Pub Date : 2022-04-28DOI: 10.31582/rmag.mg.59.2.77
L. S. Jones, C. Fenton
A well-defined type section and consistent nomenclature are fundamental requirements for successful correlation of stratigraphic units and subsequent accurate paleogeographic reconstructions. The exact location of the original type section of the Middle Jurassic Wanakah Formation of the eastern Colorado Plateau was found in Ouray County, Colorado. Two nearby reference sections are described that supplement the lithologic descriptions in the original type section. These reference sections and the original type section constitute a “principal reference section” for the Wanakah Formation. Informal names “upper shale beds”, “marl member”, and “beds at Sawpit” have been used to describe the uppermost member of the Wanakah. These terms are replaced with the formal name, “Crooked Tree Member of the Wanakah Formation”, in the reference sections to avoid confusion, foster consistent usage, and facilitate litho-stratigraphic correlation.
{"title":"Revisions to the Wanakah Formation in and around Ouray County, Colorado","authors":"L. S. Jones, C. Fenton","doi":"10.31582/rmag.mg.59.2.77","DOIUrl":"https://doi.org/10.31582/rmag.mg.59.2.77","url":null,"abstract":"A well-defined type section and consistent nomenclature are fundamental requirements for successful correlation of stratigraphic units and subsequent accurate paleogeographic reconstructions. The exact location of the original type section of the Middle Jurassic Wanakah Formation of the eastern Colorado Plateau was found in Ouray County, Colorado. Two nearby reference sections are described that supplement the lithologic descriptions in the original type section. These reference sections and the original type section constitute a “principal reference section” for the Wanakah Formation. Informal names “upper shale beds”, “marl member”, and “beds at Sawpit” have been used to describe the uppermost member of the Wanakah. These terms are replaced with the formal name, “Crooked Tree Member of the Wanakah Formation”, in the reference sections to avoid confusion, foster consistent usage, and facilitate litho-stratigraphic correlation.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2022-04-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127262767","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-04-28DOI: 10.31582/rmag.mg.59.2.47
S. Wigginton, E. Petrie, James P. Evans
This study integrates the results of numerical modeling analyses based on outcrop studies and structural kinematic restorations to evaluate the mechanics of thrust fault initiation and development in mechanically layered sedimentary rocks. A field-based reconstruction of a mesoscopic thrust fault at Ketobe Knob in central Utah provides evidence of thrust ramp nucleation in competent units, and fault propagation upward and downward into weaker units at both fault tips. We investigate the effects of mechanical stratigraphy on stress heterogeneity, rupture direction, fold formation, and fault geometry motivated by the geometry of the Ketobe Knob thrust fault in central Utah; the finite element modeling examines how mechanical stratigraphy, load conditions, and fault configurations influence temporal and spatial variation in stress and strain. Our modeling focuses on the predicted deformation and stress distributions in four model domains: (1) an intact, mechanically stratified rock sequence, (2) a mechanically stratified section with a range of interlayer frictional strengths, and two faulted models, (3) one with a stress loading condition, and (4) one with a displacement loading condition. The models show that early stress increase in competent rock layers are accompanied by low stresses in the weaker rocks. The frictional models reveal that the heterogeneous stress variations increase contact frictional strength. Faulted models with a 20° dipping fault in the most competent unit result in stress increases above and below fault tips, with extremely high stresses predicted in a ‘back thrust’ location at the lower fault tip. These findings support the hypothesis that thrust faults and associated folds at the Ketobe Knob developed in accordance with a ramp-first kinematic model and development of structures was significantly influenced by the nature of the mechanical stratigraphy.
