地下の圧力構造の把握と石油地質学的解釈

抄録

Distribution of overpressures in a sedimentary basin is recognized as an important factor in the evolution of hydrocarbon provinces. Theoretical consideration of the mechanism of overpressuring reveals the importance of pressure seals, which prevent flow of formation fluids, for generating them. From this point of view, pressure differences in pressure regimes can be more logically interpreted as indicating lack of hydraulic connection and no fluid flow between them. This leads us to the concept of pressure compartments characterized by pressure seals in three dimensions. Pressure compartments are found worldwide and their distribution (i.e. compartmentation) affects the fluid movements within a sedimentary basin.
Because pressure seals occur in fine-grained, extremely low permeability rocks, they are basically excellent caprock in hydrocarbon traps. However, when the internal fluid pressure in a compartment is close to the fracture pressure of the seal, it is interpreted that the seal had fractured and the trapped hydrocarbons had leaked from the compartment during geologic time.
Some methods predicting formation pressure by using wireline log data such as sonic transit time are applicable to evaluate the above overpressure conditions. In this study, a modified method is developed in order to obtain more accurate estimations and applied to a sedimentary basin in Niigata area, Japan. The results of pressure analysis clearly illustrated the state of compartmentation of the basin. They also reveal that seal failure by hydraulic fracturing possibly controls the stratigraphic distribution of hydrocarbon accumulations.
Thus, the knowledge and understanding of subsurface pressure regimes and basin compartmentation give us a new point of view to evaluate petroleum systems, especially hydrocarbon migration and entrapment.