Sequence Architecture and Depositional Model for the Tortonian Succession, in the Jeth Oilfield, Niger Delta.

Abstract

The Jeth oilfield forms part of a group of oil fields in the coastal swamp depo-belt of the Niger Delta province that holds abundant oil, and reserves of both associated and non-associated gas. Among the key risks associated with reservoir development in the oil field are those pertaining to uncertainties of reservoir characteristics, depositional environments of the reservoirs, and reservoir continuity/and connectivity. This study adopted a sequence stratigraphic approach based on biostratigraphic/palynological data, wireline logs, and core data, to identify and characterize the reservoirs in terms of lithofacies, depositional environments, and reservoir distribution, in order to provide a framework that ensures optimum hydrocarbon production from the oilfield. Results of the analyses of wireline logs, and lithofacies data revealed that the late Middle-Miocene (Tortonian) succession penetrated in the Jeth oilfield contains five reservoir lithofacies including (i) fine to coarse grained cm-scale cross bedded sandstone (Sx), (ii) Very fine-grained mm-scale laminated sandstone (Sc), (iii) very fine-grained mm-scale laminated sandstone with clay drapes/laminae (Sl), (iv) bioturbated sandstone (Sb), (v) wavy-bedded sandy heteroliths (SMw), and (vi) bioturbated sandy heteroliths (SMb). These reservoir lithofacies cumulatively make up 60% of the succession. The non-reservoir lithofacies include (i) lens-bedded muddy heterolith (Mst), (ii) bioturbated muddy heterolith (Msb), (iii) bedded/ massive mudstone (Mb/Mm), and (iv) Sideritic-bedded mudstone (Msd). These reservoir and non-reservoir lithofacies are further grouped into five facies associations that are interpreted to have accumulated in either (i) distributary channel, (ii) tidal channel, (iii) tidal flat (iv) lagoon or (v) distributary mouth bar environments. Sequence stratigraphic analysis revealed that these deposits belong to two major 3rd-order depositional cycles dating 10.35 Ma -8.5 Ma and 8.5 Ma -6.7 Ma respectively. Each depositional cycle begins with a transgressive systems tract comprising distributary channel, tidal channel, tidal flat, and lagoon (fluvio-estuarine) deposits, and ends in a highstand systems tract (HST) composed of delta front deposit. The transgressive systems tracts (TST) contain high quality reservoir sands that hold the bulk of the total original oil in place in the Jeth oilfield, while the highstand systems tract (HST) reservoirs hold most of the less developed reserves in the field. The distribution and internal architecture of the sequences reflect the complex interplay between depositional style, sea level change and basin structures. Reservoir sands are better developed within incised valleys (TST) in the distal areas, and are less developed in the more proximal areas. These sand units grade laterally into more mud-dominated facies that possibly reflect basinward decline in clastic influx. This research thus provides a reliable framework for predicting the spatial distribution and continuity of reservoirs within the Tortonian succession in the Niger Delta, especially in areas where well coverage is poor. Production strategy in the Jeth oilfield should therefore be reviewed in line with the new knowledge of the sequence architecture and depositional environments of the reservoir sands.