Structural and Seismic Facies Interpretation of Fabi Field, Onshore Niger Delta, Nigeria

Abstract

The Niger Delta is a prolific oil province within the West African subcontinent. Exploration activities have been concentrated in the onshore part of this basin, but as the delta becomes better understood, exploration influences are gradually being shifted to the offshore. Although the geology, tectonics and evolution of the Eocene-Pliocene sequence of the Niger Delta are fairly well known, these are expected to increase as new analytical tools and concepts evolve. This work was an integrated structural, seismic facies and stratigraphic study conducted in the Fabi Field, onshore western Niger Delta, and targeted at improving the present understanding of the structural development, sequence stratigraphic history, paleo-depositional environments and hydrocarbon reservoir potential of the field.

Five wireline logs, biostratigraphic data, 3-D seismic section, check shot data and core data were analysed and utilized in this study. Well logs were used to determine the different lithologies, system tracts, stacking patterns and reservoir potentials of the field. Sequence stratigraphy and seismic facies were used to identify the reflection packages in order to determine the environment of deposition. Structural and horizon mapping results were used to generate time and depth structural map with the aid of a derived function calculated from the check shot data.

The base of these sequences consists of massive and monotonous marine bioturbated shales, which grade into inter-bedding shallow marine fluvial sands with parallel-cross bedding laminations, silt and clays, while the upper part is a massive marine sandstone section. The gross reservoir thickness ranges from 150ft-700ft with net thickness of 20ft-175ft. Sequence stratigraphic analysis revealed that the succession consists of two sequence boundaries, dated 10.35Ma and 10.6Ma and two maximum flooding surfaces, dated 9.5Ma and 10.4Ma.

The high percentage of the reflections with low to moderate amplitude/continuity of the parallel/divergent configuration is identified as a feature of delta platform facies, while the sigmoidal-hummoky reflections indicate a slope facies. The system tracts from the log are the trangressive and highstand system tracts, while growth faults(F1), antithetic faults(F3,F4) and synthetic faults (F2,F5,F6,F7) are the identified structures which are typical of the Niger Delta reservoir sandstone.

Depositional setting of the Middle-Upper Miocene strata were influenced by fluvial, tidal and marine systems. The up dip areas on the depth structure maps with closure signify possible anticlinal structures where hydrocarbons could be entrapped. These could serve as possible appraisal locations where wells could be sited to optimize the development of the reservoir sands within the field using the structural model generated.