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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.

Interpreting shale lithofacies is an important step in identifying productive zones in the Marcellus Shale gas play; the target reservoir can be less than 3 m (10 ft) thick with overlying shales that appear similar on certain petrophysical well logs and in the core. However, these nonreservoir-quality shales contain widely varying organic and mechanical properties. To distinguish between reservoir- and nonreservoir-quality shale facies, this classification method applies a pattern-recognition algorithm, expectation maximization, to a set of commonly available petrophysical and elastic well logs to create 1D shale facies models along the wellbore. The statistical algorithm classifies the petrophysical and elastic well-log data into defined facies using the theory that each well log contains a distribution of Gaussian curves for each facies. The method is applied to 12 wells across Pennsylvania and northern West Virginia, and it is able to discriminate between shale facies based on organic content and brittleness, characteristics which are not always evident in core. To verify the geologic accuracy of the facies models, the results are compared with the core data, well logs, mudlogs, and regional stratigraphic studies when available. In addition, we were able to designate a lithofacies-defined top to the Marcellus Formation in parts of the Appalachian Basin where the gradational contact with the overlying Mahantango Formation is poorly defined on petrophysical logs.

Log-facies classification at the well location allows determination of the number of facies, the facies definition, and the correlation between facies and rock properties along the well profile. In unconventional reservoirs, because of the necessity for hydraulic fracturing in shale gas and shale oil reservoirs, facies classification should account for petroelastic and geomechanical properties. We developed a facies classification methodology based on the expectation-maximization algorithm, a statistical method that allows finding the most likely facies classification and the associated probability distribution, given the set of geophysical measurements in the borehole. We applied the proposed workflow to a complete set of well logs from the Marcellus shale and developed the corresponding facies classification from log properties measured and computed in three different domains: petrophysics, rock physics, and geomechanics. In thne preliminary well-log and rock-physics analysis, we identify three main lithofacies: limestone, shale, and sandstone. The application of the classification method provided the vertical sequence of the three lithofacies and their pointwise probability of occurrence. A sensitivity analysis was finally evaluated to investigate the impact of the number of input variables on the classification and the effects of cementation and kerogen.

Pennsylvanian rocks of the northern U.S. Rocky Mountains are mature quartzose sandstones. This paper uses detrital zircon geochronology on seven samples from the Wood River Formation, Tensleep Sandstone, and Weber Sandstone to determine if these sandstones have a common provenance, representing eastern Laurentian and Appalachian sand reworked within shallow-marine and eolian environments from the Wyoming craton westward to the Pioneer thrust plate of south-central Idaho. Our work suggests that this continental sand blanket was mixed with local sources on the south in the Yavapai-Mazatzal provinces of the Ancestral Rocky Mountains and in samples from the western Cordilleran thrust belt in south-central Idaho. In total, these Pennsylvanian sandstones contain a broad spectrum of detrital zircon U-Pb ages including, from old to young: A) minor Archean-age (3300–2550 Ma) populations; B) Paleoproterozoic (2000–1600 Ma), Mesoproterozoic (1470–1350 Ma), and major “Grenvillian” (1250–950 Ma) populations; and C) Cryogenian- to Ediacaran-age (665–565 Ma) and minor Paleozoic (495–410 Ma) populations. We interpret these detrital zircon ages to represent provenance mainly from the Appalachian mountain belt of eastern North America; however, central Appalachian versus northern Appalachian derivation is not clearly distinguished. The Weber Sandstone from the north flank of the Uinta Mountains in northeast Utah contains a strong 1700–1640 Ma age population derived from the Yavapai-Mazatzal provinces in the adjacent Ancestral Rocky Mountains. The shallow-marine Hailey Member of the Wood River Formation in south-central Idaho yields a population of >1800 Ma detrital zircons reworked from the uplifted Mississippian Copper Basin Formation. Both the Hailey and Wilson Creek Members of the Wood River Formation contain unique 640–490 Ma grains that may represent provenance from the Big Creek-Beaverhead plutonic belt of east-central Idaho and/or eastern Klamath terrane in the Klamath Mountains of northwest California and southwest Oregon. These new data support published models for Pennsylvanian–Permian transport of siliciclastic sediment with sources mainly from the North American craton, north of the Ancestral Rocky Mountains, into continental margin basins.

