Abstract:

Observed pressure reveals that overpressure is widely developed in the Paleocene in DongyingDepression, Eastern China. According to logging responses of overpressure mudstones, the vertical distribution of overpressure seal is identified. On this basis, its petrological characteristics and sealing ability were systematically studied. The result shows that the burial depth for the Paleocene overpressure seal top is mainly 2 400～2 800 m and became shallower from the center to edge of the sag and the central uplift in the plane. The lithological composition of the overpressure seal is formed by a group of dark graymudstone, calcareous mudstone and sandymudstone, ofwhich the average sand-clay ratio is 0.092, the thickness is 500～900 m and the maximal thickness of the single mudstone is greater than 10 meters. Whole rock X-ray diffraction shows that the mudstones of the pressure seal contain large amounts of carbonate minerals and clay minerals. Mudstone diagenesis reaches the rapid transformation of the illitemontmorillonite interlayer minerals, and corresponds to the mass carbonate minerals during the first and second stage. The overpressure seal could close surplus pressure greater than 14 MPa, which had a close relation with high displacement pressure （average 5.45 MPa）. Besides, thickness and diagenetic grade of the overpressure seal are important influencing factors for its sealing ability.

Key words: cap rocks, capillary pressure, physical property trap, bottom-water reservoir, top-water reservoir, heavy oil