Abstract: In the eastern slope area of Shijiutuo uplift of Bohai Sea, the main oil-bearing strata is the Lower Member of Neogene Minghuazhen Formation. Conventionally, it is believed that the hydrocarbons only pass through the transport layer in Neogene Guantao Formation, without any retentions. Moreover, the Lower Member of Minghuazhen Formation is dominated by small-scale river channel deposits, leading to poor exploration efficiency. Based on the drilling, seismic, and geochemical data, combined with physical simulation experiments of hydrocarbon migration, studies were carried out on the hydrocarbon enrichment patterns in the Lower Member of Minghuazhen Formation in the slope area of the uplift. The results show as follows. (1)The eastern Shijiutuo uplift is adjacent to Bozhong depression and Qinnan depression, of which the source rocks are of good organic matter type and high maturity. Shahejie Formation in Bozhong depression is the main hydrocarbon source rock strata. (2)The activity rate of Shijiutuo No.1 fault was relatively large in the later stage, and the Paleogene fan bodies were generally developed on the downthrown side of faults, and playing the role of hydrocarbon transfer station. The assemblage of faults and fans bodies promotes the large-scale hydrocarbon vertical migration. (3)The transport layer of Neogene Guantao Formation in the eastern Shijiutuo uplift is located on a wide and gentle slope, with the slope angle generally less than 1.0°, providing favorable conditions for the retention of crude oil in transport layer. (4)Under the control of the pre-existing No.1 and No. 2 strike-slip faults, the strike-slip adjusted faults were formed in the shallow layers of Shijiutou uplift in the later period. By contrast, No. 2 strike-slip fault has relatively stronger activity, and the corresponding strike-slip adjusted faults have longer extension distances thus facilitating the vertical migration of crude oil from Guantao Formation to the Lower Member of Minghuazhen Formation. (5)Under the background of the arid climate in the Neogene, the supply capacity of the sediment source was enhanced, and the river channel was prone to frequently burst, resulting in the formation of large-scale and connected sand bodies dominated by river channel deposits. Guided by the above understandings, the paper establishes a hydrocarbon enrichment mode of "oil transported through Guantao Formation and retained in the gentle slope, oil migration controlled by strike-slip faults, and oil trapped in the Lower Member of Minghuazhen Formation due to channel avulsion" in the slope area of Shijiutuo uplift. The Qinhuangdao27-3 oilfield was successfully discovered in the Lower Member of Minghuazhen Formation. The key exploration technology for discriminating the connectivity of the large-scale connected sand bodies in the Neogene provides important technical support for the efficient evaluation of Qinhuangdao27-3 oilfield. The new geological understandings and key technologies formed during oilfield exploration offer a favorable reference for the exploitation of extra-source strata in the slope of Bohai Bay Basin.

Key words: Bohai sea, Neogene, Minghuazhen Formation, Guantao Formation, Lithologic reservoir, Oil retention, Qinhuangdao27-3 oilfield

摘要： 渤海海域石臼坨凸起东段斜坡区的主要含油层系为新近系明化镇组下段(明下段)。传统认为,新近系馆陶组输导层中的油气"过而不留",且明下段以小型河道沉积为主,勘探成效较差。基于钻井、地震和地球化学等资料,结合油气运移物理模拟实验,对石臼坨凸起东段斜坡区明下段的油气富集规律开展了研究。研究结果认为:①石臼坨凸起东段紧邻渤中凹陷和秦南凹陷,凹陷内烃源岩的有机质类型好、成熟度高,其中,渤中凹陷古近系沙河街组为主力供烃层系;②石南1号断裂在晚期的活动速率较大,断层下降盘普遍发育的古近系扇体成为油气"中转站",断层与扇体的配置促进了油气在垂向上的规模运移;③在石臼坨凸起东段,馆陶组输导层处于宽缓斜坡背景,其地层坡角普遍小于1.0°,这为输导层中的原油滞流提供了良好的条件;④在先存的1号、2号走滑断裂控制下,石臼坨凸起的浅层形成了晚期走滑调节断层,其中,2号走滑断裂的活动强度较大,其对应的走滑调节断层长度也较大,有利于原油由馆陶组向明下段的垂向运移;⑤明下段沉积期,在干旱气候背景下物源的供给能力增强,河道易于频繁决口,河道冲决作用形成了以河道沉积为主的大型连片砂体岩性圈闭。基于上述认识,建立了石臼坨凸起斜坡区"馆陶组输导层缓坡原油滞流、走滑调节断层控制运移、明下段河道冲决成圈"的油气运聚模式,在明下段成功发现了亿吨级的秦皇岛27-3油田。针对新近系明下段大型连片砂体连通性判别的关键勘探技术为秦皇岛27-3油田的高效评价提供了重要的技术支持。油田勘探过程中的地质新认识和关键技术可为渤海湾盆地斜坡背景下的源外层系勘探提供良好的借鉴。

关键词: 渤海海域, 新近系, 明化镇组, 馆陶组, 岩性油藏, 原油滞流, 秦皇岛27-3油田