The state of Wyoming, in the northwestern United States, produces 40% of the nation’s coal, most of which is transported out of the state. The remainder is used at power plants within Wyoming to generate approximately 7% of U.S. electricity. Carbon capture and storage from these power stations could significantly reduce U.S. carbon emissions. Wyoming statutes and rules proposed by the U.S. Environmental Protection Agency and Wyoming Department of Environmental Quality regarding subsurface carbon storage require that CO2 injection must not affect established or potential drinking water aquifers, oil and gas fields, or other mineral estates. Wyoming has several potential large-scale geologic carbon storage reservoirs that meet these criteria, in the form of saline aquifers in regional basins and uplifts. The Wyoming Carbon Underground Storage Project has recently been funded by the U.S. Department of Energy and the state of Wyoming to 1) assess the CO2 storage potential of two possible locations in southwestern Wyoming: The Moxa Arch and the Rock Springs Uplift, 2) develop a system for displaced fluid management, 3) plan monitoring and verification activities, and 4) design infrastructure in preparation for geologic carbon sequestration. The Wyoming Carbon Underground Storage Project represents collaboration between the University of Wyoming, the Wyoming State Geologic Survey, ExxonMobil Corporation, Los Alamos National Laboratory, and Baker Hughes Incorporated. The authors are involved primarily in the geologic stratigraphic and structural characterization of the Moxa Arch and Rock Springs Uplift. The Moxa Arch is an anticline that trends from the Uinta Mountains, 200 km north-northwest to the eastern front of the Wyoming fold-and-thrust belt. Potential storage reservoirs on this large geologic structure include the Jurassic Nugget Sandstone, the Mississippian Madison Limestone, and the Ordovician Bighorn Dolomite. The Nugget Sandstone is a heterogeneous and anisotropic eolian deposit that has been extensively exploited for oil and gas at certain locations on the Moxa Arch, which complicates its usage as a repository for carbon dioxide. The Madison Limestone is a proven storage reservoir; ExxonMobil has been injecting CO2 (up to 25 MMCFD) and H2S (up to 65 MMCFD) into the Madison Limestone on the Moxa Arch for seven years at the Shute Creek Gas Plant. The Bighorn Dolomite is stratigraphically complex with large variations in porosity and permeability due to primary burrowing and repeated dolomitization and dedolomitization. Depending on location on the anticline, the Nugget Sandstone lies 3 to 6.5 km below the surface (-1 to -4.5 km subsea), and the Bighorn Dolomite and Madison Limestone range from 4.5 to 8 km below the surface (-2.5 to -6 km subsea). The Moxa Arch is structurally uncomplicated; it is a basement-involved anticline formed by a west-vergent Late Cretaceous-age thrust fault, with gently-dipping limbs (0 to 5 degrees). Leakage risk is extremely low because impermeable evaporite (anhydrite) intervals overly the potential reservoirs, and preliminary interpretation of seismic data reveal that few faults exist other than the main thrust. The Naughton Power Plant, a 707 MW coal-fired power station emitting up to 6 Mt of CO2 per year, lies 30 km west of the crest of the anticline. The Rock Springs Uplift, 100 km east of the Moxa Arch, extends 80 km north from the Wyoming-Utah border. The target storage reservoirs are the Pennsylvanian Weber Sandstone (correlative to the Tensleep Sandstone) and the Mississippian Madison Limestone. The Weber Sandstone exhibits wide variations in reservoir properties (porosity and permeability) due to dune/interdune/intradune facies changes, and appears to have experienced local secondary diagenesis that further reduced porosity. The Madison Limestone is expected to have similar reservoir properties to its lithologic correlative on the Moxa Arch. The Weber Sandstone and Madison Limestone range in depth from 2 to 6 km below the surface (0.3 to -4.5 km subsea), depending on location on the anticline. The Rock Springs Uplift offers challenges for structural analysis. Like the Moxa Arch, the Uplift was formed by a Late Cretaceous-age west-vergent basement-involved reverse fault, but the limbs of the fold are at steeper dips (approximately 15 degrees on the west limb, shallower on the east limb), and these limbs are cut at depth by additional reverse faults. In a hydrocarbon field on the southeastern flank of the uplift there is a possibility that condensate is migrating from the Weber Sandstone along one of these reverse faults, suggesting that the trap is breached. In addition, a series of east-west trending normal faults cut Cretaceous shales at the surface, possibly with throws that exceed the thickness of the uppermost regional seal. It is necessary to determine if these east-west faults also compromise the Triassic units that could provide a seal above the Weber Sandstone. The Jim Bridger Power Plant (coal-fired) is located on the east flank of the Rock Springs Uplift; it has 2200 MW capacity and emits up to 18 Mt of CO2 per year. Future U.S. energy demands will draw heavily on Wyoming’s coal-fired power plants, and the state is taking steps to sequester the produced carbon. Wyoming hosts several large geologic traps that if properly risked and evaluated have promise as long-term, stable repositories for anthropogenic carbon dioxide. Based upon our preliminary assessment of the multiple clastic and carbonate receiving formations in the Moxa Arch and Rock Springs Uplift, and the experience of successful injection at ExxonMobil’s Shute Creek Gas Plant, these geologic structures in southwestern Wyoming are among the most promising large CO2 geologic storage sites in the United States.

