MULTIPLE S-ISOTOPIC CONSTRAINTS ON SEAWATER INCURSION EVENTS IN THE UPPER CRETACEOUS SONGLIAO BASIN

Songliao Basin in northeast China developed one of the most productive oilfields in the world. It was traditionally thought that the organic-rich source rocks of these oilfields came from terrestrial rocks. However, recent studies suggest that episodic marine incursions may have occurred in member 1 and 2 of Nenjiang Formation and member 1 of Qingshankou Formation. However, there is little constraints on the scale of these seawater incursion events. In this study, we report multiple sulfur isotopic compositions of pyrite from member 1 and 2 of Nenjiang Formation in the Songliao Basin. Our isotopic results show that the upper Nenjiang Formation are characterized by positive δ³⁴S values (mostly +15‰), however, the lower Nenjiang Formation are characterized by extraordinarily negative δ³⁴S values (mostly -39‰). These negative δ³⁴S data provide strong evidence for the input of seawater that may have increased the sulfate concentrations in the paleo-lake. We estimated the scale of this seawater incursion event by using steady-state isotope box model. By modelling the δ³⁴S and Δ³³S data of pyrite, we calculate the ratio of marine sulfate fluxes to the total input fluxes of sulfate when the marine transgression occurred, as well as the changing sulfate concentrations after the seawater incursion event.

We conclude that the marine incursions events and the redox conditions may have played an important role in the burial of organic carbon and the formation of hydrocarbon source rocks in Songliao Basin. Our study indicates that multiple S-isotopes are an effective tool for the reconstruction of transgression events in the paleo-lake.