Entrada Sandstone Research Articles

New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA

Based on a methodic sedimentological analysis, the Late Jurassic (Oxfordian) Curtis Formation unravels the intricate facies variability which occurs in a tide-dominated, fluvially starved, low-gradient, semi-enclosed epicontinental basin. This unit crops out in east-central Utah, between the eolian deposits of the underlying Middle Jurassic (Callovian) Entrada Sandstone, from which it is separated by the J-3 unconformity, and the conformable overlying supratidal Summerville Formation of Oxfordian age. A high-resolution sedimentary analysis of the succession led to the recognition of eight facies associations (FA) with six sub-facies associa­tions. Based on the specific three-dimensional arrangement of these eight facies associations, it is proposed to separate the Curtis Formation into three sub-units: the lower, middle and upper Curtis. The J-3 unconformity defines the base of the lower Curtis, which consists of upper shoreface to beach deposits (FA 2), mud-domi­nated (FA 3a) and sand-dominated heterolithic subtidal flat (FA 3b), sand-rich sub- to supratidal flat (FA 4a) and correlative tidal channel infill (FA 4c). It is capped by the middle Curtis, which coincides with the sub- to intertidal channel-dune-flat complex of FA 5, and its lower boundary corresponds to a transgressive surface of regional extent, identified as the Major Transgressive Surface (MTS). This surface suggests a potential correla­tion between the middle and the upper Curtis and the neighboring Todilto Member of the Wanakah Forma­tion or Todilto Formation. The upper Curtis consists of the heterolithic upper sub- to intertidal flat (FA 6) and coastal dry eolian dunes belonging to the Moab Member of the Curtis Formation (FA 7), and it conformably overlies the middle Curtis.

Effect of CO2–brine–rock interaction on fracture mechanical properties of CO2 reservoirs and seals

The effect of CO2–water–rock interactions on fracture mechanical properties of reservoir and seal rocks due to injection of CO2 in the subsurface may impact long-term (102–104 yr) storage security. At the Crystal Geyser and Salt Wash field sites near Green River, Utah, sandstone of the Salt Wash Member of the Morrison Formation and Mancos Shale have undergone chemical alteration associated with flow of CO2-bearing water under natural conditions over geologic time scales of 103–105 yrs. To quantify the effects of these diagenetic changes on rock fracture mechanical properties we conducted opening-mode fracture mechanics tests using the double torsion method on a suite of CO2-altered and unaltered siliciclastic samples. We show that dissolution of hematite and carbonate cement in Entrada Sandstone by CO2-rich brine lowers fracture toughness by nearly 40% relative to adjacent, unaltered samples from the same unit. In contrast, precipitation of calcite pore cement in a sandstone of the Salt Wash Member of the Morrison Formation, attributed to CO2 degassing during upward fluid flow along the Little Grand Wash Fault, results in a 100–700% increase in fracture toughness. Similarly, precipitation of carbonate mineral cements and replacement of matrix measurably strengthens Mancos Shale, considered a regional top seal. Subcritical fracture growth index (SCI) which quantifies reaction-assisted subcritical fracture propagation is also affected by CO2-related alteration. Based on previous numerical simulations of fracture network growth with varying fracture mechanical parameters, we find that the measured variations in fracture mechanical properties qualitatively match observed differences in opening-mode fracture distributions along the fault, with short, non-connected fractures in CO2-bleached rock, long, throughgoing fractures in calcite-cemented shale, and short, closely spaced, interconnected fracture networks in highly altered rock directly adjacent to fault conduits. Our fracture mechanics tests demonstrate that fracture mechanical properties of reservoir and seal rock can change as a result of chemical CO2–brine–rock interaction. These chemical interactions may favor or inhibit subcritical fracture growth processes and thus reservoir and seal flow properties. In regions of CO2-related mineral dissolution, subcritical fracture growth may reduce seal integrity over time scales beyond those of active injection and reservoir monitoring, thus negatively affecting seal integrity over the expected design lifetime of a CO2 reservoir.

