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

摘要： 渤海湾盆地石臼坨凸起东段新近系是重要的勘探层系,但在南侧与其邻近的渤中凹陷北部,新近系勘探成效却持续不佳。为了揭示石臼坨凸起东段与渤中凹陷北部新近系的油气差异富集机理,基于钻井、测井、岩石热解、黄金管热模拟、气相色谱-质谱和盆地模拟,对渤中凹陷北部的烃源岩地球化学特征、生烃动力学、烃源灶演化、油源特征、断裂体系、油气运移路径和超压系统开展了综合分析。研究结果表明:①渤中凹陷北部古近系发育东营组三段(东三段)、沙河街组一段(沙一段)和沙河街组三段(沙三段) 3套好—优质Ⅱ₁型倾油型烃源岩。②烃源岩的生烃动力学与埋深协同控制了烃源灶的演化。东三段烃源岩埋藏浅且生烃活化能高,晚期(5.3 Ma以来)生油强度的增幅有限;沙一段烃源岩的生烃活化能低且厚度薄,在距今5.3 Ma时其烃源灶的生油强度基本定型;凹陷边缘沙三段烃源岩在晚期的生油强度大幅增加至400×10⁴t/km²以上,是渤中凹陷北部的主力烃源灶。③石臼坨凸起的新近系原油主要来自沙三段烃源岩和沙一段烃源岩,原油的成熟度稍低且普遍发生了生物降解;渤中凹陷北部的新近系原油以来自东三段烃源岩为主,混合少量沙一段烃源岩供烃,烃源岩成熟度稍高且未发生生物降解。④东三段烃源岩的地层压力系数高达1.5且存在明显生烃增压,指示排烃受阻;沙一段和沙三段顶部烃源岩为常压—弱超压状态,指示排烃顺畅。⑤石臼坨凸起东段亿吨级油田的成藏得益于陡坡带扇体、同沉积边界断裂和馆陶组砂体构成的长距离运移体系;渤中凹陷北部受仅断至东三段的晚期非同沉积断裂、东三段生油转化率低和排烃效率低等因素控制,仅发现数个含油构造。⑥石臼坨凸起东段的原油未见东三段烃源岩的贡献,证实渤中凹陷北部新近系馆陶组细砂岩并不具备较强的横向分流运移能力,与馆陶组在渤中凹陷北部转变为湖相沉积有关。⑦受东三段烃源岩供烃的油藏,其原油成熟度较高且成藏期较晚,指示东三段烃源岩仅在高成熟—过成熟阶段可以有效排烃。建议加强东三段烃源岩在生油窗晚期—湿气窗阶段烃源灶的精细表征以及超压背景下岩石破裂排烃机理研究。凹陷内具有高成熟—过成熟的东三段烃源岩供烃、重力流砂体深层汇聚、晚期垂向运移调整成藏模式的新近系成藏区,有望成为渤中凹陷新近系储量持续增长的战略接替领域。

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