Buried-Hill Discoveries of the Damintun Depression in North China ¹

Several buried-hill oil fields recently have been found in the Damintun depression. All the buried hills except one consist of Archean metamorphic rocks. Of these, the Dongshenpu buried hill is the largest. The metamorphic rocks have no original porosity but have secondary porosity, such as weathering fractures, structural fractures, brecciated pores, secondary corrosion pores, and secondary replacement pores. Fractures, especially structural fractures, form the only migration passages and main reservoir spaces for hydrocarbons in the buried hill. Crude oil comes from Paleogene continental source rocks and is very waxy. Based on the interpretation of log data, the porosity averages 3.24%. According to calculations using pressure build-up curves, the effective permeability averages 0.140 μm².