Mancos Shale Research Articles

Effect of water imbibition on hydration induced fracture and permeability of shale cores

During a hydraulic fracturing operation with water-based fracturing fluids, in-situ compressive stress was applied on shale. To study whether or not hydration induced fracture can be created to improve oil and gas recovery under isotropic compressive stress conditions, time-elapsed computerized tomography (CT) was used to obtain cross section images of shale cores with confining pressures loaded. Based on CT data, cut faces parallel to the core axial through the middle of core and 3D fracture images were reconstructed. To study the effects of hydration on shale pore fluid flowing under isotropic compressive stress conditions, shale permeability was measured with Nitrogen (N2), distilled water, 4% KCl solution, and 8% KCl solution. Hydration induced fractures came into being for Mancos shale with low confining pressure (15 psi) loaded. However, with high confining pressure (3000 psi) loaded, fractures tended to close eventually due to hydration. For the Eagleford shale, with either a low or a high confining pressure loaded, fractures eventually became nearly closed. In the Mancos shale which contained more swelling clay minerals, larger reduction of fracture apertures or permeability was observed than that in the Eagleford shale with confining pressure applied. Adding KCl into the water-based fracturing fluid during fracturing could decrease shale hydration and reduce shale permeability damage. Our study shows that 8% KCl solution could help reduce the permeability damage of Mancos shale, and 4% KCl solution was feasible to reduce the permeability damage of the Eagleford shale under isotropic compressive stress conditions.

A depositional model for offshore deposits of the lower Blue Gate Member, Mancos Shale, Uinta Basin, Utah, USA

AbstractDepositional models that use heterogeneity in mud‐dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. Six cored intervals of the main body of the Mancos Shale, the lower Blue Gate Member, Uinta Basin, were examined sedimentologically, stratigraphically and geochemically in order to evaluate facies heterogeneity and depositional mechanisms. Unique sedimentological and geochemical features are used to identify three offshore environments of deposition: the prodelta, the mudbelt and the sediment‐starved shelf. Prodelta deposits consist of interlaminated siltstone and sandstone and exhibit variable and stressed trace fossil assemblages, and indicators of high sedimentation rates. The prodelta was dominated by river‐fed hyperpycnal flow. Mudbelt deposits consist of interlaminated siltstone and sandstone and are characterized by higher bioturbation indices and more diverse trace fossil assemblages. Ripples, scours, truncations and normally graded laminations are abundant in prodelta and mudbelt deposits indicating dynamic current conditions. Mudbelt sediment dispersal was achieved by both combined flow above storm wave base and current‐enhanced and wave‐enhanced sediment gravity flows below storm wave base. Sediment‐starved shelf deposits are dominantly siltstone to claystone with the highest calcite and organic content. Bioturbation is limited to absent. Sediment‐starved shelf deposits were the result of a combination of shelfal currents and hypopycnal settling of sediment. Despite representing the smallest volume, sediment‐starved shelf deposits are the most prospective for shale hydrocarbon resource development, due to elevated organic and carbonate content. Sediment‐starved shelf deposits are found in either retrogradational to aggradational parasequence sets or early distal aggradational to progradational parasequence sets, bounding the maximum flooding surface. An improved framework classification of offshore mudstone depositional processes based on diagnostic sedimentary criteria advances our predictive ability in complex and dynamic mud‐dominated environments and informs resource prospectivity.

A large pterodactyloid pterosaur from the Late Cretaceous Ferron Sandstone of Utah

Abstract Three associated incomplete wing bones of a large pterodactyloid pterosaur from the Ferron Sandstone Member of the Mancos Shale Formation of Utah are described. Based on the morphology of the bones’ articular ends, the specimen is referred to the Pteranodontoidea. Associated ammonites indicate that the specimen's age is Middle Turonian, so it adds to the sparse worldwide record of Turonian pterosaurs. The record of pteranodontoid pterosaurs in the Western Interior Seaway of North America is reviewed, and it is suggested that pteranodontoids were not diverse.

