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The Akaso Field [STOIIP 78 × 106 m3 (401 Million STB)] of The Eastern Niger Delta is separated from the large Cawthorne Channel field [STOIIP 208 × 106 m3 (1307 million STB)] by a major antithetic (counterregional) normal fault. In 1989, three-dimensional (3-D) seismic data acquired and interpreted over the field revealed a subtle splinter or backsplit fault off this major antithetic fault west of the main Akaso field. Behind this backsplit fault we observed stacked amplitude anomalies. These anomalies extended to the structural spillpoint defined against the Akaso backsplit fault. Fault slicing and clay smear studies of the backsplit and main antithetic fault indicated strong seal potential with possible "leak windows along the fault planes. In 1989, the sealing potentials of the western end of the Akaso boundary fault were fully confirmed by a deviated appraisal well, which found a STOIIP of 29 × 106 m3 (183 million STB) in a series of stacked footwall closures.
The initial trapping potential of this major antithetic normal fault can be contrasted with its subsequent behavior during depletion by using RFT data acquired in a series of development wells drilled in 1990 in the main Akaso structure at the eastern end of the antithetic fault. Here, major stacked columns of oil and gas are trapped and sealed by the antithetic fault, but we identified evidence of subsequent leakage due to production effects from the adjacent Cawthorne Channel field. Depletion of the unproduced Akaso G sands was found to be due to production from the juxtaposed E sands of the adjacent Cawthorne Channel field, thus we inferred dynamic fault leakage. We observed a dynamic pressure drop of 400-2144 kPa (58-311 psi) between the juxtaposed Akaso and Cawthorne Channel sand due to active crossflow. The Akaso field, therefore, represents a prime example of the trapping potential of a clay-smeared fault and its subsequent leakage during differential depletion. Our observations, methodology, and implications described herein may have an important bearing on reservoir management philosophies in similar structural settings.
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