ADVANCES IN PETROLEUM GEOSCIENCE RESEARCH IN NIGERIA

Abstract

Mature petroleum fields, also called old fields, are fields within which petroleum production is declining after an earlier phase of peak production. In the Niger Delta, old fields also comprise unevaluated fields discovered by horizon mapping of 2D seismic data. Afenmai Field was chosen for illustrative study it is an unapprised 2D seismic discovery, and therefore an old field. Opportunities for new plays and prospects at shallower depths in this field are subtle unconformity traps related to relative sea level changes, and deep seated structures. Correlative horizons are conventionally chosen as lithostratigraphic surfaces. These often fail to provide reliable prediction away from well control. Drilling prospects revealed on depth structure maps of such horizons commonly results in dry wells. Structurally mapped seismic horizons were generated in this study by sequence stratigraphic interpretation of lithologic data, foraminifera information, open- hole geophysical well logs, fluid information and 3D seismic data. Known petroleum accumulation in the field are characterised by medium values of root mean square (RMS) and instantaneous frequency. Agbada Formation top was found to constitute a regional unconformity surface related to lowstand relative sea level, and therefore a new play. An unconformity play was identified below the wells’ bottom depth. New subtle unconformity prospects on depth structure maps generated from these plays are structural high positions that are associated with medium RMS and instantaneous frequency values. A new structural prospect was generated within the footwall of the NW-SE trending structure building fault on the eastern part of E8000 sand’s depth structure map. The E1000 sand contains petroleum in the rollover anticline on its hanging wall block. A new prospect is constituted by a 3-way closure on the NW of this anticline. New 3-way closure prospects were also revealed on the SE and NW of the footwall block.