Pemberton, S. George
University of Alberta, Edmonton, Alberta, Canada

Abstract: The Application of Ichnofacies Analysis to the Evaluation of Marginal Marine Reservoirs

Marginal marine reservoirs, by their very nature, consist of a complex array of facies reflecting the juxtaposition of several sub-environments. The superimposition of freshwater, brackish, and fully marine settings across key sequence stratigraphic surfaces is a common response to changes in relative sea level in the marginal marine realm. Consequently, distinguishing between freshwater, brackish water, and fully marine deposits is of paramount importance in the exploration and exploitation of such reservoirs. Trace fossil assemblages can be used to assist in the differentiation of these settings. In general, brackish water trace fossil suites are characterized by (1) low diversity; (2) an impoverished marine suite, rather than a mixture of marine and nonmarine traces; (3) simple structures constructed by trophic generalists; (4) suites that are commonly dominated by a single ichnogenus; (5) diminutive forms; and (6) vertical and horizontal ichnofossils that are common to both the Skolithos and Cruziana ichnofacies.

The concept of incised valley complexes is one of the more important aspects to emerge from genetic stratigraphy and the recognition of these features is partly dependent upon distinguishing between fully marine and brackish water environments. Although this distinction is best made using an integrated approach, trace fossils can often provide the least equivocal, and most readily accessible, data on the recognition at brackish water deposits. The Upper Albian Viking Formation in Alberta produces hydrocarbons from anomalous deposits in the Crystal, Willesden Green, Cyn-Pem, and Edson fields. These unique deposits are interpreted to represent lowstand incised valley systems, and are erosionally juxtaposed against regionally extensive offshore to lower shoreface parasequences. The contained trace fossil suites have been instrumental both in the delineation of the valley margins and in the delineation of paleoenvironmental interpretations of facies within the estuarine, incised valley complexes.

AAPG Search and Discovery Article #90929©1998-1999 AAPG Distinguished Lecturers