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Niger Delta, Serravallian, Palynostratigraphy, Palynomacerals, Palaeoenvironments, Palaeoclimates, Sequence Stratigraphy.

PalynomorPalynomorph and palynofacies distributions from the Alakiri Field wells X and Z, Coastal Swamp Depobelt, onshore Niger Delta, were investigated to interpret stratigraphic successions and reservoir-scale palaeoenvironment variations in order to understand the impact of sea level change and palaeoclimate synchronicity during the deposition of Serravallian aged sediments. The resulting observed climate and eustacy changes supported by the notional absence of marine dinoflagellate cysts, during a period of 2.19 m.y. within the Serravallian enables key proxies to be interlinked better to understand a dynamic depositional system for the key hydrocarbon producing Agbada Formation across the Niger Delta basin.

Terrestrial pollen and spores in lieu of the noted absence of dinoflagellate taxa in the wells were used to delineate two (2) palynozones per well based on the following bioevents, namely: Top Consistent Occurrence (TCO) of Verrutricolporites rotundiporus and Top Regular Occurrence (TRO) of Belskipollis elegans. These bioevents corresponded to boundaries between well-established biozones used across the Niger Delta and demonstrate that the studied intervals penetrate two biozones, namely the P770 and P750 zones of Evamy et al. (1978) and zones F to G of Legoux (1978). The palynostratigraphic framework shows that Verrutricolporites rotundiporus, Belskipollis elegans, Verrucatosporites usmensis, Zonocostites ramonae, Brevitricolporites molinae, Crassoretitriletes vanraadshooveni, and Retibrevitricolporites obodoensis, in association with other palynomorphs suggest a Middle Miocene (Serravallian) age linked to a relative age range between 13.82 Ma (within the P750 Zone) to approximately 11.63 Ma within the P770 Zone for the studied Agbada Formation based on recent calibration with the Global Time Scale 2020 of Gradstein et al. (2020). Palynofacies distributions for the Alakiri-Z Well show a dominance of Palynomaceral type 2, low recoveries of Palynomaceral type 4 debris and Palynomaceral type 3, while the palynofacies assessment of the Alakiri-X Well reflects an overwhelming dominance of Palynomaceral type 4 with associated Palynomaceral type 1 sensu Whitaker (1982, 1984). This palynomaceral scheme was initiated to elucidate the palynomorph and palynomaceral microflora hydrodynamic controls of the Niger Delta basin for the first-time. Palynomaceral provenance mechanics in the palynological analysis of the Agbada Formation sediments substantiates the depositional environment and fluctuating marine realm dynamics.

The palynomorph and palynofacies microfloral evidence reflects variable palaeoenvironmental settings suggestive of fluvio-deltaic systems transgressing coastal swamps with other low energy fluvial settings consistent with swamps, marshes and crevasse splays suggestive of a coastal plain domain. The high abundance and diversity of pollen and spores with the non-occurrence of marine taxa indicates continued regression suggesting a causal link between climate change and eustasy during the Serravallian. The wet and warm climate events linked with marine sequence stratigraphy signature are characterised by arid savannah, and humid/ever-wet mangrove taxa, with a well-developed intermediate canopied rain forest as sourcing terrane for many of the sub-tropical terrestrial composition types; indicating a strong connection between mean sea-level changes, variable inferred sequence stratigraphy and sedimentary distributary facies and palaeoclimate-linked palynostratigraphic events across the Niger Delta basin.ph