Hydrocarbon accumulation and models for the Palaeozoic reservoirs in the peripheral areas of the Nanpu depression

The investigation of the mechanisms for the hydrocarbon accumulation in the carbonate rocks in the peripheral areas of the Nanpu depression is made by means of the analysis of thin section,chemical property,source rocks,physical property and mercury-injection experiments of cast sections.The results show that the carbonate rocks in these areas are not capable of oil generation,and oil is mostly derived from the Palaeogene source rocks which have better types of organic matter,high maturity and tremendous thickness.The carbonate rocks are characterized by lower porosity of matrix and less-developed intercrystal pores.The early structural fractures have almost been filled,and only late ones may serve as the dominant reservoir spaces for the accumulation and migration of hydrocarbons.No vertical zoning is observed for the development of palaeokarsts,and the solution openings are isolated and poorly connected on the whole.There are four types of cap rocks dominated by mudstone(shale),constituting excellent seal-reservoir associations together with the reservoir rocks.The reservoir types consist of unconformable reservoirs and structural reservoirs.