Carolyn P. Peddy

Some AVO failures and what (we think) we have learned

Amplitude‐versus‐offset (AVO) analysis has had a dramatic impact on exploration in the Texas Gulf Coast. It has opened new trends in mature areas as well as lowering drilling risk, so that even some small one‐well targets are more economically attractive. From 1987–91, an estimated 300 billion ft 3 gas were found in the Eocene Yegua and Cook Mountain trends in stratigraphic traps by using AVO analysis (see Seismic hydrocarbon indicators lower risks by O. Welper et al., Oil & Gas Journal, November 4, 1991).

Deep seismic reflection profiling: How far have we come?

How much would you know about your oil or gas prospect and its surrounding geology without seismic reflection data? The odds are not very much, especially if you work in an offshore area inaccessible to surface mapping. The deep crust is also inaccessible to mapping and, until the past decade when seismic reflection techniques were used to study deep crustal processes, little was known about the detailed structures of the crystalline continental crust. Deep seismic reflection profiling — an adaptation of techniques used by the petroleum industry — allows the entire crust and part of the upper mantle to be imaged with something like the same detail that an exploration geophysicist obtains for prospect mapping (Figure 1). The results obtained by deep seismic reflection profiling are exciting, and are beginning to revolutionize our understanding of crustal geology.

Modeling Variations In the AVO Response of Wet And Gas Sands Using Calculated Shear-Wave Velocities

The resulting shear wave velocity profiles contain variations in absolute shear wave velocities, but more importantly, contain variations in the relative change in shear wave velocity between sands and shales. The relative changes in shear wave velocity produce significant variations in AVO response of reflections from sand/shale interfaces. The variation in AVO response is greatest for those model layers containing wet sands. The variation in AVO response is smaller for gas sands, as the AVO response is dominated by the decrease in the Poisson’ s ratio of the gas sands.