Evidences of Overpressure Caused by Oil Generation in Dongying Depression

Overpressures are common in the third member and the fourth member of Shahejie (Es₃ and Es₄) Formations of Dongying depression. The sonic log displays an obvious response to the overpressure from which the overpressured mudstones have higher acoustic travel time and the acoustic travel time of the normally pressured and overpressured mudstones is reduced with increasing vertical effective stress. In this paper, large numbers of well-log, tests and geological data in Dongying depression are used to determine the origin of overpressure. We put forward that the oil generation is the major overpressure mechanism based on the confirmation of that the overpressured sediments are normal compaction. It is supported by the following evidences: (1) The overpressured mudstones exhibit no anomalous low density; (2) There is apparent lack of correlation between mudstone densities and effective vertical stress; (3) The overpressured reservoir sandstones show neither anomalous high matrix porosities nor anomalous geothermal gradient; (4) The depths to the top of the overpressure range from 2000m to 3000m, increasing with the increased source rocks depths and the temperatures for the top of overpressure zones are approximately 87-123℃ corresponding to an estimated vitrinite reflectance (R_o) of 0.5%-0.75%; (5) The overpressured source rocks are still capable of oil generation; (6) There are good conditions for overpressure seals because of the calcareous mudstones; (7) There are large numbers of microfractures in the source rocks; (8) The overpressured reservoirs are predominantly oil saturated or oil-bearing and overpressured water reservoirs are rare. It is concluded that overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks.