Genetic Mechanism of Carboniferous Low-Oil Saturation Reservoirs in Tazhong-16 Well Block

The Tazhong-16 Carboniferous oil reservoir, located in the central Tazhong area, has the characteristics of low-oil saturation layers preferentially accumulated in the low porosity and permeability sandstone reservoirs and water layers exist in high porosity and permeability reservoirs. Grains containing Oil Inclusion (GOI) method was applied to further understand the mechanisms of the low-oil saturation layers in this field. Most GOI values in both current oil layers and water layers range from 5.6% to 32%, suggesting abundant paleo-oil charge and large amount of oil leak from filled oil layers. The GOI values have positive correlation with high porosity and permeability, indicating oil preferentially charge reservoirs with high porosity and permeability forming oil layers with high-oil saturation. When oil leaks from a trap, the larger pores are drained more easily than the smaller ones, resulting in a decrease of residual oil saturation with increasing porosity and permeability in drained reservoirs. The framework of the Tazhong-16 Carboniferous trap changed during the Late Triassic can be responsible for the large amount of oil leak. Preferential oil charge and preferential leak of the accumulated oil in reservoirs with different porosity and permeability during oil remigration is the mechanism of the formation of water layers with high GOI values and the low-oil saturation layers. Petroleum reservoirs in basins with complex evolution history usually experienced multiple phases of petroleum charge and post-accumulation alteration process. Thus, understanding the origin, charge history and physicochemical alteration processes associated with oil accumulation and preservation become critical for successful petroleum exploration in basins with complex evolution history.