Impact of seismic imaging constraints and structural ambiguity on the historical non-commercial categorization of the Magna Field western Niger Delta

The limitations of seismic imaging in growth-fault dominated deltaic environments are a major, unrecognized source of uncertainty when assessing hydrocarbon traps and are rarely investigated critically in the context of historical appraisal decisions in legacy fields. This study bridges this gap by conducting a diagnostic assessment of the seismic imaging constraints in the Magna Field, western Niger Delta, which has been classed as non-commercial in the past from two wells and a set of legacy 2D seismic data from the late 1970s to early 1990s. Notably, the 3D seismic survey acquired over the field during 1991–1992 was never integrated with the appraisal drilling decision, meaning the non-commercial classification predates the best available imaging data. A combination of the fault shadowing effects across the rollover crest, reflector discontinuities across the main fault zones, velocity uncertainty due to the lack of checkshot or VSP calibration data, and the poor resolution of the legacy data all contribute to the resultant subsurface image, which is not well suited to determining the geometry of the traps, the overall area of closure, and the behaviour of the fault-seals. Any volumetric assessment could be significantly different from the second-order polynomial velocity function derived without checkshot calibration because of the potential crestal depth uncertainty introduced at the D-5 and E-1 reservoir horizons of several hundred feet. The prevailing data quality in this study is inadequate to warrant a confident determination of the commercial potential of the Magna Field with reasonable certainty. Hence, the non-commercial classification is better regarded as a data quality verdict rather than a geological one. It is proposed that a prioritised imaging improvement workflow, starting with reprocessing of the existing 2D and 3D data to enhance the quality, and followed by checkshots targeted at the necessary areas, is the necessary precondition for a reliable appraisal decision.