Reservoir rock chemo-mechanical alteration quantified by triaxial tests and implications to fracture reactivation

CO2 injection into geological formations shifts the geochemical equilibrium between the minerals and resident brine. The induced mineral-brine-CO2 reactions can alter the reservoir rock strength and deformational behavior, which subsequently affects CO2 storage mechanical integrity. This study attempts to investigate quantitatively the effect of mineral cement and particle dissolution through numerical modeling and validation of triaxial tests performed on unaltered and geologically altered Entrada Sandstone. We utilize a numerical model that couples the discrete element method (DEM) and the bonded-particle model (BPM) to perform simulations of triaxial tests on synthetic rocks that mimic tested rock samples under various confining stresses. Numerical results, in agreement with experimental evidence, show that both cement and particle dissolution significantly contribute to rock weakening in sandstone samples with carbonate/hematite cements and pore-filling carbonate. Sensitivity analyses show that the brittleness of DEM numerical samples is mostly conditioned by the cement bond size among all cement microscopic parameters. An alteration path that mimics the mineral dissolution under subsurface boundary conditions leads to (1) vertical compaction and horizontal stress relaxation in the reservoir rock and (2) transfer of stresses to adjacent strata that results in bond breakage and potentially natural fracture reactivation at a large scale.

CO2 charged brines changed rock strength and stiffness at Crystal Geyser, Utah: Implications for leaking subsurface CO2 storage reservoirs

CO2 geological storage in saline aquifers results in acidification of resident brine. Chemical reactions between acidified brine and rock minerals lead to dissolution and precipitation of minerals at various time scales. Mineral dissolution and precipitation are often neglected in assessing the mechanical integrity of target storage formations, yet, changes in rock strength and deformational behavior can impact trapping mechanisms. This study shows the impact of exposure to CO2-charged brine on shear strength and stiffness of various outcrop rocks evaluated through triaxial testing. The tested rocks were exposed to CO2-charged brine over geological time at a naturally occurring near-surface seepage along the Little Grand Wash Fault and Salt Wash Grabens, which include the Crystal Geyser site near the town of Green River, Utah. Prior work suggests that this site provides a near-surface structural analog for possible fault-controlled CO2 leakage over time scales that exceed expected injection time scales (10–100 years). Results show mechanical alteration in various aspects: (1) CO2-charged brine alteration at near-surface conditions results in mineral dissolution/precipitation and reduction of shear strength and brittleness of Entrada sandstone and Summerville siltstone samples, and (2) carbonate precipitation in fractured Mancos shale leads to matrix stiffening and fracture mineralization resulting in overall stiffer and likely tighter shale. Additional discrete element simulations coupled with a bonded-particle-model confirm the role of cement bond size alteration as one of the main controls for rock chemo-mechanical alteration in sandstones. The chemo-mechanical alteration path that mimics cement dissolution (under stressed subsurface conditions) results in vertical compaction and lateral stress relaxation. Overall, results show that rock exposure to CO2-charged brine can impart distinct petrophysical and geomechanical changes according to rock lithology and location with respect to major CO2 conduits. While mineral dissolution in the storage rock may result in undesired reservoir strains and changes of stresses, mineral precipitation downstream from a leakage path can help seal potentially induced fractures.

What is preserved in the aeolian rock record? A Jurassic Entrada Sandstone case study at the Utah–Arizona border

AbstractAeolian dune fields evolve from protodunes and small dunes into a pattern of progressively fewer, larger and more widely spaced dunes within limits defined by boundary conditions. However, the allogenic boundary conditions that promote aeolian dune‐field development, accumulation of strata and preservation of accumulated strata are not the same. Autogenic processes, such as dune interactions, scour‐depth variation along migrating dunes and substrate cannibalization by growing dunes, result in removal of the stratigraphic record. Moreover, dune‐field events may be collapsed into major erosional bounding surfaces. The question is what stages of evolving dune fields are represented in the rock record? This case study of ca 60 m of Jurassic Entrada Sandstone on the Utah/Arizona border (USA) defines stratigraphic intervals by gross architecture of bounding surfaces and sets of cross‐strata. The interpreted intervals in stratigraphic order consist of: (i) a lower sabkha bed that transitions upward into erosional remnants of small sets representing an initial wet aeolian system; (ii) large, compound cross‐strata representing a mature dune field; (iii) isolated scour‐fill representing negatively climbing dunes that produced ca 25 m of palaeo‐topographic relief; (iv) downlapping sets that fill the landscape‐scale relief; (v) four intervals of stacked climbing sets that each represent short periods of time; and (vi) an upper sabkha bed that again transitions into small sets representing a wet system. Accumulations appear to be associated with sediment pulses, a rising water table, and filling of scoured troughs and landscape‐scale depressions. Preservation of the accumulations is selective and associated with a rising water table, burial and subsidence. The preserved record appears remarkably incomplete. Speculation about missing strata gravitates towards cannibalization of the record of early dune‐field construction, and strata removed during the formation of bounding surfaces. This local Entrada record is thought to represent a point in the spectrum of preservation styles in the rock record.

