Abstract:

Bozhong A oilfield is the first region in the Bohai Sea where deep metamorphic buried hills are covered by Mesozoic strata. Affected by multiple tectonic movements, the distribution of fractured reservoirs is complex. Accurately characterizing the fractured reservoir distribution and mitigating overall development risks are urgent issues. To address these challenges such as the complex distribution and unclear development models of fractured reservoirs, this study integrated logging and seismic data and incorporated key controlling factors such as weathering and paleogeomorphology. The buried hill reservoirs were vertically divided into three zones: a strongly weathered zone, a moderately weathered zone, and an inner zone, further revealing the distribution characteristics and development model of the buried hill reservoirs in the study area. The results showed that favorable reservoirs developed near long-active faults on the plane. Vertically, fractures exhibited zonal development. The upper strongly weathered zone developed network fractures and had relatively good lateral connectivity, forming a “quasi-layered” distribution pattern controlled by paleogeomorphology. The central moderately weathered zone, which had better lateral connectivity than the strongly weathered zone, developed both network and structural fractures. This zone was controlled by fold cores, faults, and paleogeomorphology, also showing a “quasi-layered” distribution pattern. The lower inner zone, which had poor lateral connectivity, developed structural fractures, forming a "funnel-shaped" distribution pattern controlled by fold cores and faults. To improve oil displacement efficiency and vertical sweep, this study analyzed the geological model of the buried hill and the distribution pattern of fractured reservoirs. Gas injection wells were deployed in the upper part of the hill, and oil production wells were placed in the middle and lower parts, following the principle of “prioritizing the moderately weathered zone, with supplementary development of favorable areas in the strongly weathered and inner zones”. This method provides a solid foundation for the development of the buried hill reservoirs in the region. By promoting integrated evaluation and construction, a synchronized model and workflow for deep buried hill exploration, evaluation, and pilot production were established. It facilitated the efficient development of newly discovered reserves, with three trial production wells near a high position producing an average of 300 tons of oil and gas equivalent per day. This achievement lays a foundation for mitigating regional risks and realizing rapid conversion of reserves to production. The research findings and practical experience provide valuable guidance and reference for the development of similar oilfields.

Key words: Bohai Sea, deep layer, metamorphic rock, covered buried hill, fractured reservoir, development model, integrated evaluation and construction