The Abnormal Tectonothermal Event in the Shanan Sag at the End of the Paleogene and its Controlling on the Hydrocarbon Generation Evolution of the Paleogene Source Rocks

Abstract

The Shanan sag in the central–western Bohai Bay Basin hosts high-quality Paleogene source rocks within the Shahejie Formation's third member (E₂s₃). Despite hydrocarbon indications in Cenozoic strata, no commercial accumulations have been discovered. An integrated approach combining geochemical analysis, fluid inclusion thermometry, apatite fission-track (AFT) thermochronology, and basin modeling was employed to unravel the paleogeothermal regime and hydrocarbon generation history of E₂s₃ source rocks. AFT data from the Shahejie Formation's second member (E₂s₂) reveal a tectonothermal event at 25 Ma that accelerated E₂s₃ maturation. Outside three sub-sag depocenters, current E₂s₂ reservoir temperatures remain below the 25 Ma paleo-geothermal maxima despite subsequent Neogene burial. Hydrocarbon-bearing brine inclusions in E₂s₂ reservoirs exhibit peak homogenization temperatures (Th) at 25 Ma, with minimal high-temperature signals, indicating that E₂s₃ hydrocarbon generation peaked during the Paleogene thermal event, with limited late-stage accumulation. The regional effects of the Dongying Movement necessitate thick Neogene sedimentation to compensate for the 25 Ma paleo-geothermal anomaly. Our findings emphasize targeting Neogene depocenters in petroleum exploration to mitigate the inhibitory effects of high paleo-heat flow on late hydrocarbon generation, thereby enhancing current accumulation potential.