{"title":"The mechanics of initiation and development of thrust faults and thrust ramps","authors":"S. Wigginton, E. Petrie, James P. Evans","doi":"10.31582/rmag.mg.59.2.47","DOIUrl":"https://doi.org/10.31582/rmag.mg.59.2.47","url":null,"abstract":"This study integrates the results of numerical modeling analyses based on outcrop studies and structural kinematic restorations to evaluate the mechanics of thrust fault initiation and development in mechanically layered sedimentary rocks. A field-based reconstruction of a mesoscopic thrust fault at Ketobe Knob in central Utah provides evidence of thrust ramp nucleation in competent units, and fault propagation upward and downward into weaker units at both fault tips. We investigate the effects of mechanical stratigraphy on stress heterogeneity, rupture direction, fold formation, and fault geometry motivated by the geometry of the Ketobe Knob thrust fault in central Utah; the finite element modeling examines how mechanical stratigraphy, load conditions, and fault configurations influence temporal and spatial variation in stress and strain. Our modeling focuses on the predicted deformation and stress distributions in four model domains: (1) an intact, mechanically stratified rock sequence, (2) a mechanically stratified section with a range of interlayer frictional strengths, and two faulted models, (3) one with a stress loading condition, and (4) one with a displacement loading condition. The models show that early stress increase in competent rock layers are accompanied by low stresses in the weaker rocks. The frictional models reveal that the heterogeneous stress variations increase contact frictional strength. Faulted models with a 20° dipping fault in the most competent unit result in stress increases above and below fault tips, with extremely high stresses predicted in a ‘back thrust’ location at the lower fault tip. These findings support the hypothesis that thrust faults and associated folds at the Ketobe Knob developed in accordance with a ramp-first kinematic model and development of structures was significantly influenced by the nature of the mechanical stratigraphy.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2022-04-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133329852","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Onion Creek salt diapir lies within the Paradox Basin of southeast Utah where it forms part of a group of salt structures that separate the Paradox Basin into smaller sub-basins. A series of anomalous, tight folds occur on the northern side of the Onion Creek diapir within the Permian Cutler Group. These folds are thought to be associated with a shallow detachment horizon with three possible origins: 1) a weak shale layer within the Cutler Group; 2) a salt namakier; or 3) a salt shoulder. We collected and analyzed gravity and magnetics data across a portion of the concealed Onion Creek salt body. Since the salt is less dense and less magnetic than the Cutler Group siliciclastics, these geophysical data aid in defining the extent of subsurface salt. Our gravity data show a free-air anomaly low over the diapir with a gradual increase in values as more of the Cutler Group covers the subsurface salt. Magnetic data display a similar trend, but also suggest more complicated 3-D structure exists beneath the study area. Forward and inverse modeling indicated a salt shoulder model best fit the geophysical data. These results suggest gravity and magnetic methods are a low-cost method to evaluate plausible subsurface salt structure for oil and gas exploration studies.
{"title":"Application of gravity and magnetic techniques to model the geometry of the northern margin of the Onion Creek salt diapir, Paradox Basin, Utah","authors":"Julia Astromovich, M. Baker, D. Doser, W. Houston","doi":"10.31582/rmag.mg.59.1.5","DOIUrl":"https://doi.org/10.31582/rmag.mg.59.1.5","url":null,"abstract":"The Onion Creek salt diapir lies within the Paradox Basin of southeast Utah where it forms part of a group of salt structures that separate the Paradox Basin into smaller sub-basins. A series of anomalous, tight folds occur on the northern side of the Onion Creek diapir within the Permian Cutler Group. These folds are thought to be associated with a shallow detachment horizon with three possible origins: 1) a weak shale layer within the Cutler Group; 2) a salt namakier; or 3) a salt shoulder. We collected and analyzed gravity and magnetics data across a portion of the concealed Onion Creek salt body. Since the salt is less dense and less magnetic than the Cutler Group siliciclastics, these geophysical data aid in defining the extent of subsurface salt. Our gravity data show a free-air anomaly low over the diapir with a gradual increase in values as more of the Cutler Group covers the subsurface salt. Magnetic data display a similar trend, but also suggest more complicated 3-D structure exists beneath the study area. Forward and inverse modeling indicated a salt shoulder model best fit the geophysical data. These results suggest gravity and magnetic methods are a low-cost method to evaluate plausible subsurface salt structure for oil and gas exploration studies.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2022-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"121242657","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-02-01DOI: 10.31582/rmag.mg.59.1.25
Jacqueline J. Epperson, J. Malone, Danika F. Mayback, D. Malone
Here we present the results of detailed (1:24,000 scale) mapping of the Woodrock 7.5 Minute Quadrangle Wyoming, which mainly consists of Archean basement rocks of the Laramide Bighorn uplift. Our focus was on the Archean geology of the Laramide age Bighorn uplift. Isotopic age determinations (U-Pb on zircon) were conducted at the University of Arizona LaserChron Center. Our work revealed the presence of four different components of the batholith. The oldest unit is a ~2880 Ma foliated Lookout Mountain Granodiorite that occurs in the southern part of the quadrangle in the vicinity of Bruce and Lookout Mountains. The ~2775 Black Mountain Tonalite occurs in the northeastern part of the quadrangle. The age of the Black Mountain Tonalite and Lookout Mountain Granodiorite overlap but the units are distinct in terms of structure and lithology so they were mapped separately. The central part of the quadrangle is underlain by the massive, ~2860 Ma Taylor Mine Granite. This unit is poorly exposed and variable in texture. The youngest unit is the ~2850 Ma Owen Creek Alkali Feldspar Granite, which occurs in the western part of the quadrangle. Mafic dikes of variable geometry, texture and age cross-cut the quartzofeldspathic rocks. Less than 25 m of poorly exposed Cambrian Flathead Sandstone occurs along the extreme western margin of the quadrangle. We discovered as much as 30 m of Oligocene White River Formation strata occur as terrace deposits along the western side of the Tongue River in the central part of the quadrangle. The White River strata are poorly exposed and consist of thin bedded tuffaceous sandstone and massive conglomerate that are light colors and include as clasts Paleozoic carbonate and Archean basement rocks. Less than 50 m of Pinedale-age glacial tills occur along the upper Tongue River in the southern part of the Quadrangle, forming hummocky, poorly-drained topography. Quaternary alluvium occurs along some of the larger streams.
