QUANTIFYING PENETRATIVE STRAIN RELATED VOLUME LOSS IN CLASTIC RESERVOIR UNITS OF THE DENVER-JULESBURG BASIN

Contractional strain is associated with regional shortening events and is expressed in the formation of macroscale structures, as well as volume loss coeval with deformation. In brittle systems, prior to the development of folds and thrusts, shortening is accommodated through intergranular deformation which constricts porosity and limits subsurface fluid pathways. This volume loss is characterized as penetrative strain, and constitutes the total amount of shortening represented by the development of microscale structures and sub-seismic brittle deformation. The magnitude and location of this internal deformation is difficult to quantify and is often ignored in regional cross-section restorations. This study intends to constrain the amount of penetrative strain accommodated in major reservoir units within the Denver-Julesburg basin, and use that information to make broader inferences about where and when penetrative strain may be occurring in similar stress regimes.

For this study, samples were taken from quartz arenites of the Cretaceous Dakota and Jurassic Morrison formations of the eastern Colorado Front Range. Petrographic analysis was used to restore compromised grain boundaries syn-deformational with Laramide compression. Best fit ellipses and irregular polygons were used to delineate pre-deformation grain boundary contacts altered by grain impingement, grain rotation and pore-scale compaction processes. The Onasch method was applied to estimate microstructural strain attributed to pressure solution shortening. The ellipse method placed an upper limit on penetrative strain related volume loss at 14.7%, while the polygon method placed a lower boundary at 10.5%. These values are within the range consistent with previous studies conducted in the Appalachians, Wyoming Salient, and via analog modeling.

This research suggests penetrative strain is a significant reducer of reservoir quality in clastic sedimentary units, and impacts aquifer mechanics and permeability patterns through the closure of pore throat spaces by penetrative strain related volume loss. Increasing our knowledge of structural controls on subsurface deformation will help geologists better predict reservoir quality, fluid flow, porosity and permeability in target units.