Southwest Wyoming Research Articles

Sequence Stratigraphic and Synsedimentary Tectonic Controls on Reservoir Compartmentalization in a Transgressive Sequence Set: Almond Formation, Southwest Wyoming: ABSTRACT

The Campanian upper Almond Formation in Southwestern Wyoming contains at least 15 aggradational to backstepping microtidal to low mesotidal barrier/shoreline complexes laid down during a period of net transgression from 72 to 70.5 million years ago. Reservoir compartmentalization in the upper Almond occurs at several scales, including an aggradational to retrogradations sequence set composed of 3 retrogradational parasequence sets; numerous parasequences, and diverse barrier sub-facies units. The lowstand shorelines of these sequence sets stack aggradationally prior to transgression by a really extensive, marine mudstone horizons which separate the sequences. Highstand systems tracts are poorly preserved, often completely removed below fourth of fifth order sequence boundaries which cause seaward jumps of facies in excess of 30 Km and place fluvial sediment, coal and lagoonal deposits abruptly over marine mudstone. Each sequence in the upper Almond is composed of several parasequences (sanding-upward, storm-dominated barrier shorefaces) which intercalate with marine mudstone to the east and grade into oyster-bearing, organic-rich lagoonal mudstone to the west. Compartmentalization in the barrier complexes occurs at most parasequence boundaries and in association with major sub-facies; boundaries (barrier margins, tidal inlets, flood-tidal deltas, washover fans). Further reservoir compartmentalization is induced by synsedimentary faulting and subsidence which locally preserve isolatedmore » reservoir-quality barrier/shoreline sandstone bodies by dropping them below the depth of ravinement (5-30 m). The recognition of synsedimentary faulting and subsequent ravinement is critical to accurate sequence stratigraphic analysis and for prediction of reservoir compartments.« less

Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt

Research Article| January 01, 1996 Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt Kurt N. Constenius Kurt N. Constenius 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information Kurt N. Constenius 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1996) 108 (1): 20–39. https://doi.org/10.1130/0016-7606(1996)108<0020:LPECOT>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kurt N. Constenius; Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt. GSA Bulletin 1996;; 108 (1): 20–39. doi: https://doi.org/10.1130/0016-7606(1996)108<0020:LPECOT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle Eocene to early Miocene (ca. 49–20 Ma) episode of crustal extension. The sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern Canada to central Utah. Extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy, structural relief, and sole faults of preexisting thrust-fold structures. The sole faults dip 3°–6° west above an undeformed Precambrian crystalline basement and accommodated tectonic transport of a thick (up to 20+ km) eastward-tapering hanging wall during regimes of crustal shortening and extension. The chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces. Dated movement on two extensionally reactivated thrusts, the Lewis thrust of northwest Montana and southeast British Columbia and the Medicine Butte thrust of southwest Wyoming and northeast Utah, suggests that the hiatus between the end of crustal shortening in the early or early middle Eocene and the start of extension in the early middle Eocene was brief.Lateral spreading and extensional basin formation in the Cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism. Conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late Paleogene Cordilleran orogenic wedge is accomplished by applying the extensional-wedge Coulomb critical-taper model. The rapid drop in North America–Pacific plate convergence rate and/or steepening of the subducted oceanic slab at ca. 50 Ma resulted in a large reduction in east-west horizontal compressive stress in the Cordillera. As a result, the Cordilleran orogenic wedge was left unsupported, and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca. 20 Ma. Subsequently, crustal extension and magmatism during the Basin and Range event (ca. 17–0 Ma) overprinted much of this earlier phase of extension. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Trona resources in southwest Wyoming

Bedded trona (Na2CO3·NaHCO3·2H2O) in the lacustrine Green River Formation of Eocene age in the Green River Basin, southwest Wyoming, constitutes the largest known resource of natural sodium carbonate in the world. In this study, 116 gigatons (Gt) of trona ore are estimated to be present in 22 beds, ranging from 1.2 to 11 meters (m) in thickness. Of this total, 69 Gt of trona ore are estimated to be in beds containing less than 2 percent halite and 47 Gt in beds containing 2 or more percent halite. These 22 beds underlie areas of about 130 to more than 2,000 km2 at depths ranging from about 200 m to more than 900 m below the surface. The total resource of trona ore in the basin for which drilling information is available is estimated to be about 135 Gt.

