VELOCITY VARIATIONS AROUND LEDUC REEFS, ALBERTA

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Abstract
Subsurface and surface velocity determination methods in south‐central Alberta have revealed a significant velocity increase in the geologic section adjacent to and above Upper Devonian Leduc reefs. Through the use of well‐velocity survey data, velocity anomalies have been measured within individual stratigraphic units over Leduc reefs including Strachan, Chedderville, Rimbey‐Homeglen, Westerose, Bonnie Glen, and Innisfail. These anomalies indicate an above‐reef average interval velocity increase of 9 percent for the Upper Devonian, 3 percent for the Mississippian, 5 percent within the basal Mesozoic and comprise a total Blairmore‐to‐base‐of‐Ireton interval average change of about 7 percent. Velocity determinations through seismic‐reflection velocity analyses in the Strachan area are consistent with these results, indicating that the use of seismic reflection velocity analysis may prove to be an effective tool for delineating and confirming reef presence in problem areas. Geophysical and geological studies concerning the nature of the velocity variations reveal the existence of density contrasts above a Leduc reef similar in distribution to the observed velocity anomalies. These contrasts appear to be sufficient to account for the gravity anomalies observed above Leduc reefs. Within at least part of the Upper Devonian section above‐reef, these anomalies are believed to be due to primary lithologic facies changes that localized post‐depositional processes including dolomitization. In addition, these post‐depositional processes may have involved the Upper Devonian to Lower Cretaceous interval above Leduc reefs; however, no complete geologic model can be offered at this time.

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