Hydrocarbon generating potential of the Upper Paleozoic section of the Huanghua Depression, Bohai Bay Basin, China

The origin of the Upper Paleozoic hydrocarbon accumulations in the Huanghua Depression of Bohai Bay Basin was studied by petrography and Rock-Eval pyrolysis on more than 90 coal, dark mudstone, and carbonaceous mudstone samples, with 4 samples of hydrous pyrolysis. Additionally, biomarker analysis of oils produced from the Upper Paleozoic reservoirs were conducted. The source rocks were evaluated for their amounts of type III kerogen and content of hydrogen-rich macerals. The samples can be graded between fair and excellent quality, showing that the source rocks possess good potential for gas and oil generation. The main stage of oil generation occurs around 2600-4000 m [with random vitrinite reflectance (R-o) values of 0.7-1.3%], and gas generation happens in a wide depth range (R-o up to 2.0%). Hydrous pyrolysis under a closed system exhibits large potential for gas generation, and the dark mudstone can generate 500-1000 mL/g of total organic carbon (TOC) at a temperature between 350 and 550 degrees C. Coal and carbonaceous mudstone exhibit a clear capacity for oil generation (5-25 mg/g of TOC), with dark mudstone showing the highest values (up to 84 mg/g of TOC at 400 degrees C). Two categories of crude oils produced from the Upper Paleozoic reservoirs can be recognized, indicating that the oils are mainly migrated from strata. The studied source rocks and its produced oil are of high concentrations of diasterane and gammacerane, Ts < Tm, and Pr/Ph ratios of 1.01-3.44, indicating the input of coaly organic matter under a marine to continental transitional environment. The results illustrate good gas generation and limited oil generation capacity for the Upper Paleozoic source rocks, and more attention should be focused on the natural gas exploration in the study area and also other coal-bearing basins.