Sequence stratigraphy and depositional systems of late Miocene to early Pliocene deep offshore Niger Delta, southeastern Nigeria

Abstract

The Niger Delta of Nigeria is one of the world’s most prolific Cenozoic hydrocarbon provinces. The Sample_B well from Niger Delta Basin encountered the upper Miocene-lower Pliocene Agbada Formation. This present study analyzed 125 ditch cutting samples for palynomorphs and foraminifera in conjunction with well log data to illustrate the sequence stratigraphic framework for the understanding of the distribution of system tracts, key surfaces, and depositional systems. Three lithologies (sandstones, siltstones, and mudstones) were recorded. The recorded palynomorph assemblages that enabled the delineation of two palynological subzones, ten palynozones, and five palynocycles of transgressive and regressive depositional sequence were recognized. A detailed biostratigraphic based on foraminifera allowed the subdivision of Sample_B well into two zones. Integration of lithologic, palynologic, biostratigraphic, and well log data were used to constrained 9.2Ma, 7.0Ma, 5.8Ma, 5.49Ma, 5.0Ma, 4.15Ma, 4.0Ma, and 3.2Ma maximum flooding surfaces, and 8.2Ma, 6.3Ma, 5.5Ma, 4.2Ma, and 3.8Ma sequence boundaries that were correlated with dating of Haq et al. (1988). The alternation of the HST sands and TST shales provided a seal for reservoir facies, which are essential for the hydrocarbon accumulation and its stratigraphic trapping. The LST sands served as the reservoir rocks. Near-shore to marginal marine (coastal deltaic) and inner to middle neritic paleoenvironments were recorded. Two distributary channels, two tidal deltas, and four tidal channels were revealed to fluctuating from littoral to shallow marine environments. The results of this study will serve as a predictive tool for determining the likely presence and distribution of source, seal, and reservoir rocks and optimizing production prospective in the Niger Delta region of Nigeria.