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Reservoir Architecture and Stratigraphic Evolution of Channelized Deep-Water Depositional Systems, Offshore West Africa: Implications for Predictive Stratigraphic Modeling


The stratigraphic evolution of channelized deep-water reservoirs results in a heterogeneous arrangement of facies, which is commonly below the resolution of industry seismic-reflection data. Reservoir heterogeneity, coupled with drilling in water depths >1,000 m, presents significant challenges to exploration and development of oil and gas. We interpret the field-scale (225 sq. km) stratigraphy of a Miocene subsurface interval, Block 14, offshore Angola. Our interpretation of seismic stratigraphy (dominant frequency 40 Hz) and wireline logs suggests that the subsurface interval predominantly represents channelized turbidite deposition, confined within a deep-water slope valley (∼200 m thick; 2-3 km wide). We interpret a stratigraphic evolution from amalgamated, high net-to-gross, laterally discontinuous (in dip direction) channel elements and mass-transport deposits to semi-amalgamated, lower net-to-gross, laterally continuous (in dip direction), vertically stacked channel elements. A similar model of stratigraphic evolution has been documented in the subsurface of continental margins worldwide. This evolution has been interpreted to represent a combination of changing sea level, evolving sediment source characteristics, and the dynamic geomorphology of tectonically active continental margins. We employ synthetic seismic modeling to evaluate the stratigraphy and degree of diachroneity of large-scale, concave-up seismic reflections (up to 50 ms [70 m] relief) within the slope valley. Analogous reflections have been interpreted by previous workers in sequence stratigraphic terms to be 4-5th-order (<1 Ma) erosional surfaces cut during a sustained period of incision, followed by episodes of filling and abandonment. Importantly, these surfaces are thought to partition reservoirs. We question this previous interpretation and show that these large-scale reflections could be a highly diachronous expression of multi-phase and multi-scale channel element cutting, filling, and stacking. This alternative interpretation is significant in that it suggests that reservoir compartmentalization is controlled by more composite amalgamated erosional surfaces.