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At least 37 records · Page 2Linked to original sources
USGS1985

Mineral, chemical and textural relationships in rhythmic-bedded, hydrocarbon-productive chalk of the Niobrara Formation, Denver Basin, Colorado ( USA).

The types of hydrocarbons produced from these chalks are determined by the level of thermal maturity associated with present-day burial or paleoburial conditions. Detailed analyses of deeply-buried chalk from core of the Smoky Hill Chalk Member of the Niobrara Formation in the Champlin Petroleum 2 Boxelder Farms well combined with core data from other Niobrara wells have helped identify many depositional and diagenetic relationships. Porosity of the chalk is proportional to maximum burial depth and inversely proportional to the amount of non-carbonate material (acid- insoluble residue content) in the chalk. Total organic carbon content in the chalk is proportional to the amount of acid-insoluble residue and relative abundance of pyrite in the acid-insoluble fraction. Quartz is inversely proportional to the amount of insoluble material, and the amount of clay tends to increase as insolubles increase, suggesting that detritus in these chalks is greatly influenced by reworked, altered, volcanic products rather than siliceous clastics.-from Authors

Mountain Geologist↗
USGS1988

Facies composition calculated from the sonic, neutron, and density log suite, upper part of the Minnelusa Formation, Powder River basin, Wyoming

Sandstones and dolomites of the Permian upper part of the Minnelusa Formation are treated here as four-component systems consisting of fluid-filled pore space, quartz, dolomite, and anhydrite. Response equations of sonic, neutron, and density logs form a system of four simultaneous equations. With four equations and four unknowns, the composition of upper Minnelusa facies is defined by the three-log suite and can be calculated by solving a 4 ?? 4 matrix. Such calculations of facies composition help in establishing subsurface correlations and yield information on the diagenesis and physical character of upper Minnelusa sandstones and dolomites. Applications of composition calculations are illustrated by examples drawn from the area of the West Mellott field (T52N, R68W), where the upper Minnelusa is at depths of about 7000 ft (2100m). -from Authors

Mountain Geologist↗
USGS1991

Sedimentology and cyclicity in the Lower Permian De Chelly Sandstone on the Defiance Plateau: Eastern Arizona

The Lower Permian (Leonardian) De Chelly Sandstone crops out along a north-south trend on the Defiance Plateau of eastern Arizona. It is divided into lower and upper members separated by a tongue of the Supai Formation that pinches out to the north. Lithofacies in the De Chelly Sandstone consist of (1) a large-scale trough to tabular- and/or wedge-planar cross-stratified sandstone facies of large-scale eolian dune origin, (2) a small- to medium-scale, trough cross-stratified sandstone also of eolian dune origin, (3) a horizontally stratified, wind-rippled sandstone of sand sheet origin, (4) a wavy, horizontally stratified, wind-rippled sandstone of sabkha origin, and (5) a mud-draped ripple-laminated sandstone of mud-flat origin. The De Chelly Sandstone in the northern Defiance Plateau consists mainly of large-scale dune deposits. Stratigraphic sections in the middle of the plateau are dominated by small- to medium-scale dune and sand sheet deposits whereas those along the southern end of the plateau are composed largely of sabkha and supratidal mud-flat deposits. The lateral distribution of these facies suggests a north-south juxtaposition of central-erg, fore-erg, and mixed sabkha-supratidal depositional environments. Repetitive interbedding of facies in the De Chelly indicates at least twelve depositional cycles in which sabkha and/or supratidal to coastal-plain mud-flats were sequentially overridden by eolian sand sheets and cross-stratified dunes. Lateral and vertical facies relations within the lower and upper members of the De Chelly Sandstone record episodic expansion of the De Chelly erg southward, The comparative abundance of large-scale dune deposits in the upper member suggests that progradation was more extensive during latter stages of deposition. The intervening tongue of Supai Formation and the redbeds that overlie the upper member of the De Chelly at Bonito Canyon document northward transgression of sabkha and supratidal to coastal-plain mud-flat environments. Eolian dune deposition was restricted to the northern Defiance Plateau during deposition of these units.

Arizona↗
USGS1987

Government Draw Bentonite Beds: a newly identified stratigraphic marker in the Virgin Creek Member of the Pierre Shale, central South Dakota ( USA).

