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Overpressures and Their Significance in Offshore Bohai Bay Basin, China

Abstract

The Bohai Bay Basin is a major continental hydrocarbon basin located in eastern China, of which the offshore part of the basin contains more than one-third of the total reserves. The research focuses on two main depressions in the offshore Bohai Bay Basin: the Bozhong and Liaodongwan Depressions. The stratigraphy in the basin is dominated by thick braided river, lacustrine and fan delta sediments of the Eocene Shahejie (Es) and Oligocene Dongying (Ed) Formations. The distribution of overpressure in the basin was evaluated by integrating direct pressure measurements and drill stem tests (DST) and pressures calculated from wireline logs. In total, 1433 SDT from 143 wells has been incorporated into this study from the overpressured Es and Rd Formations. Overpressure tends to be restricted to the Bozhong and Liaodongwan Depressions that encountered the highest sedimentation rates. Compaction disequilibrium is favoured as the main mechanism to explain overpressure generation in much of the offshore Bohai Bay Basin, primarily due to the rapid sedimentation rates (~500m/m.y) of fine-grained sediments. However, our data indicates that the highest-magnitude overpressures are caused by the addition of hydrocarbon generation from the source rocks within the Es Formation. The depth to the top of the overpressure intervals ranges from 2000 to 2800m and in several portions of the basin bears a close correlation to source rock depth of the Es Formation. All of the overpressured reservoirs and source rocks have a minimum temperature of ~87°C and overpressured source rocks have a vitrinite reflectance (Ro) values of 0.6% or higher. Many of the overpressured source rocks contain microfractures that may be related to episodic expulsion of hydrocarbons or overpressure dissipation. Overpressure in the sandstone intervals of the Es Formation and to a lesser extent of the younger Ed formation corresponds to anomalously high porosity of up to 38% compared to the regional porosity-depth trend for the basin. In many cases the overpressure reservoir sandstones reflect an undercompacted state of burial and show a close association with tight calcareous mudstones that form an effective pressure seal in the basin. The distribution of overpressure, vertical effective stress, porosity and general reservoir quality of the reservoir sandstones are investigated as part of this study to more accurately guide the exploration in the offshore Bohai Bay Basin, China.