This value can be chosen by inspection of a SNA histogram. We can
hope for clean separation of roughly Gaussian distributions of
shale-wet sand reflections and shale-oil sand (and/or shale-gas sand)
resulting in a bi- or tri-modal distribution of reflections. As
discussed above, the less dense the pore-filling fluid, the higher the
SNA value. Therefore, assuming only that most intervals are
water-bearing, we can confidently pick the first mode as indicative of
water reflections. **Figure 4** shows an ideal distribution of
SNA, while **Figure 5** shows the histogram for the synthetic
data.

ideal
Ideal distributions of
SNA attribute. Threshold values for color tables or clipping can be
set with these in mind for fast and furious prospecting or horizon
mapping and volumetrics.
Figure 4 |

histo
Histogram of SNA from
modeled then migrated from the density/velocity grid shown in
6. The extreme preponderance of zero values in the model
makes the plot ugly, but the tri-modal nature of the histogram is evident.
Figure 5 |

Figure 6

An even more powerful approach is to extract the SNA attribute along an event and
plot it as a function of depth or time. As the event crosses fluid
levels, we will see nice step-changes in the SNA value.^{}

event
While fault blocks
complicate the plot somewhat, fluid level remains obvious, and a
prospecting threshold of 12-14 is comfortably assigned. Values were
not normalized by .Figure 7 |

To use this type of analysis on well or seismic data (integration!),
one is only required to perform the trivial calculations of equations
(1) and/or (3) to generate the plane.
Further, after estimating the Shale trend angle on the
plot,^{}
the SNA attribute can be calculated and used as a prospecting or reservoir
characterization tool.

The next step in proving the value of this concept will be to apply these simple transformations to single events known to have fluid contacts. Such a data set is thought to exist in-house, and a thorough description of our success/failure with this analysis will be included in the next report.

9/18/2001