{"title":"Geologic Map of the Woodrock Quadrangle, Sheridan and Big Horn Counties, Wyoming","authors":"Jacqueline J. Epperson, J. Malone, Danika F. Mayback, D. Malone","doi":"10.31582/rmag.mg.59.1.25","DOIUrl":"https://doi.org/10.31582/rmag.mg.59.1.25","url":null,"abstract":"Here we present the results of detailed (1:24,000 scale) mapping of the Woodrock 7.5 Minute Quadrangle Wyoming, which mainly consists of Archean basement rocks of the Laramide Bighorn uplift. Our focus was on the Archean geology of the Laramide age Bighorn uplift. Isotopic age determinations (U-Pb on zircon) were conducted at the University of Arizona LaserChron Center. Our work revealed the presence of four different components of the batholith. The oldest unit is a ~2880 Ma foliated Lookout Mountain Granodiorite that occurs in the southern part of the quadrangle in the vicinity of Bruce and Lookout Mountains. The ~2775 Black Mountain Tonalite occurs in the northeastern part of the quadrangle. The age of the Black Mountain Tonalite and Lookout Mountain Granodiorite overlap but the units are distinct in terms of structure and lithology so they were mapped separately. The central part of the quadrangle is underlain by the massive, ~2860 Ma Taylor Mine Granite. This unit is poorly exposed and variable in texture. The youngest unit is the ~2850 Ma Owen Creek Alkali Feldspar Granite, which occurs in the western part of the quadrangle. Mafic dikes of variable geometry, texture and age cross-cut the quartzofeldspathic rocks. Less than 25 m of poorly exposed Cambrian Flathead Sandstone occurs along the extreme western margin of the quadrangle. We discovered as much as 30 m of Oligocene White River Formation strata occur as terrace deposits along the western side of the Tongue River in the central part of the quadrangle. The White River strata are poorly exposed and consist of thin bedded tuffaceous sandstone and massive conglomerate that are light colors and include as clasts Paleozoic carbonate and Archean basement rocks. Less than 50 m of Pinedale-age glacial tills occur along the upper Tongue River in the southern part of the Quadrangle, forming hummocky, poorly-drained topography. Quaternary alluvium occurs along some of the larger streams.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2022-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123938070","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-10-27DOI: 10.31582/rmag.mg.58.4.433
J. Bader
Casper Mountain is an E–W trending anticlinal structure that is bound on the north by the oblique-slip Casper Mountain fault. The fault is postulated to reflect preexisting Precambrian structure/fabrics that were reactivated and/or guided deformation during the Laramide orogeny. A structural analysis of the fault zone and surrounding area was conducted to confirm this hypothesis, and to garner insight into both Precambrian origins and Laramide kinematics. Surface and subsurface data for structural analysis was collected and synthesized from numerous published sources along the proposed deformation corridor that roughly coincides with the Oregon Trail structural belt of central Wyoming. The Casper Mountain fault zone is characterized by an E–W rectilinear zone of en échelon, steeply inclined faults. The Casper Mountain fault strikes E–W with smaller faults in the zone striking N65°E. Folds trend to the WNW and are left-stepping. Foliations in Precambrian rocks of Casper Mountain are oriented subparallel to the Casper Mountain fault. The North Granite Mountains fault zone is located due west of Casper Mountain and is similarly oriented E–W with associated faults striking NE, NW/SE, and ENE/WSW, off the dominant master fault. Curvilinear, left-stepping, en échelon folds trend to the northwest and are truncated on the south by the North Granite Mountains fault. Faults in basement rocks of the Popo Agie Primitive Area of the central Wind River Mountains are characterized by moderate to high-angle faults striking E–W, NNW, and NE that coincide with mapped surface lineaments and fabric data. Fabric data suggest that Laramide deformation along the Casper Mountain fault was guided by Precambrian anisotropies. Surface and subsurface mapping of the fault zone and the deformation corridor to the west indicate that the Casper Mountain and North Granite Mountains faults are part of a basement-rooted system (wrench fault) that likely extends westward into the Popo Agie Primitive Area. Here, the steeply inclined (75–90°) proposed master fault is exposed within a WNW-striking corridor of faults that sinistrally offset steeply dipping, NE-striking Proterozoic diabase dikes. The dikes likely intruded older faults that are antithetic to the WNW-striking faults. Other faults strike to the NNW and have shallower dips of 45–65°. These three directions of anisotropy (WNW, NE, and NNW) are proposed to have formed from SW–NE-directed subduction along a long-lived, Neoarchean, active continental margin.