The Moxa Arch in the Greater Green River Basin, southwestern Wyoming, hosts two potential reservoirs for CO2 sequestration. The Bighorn Dolomite and Madison Limestone are interpreted to be independent reservoirs based on differing CO2 compositions and production histories; the two reservoirs are separated by Devonian carbonates, siliciclastic rocks, and evaporites. On the Moxa Arch, the Bighorn ranges in thickness from 67 to 120 m with porosities from 3 to 15 percent. The massive buff-colored Steamboat Point Member comprises the bulk of the subsurface Bighorn in southwest Wyoming. Mottled dolostone (light-colored patches with higher porosity and dark-colored patches with lower porosity) is very common and is presumably the result of preferential early dolomitization of bioturbation. Core study suggests that this factor affects gas saturation and storage in this extensive reservoir. The Lower Member of the Jefferson Formation is the most probable seal within the Devonian stratigraphy. Strata of this member are interpreted to have been deposited in a shallow basin semi-isolated from the deeper marine environment to the west. 87Sr/86Sr isotopic analyses of anhydrite sampled from Moxa Arch well cuttings support the interpretation of a depositional environment exposed to a mixture of seawater and freshwater. High-frequency relative sea level fluctuations superimposed on a gently sloping shelf produced alternating layers of marine carbonates, peritidal siliciclastic rocks, and evaporites. The evaporites are interpreted to seal CO2 in the Bighorn Dolomite from the overlying Madison Limestone. The Lower Paleozoic strata on the Moxa Arch provide an effective trap-reservoir-seal combination for naturally occurring CO2 with potential applications to future studies at analogous locations in the central Rocky Mountain Region.

Summary Facies classification at the well location is generally based on sedimentological models, however, the extension of the classification of an individual well into the entire reservoir model is contingent on the calibration of a rock physics model that links rock and fluid properties with geophysical measurements such as seismic velocities and/or electromagnetic-derived resistivities. The goal of this work is to present a workflow to define a geologically consistent facies classification at the well location, to accurately reconstruct this classification using elastic and electrical properties, and to extend the classification to the 3D reservoir model. The initial classification at the well location is obtained using traditional statistical methods applied to computed rock properties such as mineralogical volumes, porosity, density and permeability. The facies reconstruction based on elastic/electrical properties is obtained using a Bayesian approach that combines rock physics with statistical models. The workflow is illustrated through the application to the Rock Springs Uplift field, Wyoming, which hosts several potential CO2 storage reservoirs.

The Colorado River Extensional Corridor (CREC) of southeastern California, southern Nevada, and western Arizona experienced up to 100% extension between ~23 and 12 Ma. Extension was accommodated by low-angle normal faulting in the upper and middle crust, subvertical brittle failure of the lower crust, emplacement of mantle-derived magma in a crustal-scale fracture, and, later, crustal flow. Evidence for brittle, whole-crustal failure includes geophysical observations, structural analysis, geochronology, thermochronology, geobarometry, and paleomagnetic studies of surface exposures of syntectonic, mantle-derived mafic plutonic sheets emplaced as subvertical, crustal-scale intrusions tracking the zone of maximum extension. Their subsurface extent and geometry are interpreted from the shape and magnitude of the Colorado River gravity high, a 10-20 mGal positive gravity anomaly that follows the belt of core complexes west of the Colorado River for ~150 km along strike. In the southern Sacramento Mountains (la...