Study of the propagation of hydration-induced fractures in mancos shale using computerized tomography

The simulation study of the effect of clay swelling on fracture generation and porosity change in shale rocks is of great significance for applying hydraulic fracturing in the development of shale reservoirs in the petroleum industry. However, previous simulations did not consider the heterogeneous characteristics of shales. Besides, it is not clear how the swelling stresses are distributed owing to swelling inside shales. In this paper, we employ a discrete element method (DEM) modeling, which is combined with the CT images and MATLAB coding, to predict the effect of clay swelling on fracture generation and porosity change in shales under stress anisotropy. We are able to successfully simulate the effect of clay swelling on fracture generation and porosity change in heterogeneous shales under stress anisotropy by employing the discrete element method (DEM) modeling. Clay content is the key factor in fracture generation and the resulted porosity change. For shales with high clay content, clay swelling has contributions to the recovery of shale's porosity. The von Mises stress maps show that the stress inside the model is highly anisotropic and the maximum effective stresses concentrate on the fractures' areas. The larger the pressure is applied to the sample, the larger the von Mises stress can be observed. Stress anisotropy is a positive factor in fracture generation in the process of water-shale interaction. The degree of the clay swelling and the heterogeneity of samples are two factors that could influence the results.

Mecaster batnensis (Coquand, 1862), a late Cenomanian echinoid from New Mexico, with a compilation of Late Cretaceous echinoid records in the Western Interior of the United States and Canada

Abstract Echinoids are rare in the Upper Cretaceous of the Western Interior, where fewer than 60 unique occurrences are known to date, most of these represented by only a few tests or isolated spines. A notable exception is the Carthage coal field (Socorro County, New Mexico), where more than 200 specimens of Mecaster batnensis, previously referred to as Hemiaster jacksoni Maury, 1925, have been collected from the basal Bridge Creek Limestone Beds of the Tokay Tongue of the Mancos Shale. Prolific occurrences from the same beds are known from elsewhere in west-central and southwest New Mexico. Recorded originally from the Upper Cretaceous of Algeria, M. batnensis is a small- to medium-sized, irregular echinoid that is confined to the upper Cenomanian Euomphaloceras septemseriatum Zone in New Mexico. Measurements on 169 well-preserved specimens from two localities in New Mexico document a species that is, on average, 21.0 mm long, 19.8 mm wide, and 15.1 mm tall, yielding a width/length ratio of 0.94 and a height/length ratio of 0.72. Graphs plotting width against length and height against length are strongly linear. The Western Interior echinoid record spans the entire Late Cretaceous, although there are no records from rocks of Santonian age. Localities are spread from New Mexico on the south to Alberta on the north. Preservation ranges from coarse internal molds in high-energy sandstones to original tests in low-energy limestones.

Plan-view Paleochannel Reconstruction of Amalgamated Meander Belts, Cretaceous Ferron Sandstone, Notom Delta, South-central Utah, U.s.a.