{"title":"Structural analysis of the Casper Mountain fault zone and area, Wyoming surrounding area, Wyoming: Implications for Laramide kinematics and structural inheritance across the Wyoming Province","authors":"J. Bader","doi":"10.31582/rmag.mg.58.4.433","DOIUrl":"https://doi.org/10.31582/rmag.mg.58.4.433","url":null,"abstract":"Casper Mountain is an E–W trending anticlinal structure that is bound on the north by the oblique-slip Casper Mountain fault. The fault is postulated to reflect preexisting Precambrian structure/fabrics that were reactivated and/or guided deformation during the Laramide orogeny. A structural analysis of the fault zone and surrounding area was conducted to confirm this hypothesis, and to garner insight into both Precambrian origins and Laramide kinematics. Surface and subsurface data for structural analysis was collected and synthesized from numerous published sources along the proposed deformation corridor that roughly coincides with the Oregon Trail structural belt of central Wyoming. The Casper Mountain fault zone is characterized by an E–W rectilinear zone of en échelon, steeply inclined faults. The Casper Mountain fault strikes E–W with smaller faults in the zone striking N65°E. Folds trend to the WNW and are left-stepping. Foliations in Precambrian rocks of Casper Mountain are oriented subparallel to the Casper Mountain fault. The North Granite Mountains fault zone is located due west of Casper Mountain and is similarly oriented E–W with associated faults striking NE, NW/SE, and ENE/WSW, off the dominant master fault. Curvilinear, left-stepping, en échelon folds trend to the northwest and are truncated on the south by the North Granite Mountains fault. Faults in basement rocks of the Popo Agie Primitive Area of the central Wind River Mountains are characterized by moderate to high-angle faults striking E–W, NNW, and NE that coincide with mapped surface lineaments and fabric data. Fabric data suggest that Laramide deformation along the Casper Mountain fault was guided by Precambrian anisotropies. Surface and subsurface mapping of the fault zone and the deformation corridor to the west indicate that the Casper Mountain and North Granite Mountains faults are part of a basement-rooted system (wrench fault) that likely extends westward into the Popo Agie Primitive Area. Here, the steeply inclined (75–90°) proposed master fault is exposed within a WNW-striking corridor of faults that sinistrally offset steeply dipping, NE-striking Proterozoic diabase dikes. The dikes likely intruded older faults that are antithetic to the WNW-striking faults. Other faults strike to the NNW and have shallower dips of 45–65°. These three directions of anisotropy (WNW, NE, and NNW) are proposed to have formed from SW–NE-directed subduction along a long-lived, Neoarchean, active continental margin.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2021-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128758429","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-10-27DOI: 10.31582/RMAG.MG.58.4.375
J. Lorenz, S. Cooper
Fractures in Cretaceous and early Tertiary strata record several deformation events that were imposed on the formations that fill the Raton Basin in Colorado and New Mexico. A regional, generally WNW-ESE striking extension-fracture set is present across much of the basin, but fracturing also includes both dip-slip and strike-slip conjugate shear-fracture sets as well as irregular deformation-band shear fractures. In some areas of the basin, the extension fractures are dynamically-compatible with associated conjugate shear fractures, both recording a maximum compressive stress that was horizontal, trending predominantly WNW-ESE. Fracture strikes vary from NW-SE to ENE-WSW but are approximately normal to the front of the Laramide thrust-fault system that forms the western edge of the basin, implying that fracturing was the result of a horizontal compressive stress anisotropy created by indentation of the thrust system into the basin margin. Fracture anomalies occur over local structures including a N-S basement wrench-fault system that connects two large anticlines within the basin, the Tercio and Vermejo Park anticlines, where N-S strike-slip offset along the basement wrench faults caused folding and fracturing in the overlying strata. The Laramide stress system in the basin changed from thrust-related WNW-ESE horizontal compression to the present-day N-S maximum horizontal compressive stress in mid-Tertiary time as the thrust system became inactive and was replaced by regional E-W extension. No new fracture sets were formed by the re-oriented stress system, although stress-release fractures normal to the regional set formed in outcrops as overlying strata were eroded. Fracture datasets were derived from three sources during this study: outcrops, image logs, and cores. Each source provides a somewhat different perspective on the fractures that enhance permeability in Raton Basin reservoirs. Taken together, the three datasets provide the basis for a relatively complete conceptual model of the Raton fracture system. Fracture-controlled permeability anisotropy will be greatest in the WNW-ESE direction, parallel to the strike of the dominant set of Laramide-age natural fractures, but hydraulic stimulation fractures will propagate N-S, across the strike of those fractures under the influence of the present-day stress system. The apertures of the WNW-ESE fractures will be susceptible to closure under that stress system since the maximum horizontal compressive stress is approximately normal to fracture strike.