CO2‐induced chemo‐mechanical alteration in reservoir rocks assessed via batch reaction experiments and scratch testing

AbstractThe injection of carbon dioxide (CO2) into geological formations results in a chemical re‐equilibration between the mineral assemblage and the pore fluid, with ensuing mineral dissolution and re‐precipitation. Hence, target rock formations may exhibit changes of mechanical and petrophysical properties due to CO2 exposure. We conducted batch reaction experiments with Entrada Sandstone and Summerville Siltstone exposed to de‐ionized water and synthetic brine under reservoir pressure (9–10 MPa) and temperature (80°C) for up to four weeks. Samples originate from the Crystal Geyser field site, where a naturally occurring CO2 seepage alters portions of these geologic formations. We conducted micro‐scratch tests on rock samples without alteration, altered under laboratory conditions, and naturally altered over geologic time. Scratch toughness and hardness decrease as a function of exposure time and water salinity up to 52% in the case of Entrada and 87% in the case of Summerville after CO2‐induced alteration in the laboratory. Imaging of altered cores with SEM‐EDS and X‐ray microCT methods show dissolution of carbonate and silica cements and matrix accompanied by minor dissolution of Fe‐oxides, clays, and other silicates. Parallel experiments using powdered samples confirm that dissolution of carbonate and silica are the primary reactions. The batch reaction experiments in the autoclave utilize a high fluid to rock volume ratio and represent an end member of possible alteration associated with CO2 storage systems. These types of tests serve as a pre‐screening tool to identify the susceptibility of rock facies to CO2‐related chemical‐mechanical alteration during long‐term CO2 storage. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Discrete Element Modeling of Micro-scratch Tests: Investigation of Mechanisms of CO2 Alteration in Reservoir Rocks

The injection of CO2 into geological formations leads to geochemical re-equilibrium between the pore fluid and rock minerals. Mineral–brine–CO2 reactions can induce alteration of mechanical properties and affect the structural integrity of the storage formation. The location of alterable mineral phases within the rock skeleton is important to assess the potential effects of mineral dissolution on bulk geomechanical properties. Hence, although often disregarded, the understanding of particle-scale mechanisms responsible for alterations is necessary to predict the extent of geomechanical alteration as a function of dissolved mineral amounts. This study investigates the CO2-related rock chemo-mechanical alteration through numerical modeling and matching of naturally altered rocks probed with micro-scratch tests. We use a model that couples the discrete element method (DEM) and the bonded particle model (BPM) to perform simulations of micro-scratch tests on synthetic rocks that mimic Entrada sandstone. Experimental results serve to calibrate numerical scratch tests with DEM–BPM parameters. Sensitivity analyses indicate that the cement size and bond shear strength are the most sensitive microscopic parameters that govern the CO2-induced alteration in Entrada sandstone. Reductions in cement size lead to decrease in scratch toughness and an increase in ductility in the rock samples. This work demonstrates how small variations of microscopic bond properties in cemented sandstone can lead to significant changes in macroscopic large-strain mechanical properties.

