Reservoir hydrocarbon prospectivity and productivity evaluations of sands S-600 and S-700 of Fega field, onshore Niger Delta Basin, Nigeria

Abstract

Poor quality 3-D seismic surveys are often reprocessed into modern 3-D seismic data of better quality to improve seismic imaging and resolve structural uncertainties during seismic data interpretation. This study presents an integration of a modern 3-D seismic and composite well log data, together with petrophysical analysis to re-evaluate hydrocarbon prospectivity of a historically produced Fega field, located in the onshore part of the Niger Delta Basin. Two reservoirs, S-600, and S-700 composed essentially of shoreface deposits stacked by channel sandstones with heteroliths were evaluated. The structural interpretation shows that the study interval is characterized by antithetic and synthetic-growth faults that compartmentalized the reservoirs into blocks. Besides, the reservoir structural surface maps document fault-dependent structural closures that show possible hydrocarbon prospects and leads in some yet penetrated reservoir blocks. However, reservoir S-600 shows a better hydrocarbon prospect potential, ascribed to its relatively strong amplitude structural conformity. Petrophysical analysis indicates that the sandstones generally have very good to excellent reservoir quality. Nevertheless, most of the reservoir blocks show heterogeneity in bulk volume water distribution and they may possibly not produce water-free hydrocarbon. The volumetrics analysis shows that the identified prospects are prolific and commercially viable for an increase in oil production opportunities within the study area. However, the estimated volume suggests that the current commercial status of field is fairly viable for multinational field operators, and the field may be given out to marginal field operators. The study approach could also be applied to marginal fields in the Niger Delta and other similar petroleum provinces, where contract terms are similar to those in Nigeria, to revitalize and optimize production. The study demonstrates the usefulness of reprocessing 3-D reflection seismic data and updating reservoir information as soon as new geological and geophysical data are available, together with the revaluation of hydrocarbon fields from time to time. This may increase the chances of finding new opportunities in structurally complex settings.