Features of buried hill reservoirs in Hexiwu structural belt, Langgu Sag, Jizhong Depression

The buried hill reservoirs of Hexiwu structural belt in Langgu Sag, Bohaiwan Basin are comprehensively studied by core and outcrop observation. The reservoir lithology is dominantly carbonate rock, and the reservoir spaces include dissolved pores, dissolved cavities, cracks and bedrock micropores, forming pore-cavity type, crack type, crack-cavity type and combination pore-cavity-crack type reservoirs. Multiple-phase fossil karstification is an important factor during reservoir formation. The fossil karsts are developed in the weathering and leaching belt, submerged current belt, fracture belt and bedding direction along buried hill unconformities. The cracks formed by tectonic movement increase porosity, improve connection of pores and cavities and permeability. The reservoirs developed in different lithologies and tectonic positions have different physical properties: dolomite reservoir is better than carbonate reservoir, and reservoir not overlapped by Permo-Carboniferous stratum is better than that overlapped by Permo-Carboniferous stratum, and reservoir in the fault-developed area is better than that in the non-fault-developed area. The reservoirs are favorable areas for oil and gas exploration.