Geochemical characteristics and natural gas-oil-source correlation of the Shulu depression in the Jizhong Subbasin, Bohai Bay Basin, eastern China

Abstract

Buried-hill reservoirs in the Shulu Depression of the Jizhong Subbasin, Bohai Bay Basin have shown great exploration potential. However, the origins of natural gas and oil resources in buried-hill reservoirs are still uncertain, hindering future exploration. The present study is the first to provide a comprehensive investigation on geochemical characteristics and sources of natural gases and oils from buried-hill reservoirs in the Shulu Depression. The results show that the source rocks in the lower first member of the Paleogene Shahejie Formation (Es1L) in the Shulu Depression were deposited under anoxic brackish-water environments containing inputs of immature to low-mature organic matter (OM) derived from algae and planktons. In contrast, the lower third member of the Paleogene Shahejie Formation (Es3L) source rocks were deposited under suboxic fresh-brackish water environments with inputs of high-mature OM sourced from a mixture of planktons and land plants. The natural gases only presented in the paleo-uplifts in the East Sag of the Shulu Depression are characterized as shale-sourced gases and primary cracking gases, and mainly generated by the Es3L source rocks. Relative to the crude oils in the East Sag, the oils from the West Slope exhibit lower maturity and have experienced various degrees of biodegradation. Such spatial difference is probably due to the various depth and localities of corresponding source rocks. Low mature-mature oils in the shallow buried-hill reservoirs of the West Slope are mainly derived from the Es3L source rocks that are shallowly buried at the western edge of the East Sag. By comparison, highly mature oils in the deep buried-hill reservoirs are predominantly derived from the Es3L rocks deeply buried in central East Sag. In addition, variations in oil physical properties between the West Slope and the East Sag are probably caused by various maturations of source rocks and biodegradation of oils. Collectively, the results provide insights into the origins of hydrocarbons in the buried-hill reservoirs in the Shulu Depression and better knowledge needed for optimizing future exploration and production.