A SEISMIC ANALYSIS OF DIFFERENTIAL COMPACTION IN THE FRASNIAN-DUHAMELREEF, SOUTH-CENTRAL ALBERTA

The Duhamel reef is an isolated Upper Devonian Leduc Formation limesto ne bioherm, situated immediately NNE of the Bashaw reef complex in sou th-central Alberta. Although Duhamel has a relatively small basal area (less than 12 km(2)), this reef rises some 275 m above the platform f acies. From seismic data we interpret that there is a raised periphera l reef rim that appears to be elevated about 25 m relative to the inte rior of the reef. Raised rims are considered to be diagenetic in origi n and attributed to differential compaction within the reef complexes. Although late-stage accretionary growth about the periphery of the re ef complex may have contributed to its development, we interpret the r aised rim at Duhamel to be principally of secondary origin. This thesi s is consistent with the incorporated seismic and well-top data. The f acies within the reef rim have a lower seismic velocity than the facie s within the structurally lower reef interior, consistent with the obs ervation that the raised rim is generally more porous than the encircl ed and structurally lower lagoon and, therefore, constitute a preferre d well-completion site. Optimal hydrocarbon recovery could be realized if all production wells were drilled into the raised reef rim. The Du hamel example shows that even relatively small Leduc reefs can exhibit raised rims and that these rims may be seismically visible. Before dr illing, reflection seismic data should be acquired across similar reef s with a view to imaging and locating the complete raised rim, not sim ply its updip side, so that all parts of the rim can be tested by the drill.