A grouping of four bentonite beds, herein named the Government Draw Bentonite Beds, is identified as a stratigraphic marker within the Virgin Creek Member of the Pierre Shale. The beds are found west of Pierre, South Dakota, over an area of at least 130 mi2 (210 km2) where no other markers within the Virgin Creek Member have been identified. In this area, the Government Draw is a potential tool needed to determine the stratigraphic and structural relationships within the upper part of the Pierre Shale, heretofore little known. A better understanding of structural elements found in the Pierre Shale is needed to unravel the Late Cretaceous and younger geologic history of the area. -Authors

Mountain Geologist↗
USGS1986

Southeastern extension of the Lake Basin fault zone in south- central Montana: implications for coal and hydrocarbon exploration ( USA).

The Lake Basin fault zone consists mainly of en echelon NE-striking normal faults that have been interpreted to be surface expressions of left-lateral movement along a basement wrench fault. Information gathered from recent field mapping of coal beds and from shallow, closely-spaced drill holes resulted in detailed coal bed correlations, which revealed another linear zone of en echelon faulting directly on the extended trend of the Lake Basin fault zone. This faulted area, referred to as the Sarpy Creek area, is located 48 km E of Hardin, Montana. It is about 16 km long, 13 km wide, and contains 21 en echelon normal faults that have an average strike of N 63oE. We therefore extend the Lake Basin fault zone 32 km farther SE than previously mapped to include the Sarpy Creek area. The Ash Creek oil field, Wyoming, 97 km due S of the Sarpy Creek area, produces from faulted anticlinal structues that have been interpreted to be genetically related to the primary wrench-fault system known as the Nye-Bowler fault zone. The structural similarities between the Sarpy Creek area and the Ash Creek area indicate that the Sarpy Creek area is a possible site for hydrocarbon accumulation.-from Authors

Mountain Geologist↗
USGS1989

Paleogeothermal gradients and timing of oil generation in the Belden Formation, Eagle Basin, northwestern Colorado

Paleogeothermal gradients and timing of oil generation for the Lower and Middle Pennsylvanian Belden Formation have been estimated for four locations in the Eagle Basin of northwestern Colorado, by comparing measured vitrinite reflectance with maturity modeling. Two thermal models were made for each location: one assumes a constant paleogeothermal gradient through time while the other is a two-stage model with changing paleogeothermal gradients. The two-stage paleogeothermal gradient scenario is considered more geologically realistic and is used to estimate the timing of oil generation throughout the Eagle basin. From the data and interpretations, one would expect Belden oil to be found in either upper Paleozoic or Mesozoic reservoir rocks. -Authors

Mountain Geologist↗
USGS1992

Paired, facing monoclines in the Sanpete-Sevier Valley area, central Utah

Several major monoclines that trend northward through the Sanpete-Sevier Valley area of central Utah are paired and face one another. This pairing of monoclines may have occurred when near-horizontal sedimentary and volcanic strata subsided into voids created as salt was removed from a salt diapir concealed beneath valley fill. Removal was mostly by dissolution or extrusion during Neogene time. The paired monoclines, thus, are viewed as collapse features rather than as normal synclinal folds. -from Author

Mountain Geologist↗
USGS1993

Sedimentology of Permian upper part of the Minnelusa Formation, eastern Powder River Basin, Wyoming, and a comparison to the subsurface

Describes dolomite, gypsum, and sandstone units deposited in transgressive-regressive cycles. Three depositional cycles are partly exposed. The cycles observed in outcrop were informally labelled cycle 1, cycle 2, and cycle 3 in ascending stratigraphic order. West Mellott field represents a subsurface example of the facies and facies relationships observed in outcrop. The eolian-dune sandstone of the C cycle, which was partially reworked by the transgression of the B cycle, produces oil at West Mellott. The draping of dolomite and anhydrite of the B cycle on the eolian-dune sandstone of the C cycle is analogous to the draping gypsum on dune sand in cycle 2 in outcrop. -from Authors

Mountain Geologist↗
USGS1998

Stratigraphy and depositional environment of nonmarine facies of Frontier Formation, Eastern Pioneer Mountains, southwestern Montana

