Fault Sealing Processes at Ceiba Field, Offshore West Africa
Goode, Joseph; Clechenko, Cory; Dula, William F.; Kilsdonk, Bill; Mondziel, Steven
Faults can form seals in hydrocarbon reservoirs over geologic time and can act as barriers to fluid flow during hydrocarbon production. This study draws on well and seismic data from the Ceiba field offshore Equatorial Guinea to examine the sealing properties of faults. In particular, we develop a local calibration relating shale gouge ratio (SGR) in fault-zone rock to differential fluid pressures that the faults support. Fluid contacts observed in petrophysical logs and in seismic AVO data as well as MDT fluid pressure measurements constrain differential fluid pressures supported by faults in the field. A rock physics inversion of 3-D seismic data for the ratio of clay to sand within the reservoir interval provides an estimate of the rock type present along each fault. From these seismic inversion data, both facies juxtaposition and SGR are calculated on each fault surface. We find that two faults serve as lateral seals within the field where oil-water contacts are offset across faults. One of these faults forms a seal by juxtaposing sand against shale across the fault. The second fault seals by a combination of juxtaposition and the inferred presence of fine-grained gouge in the fault zone. These faults, at depths of 2500 to 2700 m, support differential fluid pressures of <80 psi where sand bodies are self-juxtaposed across faults. Where oil-filled sands are juxtaposed against shale by faulting, higher differential pressures may be supported. The strength of this fault-rock seal increases with the predicted shale content of the fault gouge and is comparable to the strength of other documented fault seals. The resulting relationship between SGR and fault seal capacity allows us to evaluate the sealing potential of two more faults that bound seismic amplitude anomalies found isolated from the main field.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013