Genetic Sequence Stratigraphy of the Intermontane Paleocene Fort Union Formation, Greater Green River Basin, Southwest Wyoming and Northwest Colorado: ABSTRACT

Genetic Sequence Stratigraphy of the Intermontane Paleocene Fort Union Formation, Greater Green River Basin, Southwest Wyoming and Northwest Colorado: ABSTRACT

History of the Sevier orogenic wedge in terms of critical taper models, northeast Utah and southwest Wyoming

Problems with applying critical taper models to ancient orogenic wedges are overcome in the Late Cretaceous–late Paleocene Sevier orogenic wedge in Utah and Wyoming by a symptomatic approach in which wedge behavior is inferred from the time-space distribution of thrust faulting, erosion, and synorogenic sediment accumulation in association with the orogenic wedge. In turn, the causes of wedge behavior are interpreted in terms of features that are known about the Sevier wedge, such as the locations of major decollements and the lithologic constituents of major thrust sheets. From Coniacian through late Paleocene time (∼35 m.y.), basement and cover rocks in the Sevier wedge were shortened by ∼100 km in three major and one minor events. An overall eastward progression of thrusting was punctuated by several episodes of out-of-sequence and hinterland vergent thrusting. Over the long term, wedge taper was controlled by internal wedge strength, initial taper, and changes in the durability of the upper surface of the wedge and its internal lithostratigraphy. Critical taper was maintained by the offsetting effects of basement duplexing in the rear of the wedge and forward imbrication at the front of the wedge. While the basal decollement was mainly in basement rocks, the wedge was highly tapered and thrust spacing was relatively close (∼15 km). Beginning ca. 75 Ma, the basal decollement ramped upward and eastward into weak Cambrian shale and Jurassic salt, thrust spacing increased to 25–45 km, and wedge taper decreased drastically. The highly tapered rear part of the wedge carried strong basement rocks above moderately weak mylonitic and cataclastic decollements, whereas the frontal, lower-taper part of the wedge carried sedimentary rocks above extremely weak (salt and shale) decollements. Through time, internal strength of the wedge decreased as it incorporated an increasing proportion of sedimentary rocks; the thrust belt was able to advance eastward in spite of its decreasing internal strength and decreasing taper because the basal decollement of the wedge also became weaker through time, both by strain softening (beneath the rear of the wedge) and by propagating along extremely weak rocks (beneath the front of the wedge). Provenance data from associated synorogenic conglomerates indicate that the durability of the upper surface of the wedge probably increased over the long-term history of the Sevier wedge, as an increasing proportion of the erosional surface became occupied by resistant Precambrian quartzite and basement rocks. This would have had the effect of decreasing the rate of erosion through time and maintaining a steeper surface slope, which would have helped the wedge to remain at critical to supercritical taper. The Sevier wedge also exhibits several shorter-term ( 10 km, and (3) was subjected to regional erosion (marked by major unconformities) that ultimately caught up with tectonic thickening and caused the wedge to stall. This cyclicity is attributed to the need for wedges to become supercritical before they can propagate forward over large distances and a lag-time between wedge thickening (and uplift) and wedge erosion. Erosion eventually catches up to the rate of uplift, the wedge stalls, and the locus of deformation shifts to the rear of the wedge in order to rebuild taper. This analysis suggests that orogenic wedges continually deform and “find a way” to maintain taper sufficient to allow forward propagation as long as a sufficient push from the rear exists. The actual taper value may change substantially through time as lithostratigraphy, internal strength, upper surface durability, and presence/absence of weak potential stress guides conspire to maintain a critical taper.

