An integrated surface seismic/seismic profile case study of the Leduc formation reef, Lanaway Field, Alberta, Canada

Abstract

On the basis of conventional surface seismic data, an exploratory well (referred to as the VSP well) was drilled into the updip, raised rim of the Leduc Formation (Devonian Woodbend Group) reef complex at Lanaway Field, southeastern Alberta, Canada. The well was expected to encounter an anomalous late-stage accretionary buildup at the Leduc level. It was anticipated that the Leduc at the VSP well location would be up to 80 m higher than at adjacent rim well sites. To the consternation of the geophysicists, the envisioned accretionary growth was not present; the top of the Leduc in the VSP well was consistent with other rim wells and inconsistent with the seismic interpretation. Fortunately, however, the Leduc was structurally closed and the VSP well was completed as an oil producer (from both the Nisku and Leduc Formations). In order to resolve the apparent discrepancy between the interpreted surface seismic data and geology at the VSP well, a vertical seismic profile (VSP) was conducted at the well site. The interpretation of the VSP data was expected to elucidate the geological origin of the misinterpreted seismic anomaly. Towards this end, the VSP was relatively successful. These data confirmed that the original interpretation of the surface seismic data (with respect to the Nisku, Ireton, and Leduc top) was incorrect, and that the anomaly observed on the surface seismic line was not a processing artifact. It was ultimately attributed to several superposed effects: (1) anomalous structural relief at the pre-Cretaceous subcrop; (2) stratigraphic anomalies within the Winterburn Group; (3) multiple interference; and (4) tuning effects associated with the thinning of the Ireton along the seismic profile.