GARDNER, MICHAEL H., Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: Increasing Predictability in Reservoir Characterization by Constraining Facies Models with Stratigraphy

Facies models regard different delta morphologies as products of differing sediment supply, subsidence, and oceanic current and wave regimes. Thus, end-member "wave-dominated," "fluvial-dominated," and "tide-dominated" deltas are commonly regarded as different entities reflecting different sedimentologic, geomorphic, and tectonic regimes. In the Ferron Sandstone(Turonian), different delta morphologies--and accompanying changes in facies composition, facies associations, and stratal architecture--are produced in a constant tectonic and geomorphologic setting. The only variable that controls observed differences is the change in sediment flux to the delta front, which is a response to accommodation rather than a response tochanges in climate, drainage basin size, river discharge, or oth r external conditions.

This conclusion has important implications for the use of facies models, which are constructed by comparing many facies assemblages. This comparison assumes that deposits from a particular geomorphologic environment have facies that are similar and fundamental to that environment. Consequently, most descriptive facies attributes may be generalized from multiple specific examples. In this view, the stratigraphy of deposits from a geomorphic environment is only the product of depositional processes operating in that environment and are not subject to modification or influence by accommodation changes. Facies models do not consider the possibility that entirely different geomorphological elements may occur in identical facies tracts or geomorphic environments as a function of their strat graphic position.

If facies assemblages representing a single depositional environment are collected from both base-level rise and base-level fall portions of cycles, the resulting facies model of that depositionai environment will be derived from a mixture of unrelated elements. Unless placed in a stratigraphic context, systematic changes in the facies architecture of similar

geomorphological elements may be overlooked. Although facies models are useful for paleoenvironmental interpretation, their utility in reservoir characterization is limited because potentially deterministic facies variations have been obscured by mixing statistically different facies assemblages that accumulated under different accommodation conditions.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.