Abstract Few studies reconstruct paleohydraulic parameters, such as meander amplitude and sinuosity, of ancient rivers based on plan-view outcrop exposures, but rather rely on interpolation from cliff exposures. In this study, extensive plan-view exposures of amalgamated ancient meander belts in the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, Utah, USA, allow paleohydraulic reconstruction, evaluation of grain-size variability in meander belts, and their component unit and compound bars by incorporating the variability of paleocurrents, facies architectural-element analysis, and bar migration patterns. Three channel belts were identified, marked by scour surfaces, an increase in grain size, and abrupt changes in paleocurrent orientations. The youngest channel was about 2.1 m deep and 118 m wide, and had a meander wavelength of 1 km and a sinuosity of 1.1–2.9. The middle channel was about 3.4 m deep, with a meander wavelength of 2.6 km, and a sinuosity of 1.2. The oldest channel was insufficiently exposed to document its plan-view style. Grain-size trends showed systematic upwards fining in each channel story. In plan view, coarsening occurs towards the bend apex along the bend axis, and fining occurs downstream within some meander scrolls. Plan-form grain-size trends were not obvious, partially due to grain-size variation within unit bars, perhaps also reflecting the fact that the exposures capture variable vertical position in each channel belt. Paleocurrents showed systematically varying trends within individual channel belts (from NE, to SW, to E), and abrupt changes between belts. Grain size and vertical facies associations vary as a function of the style of bar migration, as well as position within a bar (upstream vs. downstream). The outcrop shows a dominance of a meandering-river style, with significant lateral amalgamation of successive point bars within each belt. Compound braid bars, built by overlapping unit bars, constitute the youngest channel deposits, and are probably associated with channel abandonment. Independent measurements of meander wavelength, based on plan-view exposures, match results estimated from empirical equations. The Ferron rivers are small to medium in scale according to calculated paleohydraulic parameters (Qw = 1.6 ∼ 2.8 × 102 m3/s). Architectural-element analysis shows that the last stage, and the most sinuous channel belt (A1), filled largely with downstream accreting unit and compound braid bars, similar to those seen in high-sinuosity meander loops of the modern Red River at the Texas–Oklahoma border. This suggests that both braid bars and point bars may occur within a single meander channel and that braid bars may evolve into compound point bars as they migrate and attach to a channel margin.

Effect of temperature on the fracture toughness of anisotropic shale and other rocks

Abstract Fracture toughness was measured for a range of rock materials as a function of temperature between ambient temperature and 150°C. Measurements were made along all three principal crack orientations for the transversely isotropic Mancos shale and in single orientations for the more isotropic Darley Dale sandstone, Indiana limestone and Lanhelin granite. Fracture toughness was measured using a modified short-rod method with the sample and loading equipment enclosed within an elevated temperature chamber. A slight increase in K Ic was observed in Lanhelin granite with increasing temperatures up to 54°C, before a steady decrease at higher temperatures. For the sandstone and limestone, little change was observed in K Ic over the studied temperature range. In measurements on Mancos shale at elevated temperatures. Fracture toughness was seen to increase slightly with increasing temperature in the arrester orientation over this range, while remaining constant in the other two orientations. These observations can be explained in terms of the development of thermally induced microfractures parallel to the bedding planes in this material. A bimodal distribution of K Ic values in the short-transverse orientation was not observed, as it has been for previously published measurements at ambient conditions.

Provenance Signals In the Piceance Creek Basin: Unroofing of the Sawatch Range and Extent of the Early Paleogene California River System (Colorado, U.S.A.)

Abstract: New and compiled U-Pb detrital zircon ages (n = 495) and sandstone petrographic compositions (n = 45) indicate a largely similar provenance throughout deposition of the Paleocene and lowermost Eocene Wasatch Formation in the Piceance Creek basin of northwest Colorado, U.S.A. Age spectra are dominated by Cretaceous- and Precambrian-age zircons from fluvial sand bodies composed of compositional litharenites and feldspathic litharenites. Our comparison with age spectra from other geologic formations exposed in surrounding Laramide uplifts indicates the Wasatch Formation is most similar to Upper Cretaceous fluvio-deltaic and marine strata of the Mesaverde Group, Mancos Shale, and their equivalents. Age spectra and paleodrainage directions from the lower Paleogene Wasatch and underlying Ohio Creek formations suggests recycling of these strata off the Sawatch and Uncompahgre uplifts from south and southeast of the basin. The data are inconsistent with previous hypotheses that Yavapai–Mazatzal crystalline basement in the Sawatch Range and Mesozoic eolianites in the Uncompahgre uplift contributed significant sediment to the basin. Based on these provenance insights and estimates of maximum depositional ages from detrital-zircon ages and biostratigraphy it appears that exposure and erosion of sedimentary strata of the Sawatch Range began between 65 and 60 Ma. This timing is significantly younger than the previously hypothesized ∼ 72 Ma unroofing of the crystalline basement in the uplift. Furthermore, we find no provenance evidence that sediment was transported across the Douglas Creek arch into the Piceance Creek basin from the Uinta basin. Thus, we infer this subdued Laramide structure was sufficient to block sediment dispersal from the paleo–California River system, which contemporaneously deposited ∼ 3000 km 3 of sediment, transported from the magmatic arc, in the Uinta basin.

