关键词: 渤海湾盆地, 冀中坳陷, 致密油气, 地质条件, 成藏条件, 资源潜力

Abstract: Jizhong depression is the main crude oil production base in Bohai Bay Basin, boasting of rich and diversified oil and gas resources. After more than 50 years of exploration and development, the proved reserves of conventional oil and gas are very considerable. However, there is an increasing difficulty in hydrocarbon exploration. As a result, the unconventional oil and gas resources will become the key target for the next large-scale discovery. Through systematically summarizing the geological conditions for the formation of tight oil and gas in the faulted lacustrine basin of Jizhong depression, it has been determined that there are a number of sedimentary and subsidence centers and oil-generating sags and troughs in the depression, where the high-quality hydrocarbon source rocks together with fan sedimentary sands of the adjacent delta front, fan delta, and nearshore subaqueous fan or lacustrine facies carbonate reservoirs form a dominant configuration, thus providing favorable conditions for the efficient charging and accumulation of tight oil. According to the source-reservoir configuration, the tight oil and gas reservoirs in Jizhong depression can be divided into two types, i.e., extra-source and intra-source hydrocarbon reservoirs, generally showing the characteristics of large-area continuous distribution, complex lithology, non-homogeneity and near-source reservoirs. The study suggests that three types of tight oil and gas reservoirs are mainly developed in the Paleocene of Jizhong depression, including tight oil and gas conglomerate reservoirs in nearshore subaqueous fan of the steep belt (Type 1), and in fan delta of the slope belt (Type 2), as well as delta-front and shore-shallow lake facies tight oil and gas reservoirs at the peripheral of sags and troughs (Type 3), which have superior hydrocarbon accumulation conditions, demonstrating a large resources potential and a broad exploration prospect. Among them, Type 1 is mainly developed in single-faulted dustpan-like sags such as Langgu sag, Wuqing sag, Jinxian sag, and Shulu sag, with the distribution area of 476 km², the predicted oil resource of 0.53×10⁸t, and the estimated natural gas resource of 1 460×10⁸ m³. It is the main zone where breakthroughs have been made in hydrocarbon exploration of deep sags in the steep belt of Jizhong depression. Type 2 is mainly developed in Shuolu sag, Jinxian sag, and Baxian sag with narrow and steep slopes in Jizhong depression, with the distribution area of 700 km² and the predicted resources of 2.6×10⁸t. It demonstrates a realistic battleground for the realization of large-scale efficient discovery. Type 3 is mainly distributed in the lower member of the first Member of Shahejie Formation of Raoyang sag, with the distribution area of 1 214 km² and the predicted resources of 1.8×10⁸t, indicating a favorable direction for the strategic succession.

Key words: Bohai Bay Basin, Jizhong depression, tight oil and gas, geological condition, accumulation condition, resources potential