Subsurface Reinterpretation of Ordovician and Devonian Strata in Southwest Wyoming with Implications for Upwarping Across the Transcontinental Arch

A new interpretation of the subsurface geometries of the Ordovician Bighorn Dolomite and overlying Devonian strata across southwestern Wyoming arises from revising the stratigraphy in a core from the Mountain Fuel Supply UPRR #11–19–104–4 well drilled on the crest of the Rock Springs Uplift in 1962. One of only a few wells to penetrate all or part of the Lower Paleozoic succession in the subsurface of southwestern Wyoming, the well was almost continuously cored through the Devonian–Cambrian succession. From a reinterpretation of the stratigraphy in the core, 22 ft of Bighorn Dolomite is recognized based on the characteristic Thalassinoides bioturbation fabric in skeletal dolowackestone typical of Late Ordovician subtidal carbonate facies ranging from Nevada to Greenland along the western margin of the Great American Carbonate Bank. This lithology is in complete contrast with the alternating dolomitic flat-pebble conglomerate and dolomudstone of the underlying Cambrian Gallatin Limestone and the cyclical units of brecciated anhydritic dolomudstone and quartzose sandstone of the overlying Devonian Lower Member of the Jefferson Formation. Stratigraphic re-interpretation yields insights regarding Ordovician–Devonian stratal geometries across southwestern Wyoming. More widespread than previously portrayed, the Bighorn Dolomite pinches out on the eastern flank of the Rock Springs Uplift. Similar to past interpretations, Devonian strata pinch out east of the Rock Springs Uplift at Table Rock Field. A true-geometry multi-datumed stratigraphic cross section yields insights not obtainable by mapping. Regionally, top truncation of stratigraphic units below the base-Madison Limestone unconformity normally progresses stratigraphically deeper eastward. However, in southwestern Wyoming, the Devonian Lower Member of the Jefferson Formation overlaps the older Bighorn Dolomite by marked onlap across the Rock Springs Uplift and then pinches out by top truncation/onlap near Table Rock Field, forming an “abnormal” overlap relationship along the northern margin of the Transcontinental Arch. The underlying Bighorn Dolomite shows little to no onlap onto the underlying Cambrian section, but is markedly top truncated below the Lower Member of the Jefferson Formation. Comparing proportions of onlap versus top truncation for the two formations constrains the timing of two successive upwarping episodes along the northern margin of the Transcontinental Arch across southwestern Wyoming. The first is arguably Middle Devonian, and the second spans the Devonian–Mississippian boundary. Two subtle and different angular unconformities created by these two episodes imply a persistent fold or tilt axis that sequentially was reactivated along the northern margin of the Transcontinental Arch in southwestern Wyoming.

Azimuthal anisotropy analysis of P-wave seismic data and estimation of the orientation of the in situ stress fields: An example from the Rock-Springs uplift, Wyoming, USA

Estimating the orientation and magnitude of maximum and minimum horizontal in situ stress is important for characterizing naturally fractured, unconventional, and carbon-sequestered reservoirs. For naturally fractured reservoirs, they are needed to guide directional drilling; for unconventional reservoirs, they are used for optimal placements of hydraulic fractures; and for carbon-sequestered reservoirs, they are used to avoid fracturing of overlying seal rocks. In addition, a knowledge of stress fields can be used to induce fractures within the target reservoirs and enhance additional storage for carbon-sequestration experiments. The orientation and magnitude of in situ stress can be calculated at the well locations. For locations, away from the wells, analysis of the azimuthal dependence of the amplitude-variation-with-angle gradient or azimuthal angle stacks are used to quantify anisotropy, which are then related with well data and other geologic information for stress estimation. Such azimuthal analysis requires accurate conversion of offset-domain seismic data into angles. We use isotropic prestack waveform inversion for an accurate offset-to-angle transformation along different source-to-receiver azimuths followed by azimuthal analysis. Applying our method to the real seismic data from the Rock-Springs uplift, Wyoming, USA, and relating the results to the well data, we find that our results are favorably related to the orientation of the maximum in situ horizontal stress field measured at the well location.

Prestack waveform inversion of three-dimensional seismic data — An example from the Rock Springs Uplift, Wyoming, USA

Applying seismic inversion to estimate subsurface elastic earth properties for reservoir characterization is a challenge in exploration seismology. In recent years, waveform-based seismic inversions have gained popularity, but due to high computational costs, their applications are limited, and amplitude-variation-with-offset/angle inversion is still the current state-of-the-art. We have developed a genetic-algorithm-based prestack seismic waveform inversion methodology. By parallelizing at multiple levels and assuming a locally 1D structure such that forward computation of wave equation synthetics is computationally efficient, this method is capable of inverting 3D prestack seismic data on parallel computers. Applying this inversion to a real prestack seismic data volume from the Rock Springs Uplift (RSU) located in Wyoming, USA, we determined that our method is capable of inverting the data in a reasonable runtime and producing much higher quality results than amplitude-variation-with-offset/angle inversion. Because the primary purpose for seismic data acquisition at the RSU was to characterize the subsurface for potential targets for carbon dioxide sequestration, we also identified and analyzed some potential primary and secondary storage formations and their associated sealing lithologies from our inversion results.