Influences of overpressure on reservoir compaction and cementation: A case from northwestern subsag, Bozhong sag, Bohai Bay Basin, East China

Abstract

Based on data from core analysis and thin section, the influence of overpressure on the compaction and cementation of the Paleogene Dongying Formation reservoir is studied quantitatively in the northwestern subsag of the Bozhong sag, Bohai Bay Basin. Reservoir compaction is inhibited obviously by overpressure because the compaction strength of sandstones in overpressure setting is weaker than that of its overlying sandstones in normal setting. The primary porosity of sandstones is preserved about 1.1% as pore pressure is above hydrostatic pressure every 4 MPa in overpressure setting. Moreover, reservoir cementation is affected by overpressure: cementation strength is strong in overpressure setting and the adjacent inner-pressure setting, while it declines rapidly in outer-pressure setting far away from the overpressure. The thickness of zone with strong carbonate strength is thinner than that with strong authigenic clay strength. Differential carbonate cementation presents “build-up effect in fine grain”, which means that carbonate is prior to generate in sandstones of fine size, and causes that physical properties of sandy reservoir are different in the same overpressure setting. The results show that outer-pressure setting is the most favorable zone for preserving primary porosity, overpressure setting is the secondary, and iner-pressure setting is relatively poor. Key words: overpressure, compaction strength, cementation strength, differential carbonate cementation, Bozhong sag