Geochemical Characteristics and Hydrocarbon Generation and Expulsion of Oligocene Yacheng Formation Source Rocks in the Northern Yinggehai Basin

As natural gas exploration and research progress, the Yinggehai Basin has achieved notable advancements and discoveries regarding middle-deep strata in recent years. Due to insufficient exploration in the Northern Yinggehai Basin, the previous studies primarily concentrated on the Miocene reservoirs with lack of research attention on the Oligocene Yacheng Formation (E₃y) source rocks. A comprehensive evaluation of the E₃y source rocks, encompassing hydrocarbon generation, expulsion characteristics, and resource potential, is lacking, thereby hindering further exploration in the study area. Utilizing the geological and geochemical data of E₃y, the hydrocarbon generation and expulsion model was developed. For the first time, a division of hydrocarbon generation and expulsion amount into gas and oil amount is proposed, comprising generated and expelled gas amount, as well as generated and expelled oil amount. The findings suggest that the sedimentary facies of E₃y primarily consisted of delta plain swamp facies and neritic facies, influencing the development of coal-bearing source rocks and marine mudstones. The coal-bearing source rocks exhibited limited thicknesses and areas. During the early to late Oligocene, there was a gradual increase in thickness and area of the marine mudstone. The source rocks exhibit a notable presence of organic matter, with total organic carbon contents ranging from 19% to 95% for coal and 0.12% to 12.6% for mudstone, predominantly composed of type III, and display high maturity indicated by vitrinite reflectance values ranging from 2.0% to 5.0%. At 0.8%, R_o reached the hydrocarbon expulsion threshold, resulting in generated and expelled amounts of 677.73 × 10⁸ t and 441.14 × 10⁸ t, respectively. The generated gas amount, generated oil amount, expelled gas amount and expelled oil amount are 651.2 × 10⁸ t, 30.15 × 10⁸ t, 442.63 × 10⁸ t, and 5.95 × 10⁸ t, respectively. Employing the genetic method, the gas and oil resources of E₃y were estimated at 3.28 × 10⁸ t and 0.46 × 10⁸ t, respectively. This indicates that the Northern Yinggehai Basin presents favorable conditions for exploration.