Significant Oil and Gas Discoveries and Inspirations in Underexplored Areas of Old Oil Fields: A Case Study of Southwest Sag in Dongpu Depression, Bohai Bay Basin

To address the challenge of increasing reserves and preserving mining rights in old oil fields, particularly in the underexplored southwestern depression of the Dongpu Depression, this study employed saturation hydrocarbon chromatography and natural gas carbon isotope analysis to investigate the oil and gas sources and their entrapment patterns. The results revealed three types of crude oils and two types of natural gases in the area, namely, the low gammacerane index crude oil, medium gammacerane index crude oil, and high gammacerane index crude oil, as well as Upper Paleozoic coal-derived gas and a mixture of coal-derived gas and oil-type gas. Oil-source correlation indicates that low gammacerane index crude oil primarily originates from freshwater source rocks in the middle third submember of the Shahejie Formation. Medium gammacerane index crude oil mainly comes from saline source rocks in the upper third submember of the Shahejie Formation. High gammacerane index crude oil predominantly stems from hypersaline source rocks in the lower third submember of the Shahejie Formation. The oil and gas in the Southwest Sag exhibit a "multi-source hydrocarbon supply, structure-controlled reservoir, and fault-sand controlled richness" entrapment characteristic. Four distinct types of oil and gas entrapment patterns were identified: multi-level fault steps, Y-shaped, inverted Y-shaped, and negative flower-like. Major fault plays a controlling role in the entrapment of oil and gas in the Southwest Sag. The breakthrough understanding of hydrocarbon source rocks and entrapment mechanisms is critical to the successful exploration of underexplored areas in old oil fields. These findings provide valuable insights and reference significance for similar basins, particularly in optimizing strategies exploration and enhancing reserve discovery in old oil fields.