How marine incursions affect the sedimentary environment and the quality of source rocks in the Upper Cretaceous Songliao Basin, NE China

The linkage between marine incursions and lacustrine petroleum source rock deposition has been studied worldwide. A variety of interpretations have been suggested to explain the influence of marine incursions on the quality of source rocks in the Upper Cretaceous Qingshankou Formation (K2qn), Songliao Basin. In this study, high-resolution of geochemical and sedimentary results of two drilled wells from the K2qn were presented to improve the understanding on this issue. 117 samples from Well GY8HC of the Qijia-Gulong Sag (brief as Gulong Sag) and 251 samples from Well ZY1 of the Sanzhao Sag, the Central Depression of the basin were analyzed to establish the evolution of paleoenvironment and the influence of marine incursions on organic matter enrichment in the two sags. All the evidence suggests that marine incursions have a positive effect on the quality of member 1 of the K2qn source rocks (K2qn1) in the Sanzhao Sag, which was probably due to the rising lake level and increasing salinity of the watermasses. The enhanced salinities could have inhibited lake overturn and oxygenation of bottom water, and thus created a favorable condition for the preservation of organic matter. In contrast, the K2qn1 source rock in the Gulong Sag was less affected by marine incursions, which is likely due to the Daqing placanticline blocking two sags. The lake water in the Gulong Sag was characterized as fresh-brackish with less mixing of seawater. Petrographic observation suggests that organic matter in the Gulong source rocks was mainly derived from lacustrine algal and microsporinite. This research provides novel insights into the linkage between marine incursions and the depositional environment during the formation of K2qn1 in the Songliao Basin. •Petrographic and geochemical data revealed marine incursions into the Songliao Basin during the Upper Cretaceous.•Marine incursions had different effects across the Central Depression of the basin.•The sedimentary environments were different between Gulong and Sanzhao sags.•The quality of K2qn1 source rock in Sanzhao sag was primarily controlled by marine incursion.