Periodic changes in effluent chemistry at cold-water geyser: Crystal geyser in Utah

Crystal geyser is a CO2-driven cold-water geyser which was originally drilled in the late 1930’s in Green River, Utah. Utilizing a suite of temporal groundwater sample datasets, in situ monitoring of temperature, pressure, pH and electrical conductivity from multiple field trips to Crystal geyser from 2007 to 2014, periodic trends in groundwater chemistry from the geyser effluent were identified. Based on chemical characteristics, the primary sourcing aquifers are characterized to be both the Entrada and Navajo Sandstones with a minor contribution from Paradox Formation brine. The single eruption cycle at Crystal geyser lasted over four days and was composed of four parts: Minor Eruption (mEP), Major Eruption (MEP), Aftershock Eruption (Ae) and Recharge (R). During the single eruption cycle, dissolved ionic species vary 0–44% even though the degree of changes for individual ions are different. Generally, Na+, K+, Cl− and SO42− regularly decrease at the onset and throughout the MEP. These species then increase in concentration during the mEP. Conversely, Ca2+, Mg2+, Fe2+ and Sr2+ increase and decrease in concentration during the MEP and mEP, respectively. The geochemical inverse modeling with PHREEQC was conducted to characterize the contribution from three end-members (Entrada Sandstone, Navajo Sandstone and Paradox Formation brine) to the resulting Crystal geyser effluent. Results of the inverse modeling showed that, during the mEP, the Navajo, Entrada and brine supplied 62–65%, 36–33% and 1–2%, respectively. During the MEP, the contribution shifted to 53–56%, 45–42% and 1–2% for the Navajo, Entrada and Paradox Formation brine, respectively. The changes in effluent characteristics further support the hypothesis by Watson et al. (2014) that the mEP and MEP are driven by different sources and mechanisms.

Discrete element modeling of indentation tests to investigate mechanisms of CO2‐related chemomechanical rock alteration

AbstractDuring CO2 injection into geological formations, petrophysical and geomechanical properties of host formations can be altered due to mineral dissolution and precipitation. Field and laboratory results have shown that sandstone and siltstone can be altered by CO2‐water mixtures, but few quantitative studies have been performed to fully investigate underlying mechanisms. Based on the hypothesis that CO2‐water mixtures alter the integrity of rock structure by attacking cements rather than grains, we attempt to explain the degradation of cementation due to long‐term contact with CO2 and water and mechanisms for changes in rock mechanical properties. Many sandstones, including calcite‐cemented quartzitic sandstone, chlorite‐cemented quartzitic sandstone, and hematite‐cemented quartzitic sandstone, contain interparticle cements that are more readily affected by CO2‐water mixtures than grains. A model that couples the discrete element method and the bonded‐particle model is used to perform simulations of indentation tests on synthetic rocks with crystal and random packings. The model is verified against the analytical cavity expansion model and validated against laboratory indentation tests on Entrada sandstone with and without CO2 alteration. Sensitivity analysis is performed for cementation microscopic parameters including stiffness, size, axial, and shear strength. The simulation results indicate that the CO2‐related degradation of mechanical properties in bleached Entrada sandstone can be attributed to the reduction of cement size rather than cement strength. Our study indicates that it is possible to describe the CO2‐related rock alteration through particle‐scale mechanisms.

Potential seal bypass and caprock storage produced by deformation‐band‐to‐opening‐mode‐fracture transition at the reservoir/caprock interface

AbstractWe examined the potential impact on CO2 transport of zones of deformation bands in reservoir rock that transition to opening‐mode fractures within overlying caprock. Sedimentological and petrophysical measurements were collected along an approximately 5 m × 5 m outcrop of the Slick Rock and Earthy Members of the Entrada Sandstone on the eastern flank of the San Rafael Swell, Utah, USA. Measured deformation band permeability (2 mD) within the reservoir facies is about three orders of magnitude lower than the host sandstone. Average permeability of the caprock facies (0.0005 mD) is about seven orders of magnitude lower than the host sandstone. Aperture‐based permeability estimates of the opening‐mode caprock fractures are high (3.3 × 107 mD). High‐resolution CO2–H2O transport models incorporate these permeability data at the millimeter scale. We varied fault properties at the reservoir/caprock interface between open fractures and deformation bands as part of a sensitivity study. Numerical modeling results suggest that zones of deformation bands within the reservoir strongly compartmentalize reservoir pressures largely blocking lateral, cross‐fault flow of supercritical CO2. Significant vertical CO2 transport into the caprock occurred in some scenarios along opening‐mode fractures. The magnitude of this vertical CO2 transport depends on the small‐scale geometry of the contact between the opening‐mode fracture and the zone of deformation bands, as well as the degree to which fractures penetrate caprock. The presence of relatively permeable units within the caprock allows storage of significant volumes of CO2, particularly when the fracture network does not extend all the way through the caprock.

