Formation and origination of dominant reservoir in metamorphic buried hills in the southwestern Bohai Sea

The Neoarchean metamorphic buried hill is the most important oil-gas exploration target layer in the southwestern Bohai Sea, North China. Understanding the formation and origination of the metamorphic reservoir will provide important guidance for future exploration in this region. The drilling, logging, coring, and thin section observation, as well as the regional tectonic stress field and geological outcrop were analyzed, the developmental laws of the dominant reservoir in weathering zone of the metamorphic buried hills were studied, and the origination were discussed. Results show that dominant reservoirs are better developed, in horizontal direction, in ancient fold zone, strike-slip zone, paleo-highland zone, and nearby-fault zone, and the formation of dominant reservoirs was controlled by tectonic activities and weathering agents; whereas in vertical direction, the lower weathering zone has a bigger fracture opening and higher permeability, while the upper weathering zone has greater porosities, and the formation of dominant reservoirs was controlled by weathering eluviation and compaction. The dominant direction of fractures is near east-west in range of 45°-135°, which is controlled by the “reactivation” during the Himalayan orogenesis. The model of the formation of the dominant reservoir could be described as in a “fish back” pattern, which has been evolved from compressional fracturing and weather eluviation filling during the Indosinian and Yanshanian orogeneses, to the extensional “reactivation” and sedimentary re-compaction during the Himalayan orogenesis.