Controls on Stratigraphic Complexity in Eolian Bed-Set Architecture: Implications for Reservoir Heterogeneity

Abstract

Through analysis of bed-set geometries and foreset dip-azimuth relationships in a series of ancient outcropping eolian successions, a series of empirical relationships have been established that advance our ability to predict 3D eolian architecture from trends in 1D subsurface core data. The method allows for the first-order reconstruction of the likely the geometry and connectivity of effective net reservoir units, and preferred directional permeability trends. Comparison of subsurface stratal relationships observable in core to known architectures stored in a database of eolian outcrop analogs enables informed calculations of expected net reservoir volumes in subsurface reservoir intervals. Interpretations of expected original bedform morphology and style of migratory behavior are based on analysis of the distribution and order of occurrence of preserved eolian facies units and associated foreset dip-azimuth trends in core. A suite of sedimentological models are introduced to demonstrate the application of the technique for the identification of a variety of eolian bedform types. The preserved record of superimposed dunes on draa-scale parent bedforms is signified by compound cosets of strata within which a hierarchy of bounding surface types are present. Where superimposed dunes possessed active slipfaces, they tend to preserve thick, stacked sets of grainflow-dominated strata with favorable reservoir properties. By contrast, where draa-scale bedforms lacked superimposed dunes, they tend to preserve thick sets of wind-ripple-dominated strata that represent low-angle-inclined plinths of relatively poor reservoir quality. Most previous studies of directional permeability in eolian reservoirs predict increased fluid flow in orientations up-dip within sets along the length of relatively more permeable grainflow units. Although this is tends to be the case at the scale of individual bed-sets, at the larger architectural-element scale, the direction of maximum permeability is determined by the orientation of elongation of dune elements that typically comprise lozenge-shaped packages of grainflow strata encased within lower permeability packages of wind-ripple strata at the margins of the elements. Original bedform morphology, style of migration and rate of accumulation each play an important role in determining both directional permeability and connectivity and must be accounted for in eolian reservoir modeling workflows.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015