Late Cretaceous-Paleocene synorogenic sedimentation and kinematic history of the Sevier thrust belt, northeast Utah and southwest Wyoming

Integration of cross-cutting structural relationships, overlapping sedimentary units, new conglomerate provenance data, and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of Late Cretaceous through Paleocene thrust faulting in the northeast Utah-southwest Wyoming part of the Sevier thrust belt. These data indicate a general eastward progression of deformation that was punctuated by local out-of-sequence and hinterlandward-verging events. Provenance data delimit a sequential restoration of a regional cross section. The principal thrust systems in northeast Utah and southwest Wyoming are the Willard, Ogden, Crawford, Absaroka, and Hogsback thrusts. The Willard is the westernmost, structurally highest, and oldest of the thrusts; it carries a unique section of thick Proterozoic sedimentary rocks, as well as Paleozoic rocks, and was folded during displacement on younger thrust systems. The next youngest thrust system is the Ogden, which comprises several basement-rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the Wasatch Range. Growth of the Wasatch culmination took place during Coniacian through Paleocene time, contemporaneous with sequential displacement on the frontal Crawford, Absaroka, and Hogsback thrusts. Total shortening in this part of the thrust belt was ∼100 km, and structural relief of ∼25 km developed in the area of the Wasatch culmination. During Coniacian through Paleocene time, thrusting followed an overall eastward progression that was interrupted by local out-of-sequence and hinterlandward-verging events. Several episodes of synchronous displacement on two or more thrusts can be demonstrated. Shortening occurred in three main episodes. The first episode (∼89-84 Ma) involved ∼33 km of shortening on the Crawford thrust and its footwall imbricates. Approximately 19 km of structural relief on the basement-cover contact developed in the area of the Wasatch culmination. The second episode (∼84-62 Ma, with a break between ∼75-69 Ma) involved ∼30 km of shortening, mainly on the Absaroka thrust, and development of an additional ∼6 km of structural relief in the culmination. The third episode of shortening (∼56-50 Ma) took place on the Hogsback thrust, involved ∼21 km of horizontal shortening, and produced no significant increase in structural relief in the culmination. Long-term rates of shortening ranged between 3.0 mm/yr and 6.6 mm/yr. These three episodes of shortening produced three large accumulations of synorogenic conglomerate, totaling ∼3 km in thickness. The Henefer Formation and Echo Canyon and Weber Canyon Conglomerates were deposited during Crawford thrusting. The Evanston Formation was deposited during and after Absaroka thrusting, and the lower conglomeratic part of the Wasatch Formation was deposited during and after Hogsback thrusting. Most of the sediment in these synorogenic units, however, was derived from repeated uplift of the Willard thrust sheet and from the eastern flank of the Wasatch culmination in the rear part of the thrust belt. Only local, minor accumulations were derived from the frontal ramp anticlines. Sediment accumulation and structural deformation were generally out-of-phase. Periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin. Periods of structural inactivity were marked by accumulation of aerially widespread, braided-river conglomerate on top of the thrust belt. One exception to this pattern is the Henefer-Echo Canyon-Weber Canyon conglomerate deposit, which contains evidence of progressive deformation in close proximity to the tip of the Crawford thrust. Comparison of the sequential restoration with the Late Cretaceous subsidence history and isopach patterns in the distal foreland basin of western Wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the Wasatch culmination. Growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge.

Exploration and Development of Almond Tight Gas Sands Along the Wamsutter/Creston Arch, Wasahakie-Red Desert Basins, Southwest Wyoming: ABSTRACT

Exploration and Development of Almond Tight Gas Sands Along the Wamsutter/Creston Arch, Wasahakie-Red Desert Basins, Southwest Wyoming: ABSTRACT

Paleostructural Control of Dakota Hydrocarbon Accumulations on Southern Moxa Arch, Southwest Wyoming and Northeast Utah: ABSTRACT

Paleostructural Control of Dakota Hydrocarbon Accumulations on Southern Moxa Arch, Southwest Wyoming and Northeast Utah: ABSTRACT

Lagoonal deposits in the Upper Cretaceous Rock Springs Formation (Mesaverde Group), southwest Wyoming