Experimental investigation of the effect of vegetation on soil, sediment erosion, and salt transport processes in the Upper Colorado River Basin Mancos Shale formation, Price, Utah, USA

Because of concerns about salinity in the Colorado River, this study focused on saline and sodic soils associated with the Mancos Shale formation with the objective of investigating mechanisms driving sediment yield and salinity loads and the role of vegetation in altering soil chemistry in the Price-San Rafael River Basin. Rainfall simulations using a Walnut Gulch rainfall simulator were performed at two study sites (Ferron and Price, Utah) across a range of slope angles and rainfall intensities to evaluate the relationship between sediment yield, salinity transport processes, and rainfall-induced changes in soil chemistry. Soil at Ferron had substantially greater salinity than Price as expressed in evaluated sodium absorption ratio, cation exchange capacity in soil, sediment, and total dissolved solids (TDS) in runoff. Principal component analysis and t-tests revealed that the two sites have different runoff and soil chemistry ions. Greater concentrations of K+, NO3−, and Cl− were present in soil-under-vegetation microsites compared to interspace soil areas. Soil soluble phase ions generally increased with depth and underwent vertical fluxes at rates proportional to rainfall intensity. Vegetation appears to have a protective effect on the soils from increasing rainfall intensity. Mat-forming saltbush found at Ferron was related most strongly to soil protection. The dissolution of sediment particles in runoff may be a key component of salinity transport processes on the Mancos Shale. Plot-averaged sediment and TDS had a positive linear relationship. The Rangeland Hydrology and Erosion Model successfully predicted TDS in runoff derived from these upland rangelands in central Utah.

Experimental Investigation on Brazilian Tensile Strength of Organic-Rich Gas Shale

SummaryTensile strength is a critical parameter for hydraulic fracturing, predicting fracture initiation and propagation in reservoirs, especially in shale reservoirs with complex natural fractures and fissures. The tensile strength of conventional rocks, such as sandstone and limestone, has been well-studied and -documented. There are many studies of the tensile strength of laminated shale, which focus on scale effects, loading direction, and temperature effects; however, the studies on effects of mineralogy and the water content on tensile strength of organic-rich shales are very limited.The objectives of this paper are to (1) critically review the key parameters that affect the tensile strength of shale and 2) experimentally examine the effects of water content, mineralogy, and lamination on tensile strength. To do so, a rigorous workflow is followed: 1) Each 1-in.-long shale sample is cut into two subsamples, A and B, of similar length; (2) X-ray computed-tomography (CT) scan is performed to diagnose pre-existing cracks and defects inside the core plugs; (3) nuclear magnetic resonance (NMR) is then used to measure the air dry samples’ water content; (4) Sample A is placed on a load frame to measure the tensile strength; (5) Sample B is vacuumed and then saturated; (6) NMR is used to measure the water content after saturation; (7) tensile strength of the saturated Sample B is measured; and (8) after the sample fails, the pieces are used for X-ray diffraction (XRD), scanning electron microscopy (SEM), and pyrolysis to estimate the mineralogy and total organic content (TOC).A total of 70 Mancos and 48 Eagle Ford shale samples have been tested. The experimental results show that (1) bedding plane/lamination has a significant effect on Eagle Ford tensile strength, but no pronounced impact is observed for the Mancos shale; (2) the imbibed water significantly reduces the tensile strength by 4.4 to 51.7% as water content increases from 4.45 to 11.7%; (3) pre-existing detectable microfractures can significantly reduce the tensile strength by up to 66%; (4) Eagle Ford exhibits typical brittle hard-rock failure configuration, with primary fractures and secondary fractures being observed, whereas for the Mancos shale, only primary fractures are observed; (5) acoustic velocity-test results confirm that Eagle Ford is mechanically transversely isotropic, and Mancos is likely mechanically isotropic.