Isotopic analysis of sulfur cycling and gypsum vein formation in a natural CO2 reservoir

In order to assess the long-term security of geologic carbon storage, it is crucial to study the geochemical behavior of sulfur in reservoirs that store CO2. Fossil fuel combustion may produce mixtures of carbon dioxide and sulfur gases, and the geochemical effects of sulfur–CO2 cosequestration are poorly understood. This study examines sulfur mineralization from a core drilled in a stacked sequence of natural CO2 reservoirs near the town of Green River, Utah. These reservoirs include the Entrada and Navajo Sandstone, which are separated by the Carmel Formation caprock and transected by a system of CO2-degassing normal faults, through which saline CO2-charged brines discharge. Our objective in this study is to evaluate the mechanisms and timing of secondary mineral formation, particularly gypsum formation, in the CO2 reservoirs and intervening caprock. The Carmel Formation contains beds of gypsum within a fault zone. Secondary veins of gypsum exist throughout the Entrada Sandstone and Carmel Formation. We report sulfur and oxygen isotope data (δ34SSO4 and δ18OSO4, respectively) measured in gypsum and δ34S measured in pyrite, and the oxygen and hydrogen isotope composition (δ18O and δD, respectively) of gypsum hydration water. The multiple isotope approach allows us to trace the sources of sulfur in the reservoirs and, when combined with structural and petrological evidence, the progress of fluid-rock reactions and relative timing of vein mineralization.The secondary gypsum veins in the Carmel Formation derive from mixing of fluids with two isotopically distinct sulfate sources: sulfate from the gypsum beds within the Carmel Formation and sulfate-rich brines that originate from evaporites in the underlying Carboniferous Paradox Formation. The gypsum veins in the Entrada Sandstone have a relatively wide range of δ18OSO4, both isotopically more enriched in 18O and more depleted in 18O than the primary gypsum sources. We suggest that some aqueous sulfate in the Entrada Sandstone may cycle through multiple valence states as it undergoes reduction and reoxidation, resulting in the replacement of its oxygen atoms and allowing the occasional formation of gypsum with anomalous low δ18OSO4. Our data from gypsum hydration water indicates that groups of gypsum veins formed at two different times. Gypsum veins in the Entrada Sandstone and some veins in the Carmel Formation likely formed during Quaternary CO2-charged brine discharge events, while other veins located close to the gypsum beds in the Carmel Formation formed earlier, likely during cycles of dehydration and rehydration associated with the Laramide-age (40Mya) faulting. We conclude that calcium-sulfate mineral formation in brine-filled fractures may play an important role in inhibiting fluid migration in geologic reservoirs that contain CO2.

Middle Jurassic landscape evolution of southwest Laurentia using detrital zircon geochronology

The Middle Jurassic Wanakah Formation in western Colorado is a poorly understood unit in terms of depositional environment and absolute age of deposition; however, a refined interpretation of the provenance has important implications for understanding the landscape evolution of southwestern Laurentia during Mesozoic rifting of the supercontinent Pangea and the opening of the Gulf of Mexico. This study presents the first U/Pb age dating of detrital zircons from the Middle to Late Jurassic Entrada Sandstone, Wanakah Formation, and Tidwell and Salt Wash Members of the Morrison Formation. Detrital zircon geochronology results show a marked increase in ca. 523 Ma grains (compared to most Mesozoic sediments on the Colorado Plateau) that begins abruptly in the Wanakah Formation and continues into the basal Marker Bed A of the Tidwell Member of the Morrison Formation. U/Pb ages and petrography suggest that the Wanakah Formation was sourced, in large part, from the McClure Mountain syenite on the southwestern flank of the Ancestral Front Range. This abrupt change in provenance occurred due to stream capture and drainage reorganization that input a large amount of water into the basin and caused a shift in depositional environment from the eolian Entrada Sandstone to the hypersaline lake environments of the Wanakah Formation and the Tidwell Member. Additionally, stratigraphic, petrological, and detrital zircon analyses suggest that the contact between the Wanakah Formation and the Tidwell Member of the Morrison Formation is conformable, and the previously interpreted J-5 unconformity is likely not present in western Colorado. The stream capture and drainage reorganization that created the lake system recorded in the Wanakah Formation and the Tidwell Member likely evolved into the major fluvial system that deposited the Salt Wash Member of the Morrison Formation. The evolution of paleodrainages and provenance are important to understand because they help to constrain landscape evolution across southwestern Laurentia, and these insights can help to illuminate the influence of tectonic and sediment controls on depositional environment.

