Tuning Effects, Lithological Effects and Depositional Effects in the Seismic Response of Gas Reservoirs*

Abstract

Synergism of 3-D seismic data, wavelet processing, colour display and interactive interpretation has been exploited in the study of a Gulf of Mexico gas reservoir. Seismic amplitude has been used as a measure of the proportion of a sand/shale reservoir capable of producing gas. This has led to the mapping of net producible thickness of gas sand. The tuning phenomena resulting from geometric effects alone were studied in detail, and tuning curves of various levels of sophistication were used as the basis for amplitude editing. Statistical tuning curves were derived by interactive cross-plotting and deterministic curves by wavelet extraction. Multiple wavelet side lobes cause multiple maxima in the tuning curve. Depositional effects and intrareservoir communication have also been studied by interactive cross-plotting.

Simple net pay estimation from seismic: a modelling study

“…An acoustic impedance model has been convolved with a wavelet consistent with the spectral characteristics typical of coloured inversion operators. Connolly's Brown et al (1984, 1986 introduced a technique which addresses both thin and thick beds (i.e., thicker than tuning thickness). It is based on the similarity of the data cloud outline on the crossplot of composite amplitude versus seismic thickness and the simple single-layer tuning curve derived knowing the wavelet in the data (Figure 3).…”

“…The method of Brown et al (1984Brown et al ( , 1986 has been investigated using the model dataset described earlier. The results for both thin and thick beds are shown in Figure 4, together with the clean sand tuning curve.…”

“…For example, a second maximum is evident, related to the presence of two sands close to tuning thickness which are separated by a thin shale. Figure 5 illustrates the predicted versus actual values of N:G using the approach of Brown et al (1984Brown et al ( , 1986. (Note that the data points above the tuning curve in Figure 5 would give N:G values greater than 1 and have therefore been reduced to conform to the tuning curve before net calculation).…”

“…On the basis of these types of results, Neff (1990aNeff ( , 1990bNeff ( , 1993 considered the amplitude scaling method of Brown et al (1984Brown et al ( , 1986 to be an oversimplification. He highlighted the fact that the relationships between trough-topeak amplitudes, isochrones, and reservoir parameters may not only be complex but also may vary significantly between different geological scenarios.…”

“…Seismic crossplot example of thickness versus composite amplitude with simple tuning curve (red) and baseline (blue). Redrawn from Brown et al (1986). (3) to be applied to seismic data, Seismic N:G needs to be related to the band-limited impedance.…”