Astronomical time scale of a Middle Eocene lacustrine sequence from the Dongpu Sag, Bohai Bay Basin, Eastern China

Abstract

Being a part of the petroliferous Linqing depression, the Dongpu Sag is located in the SW part of the Bohai Bay Basin, eastern China, and is rich in oil and gas. As new discoveries of conventional, structural, and stratigraphic plays of oil and gas have been gradually diminished in the sag, there is an urgent requirement to explore unconventional shale oil and gas, for which the effective source rocks must be located accurately. In the Dongpu Sag, the main source rock is present in the third member (Es3) of the Middle Eocene Shahejie Formation, with the hydrocarbon generation potential being relatively variable in each sub-member. The optimum source rock is perceived to be present between the lower and middle parts of Es3. However, as the Dongpu Sag is full of complex fault blocks and salt formations, the stratigraphic subdivision and deposition timing of each member and sub-member in the Shahejie Formation is rather challenging and controversial. Based on the paleontology and wireline (esp. gamma-ray) log data of the P6-65 well in the center of the sag, a high-resolution cyclostratigraphic framework of the lower and middle parts of Es3 has been established, with sediment accumulation rates and paleoenvironment being validated by chemostratigraphy (elemental data from spot X-ray fluorescence analysis) from the nearby W-410 well. This astronomical framework is more precise than the paleontology framework established previously in the Dongpu Sag. Additionally, it provides a precise time interval to locate the effective source rocks within the sag, allowing a fine-scale stratigraphic correlation within the sag and to other sags in the Bohai Bay Basin, which is otherwise not possible due to insufficient absolute age data in the Dongpu Sag.