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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Applying Deltaic and Shallow Marine Outcrop Analogs
to the Subsurface
By
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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