Geochemistry and correlation of oils and source rocks in Banqiao Sag, Huanghua Depression, northern China

Abstract

Located in the western part of Huanghua Depression's central segment in the Bohai Bay Basin, the Banqiao Sag is a significant sedimentary basin. The crude oil produced has low density, low viscosity, low sulfur and high wax content. Crude oil types include conventional crude as well as condensate oil (gas), light oil and low-maturity oil. Four possible source rocks are present within the Banqiao Sag; however, at present the oil source is not very clear. Rock-Eval pyrolysis, stable carbon isotope composition and biomarker analysis were conducted on 26 source rock core samples in the Banqiao Sag, to evaluate their characteristics. Results indicate that the Shahejie Formation (43–32.8Ma) has source rocks that can be graded between poor to excellent quality. They were deposited in a marine-based reducing environment, and rich in organic matter. Sha-1 Member (Es1) and Sha-3 Member (Es3) are considered thermally immature to mature, based on various maturity indicators. 17 crude oil samples from the Banqiao Sag were compared with these rocks' biomarkers and stable carbon isotope parameters. The analyzed crude oils were divided into 4 groups, based on carbon isotope composition and Q-type cluster analysis (QCA). There are two sets of effective hydrocarbon source rocks in the study area's Shahejie group, i.e., Es1x and Es3, contributing to the oil found in the structural belts. Oil and source rock correlation results indicate that Group B1 oil is probably derived from highly mature Es3. Group B2 oil is most likely derived from a mixed mature Es3 and early mature Es1. Group B3 oil is derived from Es3 in Tanggu-Xingang buried-hill structural belts and from Es1 in immature or low mature stage, in Banqiao Sag. Group B4 oil is derived from mature Es3 in Banqiao Sag with little early mature Es1. This study can highly benefit future oil and gas exploration work in the Banqiao Sag.