{"title":"Natural fractures and their relationships to structure, stress, and permeability in the Raton Basin","authors":"J. Lorenz, S. Cooper","doi":"10.31582/RMAG.MG.58.4.375","DOIUrl":"https://doi.org/10.31582/RMAG.MG.58.4.375","url":null,"abstract":"Fractures in Cretaceous and early Tertiary strata record several deformation events that were imposed on the formations that fill the Raton Basin in Colorado and New Mexico. A regional, generally WNW-ESE striking extension-fracture set is present across much of the basin, but fracturing also includes both dip-slip and strike-slip conjugate shear-fracture sets as well as irregular deformation-band shear fractures. In some areas of the basin, the extension fractures are dynamically-compatible with associated conjugate shear fractures, both recording a maximum compressive stress that was horizontal, trending predominantly WNW-ESE. Fracture strikes vary from NW-SE to ENE-WSW but are approximately normal to the front of the Laramide thrust-fault system that forms the western edge of the basin, implying that fracturing was the result of a horizontal compressive stress anisotropy created by indentation of the thrust system into the basin margin. Fracture anomalies occur over local structures including a N-S basement wrench-fault system that connects two large anticlines within the basin, the Tercio and Vermejo Park anticlines, where N-S strike-slip offset along the basement wrench faults caused folding and fracturing in the overlying strata. The Laramide stress system in the basin changed from thrust-related WNW-ESE horizontal compression to the present-day N-S maximum horizontal compressive stress in mid-Tertiary time as the thrust system became inactive and was replaced by regional E-W extension. No new fracture sets were formed by the re-oriented stress system, although stress-release fractures normal to the regional set formed in outcrops as overlying strata were eroded. Fracture datasets were derived from three sources during this study: outcrops, image logs, and cores. Each source provides a somewhat different perspective on the fractures that enhance permeability in Raton Basin reservoirs. Taken together, the three datasets provide the basis for a relatively complete conceptual model of the Raton fracture system. Fracture-controlled permeability anisotropy will be greatest in the WNW-ESE direction, parallel to the strike of the dominant set of Laramide-age natural fractures, but hydraulic stimulation fractures will propagate N-S, across the strike of those fractures under the influence of the present-day stress system. The apertures of the WNW-ESE fractures will be susceptible to closure under that stress system since the maximum horizontal compressive stress is approximately normal to fracture strike.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2021-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127958269","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-08-02DOI: 10.31582/rmag.mg.58.3.305
Virginia Gent, R. Bottjer, M. Longman, J. Hagadorn
Core data from five key wells spanning the Denver Basin were tied to wireline log data and used to interpret the distribution of the Middle Turonian Codell Sandstone Member of the Carlile Formation across the Denver Basin. The character of the Codell’s upper contact is sharp with a localized top-down truncation across the basin, which is consistent with an associated unconformity surface. In contrast, the Codell’s lower contact varies from being gradational in most of the southern Denver Basin to being unconformable in the northern basin. Log correlations reveal that the Codell is absent within an elongate northeast-trending swath up to 125 miles wide in northeastern Colorado. This elongate gap is herein referred to as the ‘No Codell Zone’ abbreviated as NoCoZo. Hypotheses to explain the absence of the Codell Sandstone in the NoCoZo include a lateral facies change from sandstone to shale, non-deposition of Codell-equivalent sediments across this area, post-depositional erosion, or a combination of these processes. Correlation of wireline logs across the northern and southern limits of the NoCoZo, combined with outcrop and core observations, suggest top-down erosion of the Codell increasing into the NoCoZo. However, the overlying Fort Hays Limestone is laterally continuous and has a relatively consistent thickness across the NoCoZo, suggesting two tenable hypotheses: 1) The NoCoZo represents an area of post-Codell erosion due to short-lived growth of a broad, low relief uplift that was no longer active during Fort Hays deposition; or 2) A stepped sea level fall and forced regression resulting in non-deposition of the Codell over this broad swath. North of the NoCoZo, the Codell thickens northward to more than 40 ft into adjacent parts of Wyoming and Nebraska. In this northern area, the Codell has two main lithofacies in three laterally correlative zones, in ascending order: a lower bioturbated siltstone to very fine-grained sandstone ranging from 2 to 20 feet thick, a middle 2 to 10-foot thick laminated to bedded siltstone to fine-grained sandstone, and an upper 5 to 20-foot thick bioturbated siltstone to very fine-grained sandstone. Southeast of the NoCoZo the Codell thickens to as much as 80 feet in an east-trending belt from Pueblo, Colorado, into west central Kansas. The southern Codell can be divided into two coarsening upward parasequences, from a basal muddy coarse siltstones to very fine-grained sandstones. The siltstones and sandstones in the southern Codell are mostly bioturbated with locally developed bedded facies at the top.
