Prediction of Good Quality Reservoir Sands through Integrated Depositional Systems and Sequence Stratigraphic Framework Interpretations: Example from Late Miocene to Early Pliocene Deposits of Okan Field, Niger Delta, Nigeria

Abstract

The sequence stratigraphic framework and depositional systems of Okan Field were investigated by interpreting and integrating biozones, biofacies, and wireline logs (gamma ray and resistivity) with the aim of predicting the most favourable reservoirs within the analysed sections. Three depositional sequences bounded by their unconformities were delineated. Three sequence boundaries (SBs) named 1, 2 and 3 were delineated from the base of the studied sections to the top. The sequence boundaries 3 and 2 were dated 10.5Ma and 12.5Ma respectively. Three maximum flooding surfaces (MFSs) dated 9.2Ma, 11.6Ma and 13.4Ma were interpreted. The 9.2Ma MFS was marked by Last Downhole Occurrence (LDO) of Globigerinoides extremus and Sphaeroidinellopsis seminulina, the 11.6Ma MFS was marked by First Downhole Occurrences (FDOs) of Globorotalia continuosa, Globorotalia obese, Globorotalia mayeri and Cassigerinella chipolensis with some glauconite occurrence. The 13.4Ma MFS was marked by high gamma ray value and peak benthic and planktic foraminifera occurrence. The depositional systems interpreted were found to be associated with particular system tracts within the sequences. The late highstand system tracts (HSTs) are dominated by point bars while the early HSTs are dominated by shoreface sands. Lowstand system tracts (LSTs) are dominated by shoreface sands exhibiting a multistorey geometry. The early transgressive system tracts (TSTs) are dominated by point and barrier bars. The most favourable reservoirs were found to be asoociated with the LST shoreface deposits, possessing good reservoir - seal pairs. The shoreface reservoirs were capped by relatively thick TST deposit consisting of thick shales which serves as a seal.