NET-PAY ESTIMATES IN HETEROGENEOUS THIN BEDS

The preceding models assume that the thin bed is a homogeneous unit composed of a single, uniform rock type. In many situations, a thin sand may contain one or more shale stringers that subdivide the gross thin-sand unit into several thinner net sands. A question that needs to be considered is "What effect do shale stringers within a thin bed have on the thin-bed reflection response?" A model that provides insight into this problem is shown in Figure 5 below.

Figure 5.

The spatial distribution of sand and shale within the thin bed (red arrow in figure) is shown at the top of the model, and the calculated reflection responses are plotted below this lithological cross section (red arrow in figure). The apparent (measured) time thickness (red arrow in figure) and the peak-to-trough amplitude of the calculated reflection waveforms (red arrow in figure) are shown in graphical form at the bottom of the illustration. The apparent time thickness is essentially constant, (red arrow in figure) as it should be in a thin bed regime. However, the peak-to-trough amplitude increases and decreases in phase with the net-sand curve (red arrow in figure). The model shows that reflection amplitude is a reliable indicator of net pay within a thin bed when the thin bed is heterogeneous.

The model in Figure 6 below carries the investigation a little further in that it keeps the total amount of contaminating shale constant but distributes the shale through the thin sand as one, two, or four stringers.

Figure 6.

The peak-to-trough amplitude of the responses at positions 2, 3, and 4 isthe same, (red arrow in figure) which leads again to the conclusion that reflection amplitude is a measure of the net pay within a thin-bed regardless of how that pay is distributed within the bed. One must be cautious not to apply this interpretational guideline (that is, that reflection amplitude is directly proportional to net sand) when the gross sand interval being studied is thicker than one-fourth the dominant wavelength of the seismic wavelet.