ABSTRACT

Sandstones and mudstones of the Cardium Formation in the Pembina Field can be subdivided into a minimum of ten separate lobate, offlapping and prograding parasequences with an average southeast offlap direction. As presently mapped, the lobate parasequences cover a minimum of 400 sq. km. These mudstone to sandstone parasequences comprise a parasequence set that forms most of the reservoir at Pembina.

The upper surface of the parasequence set is modified by two, distinct, erosional surfaces. A lower erosional surface is manifested locally as a series of parallel, asymmetrical channels up to 5 m deep, 3 km wide and 10 to 20 km long. This surface is overlain by a package of well sorted, crossbedded conglomerate and bioturbated, conglomeratic, sandy mudstone. These rocks are in turn incised and overlain by poorly sorted conglomerate. Upper and lower erosional surfaces merge at the margins of the asymmetrical channels.

Within parasequence 4, current-bedded sandstones are laterally juxtaposed with bioturbated, muddy sandstones along southeast-trending lineaments. Significantly, these lineaments coincide with southeast-trending, sub-parallel troughs up to 10 m deep, 10's of kilometres long and several kilometres wide, which belong to the erosional surfaces at the top of the parasequence set.

In the study area, rocks were deposited in a marine environment during an episode of sea level oscillation. Sandstone parasequences formed during a relative sea level drop, while the overlying conglomeratic deposits formed during a relative sea level rise. The lithological characteristics and the spatial relationships of the various deposits are analogous to ones observed in modern subaqueous deltas or estuaries.

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