The preferential pathways of oil migration in the Qi-nan slope belt of Bohai Bay Basin, China: Insights from integrated analyses using geochemical and morphology tools

Abstract

The Qi-nan Slope belt is located in the southwestern Qikou Sag in the Bohai Bay Basin and has had relatively little oil and gas exploration. The mechanism of oil migration remains unclear due to the lack of understanding of the oil-oil relationship, the definition of oil migration pathways, and the controversy over the contributing source rocks. Based on hierarchical cluster analysis, the crude oils in the Qi-nan Slope are divided into three groups (A, B and C) with different geochemical compositions. Group A oils, including Class A1 and Class A2 biodegraded oils, and Group C oils are contributed by Es1x and Es3 source rocks with different organic facies, and distributions are limited to the WXZ oilfield in the western slope. Group B oils, including Class B1 and Class B2 biodegraded oils, mainly originated from Es1x source rocks and are widely distributed in the southern slope. The significant correlation between the relative migration distance (RMD) of oils and pyrrolic nitrogen compounds, rather than the parameters related to oil maturity, indicates that the pyrrolic nitrogen compounds can effectively trace oil migration directions in the Qi-nan Slope, and 1,8-DMC/2,6-DMC is the best indicator. For oil migration conduits, no large-scale faults developed in the inner part of the slope, and the oil-gas test results from major wells on the seismic profiles suggest that oils are mostly concentrated in Es1x with no obvious vertical migration in this area. Therefore, we speculate that the Es1x sand body provided the migration pathway for crude oil and controlled its uneven accumulation in the slope area. The isopleth map of the 1,8-DMC/2,6-DMC ratio, the distribution characteristics of the oil test results from major wells and the morphology of the Es1x transporting layer suggest that the overall migration direction of oil was dispersedly from the Qi-nan Sub-sag to the southern areas of the slope and that structural ridges provided preferential migration pathways for hydrocarbon accumulation.