A fundamental problem in subsurface reservoir characterization is determining the continuity of flow units and flow barriers. Flow units and barriers exist on a variety of scales from field-wide to interwell to wellbore.

Outcrop analog studies and databases are used to develop models for

• regional and field-scale reservoir and nonreservoir elements associated with shallow marine, deltaic reservoir types
• detailed 3D facies architecture of small-scale, intrawell heterogeneity (cements and “stochastic” shales) in specific depositional subenvironments (e.g., delta front facies) that may be incorporated into reservoir models
• conceptual reevaluations of shoreline and deltaic facies that may be applied by geologists interpreting or correlating seismic, well log or core data.

The subsurface geologist must use facies models and sequence stratigraphy concepts to correlate well data. Several examples of deltaic reservoirs that consist of horizontal layers are described. Outcrop examples suggest that delta front sandstones are not horizontal but dip seaward. This recognition challenges many current reservoir models that assume flat-lying beds and affects how we correlate core and well log data. On the regional scale, the analogs suggest very different exploration models are required to search for basin-distal reservoir sandstones.

From the perspective of general facies models, historically shorefaces have been assumed to form homogenous, uniform reservoirs that require little effort to produce. Production histories, however, show that this assumption is not valid for many so-called shoreface-type reservoirs. Many reservoir units identified as wave-dominated shorefaces are actually delta front deposits. This new model for wave-influenced coastlines suggests a distinct facies asymmetry, with homogenous beach and shoreface sands accumulating on the updrift side of the river mouth and significantly more heterogeneous facies on the downdrift side. The new model was used to reinterpret Cretaceous “shoreface” deposits in Wyoming, New Mexico and Utah and should be applicable to subsurface deltaic reservoirs elsewhere.

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