Paleoenvironment of Lower Member of Minghuazhen Formation in Laibei low uplift, Bohai Bay Basin

The characteristics of ancient sedimentary environment and the geological significance of oil and gas in the Lower Member of Minghuazhen Formation in the Laibei low uplift of the Bohai Bay Basin remain not clear. We utilized comprehensively the trace elements and paleontological data and analyzed the climate, salinity, redox conditions, and water depth during the time of Minghuazhen Formation to understand the paleoenvironment. Results indicate that the abundance of Li, Ga, Sr elements, and the ratios of Sr/Ba and Th/U trace elements, reflect that the sediments were formed in a freshwater environment. Data from the palynological assemblages sensitive to temperature and humidity, the alteration index (CIA) reflecting the weathering intensity of sedimentary rocks, and the Sr/Cu, Rb/Sr trace element ratios all indicate a warm and cold transition zone. The analysis of cobalt element content suggests that the ancient water depth ranges from 2.10 to 12.75 m, on average of 6.15 m, indicating a shallow water environment. All the contents or ratios of δU and U/Th, V/(V+Ni), V/Cr, , Ni/Co, Ce/Ce*, and Eu/Eu* reflect a weak redox environment. The joint working by the ancient climate background, stable geological structures, and gentle terrain favored the development of shallow water deltas, and the structural pattern of alternating depressions and uplifts provided a good support for the formation of large-scale oil reservoirs in the Minghuazhen Formation.