The Upper Cretaceous Frontier Formation in the eastern Pioneer Mountains of southwestern Montana was deposited in nonmarine environments west of the Western Interior Seaway within the Cordilleran foreland basin. These rocks have been assigned to the Frontier because they contain lithologies typical of the Frontier in the region even though they are entirely nonmarine and are thicker than the correlative marine Frontier to the east. The Frontier in the eastern Pioneer Mountains is underlain by the Vaughn Member of the Blackleaf Formation, but the upper part of the Frontier has been eroded and locally is overlain by rocks of Tertiary age. Geologic mapping was conducted and four sections were measured and described to determine facies, thickness variations, and depositional environments. In the eastern Pioneer Mountains study area, the Frontier ranges in thickness from about 1200 ft (366 m) in the south to more than 3400 ft (1036 m) in the north\. Frontier strata in the study area cannot be readily subdivided into mappable units, but two broadly-defined informal lithic units are described. The lower unit contains yellow-brown weathering siltstone, mudstone, and fine-grained quartz-rich sandstone, and is about 250 ft (76 m) thick. The upper unit is composed of yellow-brown to dark-gray siltstone and mudstone, quartz- and chert-rich sandstone, conglomeratic sandstone, and limestone, and is more than 2100 ft (640 m) thick in one measured section. The lower contact of the Frontier is placed at the top of a porcellanite bed that is associated with maroon mudstone and siltstone, limestone, and calcareous dark-gray shale in the underlying Vaughn Member of the Blackleaf Formation. The porcellanite bed directly overlies the highest maroon mudstone-siltstone bed of the upper Vaughn. The Frontier Formation in the eastern Pioneer Mountains was deposited on a broad delta plain dominated by fine-grained sediments. Sandstones exhibit characteristics of anastomosing fluvial channels, average 5% to 10% of the entire formation, and have width to depth ratios of less than 30. The Frontier in the eastern Pioneer Mountains is lithostratigraphically equivalent to the Frontier to the east in the Gravelly, Greenhorn, and Madison Ranges, but may include strata that are younger in age. Frontier strata to the south at Lima Peaks are the thickest in the region (up to 7000 ft; 2100 m) and include facies that are time-equivalent to the marine Frontier, the overlying Cody Shales, and the Telegraph Creek Formation of the Madison Range to the east. The Frontier in the eastern Pioneer Mountains is in part lithostratigraphically equivalent to the Coberly Formation in the Drummond, Montana area (50 mi [80 km] northwest of Butte, Montana) and the Marias River Shales near Great Falls (150 mi [240 km] northeast of Butte) in west-central Montana.

Mountain Geologist↗
USGS1998

Zircon geochronology of the Webb Canyon Gneiss and the Mount Owen Quartz Monzonite, Teton Range, Wyoming: Significance to dating late Archean metamorphism in the Wyoming craton

The Webb Canyon Gneiss is a strongly foliated and lineated orthogneiss intercalated with layered Archean gneisses in the northern part of the Teton Range in northwestern Wyoming. The Mount Owen Quartz Monzonite is a non-foliated or weakly flow foliated rock which forms a discordant pluton exposed in the central part of the range and that cuts the Webb Canyon Gneiss and the associated layered gneisses. U-Pb zircon geochronology reported here indicates that euhedral pink zircon grew in the Webb Canyon Gneiss at about 2680 Ma, probably during the peak of regional metamorphism and that the Mount Owen was emplaced at 2547??3 Ma. These dates provide the best constraints so far reported on the age of Late Archean regional metamorphism in the western part of the Wyoming craton.

Mountain Geologist↗
USGS2001

Late Cenozoic regional collapse due to evaporite flow and Dissolution in the Carbondale Collapse Center, West-Central Colorado