{"title":"Stratigraphic distribution of the Codell Sandstone in the Denver Basin using wireline logs and core","authors":"Virginia Gent, R. Bottjer, M. Longman, J. Hagadorn","doi":"10.31582/rmag.mg.58.3.305","DOIUrl":"https://doi.org/10.31582/rmag.mg.58.3.305","url":null,"abstract":"Core data from five key wells spanning the Denver Basin were tied to wireline log data and used to interpret the distribution of the Middle Turonian Codell Sandstone Member of the Carlile Formation across the Denver Basin. The character of the Codell’s upper contact is sharp with a localized top-down truncation across the basin, which is consistent with an associated unconformity surface. In contrast, the Codell’s lower contact varies from being gradational in most of the southern Denver Basin to being unconformable in the northern basin. Log correlations reveal that the Codell is absent within an elongate northeast-trending swath up to 125 miles wide in northeastern Colorado. This elongate gap is herein referred to as the ‘No Codell Zone’ abbreviated as NoCoZo. Hypotheses to explain the absence of the Codell Sandstone in the NoCoZo include a lateral facies change from sandstone to shale, non-deposition of Codell-equivalent sediments across this area, post-depositional erosion, or a combination of these processes. Correlation of wireline logs across the northern and southern limits of the NoCoZo, combined with outcrop and core observations, suggest top-down erosion of the Codell increasing into the NoCoZo. However, the overlying Fort Hays Limestone is laterally continuous and has a relatively consistent thickness across the NoCoZo, suggesting two tenable hypotheses: 1) The NoCoZo represents an area of post-Codell erosion due to short-lived growth of a broad, low relief uplift that was no longer active during Fort Hays deposition; or 2) A stepped sea level fall and forced regression resulting in non-deposition of the Codell over this broad swath. North of the NoCoZo, the Codell thickens northward to more than 40 ft into adjacent parts of Wyoming and Nebraska. In this northern area, the Codell has two main lithofacies in three laterally correlative zones, in ascending order: a lower bioturbated siltstone to very fine-grained sandstone ranging from 2 to 20 feet thick, a middle 2 to 10-foot thick laminated to bedded siltstone to fine-grained sandstone, and an upper 5 to 20-foot thick bioturbated siltstone to very fine-grained sandstone. Southeast of the NoCoZo the Codell thickens to as much as 80 feet in an east-trending belt from Pueblo, Colorado, into west central Kansas. The southern Codell can be divided into two coarsening upward parasequences, from a basal muddy coarse siltstones to very fine-grained sandstones. The siltstones and sandstones in the southern Codell are mostly bioturbated with locally developed bedded facies at the top.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2021-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"121900905","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-08-02DOI: 10.31582/rmag.mg.58.3.355
S. Cumella
A continuous Codell Sandstone oil accumulation is present in the northern Denver Basin downdip from water-wet Codell. The Codell oil accumulation can be defined by resistivity, spontaneous potential (SP), and density logs. Updip from the oil accumulation, average deep resistivity of the Codell decreases to below 4 ohm-m, SP response increases, and density porosity increases. Codell sandstones are continuous across the transition from downdip oil to updip water, so the updip seal does not seem to be caused by a stratigraphic trap. The transition corresponds to a change in thermal history; the area of the oil accumulation was subject to much higher heat flow than the updip wet area. This thermal maturity may have had an impact on clay diagenesis resulting in reduced porosity in the more thermally mature part of the Codell. This paper presents a wireline log-based workflow that can be used to identify and map regional changes in thermal maturity that control hydrocarbon accumulations and sweet-spots in low-permeability rocks such as the argillaceous Codell Sandstone.