Most paleogeographic reconstructions of the Rock Springs Formation show shorelines having lobate to arcuate deltas. These shorelines are oriented NE-SW, with the sea to the southeast. Brackish-water bodies are usually shown in interdistributary areas or associated with abandoned delta lobes, and are open to the sea. In this study, a sedimentary sequence 30–50 m thick is interpreted as interdeltaic deposits. Brackish-water deposits within the sequence are interpreted as interdeltaic lagoons rather than interdistributary bays. Three facies associations (units) are recognized in nine measured sections of the study interval. Unit A consists of interbedded sandstone, mudrock and coal which occur in both fining- and coarsening-upward sequences less than 10 m thick. Fining-upward sequences decrease in thickness and frequency upwards in unit A and are interpreted as distributary channels. Coarsening-upward sequences associated with the channels are interpreted as crevasse splays that filled lakes or interdistributary bays. In the upper part of the unit where only minor channels are present, the coarsening-upward sequences are interpreted as bay deltas. Unit B consists of fossiliferous silty shale and bioturbated sandy siltstone. A low-diversity fauna of bivalves, gastropods, ostracods and foraminifers indicates that brackish-water conditions existed. Unit B intertongues with unit A to the northwest and with unit C to the southeast, and is interpreted as lagoonal deposits. Unit C consists of crossbedded and burrowed sandstone in beds 0.5–9 m thick. Sandstones are laterally continuous in the southeast but become tabular bodies enclosed within unit B to the northwest. Laterally continuous sandstones are interpreted as shoreface deposits on the basis of multidirectional crossbeds, marine trace fossils and continuity. Tabular sandstones are interpreted as flood-tidal deltas on the basis of NW-oriented crossbeds, pinchouts to the northwest and enclosure within unit B. Scoured surfaces and rip-up clasts within unit C indicate the presence of tidal channels or inlets. Interpreting unit B as an interdeltaic lagoon rather than an interdistributary bay is dependent on understanding its enclosing strata. The distributary channels of unit A supplied the majority of the sediment in the lower part of the sequence. However, after the lagoon formed, tidal channels and inlets of unit C supplied more sediment to the area than channels of unit A. The shift in sedimentation from a landward to a seaward source reflects the change from a delta plain to an interdeltaic lagoon following abandonment of distributary channels.

Factors influencing microhabitat partitioning in arid-land darkling beetles (Tenebrionidae): temperature and water conservation

Four species of Eleodes darkling beetles, inhabiting different microhabitats in an arid, sagebrush–steppe ecosystem (south-west Wyoming, U.S.A.), were evaluated in the laboratory for interspecific differences in temperature preferences, high temperature tolerances, water loss and metabolic rates. Results of the laboratory tests were compared to the microclimate thermal regimes encountered by each species in its respective microhabitat in the field. Temperature preferences (Tp) of E. constrictus (Tp = 21·7°C) and E. pimelioides (Tp = 20·8°C) corresponded to the maximum diurnal summer temperatures (Tmax) recorded in their preferred microhabitat, shaded plant litter beneath large shrubs (Tmax = 21·1°C). Similarly, the Tps of E. extricatus (Tp = 27·2°C) and E. nigrinus (Tp = 27·1°C) corresponded to the maximum temperatures found in their preferred microhabitat, plant litter beneath low-growing shrubs and grasses (Tmax = 27·0°C). E. extricatus, the species most frequently encountered in sunny, sparsely vegetated areas, exhibited the greatest tolerance of high temperature; E. nigrinus, a strictly nocturnal species occupying this same microhabitat on the study site, displayed the least heat tolerance. The two shrub-dwelling species also exhibited low tolerances of high temperatures. Water loss rates and metabolic rates (both measured over a range of temperatures) differed significantly among some of the species, but did not fit predicted patterns of species vs. microhabitat distributions; however, by virtue of their cooler preferred microhabitats, shrub-dwelling species could potentially retain more water and metabolize less energy than the species inhabiting warmer microhabitats. We conclude that, on this study site, microhabitat selection by these darkling beetles may be substantially influenced by each species’ preference and tolerance of field temperature regimes, but that differences in microhabitat use appear to be independent of inherent physiological capabilities of energy metabolism or water conservation.

