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Reservoir Architecture and Connectivity of a Levee-Confined Deep-Water Complex, Upper Miocene, Offshore Angola

Mitchell, Stephen M.1; Abreu, Vitor 2
1 ExxonMobil Development Company, Houston, TX.
2 ExxonMobil Exploration Co., Houston, TX.

Well developed levee systems are common in deep water deposits. In some cases, levee systems are sand-prone and are reservoirs when hydrocarbon charged. This paper presents analysis of an exceptionally well imaged example in the Lower Congo Basin, offshore Angola that can serve as an analog in modeling reservoirs. The combination of high resolution 3-D seismic and proprietary elastic inversion allows recognition of the internal architectural and depositional patterns in unusual detail.

This shallowly buried levee-confined channel complex is the uppermost deposit of a series of Upper Miocene channel complexes. A number of low impedance units are interpreted to be sand-prone based on reflection geometries and seismic gas flags. The complex is linear, although the thalweg does show sinuosity when examined in detail. The channel turns almost 90° to the south as a result of the deflection of flows around a prominent compressional salt ridge. Sediment ponded behind this ridge, allowing sand-prone flows to spill over initial mud-prone levees and develop extensive sand-prone levees. Other channels in the area that are not deflected by the ridge do not develop extensive sand-prone levees.

Sand-prone levees are located on both sides of a deeper axial region that is 400 m wide and largely filled later with low reflectivity units, interpreted to be mud-prone. The levees are approximately 1.4 to 2 km wide and up to 70m thick on both sides, thinning away from the axis. Sand-prone portions are 0.4 to 1 km wide. Four internal units or storey sets (Sprague et al, 2007) have been mapped. Features recognized include sand-prone and shale-prone levee deposits, debrites and mass transport deposits, sand-prone terraces and splays.

Vertical permeability through the levee is likely to be low since the sand-prone units are separated by several meters of mud-prone deposits. Several mechanisms are recognized that would enhance overall reservoir connectivity. Along the inner portions of the levees, sand-prone units have eroded into lower sand-prone units. Connectivity is also expected to be enhanced along sand-prone terraces that appear to connect multiple sand-prone bodies. Although the fill of the channel is interpreted to be mud-prone, in some areas, potentially sand-prone units are located in the channel axis, providing a potential mechanism to locally connect levees on both sides of the channel.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009