{"title":"Codell continuous oil accumulation in the northern Denver and SP logs Basin as defined by resistivity, density, and SP logs","authors":"S. Cumella","doi":"10.31582/rmag.mg.58.3.355","DOIUrl":"https://doi.org/10.31582/rmag.mg.58.3.355","url":null,"abstract":"A continuous Codell Sandstone oil accumulation is present in the northern Denver Basin downdip from water-wet Codell. The Codell oil accumulation can be defined by resistivity, spontaneous potential (SP), and density logs. Updip from the oil accumulation, average deep resistivity of the Codell decreases to below 4 ohm-m, SP response increases, and density porosity increases. Codell sandstones are continuous across the transition from downdip oil to updip water, so the updip seal does not seem to be caused by a stratigraphic trap. The transition corresponds to a change in thermal history; the area of the oil accumulation was subject to much higher heat flow than the updip wet area. This thermal maturity may have had an impact on clay diagenesis resulting in reduced porosity in the more thermally mature part of the Codell. This paper presents a wireline log-based workflow that can be used to identify and map regional changes in thermal maturity that control hydrocarbon accumulations and sweet-spots in low-permeability rocks such as the argillaceous Codell Sandstone.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2021-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114753749","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-08-02DOI: 10.31582/rmag.mg.58.3.211
J. Hagadorn, M. Longman, R. Bottjer, Virginia Gent, C. Holm-Denoma, J. Sumrall
We formally assign, describe and interpret a principal reference section for the middle Turonian Codell Sandstone Member of the Carlile Shale near Codell, Kansas. This section, at the informally named Pumpjack Road, provides the thickest surface expression (9 m, ~30 ft) of the unit in Ellis County. The outcrop exposes features that typify the Codell throughout the southern Denver Basin and vicinity. At this reference section, the Codell conformably overlies the Blue Hill Shale Member of the Carlile Shale and is unconformably overlain by the Fort Hays Limestone Member of the Niobrara Formation or locally by a thin (<0.9 m, <3 ft) discontinuous mudstone known as the Antonino facies. The top contact of the Codell is slightly undulatory with possible compaction features or narrow (<30.5 m, <100 ft), low-relief (0.3-0.6 m, 1-2 ft) scours, all of which hint that the Codell is a depositional remnant, even at the type section. At Pumpjack Road, the Codell coarsens upward from a recessive-weathering argillaceous medium-grained siltstone with interbedded mudstone at its base to a more indurated cliff-forming muddy, highly bioturbated, very fine-grained sandstone at its top. The unit contains three informal gradational packages: a lower Codell of medium to coarse siltstone and mudstone, a middle Codell of muddy coarse siltstone, and an upper muddy Codell dominated by well-sorted very fine-grained sandstone. The largest grain fractions, all <120 mm in size, are mostly quartz (40-80%), potassium feldspar (7-12%), and albite (1-2%), with some chert (<15%), zircon, and other constituents such as abraded phosphatic skeletal debris. Rare fossil fish teeth and bones also occur. Detrital and authigenic clays make up 9 to 42% of the Codell at the reference section. Detrital illite and mixed layer illite/smectite are common, along with omnipresent kaolinite as grain coatings or cement. As is typical for the Codell, the sandstone at the type section has been pervasively bioturbated. Most primary structures and bedding are obscured, particularly toward the top of the unit where burrows are larger, deeper and more diverse than at its base. This bioturbation has created a textural inversion in which the larger silt and sand grains are very well sorted but are mixed with mud. Detrital zircons from the upper Codell are unusual in that they are mostly prismatic to acicular, euhedral, colorless, unpitted, and unabraded, and have a near-unimodal age peak centered at ~94 Ma. These characteristics suggest they were reworked mainly from Cenomanian bentonites; their ultimate source was likely from the Cordilleran orogenic belt to the west and northwest.
{"title":"The type section of the Codell Sandstone","authors":"J. Hagadorn, M. Longman, R. Bottjer, Virginia Gent, C. Holm-Denoma, J. Sumrall","doi":"10.31582/rmag.mg.58.3.211","DOIUrl":"https://doi.org/10.31582/rmag.mg.58.3.211","url":null,"abstract":"We formally assign, describe and interpret a principal reference section for the middle Turonian Codell Sandstone Member of the Carlile Shale near Codell, Kansas. This section, at the informally named Pumpjack Road, provides the thickest surface expression (9 m, ~30 ft) of the unit in Ellis County. The outcrop exposes features that typify the Codell throughout the southern Denver Basin and vicinity. At this reference section, the Codell conformably overlies the Blue Hill Shale Member of the Carlile Shale and is unconformably overlain by the Fort Hays Limestone Member of the Niobrara Formation or locally by a thin (<0.9 m, <3 ft) discontinuous mudstone known as the Antonino facies. The top contact of the Codell is slightly undulatory with possible compaction features or narrow (<30.5 m, <100 ft), low-relief (0.3-0.6 m, 1-2 ft) scours, all of which hint that the Codell is a depositional remnant, even at the type section. At Pumpjack Road, the Codell coarsens upward from a recessive-weathering argillaceous medium-grained siltstone with interbedded