Upon Further Consideration, US Gas Reserves Look Bigger

A second look at the size of US shale formations is revealing they hold far more natural gas, and pushed a new name up near the top of the list: the Mancos Shale. A recent reassessment of the formation in western Colorado concluded it holds 66 Tcf of shale gas that could be produced using current technology, making it second only to the prolific Marcellus Formation for unconventional gas in the US. This elevates the profile of the formation, which the US Geological Survey (USGS) had previously estimated at 1.6 Tcf in 2003. The agency also recently upped its estimate for the Barnett Shale, doubling it to 53 Tcf. “We reassessed the Mancos Shale in the Piceance Basin as part of a broader effort to reassess priority onshore US continuous oil and gas accumulations,” said Sarah Hawkins, a USGS geologist who was the lead author of the study. “In the last decade, new drilling in the Mancos Shale provided additional geologic data and required a revision of our previous assessment of technically recoverable, undiscovered oil and gas.” Based on a growing body of data, the USGS is working on assessments of two Texas plays, the Wolfcamp in the Midland Basin, and the Eagle Ford Shale; and one in Colorado, the Niobrara in the Denver Basin. For those drilling into the Mancos, the USGS evaluation confirms what they have been seeing. “There has been drilling for the last 20 years and there is decades worth of drilling location left,” said David Ludlam, executive director of the West Slope Colorado Oil and Gas Association, who said the gas potential may well exceed 100 Tcf. He said operators working there have dozens of successful exploratory wells in shale, but the current gas market does not justify development drilling. The gas sands there are commonly developed using slanted vertical wells from pad sites with multiple wells. The USGS used data from 2,000 wells drilled since its last evaluation, as well as field work and research core analysis, to divide the formation into units with similar geologic and well production characteristics, Hawkins said. That work led to an estimate of technically recoverable reserves for the Mancos ranging from 34 Tcf to 112 Tcf, with a mean of 66 Tcf. Glowing assessments of the gas in the ground are not fueling the sort of activity that transformed the Marcellus. Recently there were no rigs drilling in the Barnett, and 12 of the 16 wells working in Colorado were in the DJ Basin in the eastern part of the state, according to data by Energent Group, a provider of shale data.

Formation of point bars through rising and falling flood stages: Evidence from bar morphology, sediment transport and bed shear stress

ABSTRACTFlow processes and sediment transport in a channel bend and associated point bar have been studied in modern rivers, theoretical models and physical experiments: however, the relationship between flow process and point‐bar morphology has rarely been explained due to the complex nature of open channel flow. Plan‐view exposures of an ancient point‐bar complex, exposed at the top of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, south‐central Utah, allowed reconstruction of bar morphology, sediment transport and bed shear stress, which were used to extrapolate flow processes. Studies of these outcrops show that compound point bars and scroll bars were probably formed during falling and rising flood stages, respectively. A simulation model of plan‐view channel form shows that channel dimensions, such as radius of curvature and sinuosity of the point‐bar complex, range between 205 m and 351 m and 1·04 and 1·22, respectively, throughout the evolution of the channel bend. Variations in strength of the helical flow were interpreted as the main control on facies architecture and bar morphology. Strong helical flow was related to the deposition of the scroll bars, while strength of helical flow is decreased for compound bars. The use of cross‐beds as a common palaeocurrent indicator was found to be inconsistent with mean flow directions and channel margin orientation.