Theropod Dinosaur Ichnogenus Hispanosauropus Identified from the Morrison Formation (Upper Jurassic), Western North America

Redescription of a trackway from the Salt Wash Member of the Morrison Formation (Upper Jurassic) in eastern Utah and comparisons with other tracks known from the Jurassic and Cretaceous of the region and the Late Jurassic globally indicate that the large theropod ichnogenus Hispanosauropus Mensink and Mertmann, 1984, previously known only from Europe, occurs also in the Morrison Formation. Hispanosauropus is distinct from Megalosauripus and most likely was made by an allosauroid theropod, although the possibility that it was made by a spinosauroid (torvosaurid) or a ceratosauroid cannot be ruled out. Close geographic and stratigraphic association of Hispanosauropus with the large theropod body fossil taxa Allosaurus, Torvosaurus, and Ceratosaurus in both North America and the Iberian Peninsula strengthens the case for the track's having been made by one of these genera. Megalosauripus appears to dominate large theropod track samples from the Entrada Sandstone through the base of the Morrison Formation (Tidwell Member); Hispanosauropus and similar tracks are most common in the Salt Wash Member and its equivalents, suggesting taxonomic turnover in the large theropod faunas during the early part of Morrison Formation deposition.

Fracture corridors as seal-bypass systems in siliciclastic reservoir-cap rock successions: Field-based insights from the Jurassic Entrada Formation (SE Utah, USA)

Closely spaced, sub-parallel fracture networks contained within localized tabular zones that are fracture corridors may compromise top seal integrity and form pathways for vertical fluid flow between reservoirs at different stratigraphic levels. This geometry is exemplified by fracture corridors found in outcrops of the Jurassic Entrada Formation in Utah (USA). These fracture corridors exhibit discolored (bleached) zones, interpreted as evidence of ancient fracture-enhanced circulation of reducing fluids within an exhumed siliciclastic reservoir-cap rock succession. Extensive structural and stratigraphic mapping and logging provided fracture data for analysis with respect to their occurrence and relationships to larger faults and folds. Three types of fracture corridors, representing end-members of a continuum of possibly interrelated structures were identified: 1) fault damage zone including segment relays; 2) fault-tip process zone; and 3) fold-related crestal-zone fracture corridors. The three types exhibit intrinsic orientations and patterns, which in sum define a local- to regional network of inferred vertical and lateral, high-permeability conduits. The results from our analysis may provide improved basis for the evaluation of trap integrity and flow paths across the reservoir-cap rock interface, applicable to both CO2 storage operations and the hydrocarbon industry.

An Experimental and Analogue Study of Iron Release from Red Sandstones

The Jurassic Entrada sandstone at Salt Wash Graben, Utah, USA, a red sandstone contains significant rock bleaching. The cause of the bleaching has been thought to be associated with the modern day CO2-rich fluids in the area which present on the surface by utalising the local fractures, some of which are filled with calcite and iron rich minerals (e.g. Jarosite). An experimental study was conducted to determine the cause of the bleaching. CO2 was found not to cause sandstone bleaching. However, the CO2 was found to mobilize significant amounts of iron from the fracture minerals suggesting that this is a possible source of the iron in the modern pore fluids.

