QUANTIFYING STRESS CONDITIONS OF DEFORMATION BAND FORMATION IN THE PARADOX BASIN, UTAH: IMPLICATIONS FOR RESERVOIR QUALITY PREDICTIONS

High-porosity sandstones represent important reservoirs for hydrocarbons, aquifers and geothermal systems in many locations worldwide. However, when these units are weakly cemented or completely uncemented, they are susceptible to pervasive sub-seismic deformation that can substantially restrict fluid flow and, in the case of hydrocarbons, substantially reduce rate and ultimate recovery. Although this style of plastic deformation in sandstone has been studied, creating accurate predictions for their distribution and magnitude remains elusive. In the Paradox Basin, Utah, these structures are exposed onshore adjacent to salt diapirs, providing an ideal natural laboratory. Here, we use grain size and porosity analysis of deformation bands (DB’s) to quantify the stress conditions that may have produced these features in Jurassic sandstones in the Paradox Basin. Within deformation bands, porosities ranged from 1-10 % whereas preserved (uncemented) porosity ranges from 12-23 % in undeformed host rock. In the host rock, average grain diameters were ~0.23 mm. Using a modified critical state approach, calculations from these analyses indicate grain crushing strength (P*) values of 136-583 MPa in the Jurassic sandstone units. These P* values define the hydrostatic intercept of the critical-state yield envelope and thus allow us to constrain the stress conditions required to produce these features. These yield envelopes can be integrated with studies of basin modeling, cementation, pore pressure, and burial compaction to predict the stress conditions necessary for band formation and the consequent porosity and permeability reduction in potential reservoirs.