Influence of Paleosedimentary Environment on Shale Oil Enrichment in the Raoyang Sag, Bohai Bay Basin

A set of high-quality hydrogen-rich shale was found in the upper third submember of the Shahejie Formation (Es3U) in the Raoyang sag. After more than 40 years of exploration and development, oil and gas shows have been seen in many wells in this interval, and there are many wells to obtain industrial oil flow. However, the influencing factors of its shale oil enrichment are still unclear. Based on petrology, organic geochemistry, and elemental geochemistry analysis, the control effect of the characteristics of a paleosedimentary environment on lacustrine shale oil enrichment is revealed and the shale oil enrichment model is established in this paper through a comprehensive study of the occurrence and migration of elements in different sedimentary facies areas. The parameters of V/(V+Ni), δU, Sr/Cu, Rb/Sr, Zr/Rb, Zn/Al, and Sr/Ba indicate that lake water evaporates in hot and arid climate, and brief transgression events led to the increase of water salinity; the outburst of salt-eating organisms such as algae and nutrients from the provenance provided the material basis for the enrichment of organic matter. The quiet and strong reduction of paleowater provides good preservation conditions for the enrichment of organic matter. The Mg2+ provided by transgression moved toward the center of the lake basin under the action of gravity, and the low-magnesium and calcium-rich saline environment was conducive to the occurrence of carbonate minerals such as calcite. Calcite and aragonite laminae were generated in the center of the sedimentation of the lake basin and interbedded vertically with the organic matter carried by fine-grained materials. The areas where carbonate-rich, calcareous, and argillaceous shales with rich organic matter are relatively developed are sweet spot reservoir areas of shale oil, which are mainly located in the center of the lake basin. In addition, the total organic matter content is high enough to meet the shale oil enrichment standards. Through the above studies, shale oil enrichment is established, which can guide shale oil exploration and development in the Raoyang depression successfully.