Summary A new methodology for a log-based facies classification using a statistical algorithm, Expectation Maximization, is proposed to classify lithologic-facies utilizing commonly available wireline logs. This method was tested in the Marcellus Shale, an unconventional reservoir located in the Appalachian Basin of eastern North America. The method relies on Gaussian mixture models with the assumption that each facies has a unique Gaussian distribution of petroelastic properties. The technique was checked against mud logs, well log interpretations, and regional stratigraphy for accuracy and produced promising results.

The Colorado River extensional corridor (CREC) accommodated up to 100% crustal extension between ∼23 and 12 Ma. The southernmost Sacramento Mountains core complex lies within this region of extreme extension and exposes a footwall of Proterozoic, Mesozoic, and Miocene crystalline rocks as well as Miocene volcanic and sedimentary rocks in the hanging wall to the regionally developed Chemehuevi-Sacramento detachment fault (CSDF) system. New structural, U-Pb-zircon, Ar-Ar, and fission track geochronologic and paleomagnetic studies detail the episodic character of both magmatic and tectonic extension in this region. Extension in this part of the CREC was initiated with tectonic slip along a detachment fault system at a depth between 10 and 15 km. Magmatic extension at these crustal levels began at ∼20–19 Ma and directly account for 5–18 km of extension (10–20% of total extension) in the southern Sacramento Mountains. Three discrete magmatic episodes record rotation of the least principal stress direction, in the horizontal plane, from 55° to 15° over the following ∼3 Myr. The three intrusions bear brittle and semibrittle fabrics and show no crystal-plastic fabric development. The final 3–4 Myr of stretching were dominated by amagmatic or tectonic extension along a detachment fault system, with extension directions rotating back toward 75°. The data are consistent with extremely rapid cooling and uplift of Miocene footwall rocks; the ∼19 Ma Sacram suite was emplaced at a mean pressure of ∼3.0 kbars and uplifted rapidly to a level in the crust where brittle deformation was manifested by movement on the detachment fault at ∼16 Ma. By ∼14 Ma the footwall was exposed at the surface, with detritus shed off and deposited in adjacent hanging wall basins.

Summary Estimating the orientations and magnitudes of the maximum and minimum horizontal in-situ stress fields is important for characterizing unconventional and carbonsequestrated reservoirs. For unconventional reservoirs they are needed for optimal placement of hydraulic fractures and for the carbon sequestrated reservoirs they are needed to avoid fracturing of the overlying seal rocks during and after carbon dioxide (CO2) sequestration. The orientations and magnitudes of the in-situ stress fields can be calculated at well locations and to obtain them over a three-dimensional (3D) area, the well data are usually interpolated over the interpreted horizons of stacked 3D seismic data volumes. Such an approach may be adequate in mature exploration areas with sufficiently good well control, but could be misleading when well control is sparse. Because the in-situ stress fields induce anisotropy in observed seismic data, we analyzed the azimuthal reflection amplitude variations of real P-wave seismic data from the Rock-Springs uplift, Wyoming and calculated the directions of principal azimuthal anisotropy. By relating these calculations to wellbore breakout orientations interpreted from image logs we found that the principal anisotropy direction obtained from the seismic data correspond exactly with the direction of the maximum horizontal stress computed from the image log analyses. We conclude that azimuthally sorted P-wave seismic data once calibrated with the well data could potentially be used for predicting the in-situ stress fields over a 3D area using our method.

In this work, we present a rock physics modelling workflow for seismic reservoir characterization and monitoring feasibility analysis and its application to a carbon dioxide sequestration study in the Rock Springs Uplift, a potential storage site in Southwestern Wyoming. The study includes the calibration of a facies-dependent rock physics model using well log data and core samples in two potential reservoirs, and the application of the model to a 3D volume of seismic attributes through a Bayesian rock physics inversion method for the pre-injection reservoir characterization. The study also focuses on the derivation of a rock physics model to predict the changes in the seismic response due to saturation and pressure variations during injection for the reservoir monitoring feasibility study based on time-lapse seismic data.