ABSTRACT
The Early Pliocene (Zanclean) to latest Miocene (late Messinian) sequence in Mobil/UMC Block B offshore Equatorial Guinea is defined by the appearance (last downhole occurrence, or LDO) of Podocarpus milanjianus, and the first downhole occurrence (FDO) of typical lower Pliocene/upper Miocene foraminifera.
The reservoir horizons, which occur in the lower portion of this sequence, contain palynofloral assemblages characterised by abundant Pediastrum, common Botryococcus braunii and Gramineae as well as charred Gramineae cuticle. Rhizophora mangrove is poorly represented and marine dinocysts virtually absent. Deep marine foraminifera occur in discrete horizons. Gramineae maxima and common charred Gramineae cuticle suggest extension of savannahs and reflect a strongly seasonal, relatively dry and warm climate. This climatic shift is most likely associated with the latest Miocene (Messinian) worldwide sea level drop and associated aridity event, the so-called ‘Messinian crisis’. The abundance of Pediastrum taxa is interpreted as a freshwater transport signal in the sequence. This signal indicates the occurrence of intermittent lacustrine environments which are contemporaneous with incisement and erosion of the continental shelf during this lowstand period at the Mio-Pliocene boundary. Subsequent transport and redeposition into a deeper marine setting occurred at the continental shelf palaeoslope surface, the sediments probably bypassing the upper reaches of the continental slope. The strong freshwater transport seaward probably occurred during the short, rainy season or during short, very erosive, humid cycles of this seasonal climate. Calcareous and agglutinated benthic foraminifera are rare and restricted to encapsulating bathyal marine shale horizons and confirm that the entire sedimentary package was deposited in slope or upper bathyal water depths.
46The biostratigraphic interpretation of the reservoir packages indicates destabilisation and mobilisation of nearshore sediments in response to high accomodation space which occurred immediately after Messinian canyon cutting on the continental slope. These sediments were deposited in continental slope settings as debris flows or turbidites.
The overlying mudstone sequence, which constitutes the reservoir topseal, is characterised by significant increase of Rhizophora and the presence of common dinoflagellate cysts. This is interpreted as the landward development of mangrove under consistently humid conditions and increased sea levels.