Scientific drilling and downhole fluid sampling of a natural CO<sub>2</sub> reservoir, Green River, Utah

Abstract. A scientific borehole, CO2W55, was drilled into an onshore anticline, near the town of Green River, Utah for the purposes of studying a series of natural CO2 reservoirs. The objective of this research project is to recover core and fluids from natural CO2 accumulations in order to study and understand the long-term consequences of exposure of supercritical CO2, CO2-gas and CO2-charged fluids on geological materials. This will improve our ability to predict the security of future geological CO2 storage sites and the behaviour of CO2 during migration through the overburden. The Green River anticline is thought to contain supercritical reservoirs of CO2 in Permian sandstone and Mississippian-Pennsylvanian carbonate and evaporite formations at depths > 800 m. Migration of CO2 and CO2-charged brine from these deep formations, through the damage zone of two major normal faults in the overburden, feeds a stacked series of shallow reservoirs in Jurassic sandstones from 500 m depth to near surface. The drill-hole was spudded into the footwall of the Little Grand Wash normal fault at the apex of the Green River anticline, near the site of Crystal Geyser, a CO2-driven cold water geyser. The hole was drilled using a CS4002 Truck Mounted Core Drill to a total depth of 322 m and DOSECC’s hybrid coring system was used to continuously recover core. CO2-charged fluids were first encountered at ~ 35 m depth, in the basal sandstones of the Entrada Sandstone, which is open to surface, the fluids being effectively sealed by thin siltstone layers within the sandstone unit. The well penetrated a ~ 17 m thick fault zone within the Carmel Formation, the footwall damage zone of which hosted CO2-charged fluids in open fractures. CO2-rich fluids were encountered throughout the thickness of the Navajo Sandstone. The originally red sandstone and siltstone units, where they are in contact with the CO2-charged fluids, have been bleached by dissolution of hematite grain coatings. Fluid samples were collected from the Navajo Sandstone at formation pressures using a positive displacement wireline sampler, and fluid CO2 content and pH were measured at surface using high pressure apparatus. The results from the fluid sampling show that the Navajo Sandstone is being fed by active inflow of CO2-saturated brines through the fault damage zone; that these brines mix with meteoric fluid flowing laterally into the fault zone; and that the downhole fluid sampling whilst drilling successfully captures this dynamic process.

Geostatistical relationships between mechanical and petrophysical properties of deformed sandstone

Petrophysical and mechanical properties of sandstone reservoirs are likely to change as a result of faulting. In this paper, we investigate the distribution of deformation features (structures) such as fractures and deformation bands in the Navajo and the Entrada sandstones in the fault core and damage zones of two faults in two localities in southeast (Cache Valley) and central (San Rafael Swell) Utah. These two localities had different degree of calcite cementation and hence are of interest to study the mechanical and petrophysical properties of these localities, in order to find out the impact of cementation on these properties and their possible relations. We have performed in-situ measurements by Tiny-Perm II and Schmidt hammer to examine the distribution of permeability and strength/elasticity of rock within the damage zone of these faults. We have studied the statistical relation between (i) Tiny-Perm II measurements and Schmidt hammer values, (ii) permeability and uniaxial compressive strength, and (iii) permeability and Young's modulus of deformed rocks. The statistical results demonstrate that there are correlations between the studied parameters, but the dependencies vary with the degree of calcite cementation in mineralogically similar sandstones (quartz sandstone). Statistical results demonstrate to first approximation that an exponential law is more suitable for description of the relations (i), (ii) and (iii) of non-cemented Navajo sandstone whereas for cemented Navajo sandstone these relations are better approximated by power law.

Dinosaur Tracks and Trackways in the Escalante Member of the Entrada Formation (Middle Jurassic), Twentymile Wash, Grand Staircase-Escalante National Monument, Utah

Dinosaur Tracks and Trackways in the Escalante Member of the Entrada Formation (Middle Jurassic), Twentymile Wash, Grand Staircase-Escalante National Monument, Utah

Triassic fossils found stratigraphically above 'Jurassic' eolianites necessitate the revision of lower Mesozoic stratigraphy in Picket Wire Canyonlands, south-central Colorado

