关键词: 岩石热解, 油源对比, 生烃动力学, 超压盆地, 东营组, 沙河街组, 油气差异富集, 渤中凹陷, 渤海湾盆地

Abstract: The Neogene in the eastern Shijiutuo uplift of Bohai Bay Basin is an important exploration target. However, the exploration performance of the Neogene in the northern Bozhong sag show consistently poor. Based on drilling, logging, rock pyrolysis, gold-tube thermal simulation, gas chromatography-mass spectrometry, and basin modeling, this study conducts a comprehensive analysis of the geochemical characteristics of source rocks, hydrocarbon generation kinetics, source kitchen evolution, oil source characteristics, fault systems, hydrocarbon migration pathways, and overpressure systems in the northern Bozhong sag, aiming to reveal the mechanism of differential hydrocarbon enrichment in the Neogene both the eastern Shijiutuo uplift and the northern Bozhong sag. The results are presented as follows. (1) Three sets of good-to-excellent Type Ⅱ₁ oil source rocks have been identified in the northern Bozhong sag, i.e., the Member 3 of Dongying Formation, the Member 1 of Shahejie Formation, and the Member 3 of Shahejie Formation. (2) The evolution of the source kitchens is jointly controlled by hydrocarbon generation kinetics and burial depth. The Member 3 of Dongying Formation source rocks with shallow burial depths exhibit high activation energy for hydrocarbon generation, resulting in a limited increase in hydrocarbon generation intensity during the late stage (since 5.3 Ma). The Member 1 of Shahejie Formation source rocks, characterized by low activation energy and limited thickness, had essentially reached their final hydrocarbon generation intensity at 5.3 Ma. In contrast, the Member 3 of Shahejie Formation source rocks at the margin of the sag experienced a substantial late-stage increase in hydrocarbon generation intensity, exceeding 4.0×10⁶t/km², establishing it as the principal source kitchen in the northern Bozhong sag. (3) The Neogene crude oils in the Shijiutuo uplift are mainly derived from the Member 3 and 1 of Shahejie Formation source rocks. These oils exhibit slightly lower maturity and have generally undergone biodegradation. In contrast, the Neogene crude oils in the northern Bozhong sag are mainly derived from the Member 3 of Dongying Formation source rocks, with minor contributions from the Member 1 of Shahejie Formation, displaying slightly higher maturity and no evidence of biodegradation. (4) The source rocks in the Member 3 of Dongying Formation exhibits formation pressure coefficients up to 1.5, accompanied by pronounced generation-induced overpressure, indicating hindered hydrocarbon expulsion. In contrast, the source rocks in the Member 1 of Shahejie Formation and the top of Member 3 of Shahejie Formation are characterized by normal to weak overpressure conditions, indicating efficient hydrocarbon expulsion. (5) The accumulation of hundred-million-ton reservoirs in the eastern Shijiutuo uplift benefits from a long-distance migration system comprising steep-slope fan bodies, syn-sedimentary boundary faults, and sand bodies in Guantao Formation. In contrast, the northern Bozhong sag is constrained by late-stage non syn-sedimentary faults that only extend to the Member 3 of Dongying Formation, as well as the low oil conversion ratio and low expulsion efficiency of the Member 3 of Dongying Formation, resulting in the discovery of only a few oil-bearing structures. (6) The absence of contributions from the Member 3 of Dongying Formation source rocks for crude oil in the eastern Shijiutuo uplift confirms that the Guantao Formation fine sandstones in the north Bozhong sag lack strong lateral migration and diversion capacity, attributable to the transition of the Guantao Formation to lacustrine deposition in the sag area. (7) Crude oil from the Member 3 of Dongying Formation source rocks exhibits higher maturity and later accumulation period, indicating effective hydrocarbon expulsion occurred exclusively during the high to over-mature stage. It is recommended to focus on both the detailed characterization of the Member 3 of Dongying Formation source kitchens spanning the late oil to wet gas window, and the mechanisms of hydrocarbon expulsion via rock fracturing under overpressure conditions. The Neogene hydrocarbon accumulation zone in the sag, with the accumulation model characterized by hydrocarbon supply from the high to over-mature source rocks in the Member 3 of Dongying Formation, accumulation in in deep gravity-flow sand bodies, and vertical hydrocarbon migration and adjustment in late stage, is expected to become a strategic replacement area for sustained growth of Neogene reserves in Bozhong sag.

Key words: rock pyrolysis, oil-source correlation, hydrocarbon generation kinetics, overpressure basin, Dongying Formation, Shahejie Formation, differential hydrocarbon enrichment, Bozhong sag, Bohai Bay Basin