Applying Deltaic and Shallow Marine Outcrop Analogs to the Subsurface

Janok Bhattacharya, University of Houston, Houston, TX

A fundamental problem in subsurface reservoir characterization is determining the continuity of flow units and flow barriers (i.e. sandstones, shales and cements). In any given field, there will typically exist a combination of field wide-elements, elements that may extend between wells, but not across the entire field, and elements that do not extend between wells.

Our outcrop analog data bases provide:

1. Regional and field-scale studies of reservoir and non-reservoir elements associated with shallow marine, deltaic reservoir types.
2. Detailed 3D facies architectural studies of small-scale, intra-well heterogeneity (cements and “stochastic” shales) in specific depositional sub-environments (e.g delta front facies) that may be incorporated into reservoir models.
3. Conceptual re-evaluations of shoreline and deltaic facies models that may be applied by geologist interpreting or correlating seismic, well log or core data.

The subsurface geologist must use facies models and sequence stratigraphic concepts to correlate well data. We show several examples of deltaic reservoirs depicted as consisting of horizontal layers (layer-cake). Our outcrop examples suggest that sandstones within the delta front dip seaward. This fundamentally challenges reservoir models that invoke flat versus dipping beds and we demonstrate how this can be applied to correlation of core and well log data sets. Our regional-scale stratigraphic results study also suggest very different exploration models in the 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. These assumptions have not turned out to be valid in the production behavior of many so-called “shoreface” type reservoirs. We show that many wave-dominated shorefaces are actually delta front deposits. Our 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 with significantly more-heterogenous facies on the downdrift side. We have applied this facies model to the re- interpretation of Cretaceous “shoreface” deposits in Wyoming, New Mexico, and Utah and these examples should be applicable to other subsurface deltaic reservoirs.