Enigmatic Compressional Structures In An Extensional Province: Eku Field, OML 67, Offshore Nigeria
Marisa Quinones, Robert Evans, Kola Alofe, and Ben Onyeise
Acquisition of 3-D seismic data over OML 67-70 and a detailed reservoir description study done on the Eku field, have allowed identification of previously unrecognized compressional features. Situated within a depocenter between arcuate down-to-the-basin normal growth faults, the Eku structure consists of a shale-cored anticlinal fold and fold-and-thrust separated by a zone of lateral displacement The crests of the folds have been eroded at a major unconformity at the base of the Qua Iboe shale (Early Pliocene).
In the absence of definitive biostratigraphic data, correlations among the various fault-blocks are based on the character of sedimentary packages and sequences on wireline logs. Combined with analysis of the geometry of faults and folds, the correlations support a description of pulsatory movement of folding and faulting, that ultimately culminated in extensional reactivation of compressional deformation. The direction of earlier regional extension and the apparent direction of compression were not coincident. The effect of the compression has been uplift and erosion of anticipated reservoir sections, and elevation of sections of lesser reservoir quality.
Deformation, both compressional and extensional, was gravity-driven and on shale detachments. A working hypothesis to explain the disparity in direction of earlier extension and subsequent compression is that thermal expansion that accompanied formation of the Cameroon volcanic line to the east of the Niger Delta in Miocene time, caused a change in the direction of structuring, allowing downslope gravity-driven compression to be superimposed on pre-existing extensional features.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California