mudstone at its base to a more indurated cliff-forming muddy, highly bioturbated, very fine-grained sandstone at its top. The unit contains three informal gradational packages: a lower Codell of medium to coarse siltstone and mudstone, a middle Codell of muddy coarse siltstone, and an upper muddy Codell dominated by well-sorted very fine-grained sandstone. The largest grain fractions, all <120 mm in size, are mostly quartz (40-80%), potassium feldspar (7-12%), and albite (1-2%), with some chert (<15%), zircon, and other constituents such as abraded phosphatic skeletal debris. Rare fossil fish teeth and bones also occur. Detrital and authigenic clays make up 9 to 42% of the Codell at the reference section. Detrital illite and mixed layer illite/smectite are common, along with omnipresent kaolinite as grain coatings or cement. As is typical for the Codell, the sandstone at the type section has been pervasively bioturbated. Most primary structures and bedding are obscured, particularly toward the top of the unit where burrows are larger, deeper and more diverse than at its base. This bioturbation has created a textural inversion in which the larger silt and sand grains are very well sorted but are mixed with mud. Detrital zircons from the upper Codell are unusual in that they are mostly prismatic to acicular, euhedral, colorless, unpitted, and unabraded, and have a near-unimodal age peak centered at ~94 Ma. These characteristics suggest they were reworked mainly from Cenomanian bentonites; their ultimate source was likely from the Cordilleran orogenic belt to the west and northwest.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2021-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134013685","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-08-02DOI: 10.31582/rmag.mg.58.3.331
S. Sonnenberg, J. Zumberge, J. Curtis
Carrier-bed plays are an emerging type of unconventional oil play in which reservoirs are generally of low quality because they are characterized by: 1) thinly bedded heterolithic strata; 2) significant compaction and/or diagenesis; and 3) burrowing that has mixed sandstones and mudstone lithologies (i.e., heterogeneous lithologies). In this type of play, the carrier beds are pervasively hydrocarbon saturated and can be areally extensive (>50 mi2 or 130 km2). These low-quality reservoirs generally do not meet traditional petrophysical cutoffs and because of their high clay contents can have low resistivity, low contrast pays. The reservoirs may be composed of siliciclastics or carbonates or both. Due to reservoir quality and degree of oil migration, carrier-bed plays like the Codell are being developed with horizontal drilling and multistage hydraulic fracturing. Traditional vertical drilling yields marginal to uneconomic wells that can provide a clue to the existence of a carrier-bed play. The Codell Sandstone is a low-resistivity, low-contrast pay in parts of the northern Denver Basin. The area of oil and gas production is in the deeper part of the basin between and including Silo and Wattenberg fields of Wyoming and Colorado, respectively. The thickness of the Codell in this part of the Denver Basin ranges from 15 to 25 ft (4.5 to 7.6 m). Keys to Codell production are source rock maturity, and oil entrapment in the carrier bed. Oil in the Codell carrier-bed traps was generated in various intervals including the Niobrara (mainly the “B” marl), Sharon Springs Member of the Pierre Shale, Greenhorn/Carlile, and, rarely, the Mowry Shale.
{"title":"Codell carrier-bed play, Denver Basin","authors":"S. Sonnenberg, J. Zumberge, J. Curtis","doi":"10.31582/rmag.mg.58.3.331","DOIUrl":"https://doi.org/10.31582/rmag.mg.58.3.331","url":null,"abstract":"Carrier-bed plays are an emerging type of unconventional oil play in which reservoirs are generally of low quality because they are characterized by: 1) thinly bedded heterolithic strata; 2) significant compaction and/or diagenesis; and 3) burrowing that has mixed sandstones and mudstone lithologies (i.e., heterogeneous lithologies). In this type of play, the carrier beds are pervasively hydrocarbon saturated and can be areally extensive (>50 mi2 or 130 km2). These low-quality reservoirs generally do not meet traditional petrophysical cutoffs and because of their high clay contents can have low resistivity, low contrast pays. The reservoirs may be composed of siliciclastics or carbonates or both. Due to reservoir quality and degree of oil migration, carrier-bed plays like the Codell are being developed with horizontal drilling and multistage hydraulic fracturing. Traditional vertical drilling yields marginal to uneconomic wells that can provide a clue to the existence of a carrier-bed play. The Codell Sandstone is a low-resistivity, low-contrast pay in parts of the northern Denver Basin. The area of oil and gas production is in the deeper part of the basin between and including Silo and Wattenberg fields of Wyoming and Colorado, respectively. The thickness of the Codell in this part of the Denver Basin ranges from 15 to 25 ft (4.5 to 7.6 m). Keys to Codell production are source rock maturity, and oil entrapment in the carrier bed. Oil in the Codell carrier-bed traps was generated in various intervals including the Niobrara (mainly the “B” marl), Sharon Springs Member of the Pierre Shale, Greenhorn/Carlile, and, rarely, the Mowry Shale.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2021-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115240251","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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