Vegetation canopy cover effects on sediment erosion processes in the Upper Colorado River Basin Mancos Shale formation, Price, Utah, USA

This study investigated erosion processes on the highly erosive, saline soils of the Mancos Shale formation in the Price-San Rafael River Basin in Utah, USA. Rainfall simulations were performed at two sites using a Walnut Gulch rainfall simulator with a variety of slope angles and rainfall intensities. The Rangeland Hydrology Erosion Model (RHEM) was calibrated to provide unbiased estimates of discharge and sediment load in runoff at each site. RHEM simulated the inter-plot variability best at the site with higher slope angles, vegetation cover, and sediment loads. The calibrated surface erosion parameters in RHEM (Kss, Kω) were substantially greater than any published in prior studies from non-saline environments. The spatial distribution of vegetation canopy cover was quantified using photogrammetric modeling and landscape pattern metrics. As the patches of vegetation became more contiguous and the tortuosity of the bare soil area increased, RHEM over-predicted sediment output, suggesting that vegetation-driven spatial heterogeneity influenced erosion in a way that is not captured by the model.

A new laboratory apparatus for the measurement of seismic dispersion under deviatoric stress conditions

ABSTRACTA better understanding of seismic dispersion and attenuation of acoustic waves in rocks is important for quantitative interpretation of seismic data, as well as for relating seismic data, sonic‐log data, and ultrasonic laboratory data. In the present work, a new laboratory setup is described, allowing for combined measurements of quasistatic deformations of rocks under triaxial stress, ultrasonic velocities, and dynamic elastic stiffness (Young's modulus and Poisson's ratio) at seismic frequencies. The setup has been used mainly for the study of shales. For such rocks, it is crucial that the saturation of the samples is preserved, which requires fast sample mounting. The design of our setup, together with a technique that was developed for rapid mounting of strain gauges onto the sample and subsequent sealing of the sample, allows for sample preservation, which is of particular importance for shales. The performance of the new experimental setup and sample mounting procedure is demonstrated with test materials (aluminium and polyetheretherketone) and two different shale types (Mancos shale and Pierre shale). Furthermore, experimental results are presented that demonstrate the capability of measuring the impact of saturation, stress, and stress path on seismic dispersion. For the tests with Mancos shale and Pierre shale, large dispersion (up to 50% in Young's modulus normal to bedding) was observed. Increased water saturation of Mancos shale results in strong softening of the rock at seismic frequencies, whereas hardening is observed at ultrasonic frequencies due to an increase in dispersion, counteracting the rock softening. The Poisson's ratio of Mancos shale strongly increases with the level of saturation but appears to be nearly frequency independent. We have found that the different types of shale exhibit different stress sensitivities during hydrostatic loading and that the stress sensitivity is different at seismic and ultrasonic frequencies.

Controls on selenium distribution and mobilization in an irrigated shallow groundwater system underlain by Mancos Shale, Uncompahgre River Basin, Colorado, USA

Elevated selenium (Se) concentrations in surface water and groundwater have become a concern in areas of the Western United States due to the deleterious effects of Se on aquatic ecosystems. Elevated Se concentrations are most prevalent in irrigated alluvial valleys underlain by Se-bearing marine shales where Se can be leached from geologic materials into the shallow groundwater and surface water systems. This study presents groundwater chemistry and solid-phase geochemical data from the Uncompahgre River Basin in Western Colorado, an irrigated alluvial landscape underlain by Se-rich Cretaceous marine shale. We analyzed Se species, major and trace elements, and stable nitrogen and oxygen isotopes of nitrate in groundwater and aquifer sediments to examine processes governing selenium release and transport in the shallow groundwater system. Groundwater Se concentrations ranged from below detection limit (<0.5μgL−1) to 4070μgL−1, and primarily are controlled by high groundwater nitrate concentrations that maintain oxidizing conditions in the aquifer despite low dissolved oxygen concentrations. High nitrate concentrations in non-irrigated soils and nitrate isotopes indicate nitrate is largely derived from natural sources in the Mancos Shale and alluvial material. Thus, in contrast to areas that receive substantial NO3 inputs through inorganic fertilizer application, Se mitigation efforts that involve limiting NO3 application might have little impact on groundwater Se concentrations in the study area. Soluble salts are the primary source of Se to the groundwater system in the study area at-present, but they constitute a small percentage of the total Se content of core material. Sequential extraction results indicate insoluble Se is likely composed of reduced Se in recalcitrant organic matter or discrete selenide phases. Oxidation of reduced Se species that constitute the majority of the Se pool in the study area could be a potential source of Se in the future as soluble salts are progressively depleted.

