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Penetrative Strain Related Porosity Loss in Clastic Reservoir Units of the Denver-Julesburg Basin


Penetrative strain (PS) represents the total percentage of shortening not accommodated by the formation of map-scale geologic structures. Studies of compressive settings in the Appalachians place PS estimates in the range of 2-23% shortening. At outcrop-scale, PS is accommodated through sub-seismic slip on brittle fault planes. At the microscale, PS is accommodated through grain impingement, pore-scale compaction, and grain rotation. This study considers the PS accommodated in the Pennsylvanian-Cretaceous sedimentary sequence exposed at the foothills of the Front Range, with emphasis on the clastic reservoir units that yield oil and gas within the Denver-Julesburg (D-J) Basin.

Field observations, petrographic analysis, magnetic susceptibility (AMS) data, and analog modeling are used to estimate the amount of PS in four E-W transects along the I-25 corridor. Intergranular deformation patterns were analyzed from 46 thin sections within the basin. Best fit ellipses were applied to compressed, rotated sand grains altered by low-temperature deformation. Compromised grain boundaries were reconstructed using the Onasch method. Preliminary results from sandstones of the Cretaceous Dakota Group show strain percentages on the order of 11-15%. This is consistent with AMS studies of Triassic red-beds of the Wyoming Salient. Similar AMS techniques are deployed in this study to measure weak, concealed deformation of ferromagnetic minerals from the iron-rich Fountain and Satanka Formations. We predict elevated PS will correspond with increasing depth in the sedimentary system, reaching a maximum with the basal Fountain Formation. Analog models of Laramide deformation scenarios will be used to provide a framework for understanding the field data.

Our study aims to provide a systematic and precise dataset, as well as determining the spatial and temporal component of volume loss. We predict results from this study will indicate that PS is highest near the base of the sedimentary section. The presence of overburden accumulated during deposition, as well as the absence of a basal shale or evaporite detachment, limits strain partitioning in the Fountain Formation. Therefore, PS is accommodated into strain fabrics which significantly reduce reservoir quality by constricting porosity through dislocation processes, such as creep and glide.