Comparison of Evaporite-Related Source Rocks and Implications for Petroleum Exploration: A Case Study of the Dongying Depression, Bohai Bay Basin, Eastern China

The Dongying Depression (Bohai Bay Basin, eastern China) was widely filled with evaporite (anhydrite and halite) layers during the Paleogene period, especially the middle of the fourth member of the Shahejie Formation (Es-4). Most evaporite layers are interbedded with mudstone strata. The strata of Es-4 are divided into three sections, referred to as the upper layers, evaporite layers, and lower layers, respectively. The analysis of elemental concentrations, elemental ratios, and Pr/Ph suggests that the lower layers were deposited in an intermittent saline lake environment within a relatively dry climate. The evaporite layers were formed in a highly saline lake environment, whereas the upper layers were formed in a brackish-saline to fresh-water environment. Organic matter (OM) abundance indices, including total organic carbon (TOC), chloroform extracts, total hydrocarbon content (HC), hydrocarbon generation potential (S-1 + S-2), and OM type, show that the source rock potential for petroleum generation in the upper layers is best, that in the evaporite layers is fair, and in the lower layers it is poor. Carbon isotopes (& delta;C-13) of hydrocarbons in the evaporite and lower layers were heavier than those in the upper layers. Thermal maturity parameters show that the thermal evolution process of OM in the upper layers was faster where evaporite were present compared with evaporite-free areas, while the thermal evolution of OM in the lower layers was slower in these regions. The high thermal conductivity of evaporites may have accelerated the thermal evolution of source rocks in upper layers and allowed hydrocarbon generation at a shallower burial depth. This resulted in the earlier appearance of the petroleum generation window compared to in evaporite-free areas. Meanwhile, the thermal evolution of OM in the lower layers was restrained, and consequently the hydrocarbon generation window was widened, which implies the potential for petroleum exploration in deep strata under the evaporite sequence. This is a common phenomenon in evaporite-bearing basins, according to previous and present studies.

Automated summary

The Dongying Depression in eastern China was filled with evaporite layers during the Paleogene period. The lower layers were deposited in an intermittent saline lake environment, the evaporite layers were formed in a highly saline lake environment, and the upper layers were formed in a brackish-saline to fresh-water environment. The source rock potential for petroleum generation was best in the upper layers, fair in the evaporite layers, and poor in the lower layers. The thermal evolution of organic matter in the upper layers was faster where evaporite were present, while it was slower in the lower layers.