New Approach Tests Cement-Sheath Integrity During Thermal Cycling

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 173871, “Cement-Sheath Integrity During Thermal Cycling: A Novel Approach for Experimental Tests of Cement Systems,” by J. De Andrade, SPE, and S. Sangesland, SPE, Norwegian University of Science and Technology, and J. Todorovic, SPE, and T. Vrålstad, SPE, SINTEF Petroleum Research, prepared for the 2015 SPE Bergen One Day Seminar, Bergen, Norway, 22 April. The paper has not been peer reviewed. Well cement is placed into the annulus between casing and formation to provide structural support and zonal isolation throughout the well life cycle. Nevertheless, operators in the North Sea have been concerned about the ability of the cement sheath to maintain sealing integrity because of the increasing number of reported failures in mature wells. A new laboratory setup is designed to allow visualization of the development of possible leak paths throughout the cement sheath when exposed to pressure-and temperature-related varying loads. Experimental Setup The new laboratory setup presented in this work allows pressurization while maintaining the capability to perform a detailed study of the creation and propagation of cement-sheath failures upon thermal cyclic loads. The cell is shown in Fig. 1. The applied formation materials were Saltwash North sandstone and Mancos shale. Some of the current features of the cell are specified as follows: Cell is X-ray transparent. Temperature is controlled (from inside the casing) during cement setting and cyclic tests. There is independent control of pressure inside casing on cement and confining pressure (around the rock) during cement setting and cyclic tests. Maximum pressure is set at approximately 35 bar, while the allowable temperature range is set between −1°C and 150°C. All components of the cell can be reused for several test samples. In particular, the X-ray transparency is important because it allows for visualization of leak paths without the need to release pressure from the cell, or remove the internal test sample for a computed-tomography (CT) scan, or perform the traditional invasive method of cutting the test sample cross section. This was achieved by constructing the cell from aluminum, which has suitable radiolucent characteristics and allows the radiant energy to pass toward the test sample with little attenuation. The material also provides a lightweight cell to facilitate handling.

Plant Endemism on Mancos Shale Barrens

ABSTRACT: During the past decade, three previously undescribed vascular plant species have been documented growing on Mancos Shale barrens in southwest Colorado. Soil chemistry has been correlated with the presence of some rare endemics elsewhere, such as species growing on serpentine soils. However, research on shale barrens suggests that structural components, rather than unique chemistry, more strongly influence endemism on this substrate. The effect of the structural characteristics of the barrens soil was examined in connection with the presence of cushion bladderpod (Physaria pulvinata), a rare barrens endemic. The results suggest that P. pulvinata may be uniquely suited to the shallow soil and low moisture characteristic of the shale barrens habitat. The need for long-term monitoring and management is discussed.

Fracture toughness anisotropy in shale

AbstractThe use of hydraulic fracturing to recover shale gas has focused attention on the fundamental fracture properties of gas‐bearing shales, but there remains a paucity of available experimental data on their mechanical and physical properties. Such shales are strongly anisotropic, so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in situ stress field in the shallow crust. Here we report fracture toughness measurements on Mancos shale determined in all three principal fracture orientations: Divider, Short Transverse, and Arrester, using a modified short‐rod methodology. Experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes. Significant anisotropy is observed in shale fracture toughness measurements at ambient conditions, with values, as high as 0.72 MPa m1/2 where the crack plane is normal to the bedding, and values as low as 0.21 MPa m1/2 where the crack plane is parallel to the bedding. For cracks propagating nonparallel to bedding, we observe a tendency for deviation toward the bedding‐parallel orientation. Applying a maximum energy release rate criterion, we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed‐mode loading conditions. We find for Mancos shale that the fracture should deviate toward the plane with lowest toughness regardless of the loading conditions.

