Abstract: Deep Offshore West Niger Delta Slope, Nigeria: Structural Styles and Hydrocarbon Reservoirs

Läuferts, Hermann - Shell Nigeria Exploration and Production Co. Ltd.

In the years 1995 to 1997, the West Niger Delta Slope area ( 1) offshore Warri (Latitude 4-6ºN, Longitude 4-5ºE) has been one focus for exploration drilling activities in the deep water offshore Nigeria. A series of seven exploration wells were drilled in up to 1450 m water depth on targets in turbidite reservoir objectives. These reservoirs occur in a range of stratigraphic ages throughout the Neogene and in a variety of structural settings within the West Niger Delta Slope.

The northern portion of the basin is dominated by areas of relatively shallow basement and therefore thin mobile shale substratum. Parallel sets of NW-SE striking boundary faults provided accommodation space and structural trapping configurations in footwall closures (Fig. 2, Profile 1, left). Shale diapirism played only a minor role in providing hydrocarbon traps. During lowstand times, an erosional relief was formed and sediment bypassed into lower slope and basin floor environment. Subsequently, the relief was re-filled by backstepping fan lobe and channel/levee systems. This later sedimentary fill created reservoirs in narrow, strike confined settings of the upper slope. The reservoirs usually consist of multiple series of Upper Miocene to Pliocene sandy channel fills. A variable degree of lateral and vertical amalgamation can be observed.

In the central basin, a clear distinction can be made between a proximal area, which is dominated by shale diapirism and mini basin development, and a more distal, relatively undisturbed area. The mini basin province shows a complex shale movement pattern with faults adjacent to the diapirs and on their crests ( 2, Profile 2, left). The diapirs occur as shale walls that had continuous influence on the location and confinement of channels, as shale ?needles? that had no impact on the depositional configuration, or as a combination of both. Reservoirs can be found as channel/levees (high slope relief) or fan lobes (low slope relief) and are located in the conduits and bypass zones surrounding the shale diapirs. The distal area is located in present-day middle to lower slope setting (Fig. 2, Profile 1, center). The overburden sediment load did not induce shale diapirism and trapping configurations are the result of long wavelength undulations in the shale substratum. The oldest drilled reservoirs are of Upper Oligocene age and interpreted to be terminal fan lobes. In an overall prograding sequence, they are first followed by leveed channel complexes and later by confined channels with high degree of vertical amalgamation.

The southern portion of the West Niger Delta Slope exhibits an intermediate setting. The structural configuration is dominated by N-S trending shale ridges, oriented perpendicular to the sediment input. The area received coarse clastics pulses throughout the Neogene and reservoirs developed as laterally extensive sheets and confined channels.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil