Calibration of Seismic Attributes for Reservoir Characterization ...

thin-bed sandstone reservoirs and the closely spaced thin-bed sandstone reservoirs, respectively(compare with the well-based depositional model, Figure 18). The thicker limestones in the westwere not revealed in crossline 117 while the GRNN did recognize them (Figure 30). This alsodemonstrates that the sandstones and limestones are heterogeneous.Figure 32: Crossline 117 of the invertedimpedance volume using the PNN model.It is difficult to discriminate the sandstone from the limestone using the inverted impedancealone. However, they can be separated by knowing the depositional model already constructedusing well and seismic data (see our separate paper). In order to interpret the limestone andsandstone from the impedance cube, the cutoff of 32000 ft/s*g/cc was obtained on the basis ofwell data. The thicknesses from 37 wells are shown in Figure 33, against the thickness estimatedfrom the inversion results. This relationship demonstrates the advantage of using impedance orlayer properties over using the amplitude attribute for thin bed estimates (see Figure 27a). Theestimated sandstone reservoir distribution is illustrated in Figure 34. Note that the limestones inthe west were manually removed, but we was not able to separate them from the sandstones inthe interdelta (i.e. in the south) and western margin of the main delta system. This figure showsthat no reservoir sandstone was developed in some parts of the main delta system, supporting thelateral heterogeneity of the thin-bed reservoirs in the shale-rich Caddo sequence.Computed from well log (ft)60.0050.0040.0030.0020.0010.00y = 0.9701x + 2.9637R = 0.854(37 wells)0.000 10 20 30 40 50 60Estimated from inverted impedance (ft)Figure 33: Actual sandstone and limestone thicknessplotted against predicted thickness from seismicinversion (PNN model)Figure 34: Estimated sandstone thicknessof Caddo main delta system (PNN model)Note: the limestones in the west wereremoved manually in this example)Annual Technical Progress Report Michigan Technological University DE-AC26-98BC15135October, 2001 Page 40