Regional stratigraphy, elemental chemostratigraphy, and organic richness of the Niobrara Member of the Mancos Shale, Piceance Basin, Colorado

The elemental chemostratigraphy of the Upper Cretaceous Niobrara Member of the Mancos Shale shows that six chemostratigraphic zones can be identified in the Piceance Basin, Colorado, based on geochemical data. Chemostratigraphic correlations of nine wells spaced 20 mi (∼32 km) apart closely match lithostratigraphic correlations made using gamma-ray and deep-resistivity wireline logs. Lithologic interpretations made from wireline logs indicate that the Niobrara Member and equivalent strata consist primarily of interbedded calcareous shale and shaley limestone facies that increase in thickness to the northwest in the basin. The geochemical data suggest that during deposition of the Niobrara Member, anoxia and calcium enrichment increased to the east of the basin, whereas terrestrial input and clay enrichment increased to the northwest. Element crossplots suggest that a large part of the silicon is detrital and that the Niobrara Member becomes an increasingly more clastic than carbonate system to the west and northwest. The Δlog R–derived total organic carbon (TOC) calculated using a sonic-resistivity overlay analysis technique shows that the Niobrara Member comprises organic-rich and organic-poor deposits. Average TOC values range between 1 wt. % (in organic-poor deposits) and 2.37 wt. % (in organic-rich deposits), with higher TOC values recorded in the southern and eastern parts of the basin. Relative-rock brittleness estimates from element and TOC data show the stratigraphic variability of alternating ductile (TOC rich, Ca and Si/Al poor) and brittle (TOC poor, Ca and Si/Al rich) intervals for the Niobrara Member.

Sequence stratigraphy of the Mancos Shale, lower Tres Hermanos Formation, and coeval middle Cenomanian to middle Turonian strata, southern New Mexico, USA

AbstractSequence stratigraphic analysis of four widely spaced outcrops of middle Cenomanian to middle Turonian strata deposited in the Western Interior foreland basin in southern New Mexico, USA, defines ten sequence boundaries in a marine shale‐rich interval ca 200 m thick. The majority of sequence boundaries are based on basinward shifts in lithofacies characterized by either a non‐Waltherian contact between distal‐bar or lower shoreface sandstone and underlying lower offshore shale, or an erosional contact between distal‐bar or lower shoreface sandstone and underlying upper offshore shale. The sequence boundaries commonly correlate basinward to packages of storm‐deposited sandstone and to beds of sandy grainstone composed of winnowed inoceramid shell fragments. In several cases, however, the sequence boundaries pass basinward into presumably conformable successions of lower offshore shale. Maximum flooding surfaces within the sequences are represented by one or more beds of locally phosphatized globiginerid wackestone and packstone or exist within a conformable succession of lower offshore shale. Following initial south/south‐westward transgression into the study area, the regional trend of palaeoeshorelines was north‐west to south‐east, although isopach data indicate that lobes of sandstone periodically spread south‐eastward across the study area. The ten sequences in the study area are arranged into a third‐order composite megasequence that is characterized by overall upward‐deepening followed by upward‐shallowing of sequences. The composite megasequence is similar but not identical to the previously established T‐1 transgression and R‐1 regression in New Mexico. Based on radioisotopic dates of bentonites, the average frequency of the sequences within the study area was ca 327 kyr, which is consistent with fourth‐order cycles of ca 400 kyr interpreted in coeval marine strata elsewhere in the world.