Timothy A. Cross¹

(1) Colorado School of Mines, Golden, CO

ABSTRACT: Stratigraphic Controls on Reservoir Attributes

Numerous sedimentologic and stratigraphic attributes of all scales change systematically and predictably as a function of A/S regime, even though facies and depositional environment remain constant. Many of these attributes, from porosity to heterogeneity to volume, are important for reservoir characterization. Stratigraphically induced variations in these attributes are as great as variations induced by differences in the original depositional environment. Analysis of variations in attributes from a stratigraphic perspective manifests regular organization and unidirectional tendencies, whereas analysis from a stratigraphically insensitive facies perspective shows an incoherent scatter of values. Assessment of reservoir attributes solely on the basis of depositional environment produces unnecessarily imprecise and inaccurate predictions. A more accurate assessment arises when facies attributes are described from a stratigraphic perspective.

These principles are illustrated with examples from continental and shoreface strata. For channelbelt sandstones, as trough-cross-stratification set thickness and bed thickness increase through a stratigraphic section of increasing A/S, porosity, permeability, channelbelt sandstone width, continuity and connectivity decrease, whereas macroform diversity, facies diversity, degree of preservation and heterogeneity increase. Most of these unidirectional tendencies in multiple attributes arise from variable preservation of original geomorphic elements. Stratigraphic controls also affect the composition and morphology of the original environment. Alluvial ridge height and crevasse splay thickness increase with increasing A/S conditions, and splays become sandier and have steeper clinoforms. Shorefaces accumulated in high A/S regimes are homogeneous and lack mud, whereas shorefaces of low A/S regimes are heterogeneous, have high facies diversity and significant mud drapes and partings.

Construction of an analog ruler based upon A/S regime is recommended for determining the degree of analogy between outcrops and/or producing fields. Because reservoir properties of identical facies tracts change dramatically as a function of the A/S regime, analog comparisons based only on facies tract composition are unwise.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado