Low‐ and high‐relief Leduc formation reefs: A seismic analysis

Abstract

Leduc reefs have grown to widely varying heights and areal extents along the Rimbey‐Meadowbrook trend of central Alberta, resulting in significantly different seismic signatures. Three examples considered in this paper include two high‐relief or full reefs from the Leduc‐Woodbend field, an atoll and a pinnacle, each around 200 m in height but differing greatly in areal extent, about [Formula: see text] for the atoll and [Formula: see text] for the pinnacle. The third example, a low‐relief or basal reef from the Morinville field, is about 100 m high and [Formula: see text] in areal extent. The Leduc‐Woodbend and Morinville reefs exhibit quite different seismic signatures. For example, 25 ms of time‐structural drape along the top of the Devonian is observed across the Leduc‐Woodbend atoll but only 15 ms across the Morinville reef. There is 30 ms of pullup at the Beaverhill Lake level beneath the Leduc‐Woodbend atoll, 15 ms for the Morinville reef. Also, it is very difficult to differentiate the Leduc reflection from the Duvernay reflection, with which it merges, on the Morinville (basal‐reef) section. In contrast, the Leduc reflection can be correlated readily on the Leduc‐Woodbend atoll section; and reflections from the offreef shales (Duvernay and Ireton formations) terminate abruptly against the reef flank. In addition, the amplitude of the underlying Cooking Lake platform reflection varies laterally, depending on the velocity of the overlying formation (Duvernay shale or Leduc reef) and, to a lesser extent, the thickness of the overlying reef. This variation is not as useful in distinguishing between low‐relief and high‐relief reefs as it is in indicating the presence or absence of reef.