Jin Qiang*, Wang Weifeng*, Xu Huaimin*, Yang Shaochun* and Qian Ji**

Large quantities of immature and low-maturity oil have been discovered in the North Jiangsu Basin, East China. Intense tectonic activity and the differential accumulation of immature oils during the Palaeogene have resulted in reservoir rocks which have variable sedimentary and diagenetic characteristics. Studies of oil geochemistry and reservoir diagenesis suggest that oil generation and accumulation took place early in the basin's history (i.e. during early diagenesis), significantly inhibiting later modification of the reservoir rocks. There has been no large-scale tectonic activity since the oil pools formed, and the reservoirs' petrophysical properties and pore construction therefore reflect early diagenesis. Diagenetic modification of the reservoir rocks was more intense below the oil-water interface where oil did not accumulate. Alteration in these rocks included calcite cementation, the formation of quartz overgrowths, and the development of pressure solution.