James L. Allen

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).

A case history of velocity problems in the shadow of a large growth fault in the Frio Formation, Texas Gulf Coast

False structures can appear on seismic data in the shadow of normal growth faults, with the upthrown time structure appearing to roll into the fault where no such roll exists in depth. This phenomenon was encountered in a prospect in the Frio formation of the Texas Gulf Coast, where a large growth fault expanded a thick, low‐velocity shale above the Frio. Two check‐shot velocity surveys, one at either end of the seismic control, demonstrated a severe average velocity gradient across the prospect at the level of the objective geologic formation. Assuming first one and then the other velocity survey was valid over the entire area for conversion of well data to time produced a drastic difference in the structural interpretation of the seismic data. The more obvious interpretation required a down‐to‐the‐northwest fault with a downthrown rollover anticline at the objective formation. This interpretation had been previously tested with a dry hole on the crest of the time structure. This interpretation also cont...

Using “poor man’s 3-D” to identify distributary channel sands in the Wilcox formation, Lavaca County, Texas

Within Lavaca County, Texas, sediments of the Lower Eocene Wilcox formation are cut by shale‐filled canyons (Figure 1). Three have been identified and called channels: the Yoakum, the Smothers, and the Lavaca…but in this paper they will be referred to as canyons in order to avoid confusion. There has been an argument over the depositional setting of those canyons, whether turbiditic or fluvio‐deltaic, and a recent paper has attempted to resolve the differences. Fortunately, we seismologists have been able to work the area while the debate ensued. Our efforts are simplified in one respect because the erosional base of the Lavaca canyon is relatively easy to identify on the seismic data over most of the area (Figure 2).