American Oil and Gas Reporter - September 2016 - 46

SpecialReport: Reservoir Optimization & Modeling
FIGURE 2
Band-Pass P-Impedance
And Deterministic Inversion Results

Refined Earth Model
The second phase of the study involved detailed reinterpretation of the horizons performed on layer-based simultaneous inversion data to remove false artifacts produced by wavelet and
tuning effects. As shown in Figure 2, P-impedance is a good indicator of the top and base of the Upper Cretaceous sands. Vp/Vs
provides additional information, which assists in delineating the
base of the Upper Cretaceous sands. This approach generates a
refined stratigraphic earth model, and was developed exclusively for use in geostatistical inversion.
Based on the reinterpreted stratigraphic earth model, probability distributions of each seismic facies were determined accurately in P-impedance, Vp/Vs ratio, and density probability distribution space, as illustrated by the difference in the lithofacies
distribution in Figure 2D.

Figure 2A shows the original horizons overlaid on the bandpass P-impedance. Yellow represents high P-impedance, blue is
gamma ray, and black is resistivity at the well location. All of the
reflectivity features are preserved from the seismic and the data
are seen with far superior clarity. Figure 2B is a cross-plot of Pimpedance versus Vp/Vs colored by lithofacies, showing the original interpreted Upper Cretaceous Sand.
Figure 2C displays the reinterpreted horizons overlaid on the
band-pass P-impedance, with the horizons matching the well tops
and elastic property contrasts. The P-impedance versus Vp/Vs colored by lithofacies cross-plot in Figure 2D shows a more representative identification of lithofacies, in terms of elastic properties, derived from the now accurate interpretation of the Upper
Cretaceous Sand.
While the difference between the cross-plots in Figures 2B and
2D may appear small (it is most noticeable in seismic facies S1
shown in gray), it is important that the facies are represented accurately within the interpretation of the horizons because these
bounding surfaces control which cross-plot facies statistics are
used to derive the PDFs, which in turn, drive the geostatistical
inversion. The seismic facies S1 is the seal for the Upper Cretaceous reservoir.
Seismically Constrained Process
The third phase involved the simultaneous geostatistical inversion of the seismic 3-D partial stacks. This phase incorporated data from various disciplines, including the deterministic inversion, to provide highly detailed subsurface facies models along
with absolute reservoir rock and geomechanical properties. Associated uncertainty measurements for all properties were calculated from the 21 realizations produced by the seismically constrained geomodeling process, and each property honored all prior input information provided from well logs, core data, geology, seismic data, and geostatistical information.

FIGURE 3
Cross-Section Flattened to Top of Upper Cretaceous Sand Horizon
A

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American Oil and Gas Reporter - September 2016

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