Multiple S-isotopic evidence for seawater incursions during the deposition of the upper Cretaceous source rocks in the Songliao Basin, northeastern China

Abstract

The Songliao Basin, with an area of ∼260,000 km2 in northeast China, hosts the largest oilfield in China, which is one of the world's most productive oilfields. However, the origin of the organic-rich upper Cretaceous source rocks remains controversial. In this study, we report multiple S-isotopic compositions of pyrite (δ34S and Δ33S) from Member 1 (K2n1) and Member 2 (K2n2) of the Nenjiang Formation in the Songliao Basin. Our isotopic results reveal an extraordinarily large negative δ34S excursion of 50.54‰ (from +11.09‰ to −39.45‰), which is strong evidence for seawater incursions. The increasing positive Δ33S excursion coupled with the negative δ34S excursion strengthen the inference of a large influx of seawater to the Songliao Basin. We used a steady-state isotope box model to estimate the magnitude of the seawater incursions. Our modeling results show that the sulfate concentrations may have increased by 12–46 times when seawater invaded the Songliao Basin. We suggest that marine incursions may have facilitated the chemical stratification of the Songliao Basin and played an important role in the formation of the oil source rocks. Our findings potentially have important implications for understanding the formation of oilfields elsewhere.