The formation and preservation mechanism of a high quality reservoir in deep burial carbonate rocks in a superimposed basin is a complicated and controversial issue. A great deal of geologic and geochemical evidence shows that two phases of subaerial karstification of deep burial dolomite on the northern margin of the Sichuan basin occurred successively at the end of the fourth and second Member of the Dengying Formation. The subaerial karstification plays a very important role in the development of a great number of secondary dissolution vugs and pores beneath the kastification surface. During the late deep burial-uplift process, the epigenetic karst vugs were filled by different minerals formed at different stages, and the pre-existing vugs were transformed and destroyed inordinately, becoming smaller as they were buried deeper. At the same time, some new secondary vugs were formed by corrosive fluids such as CO2, organic acid, H,S, and multi-phased exotic acidic hydrothermal fluids. Hydrocarbon was chiefly accumulated in the residual karst vugs and new secondary pores. This study manifests that the formation and preservation of high quality reservoirs in deep burial dolomite in the Dengying Formation are controlled not only by epigenetic karstification, paleokarst topography, and sedimentary microfacies, but also by multi-phase fluid filling, dissolution-precipitation, and oil thermal cracking. Among these factors, subaerial karstification plays the most important role in the formation of the high quality reservoir, the impact of this process can reach as deep as 500m downward. The other factors of paleokarst topography and sedimentary microfacies are responsible for the distribution of high quality reservoirs. The pre-existing epigenetic karst vugs could be preserved on account of multi-phased corrosive fluid and overpressure caused by oil thermal cracking.
