Chapter 13: Green Canyon Block 205 Lithofacies, Seismic Facies, and Reservoir Architecture

The Green Canyon Block 205 prospect has five primary reservoirs, which range in age from late Pliocene to early Pleistocene. The N1 and N3 sands, which contain 60% of the resource in the prospect, were deposited primarily as turbidites in a middle- to lower-slope environment within bathyal water depths. Key uncertainties which affect fluid displacement paths and displacement efficiencies were defined during a detailed reservoir characterization. They include (1) facies type and distribution, (2) reservoir architecture, and (3) reservoir continuity. A probabilistic approach allowed the manipulation of a facies based geologic model to quantify the range of these and other reservoir uncertainties. Geologic and geophysical data indicate that the N1 and N3 sand lithofacies and their associated subenvironments are distinctly different. The N1 sand was deposited as a sand-rich, early lowstand fan by a series of low-density turbidity flows. Facies range from massive sand to thin-bedded turbidites, and subenvironments include channel, channel margin, levee, interchannel/overbank, and possibly fan fringe. The N3 sand was deposited as a very sand-rich, middle-to-late lowstand fan by a series of high-density turbidity flows. It is composed of a massive sand facies within an amalgamated channel complex. A lateral offset stacking pattern, which became progressively younger to the west, is the dominant architectural element for both the N1 and N3 reservoirs. Relative to the N1 sand, the N3 sand exhibits a greater degree of overlap between successive depositional units and therefore has a higher probability of amalgamation. Drill stem test (DST)-type curve analyses indicate the presence of transmissibility restrictions in both sands, which correspond to discontinuities seen in the seismic data. In the N1 sand, the analyses indicate that fluid flow may not be inhibited between channel and thin-bedded facies but may be restricted between laterally offset depositional units. Seismically defined amalgamation surfaces in the N3 sand do not impede fluid flow within the radius of investigation of the DST.