Dissolution and flow of Pennsylvanian evaporitic rocks in west-central Colorado created the Carbondale Collapse Center, a 450 mi2 structural depression with about 4,000 ft of vertical collapse during the late Cenozoic. This paper describes evidence of collapse in the lower Roaring Fork River valley. Both the lateral extent and amount of vertical collapse is constrained by deformed upper Cenozoic volcanic rocks that have been correlated using field mapping, 40Ar/39Ar geochronology, geochemistry, and paleomagnetism. The Carbondale Collapse Center is one of at least two contiguous areas that have experienced major evaporite tectonism during the late Cenozoic. Historic sinkholes, deformed Holocene deposits, and modern high-salinity loads in the rivers and thermal springs indicate the collapse process continues today. Flow of evaporitic rocks is an important element in the collapse process, and during initial stages of collapse it was probably the primary causative mechanism. Dissolution, however, is the ultimate means by which evaporite is removed from the collapse area. As the Roaring Fork River began to rapidly down-cut through a broad volcanic plateau during the late Miocene, the underlying evaporite beds were subjected to differential overburden pressures. The evaporitic rocks flowed from beneath the upland areas where overburden pressures remained high, toward the Roaring Fork River Valley where the pressures were much lower. Along the valley the evaporitic rocks rose upward, sometimes as diapirs, forming or enhancing a valley anticline in bedrock and locally upwarping Pleistocene terraces. Wherever the evaporites encountered relatively fresh ground water, they were dissolved, forming underground voids into which overlying bedrock and surficial deposits subsided. The saline ground water eventually discharged to streams and rivers through thermal springs and by seepage into alluvial aquifers.

Mountain Geologist↗
USGS2001

Geochemical and mineralogical studies of dinosaur bone from the Morrison Formation at Dinosaur Ridge

The dinosaur bones first discovered in 1877 in the Upper Jurassic Morrison Formation at Morrison, Colorado were the first major find of dinosaur skeletons in the western U.S. and led to the recognition of four new dinosaur genera (Apatosaurus, Allosaurus, Diplodocus, and Stegosaurus). Eight articles dealing with these bones which appeared as research reports in the annual reports of the Friends of Dinosaur Ridge from 1990-1999 are condensed and summarized with some additional comments. Two of the articles are about the mineralogy and preservation of the bones; two are about the physical description of the bone occurrence; two are about the history of the site, and two are about use of novel instrumental methods (ground-penetrating radar and a directional scintillometer) to search for new bones.

Mountain Geologist↗
USGS2002

Subsurface fluid pressures from drill-stem tests, Uinta Basin, Utah

High fluid pressures are known to be associated with oil and gas fields in the Uinta Basin, Utah. Shut-in pressure measurements from drill-stem tests show how pressure varies with depth and by area within the basin. The data base used in this report incorporates over 2,000 pressure measurements from drill-stem tests in wells completed prior to 1985. However, the number of useful pressure measurements is considerably less, because many drill-stem tests fail to stabilize at the actual formation pressure if the permeability is low. By extracting the maximum pressure measurements recorded in a collection of wells within an area, the trend of formation pressure within that area can be approximated. Areal compilations of pressures from drill-stem tests show that overpressured rock formations occur throughout much of the northern and eastern areas of the Uinta Basin. In particular, significant overpressuring (0.5 < pressure gradient < 0.8 psi/ft) is found throughout much of the Altamont-Bluebell field at depths ranging from 10,000 to 13,000 ft, equivalent to 5,000 to 8,000 ft below sea level. Limited data indicate that the pressure gradient declines at depths greater than 13,000 ft. An underpressured zone appears to exist in the Altamont-Bluebell field at depths shallower than 5,000 ft. Throughout the eastern Uinta Basin, moderately overpressured zones (0.46 < pressure gradient < 0.5 psi/ft) are common, with local evidence of significantly overpressured zones, but pressure gradients greater than 0.6 psi/ft are rare.

Mountain Geologist↗
USGS2002

A review of the multiwell experiment in tight gas sandstones of the Mesaverde Group, Piceance Basin, Colorado

