Turbidite Depositional Environment In Deepwater Of Nigeria
Authors: Ofurhie M.A. , Lufadeju A.O. , Agha G.U. , Ineh G.C. 
Published: October 2008

Following the award of deepwater blocks by government in 1993, active exploration activities in Nigeria deepwater acreage, which is about 35, 000KM2 had been carried out between 800 and 2000m water depth in the Gulf of Guinea.

The first commercial oil discovery in 1996 from Bonga prospect of recoverable reserves of well over 500 Mmbo, provided the direct evidence of a giant oil field in the thick Tertiary clastic sediments. Petroleum is found in combination structural and stratigraphical paytypes formed by the folding Slope Channel complexes and Basin Floor fans facies onfaulted four-way dipping anticlines in Miocene Times.

As thick sediments prograded and are deposited on the continental margin, extensive deformation of the slope by gravity-tectonic processes created complex intrabasins, structures and diapirs. The sediment transport such as slumping, debris flows and turbidity accounted for the bulk of resedimented deep-sea clastics and isolated channels sometimes amalgamated up-dip resulting into ponded turbidite and/or Levee/overbank deposits of different facies models.

The development of point and longitudinal bars and their lithological characteristics within the channel complexes pinpointed common shoestring and sheet-like reservoir geometry diagnostic with depositional environment.

As Nigeria targets first oil from the Deepwater Petroleum Resources by the end of 2003, this paper therefore focuses on the geological characterization of reservoirs for development drilling to adequately define reservoir facies, geometry and geologic model using integrated exploration database.

Classic integration of stratigraphic seismic interpretation based on 3-D seismic data and well information cannot be over emphasized, therefore this will; give a clearer picture of the geological model and comprehensive reservoir characterization necessary for optimal field development program. The success of development drilling in highly complex reservoirs can be achieved using the integrated approach.

Introduction

The Tertiary geologic section of the deep offshore Niger Delta is predominantly comprised of a mud dominated slope-basin depositional system. These Tertiary/ Quaternary successions are developed into thick clastic sediments, which are slowly moving down slope by gravity sliding on top of a regional unconformity that marks the onset of passive margin and forms the prominent seaward bulge in the continental margin off Southern Nigeria.

Hitherto, oil exploration and exploitation has been over the years limited to the hinterland where the first oil discovery was made by Shell in Oloibiri-1 well in 1956 with production starting with a modest output of 5000 barrels in 1958. After this discovery, the potential of Nigeria as oil and gas producer has continued to increase tremendously. Sequel to the allocation of Deepwater Blocks on Production Sharing Contract and Sole Risks bases by Nigeria Government between 1991 and 1993, there has been a steady growth in the Exploration and Production activities in Nigeria Deepwater. The increase in the E&P activities is attributable to Government's robust deepwater fiscal regime and major successee by some E&P companies.

As with other worldwide deepwater sands, the Nigerian deepwater sands are characterized by sequences of conventional turbidity current origin in submarine fan settings.