The Akaso Field, Nigeria: Use of Integrated 3-D Seismic/Fault Slicing/Clay Shearing on Fault Trapping and Dynamic Leakage

Abstract

The Akaso field on the Eastern Niger delta (STOIIP 401 MMstb) is separated from the large Cawthorne Channel field (STOIIP 1307 MMstb) by a major antithetic fault. In 1989 3-D seismic data revealed a subtle back-split off this antithetic to the west of the main Akaso field, behind which stacked amplitude anomalies occurred extending to the tip line of the back-split. Fault slicing and clay smear studies of the back-split and main antithetic indicated strong seal potential with occasional leak windows. These results were fully confirmed by a deviated appraisal well in late 1989 that found 183 MMstb STOIIP in a series of stacked footwall closures. The trapping potential of this fault can be contrasted with its behavior during depletion based upon RFT data acquired in a series of Akaso development wells in 1990. Depletion of the Akaso E and G sands was observed due to production from the juxtaposed C-D and E sands of Cawthorne Channel and subsequent fault breakdown. Dynamic pressure drops of 120 psi and 311 psi observed in the Akaso field therefore represent a prime example of the trapping potential of clay-smeared faults and their subsequent leakage during differential depletion.