The Cretaceous Iles and Williams Fork Formations of the Mesaverde Group contain important reservoir and source rocks for basin-centered gas accumulations in the Piceance Basin of northwestern Colorado. The sandstones in these formations have very low permeability, so low that successful production of gas requires the presence of fractures. To increase gas production, the natural fracture system of these "tight gas sandstones" must be augmented by inducing artificial fractures, while minimizing the amount of formation damage due to introduced fluids. The Multiwell Experiment was undertaken to provide geological characterization, obtain physical property data, and perform stimulation experiments in the Iles and Williams Fork Formations. Three vertical wells and one follow-up slant well were drilled, logged, partially cored, tested for gas production, stimulated in various manners, and tested again. Drawing from published reports and papers, this review paper presents well log, core, and test data from the Multiwell Experiment while emphasizing the geological controls on gas production at the site. Gas production is controlled primarily by a set of regional fractures trending west-northwest. The fractures are vertical, terminating at lithologic boundaries within and at the upper and lower boundaries of sandstone beds. Fractures formed preferentially in sandstones where in situ stress and fracture gradients are lower than in shales and mudstones. The fractures cannot be identified adequately in vertical wellbores; horizontal wells are required. Because present-day maximum horizontal stress is aligned with the regional fractures, artificial fractures induced by pressuring the wellbore form parallel to the regional fractures rather than linking them, with consequent limitations upon enhancement of gas production.

Mountain Geologist↗
USGS2004

Vertical hydraulic conductivity measurements in the Denver Basin, Colorado

The Denver Basin is a structural basin on the eastern flank of the Rocky Mountain Front Range, Colorado, containing approximately 3000 ft of sediments that hold a critical groundwater resource supplying many thousands of households with water. Managing this groundwater resource requires understanding how water gets into and moves through water-bearing layers in a complex multiple-layered sedimentary sequence. The Denver Basin aquifer system consists of permeable sandstone interbedded with impermeable shale that has been subdivided into four principle aquifers named, in ascending order, the Laramie-Fox Hills, Arapahoe, Denver, and Dawson aquifers. Although shale can dominate the stratigraphic interval containing the aquifers, there is very little empirical data regarding the hydrogeologic properties of the shale layers that control groundwater flow in the basin. The amount of water that flows vertically within the basin is limited by the vertical hydraulic conductivity through the confining shale layers. Low vertical flow volumes translate to low natural recharge rates and can have a profound negative impact on long-term well yields and the economic viability of utilizing the resource. To date, direct measurements of vertical hydraulic conductivity from cores of fine-grained sediments have been published from only five locations; and the data span a wide range from 1??10-3 to 1??10-11 cm/sec. This range may be attributable, in part, to differences in sample handling and analytical methods; however, it may also reflect subtle differences in the lithologic characteristics of the fine-grained sediments such as grain-size, clay mineralogy, and compaction that relate to position in the basin. These limited data certainly call for the collection of additional data.

Mountain Geologist↗
USGS2004

Aquifers of the Denver Basin, Colorado

Development of the Denver Basin for water supply has been ongoing since the late 1800s. The Denver Basin aquifer system consists of the water-yielding strata of Tertiary and Cretaceous sedimentary rocks within four overlying formations. The four statutory aquifers contained in these formations are named the Dawson, Denver, Arapahoe, and Laramie-Fox Hills. For water rights administrative purposes, the outcrop/subcrop of the Laramie-Fox Hills aquifer defines the margins of the Basin. Initial estimates of the total recoverable groundwater reserves in storage, under this 6700-mi2 area, were 295 million acre-ft. Recent geologic evidence indicates that the aquifers are very heterogeneous and their composition varies significantly with distance from the source area of the sediments. As a result, available recoverable reserves may be one-third less than previously estimated. There is no legal protection for pressure levels in the aquifer, and water managers are becoming increasingly concerned about the rapid water level declines (30 ft/yr). Approximately 33,700 wells of record have been completed in the sedimentary rock aquifers of the Denver Basin for municipal, industrial, agricultural, and domestic uses.

Mountain Geologist↗
USGS2004

Timing of oil and gas generation of petroleum systems in the Southwestern Wyoming Province