The recent discovery of Triassic tetrapod fossils in the Picket Wire Canyonlands of southeastern Colorado necessitates large-scale modification of the currently accepted stratigraphy of the area. The bone-bearing strata lie stratigraphically above a thick (∼80 meter [m]) eolianite historically identified as the Middle Jurassic Entrada Sandstone. The identifiable fossils include teeth and bone fragments of Late Triassic tetrapods, including metoposaurs, phytosaurs, and aetosaurs, recovered from thin (m-scale) discontinuous channels of limestone-pebble conglomerate deposited in a high-energy fluvial environment. Metoposaur bones consist of characteristically textured dermal bone fragments of the skull and pectoral elements, as well as a tooth. Phytosaur fossils consist of type C and B teeth, skull and jaw fragments, and some osteoderms. Aetosaurs are represented by several distinctive osteoderms, including some with evidence of prominent eminences and anterior bars. All identifiable tetrapods pertain to taxa known only from strata of Late Triassic age elsewhere, but none constrains the age of the fossil assemblage more precisely, although the assemblage is similar to lower Chinle Group assemblages of Carnian age (Otischalkian–Adamanian). The two most reasonable solutions to the discovery of Late Triassic index fossils stratigraphically above “Jurassic” beds are that the Triassic strata of this area have been mistakenly correlated with the Middle Jurassic Entrada Sandstone, or else the fossils are reworked into dramatically younger (Middle to Upper Jurassic) beds. The conglomerates are lithologically dissimilar from other Jurassic units regionally, but similar to Upper Triassic conglomerates of Wyoming (Gartra Formation) and New Mexico (Cobert Canyon Bed). Therefore, we consider the fossils to be in Upper Triassic strata. New lithostratigraphic data, including a composite measured section from the Picket Wire Canyonlands—as well as analysis and correlation of newly measured sections and others in the literature from south-central Wyoming, Colorado, Oklahoma, and New Mexico—suggest that the eolianite below the bone-bearing horizon and the finer clastic strata directly beneath the eolianite are best correlated to the Red Draw Member of the Jelm Formation. We correlate the bone-bearing conglomerates with the Cobert Canyon Bed at the base of the Chinle Group, described by previous authors as limestone and lithic-pebble conglomerate underlying the Travesser Formation in northern New Mexico. The gypsiferous and clastic strata overlying the conglomerates and below the Morrison Formation, ∼30 m higher in Picket Wire Canyon, are referred to the Middle Jurassic Ralston Creek (= Bell Ranch) Formation, a correlative of the Summerville Formation. These correlations extend the known distribution of Jelm Formation strata southeastward from north-central Colorado and south-central Wyoming and highlight the need for a major, modern restudy of this unit.

Reservoir-scale CO2 -fluid rock interactions: Preliminary results from field investigations in the Paradox Basin, Southeast Utah

Abstract Despite a long history of detailed study, the extensive Jurassic sandstone outcrops of the Colorado Plateau in Southeast Utah, USA continue to provide opportunities to examine reservoir-scale processes. There are a number of large-scale CO 2 accumulations in these reservoirs and locally also natural and man-induced CO 2 -rich springs and geysers, often producing or associated with travertine deposits. These rocks have therefore been exposed to CO 2 and/or CO 2 -rich waters over a substantial period of time, and as such may provide information on resulting geochemical and geomechanical processes. Salt Wash Graben is a WNW-ESE trending structure that cuts across a northerly plunging anticline to the south of Green River, Utah. The Salt Wash Graben lies to the south of the well-studied ’Crystal Geyser’, and itself contains several CO 2 -rich springs and abundant travertine deposits. Rocks of the Slick Rock and Earthy members of the Jurassic Entrada Sandstone outcrop in the core of the anticline immediately to the north of the graben. These are locally extensively bleached, the bleaching being most pronounced towards the base of the stratigraphically lowest exposed strata, forming a light coloured area in aerial photos. Field investigations of the nature of this bleaching and its relationship to faults and fractures, travertines and CO 2 springs, have been supported by detailed analysis of aerial photographs and other satellite-based remote sensing data. The remote sensing data provides a geological context that serves to highlight the different structural and stratigraphic controls which were observed between the regionally extensive bleaching and the bleaching in the Salt Wash Graben. Diagenetic analyses have identified the geochemical processes responsible for bleaching (loss of iron-staining) and other fluid-rock interactions, notably increased porosity in bleached sandstones and differences in carbonate cementation. Simple laboratory experiments have also attempted to replicate these processes by reacting samples of unbleached Entrada Sandstone with CO 2 -rich formation waters. In this paper, we present a preliminary hypothesis that attempts to link the features described in the Salt Wash Graben, which suggest that CO 2 -rich fluids may have produced the observed features. If this hypothesis is proved correct, the Salt Wash Graben offers considerable potential to study in detail the flow-constrained geochemical processes at reservoir scale.