mild and preservation conditions are superior. Hence, this finding holds significant scientific importance for evaluating the hydrocarbon resource prospects of the Qiangtang petroliferous basin.