Burial history, thermal maturity, and timing of petroleum generation were modeled for eight key source-rock horizons at seven locations throughout the Southwestern Wyoming Province. The horizons are the bases of the Lower Permian Phosphoria Formation, the Upper Cretaceous Mowry Shale, Niobrara Formation, Baxter Shale (and equivalents), upper part of the Mesaverde Group, Lewis Shale, Lance Formation, and the Tertiary (Paleocene) Fort Union Formation. Burial history locations include three in the deepest parts of the province (Adobe Town in the Washakie Basin, Eagles Nest in the Great Divide Basin, and Wagon Wheel in the northern Green River Basin); two at intermediate basin depths (Federal 31-1 and Currant, Creek in the central and southern parts of the Green River Basin, respectively); and two relatively shallow locations (Bear 1 on the southeastern margin of the Sand Wash Basin and Bruff 2 on the Moxa arch). An overall ranking of the burial history locations in order of decreasing thermal maturity is Adobe Town > Eagles Nest > Wagon Wheel > Currant Creek > Federal 31-1 > Bear-1 > Bruff 2. The results of the models indicate that peak petroleum generation from Cretaceous oil- and gas-prone source rocks in the deepest parts of the province occurred from Late Cretaceous through middle Eocene. At the modeled locations, peak oil generation from source rocks of the Phosphoria Formation, which contain type-IIS kerogen, occurred in the Late Cretaceous (80 to 73 million years ago (Ma)). Gas generation from the cracking of Phosphoria oil reached a peak in the late Paleocene (57 Ma) only in the deepest parts of the province. The Mowry Shale, Niobrara Formation, and Baxter Shale (and equivalents) contain type-IIS or a mix of type-II and type-III kerogens. Oil generation from these units, in the deepest parts of the province, reached peak rates during the latest Cretaceous to early Paleocene (66 to 61 Ma). Only at these deepest locations did these units reach peak gas generation from the cracking of oil, which occurred in the early to late Eocene (52 to 41 Ma). For the Mesaverde Group, which also contains a mix of type-II and type-III kerogen, peak oil generation occurred only in the deepest parts of the province during middle Eocene (50 to 41 Ma). Only at Adobe Town did cracking of oil occur and gas generation reach peak in the earliest Oligocene (33 Ma). Gas-prone source rocks (type-III kerogen) of the Mowry and Baxter (and equivalents) Shales reached peak gas generation in the latest Cretaceous (66 Ma) in the deepest parts of the province. At the shallower Bear 1 location, the Mancos Shale (Baxter equivalent) source rocks reached peak gas generation at about this same time. Gas generation from the gas-prone Mesaverde source rocks started at all of the modeled locations, but reached peak generation at only the deepest locations in the early Eocene (54 to 49 Ma). The Lewis Shale, Lance Formation, and Fort Union Formation all contain gas-prone source rocks with type-III kerogen. Peak generation of gas from the Lewis Shale occurred only at Eagles Nest and Adobe Town in the early Eocene (52 Ma). Source rocks of the Lance reached peak gas generation only at the deepest locations during the middle Eocene (48 to 45 Ma) and the Fort Union reached peak gas generation only at Adobe Town also in the middle Eocene (44 Ma).

Mountain Geologist↗
USGS2005

Bedrock erosion surface beneath the rocky flats alluvial fan, Jefferson and Boulder counties, Colorado

The early Pleistocene Rocky Flats alluvial fan formed at the mouth of unglaciated Coal Creek Canyon along the eastern flank of the Colorado Front Range. The fan consists of boulder, cobble, and pebble gravel deposited on an erosional surface cut on tilted Mesozoic sedimentary strata. A north-trending hogback of steeply dipping Cretaceous Laramie Formation and Fox Hills Sandstone is exposed through the gravel across the central portion of the fan. Elevations on the gravel-bedrock contact were used in a GIS to reconstruct the bedrock surface at the base of the gravel, providing a glimpse of the geomorphology of the early Pleistocene Colorado Piedmont. The reconstructed erosional bedrock surface portrays a landscape carved by a series of easterly flowing streams that eroded headward to the resistant hogback units, creating a bedrock step up to 37 m high. East-trending ridges on the bedrock surface are remnants of drainage divides between the Pleistocene streams. Water gaps in the bedrock step allowed the streams access to the upper surface of the step. This entire surface, except the hogback, was covered by gravel about 1.35 to 1.5 Ma ago. Subsequent erosion of the alluvial fan has been by headward (westward) erosion of easterly flowing streams incising into the eastern portion of the fan. Because the gravel is more resistant than the underlying bedrock, modern streams are established over the Pleistocene drainage divides, where the gravel was thinnest. Thicker gravel in the Pleistocene paleovalleys now caps modern drainage divides, producing an inverted topography.

Mountain Geologist↗