Abstract: A great breakthrough of gas exploration has been gained in deep Paleozoic carbonate reservoirs of Buried Hill in the southwestern Bozhong Sag, Bohai Bay Basin. Drilling wells reveal that the vertical zonation of the Paleozoic gas layers was mainly controlled by karstification in this region. Recognizing the characteristics of vertical karst zoning in the study area is the key to predict the distribution of high-quality reservoirs. According to cores, thin sections, logging characteristics, spectrum logging U values, C-O-Sr isotope compositions, combined with the hydrodynamic magnitude and direction, karst reservoirs can be vertically divided into four zones from top to bottom: surface weathering zone, vertical percolating zone, horizontal underflow zone, and deep slow flowing zone. The differences of lithologic-logging properties and denudation intensity were closely related to the development of high-quality reservoirs. It is showed that the high-quality reservoirs are mainly distributed in the surface weathering zone (20 m under unconformity) and horizontal underflow zone (50 to 200 m beneath unconformity). In addition, the potentially high-quality reservoirs are developed in dolomiteexisting area of deep slow flowing zone with weak denudation. Therefore, clarifying the characteristics of karst vertical zoning and the patterns of reservoir development have important guidance for exploration of oil and gas in the future.

Key words: Bozhong Sag, Paleozoic, carbonate, karst zoning, high-quality reservoirs