SOURCE:

Faculty: Physical Sciences
Department: Applied Geophysics

CONTRIBUTORS:

Ajaegwu, N. Ejike
Onwuemesi A.G.
Anike O.L.

ABSTRACT:

Okan-65, 75RD and 78 wells located in Okan field were selected across strike in a NW-SE direction for high resolution biostratigraphic interpretation in a sequence stratigraphic context. The studied intervals range from 6, 560 – 11, 200ft in Okan-65, 5, 980 – 13, 600ft in Okan-75RD and 5, 990 – 11, 855ft in Okan-78. The study was carried out for the purposes of subdividing the geological sections into chronostratigraphically zones/subzone and determining the ages and environments of deposition of all sequences penetrated by the wells. The biozones, biofacies, lithological descriptions and environments of deposition identified were used in establishing the sequence stratigraphic framework. The key sequence stratigraphic surfaces (sequence boundaries and maximum flooding surfaces) and system tracts were correlated across the wells. Ditch-cutting samples, wireline logs (gamma ray and resistivity), and base map were available for the study. Ditch-cutting samples from the three wells were subdivided for sedimentological, palynological, micropalaeontological and nannopalaeontological analyses. The studied sections which fell within the Agbada Formation were sedimentogically characterized by integration of textural, accessory minerals/materials and lithological descriptions of the sediments. Sedimentological results indicate a cyclic pattern of sedimentation which changes from regressive to transgressive phases. The regressive phase varies from channel fill deposits (point bars) to barrier bars/foot sands while the transgressive phase varies from fluviomarine sand to fully marine shales. Three palynological zones: Verrutricolporites rotundiporus, Racemonocolpites hians and Multiareolites formosus Zones were identified from base to top of the well sections analysed. Micropalaeontological interpretations identified three zones: Globorotalia mayeri (N14), Globorotalia obesa / Globorotalia acostaensis (N15)/ (N16) and Globigerinoides extremus (N17) Zones. The three well sections were dated Middle Miocene to Late Miocene based on integrated results from palynology and micropalaeontology. The depositional environments depicted by benthic foraminifera ranges from coastal deltaic to outer neritic with exception of Okan-75RD in which the deepest environment is the middle neritic. Two complete and one incomplete depositional sequences defined by their sequence boundaries were identified in the three wells. Two Sequence Boundaries (SBs) dated 12.5Ma and 10.5Ma and three Maximum Flooding Surfaces (MFSs) dated 13.4Ma, 11.6ma and 9.2Ma were interpreted. The SBs, MFSs and gamma-ray log stacking patterns were used to delineate the sequences into system tracts. In Sequence 1 only the Highstand System Tract (HST) was identified in Okan-65 and Okan-78 while in Okan-75RD, the Transgressive System Tract (TST) and HST were identified. Sequence 2 comprises a TST and HST while Sequence 3 comprises a Lowstand System Tract (LST), TST and HST. The LST in Sequence 3 is interpreted as an incised valley fill (IVF) deposited in a tidal-dominated estuarine environment. Sequence stratigraphic result has revealed that the LSTs and late HSTs are the most sand-prone system tracts and therefore the most prospective and attractive targets for future exploration effort in the Okan Field.