Paleo Tethys marine incursion controlling lacustrine source rocks in the Ordos Basin (Yanchang Formation)

The Tethyan tectonic domain is one of the most hydrocarbon-rich regions worldwide. This paper reports on a newly identified marine incursion event in the Paleo-Tethys ocean and its effects on the formation of high-quality continental hydrocarbon source rocks. We investigated the Middle Triassic Chang 7 oil shale in the Tongchuan area of the Ordos Basin, China. The marine incursion is particularly well recorded by the occurrence of red algae. The effects of the marine incursion decrease upward through the section. The basal incursion intervals are dominated by red algae and cyanobacteria, while in the upper part of the section, red algae decrease gradually in abundance and planktonic algae become dominant. The basal incursion interval was deposited under relatively brackish to saline conditions (Sr/Ba > 0.5; mean = 0.94; occasional values of >1) and elevated β-carotane abundances (β-carotane/n-alkane main peak carbon ratio; mean = 0.042). This interval also has high total organic carbon contents (mean = 16.8 wt.%) and was deposited under moderate paleo-productivity (Babio; mean = 340.62 μg/g) and dysoxic to anoxic depositional conditions. These results indicate that the marine incursion of the Paleo-Tethys ocean introduced marine organisms (e.g., red algae), delivered nutrients, enhanced the primary productivity, and promoted the development of anoxic conditions. This facilitated the enrichment and preservation of organic matter and led to the formation of high-quality hydrocarbon source rocks. The closure of the Paleo-Tethys ocean along its eastern margin in northwestern China proceeded from east to west. The retreat of seawater was not a single and continuous process, but occurred along multiple channels. Our study highlights the positive effects of a marine incursion on the deposition of lacustrine hydrocarbon source rock, which refines our understanding on the mechanisms. This might be general throughout geological history. The formation of lacustrine source rocks is generally related to marine influence, and coastal, nearshore lacustrine basins provide favorable conditions for hydrocarbon generation. This advances the development of the terrestrial petroleum generation theory.