Origin and petrophysical log response of overpressures in the Baram Delta province, Brunei.

Abstract

The ‘window’ of safe mud weights between pore pressure and fracture pressure is narrower in overpressured sediments than in normally pressured sediments. This ‘window’ also controls the maximum buoyancy pressure, and hence the maximum height of hydrocarbon columns. Therefore, accurate pore pressure prediction is of critical importance for hydrocarbon exploration. Accurate pore pressure prediction is especially important in the rapidly depositing (3000 m/Ma) Tertiary Baram Delta Province where all economic fields exhibit overpressures, often of high magnitude and with narrow transition zones. A database of pore pressure information was compiled for 157 wells in 61 fields throughout Brunei. Overpressures are observed in 54 fields both in the inner shelf deltaic sequences and the underlying pro-delta shales. Porosity-vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the pro-delta shales, but have been vertically transferred into the inner shelf deltaic sequences.Sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer and hence, different pore pressure prediction strategies need to be applied in the prodelta shales and inner shelf deltaic sequences. Sonic and density log data detects overpressures generated by disequilibrium compaction and pore pressures are accurately predicted using an Eaton exponent of 3.0. Sonic log data detects vertically transferred overpressures, even in the absence of a porosity anomaly, and pore pressures are reasonably predicted using an Eaton exponent of 6.5.