Paleostructural Control of Dakota Hydrocarbon Accumulations on Southern Moxa Arch, Southwest Wyoming and Northeast Utah: ABSTRACT

Production from the Lower Cretaceous Dakota Formation along the southern Moxa arch shows a number of anomalies in distribution and hydrocarbon character that can best be explained by complex interplay of the arch's depositional, structural, diagenetic, and thermal histories. The development of the Western Overthrust belt and its foreland basin largely controlled the orientation of the Dakota fluvial and deltaic systems. Differences in sandstone trends between the upper and lower Dakota resulted in different migration pathways an loci of oil and gas accumulations. Isochore mapping indicates that the onset of major structural growth of the Moxa arch began contemporaneous with deformation in the Overthrust belt during the Cretaceous Campanian. After early migration of hydrocarbons into the resulting structural/stratigraphic traps, subsequent diagenesis of Dakota sandstones, particularly in the downdip water legs of the reservoirs, effectively sealed the accumulations in their initial paleostructural position. Paleocene and Eocene structural compression reversed the original northward plunge of the arch and rotated it slightly to the east into its present structural configuration. Hydrodynamically driven convective heat flow has resulted in varying geothermal regimes within the area, increasing heat flow along most of the arch's crest, while cooling its southern portion. This phenomena has affected gas-oilmore » ratios and API gravities by enhancing or retarding thermal maturation of the trapped hydrocarbons.« less

Edaphic and Reclamation Aspects of Vesicular‐Arbuscular Mycorrhizae in Wyoming Red Desert Soils

AbstractThree Aridisols and an Entisol in the Red Desert of southwest Wyoming were surveyed for vesicular‐arbuscular mycorrhizal (VAM) fungal biomass. The survey was conducted to determine attenuation of VAM fungal biomass in profiles of these arid soils, to examine possible edaphic influence on VAM fungal distributions and to evaluate the potential of these soils in providing VAM fungal biomass for use in reclamation. Fungal biomass levels—as indicated by fungal spore densities and bioassay percent infections—was apparently related to morphological and chemical properties that differ between individual soils and their associated vegetation. While fungal spore densities generally attenuated with depth in the Aridisols, the Torrifluvent retained high densities of spores (up to 81 spores g−1 soil) to a depth of 140 cm. The VAM fungal biomass attenuation in relatively deep horizons of these arid soils may still allow adequate formation of propagules for use in reclamation of disturbed lands.

Seeds, Weeds, And Prehistoric Hunters And Gatherers:

Little information is available on the role of plants in the prehistoric hunter and gatherer societies of the Great Plains and Rocky Mountain regions. The recovery of thousands of chaced seeds from the fill of 152 hearths or pit features at eight prehistoric sites provides an excellent opportunity to study the importance of seed foods in the subsistence of prehistoric inhabitants in southwest Wyoming, an area between the Great Plains, northern Colorado Plateau, and Great Basin. The type and season of represented activities are inferred partly from ethnographic and historic literature. Comparisons with other macrofossil studies in the area indicate a greater reliance on seeds from weedy annuals during the Late Prehistoric than in earlier Archaic periods. This greater reliance may have resulted from hunters and gatherers intentionally or accidentally manipulating their environment. They may have burned off the vegetation or actually broadcasted seeds of desired plants.

Taphonomy and preservation of a monospecific titanothere assemblage from the Washakie formation (Late Eocene), southern Wyoming. An ecological accident in the fossil record

More than twenty partial skeletons, all cf. Mesatirhinus sp., were discovered in overbank deposits associated with sandstone channels within the Adobe Town Member of the Washakie Formation (Washakie Basin, southwest Wyoming). Taphonomic and sedimentologic data indicate a mass mortality event with reduced post-mortem transport of the carcasses. Such a mass-death assemblage of a large mammal taxon is a rare occurrence for the Paleogene record. Accordingly, position and orientation of all the bones were recorded to make possible an accurate detailed map of the deposit. The remains principally include disarticulated skeletons of both sexes, from juveniles to old adult. Some axial portions and limbs remained articulated; others suggest partial articulation. The collected remains were scattered in loose associations within an area of about 7.5 × 9 m. Recent erosion (sheetwash) has destroyed an unknown extent of the original mass burial to the east and south of the excavation, but the 68 oriented bonebearing blocks collected represent about 50% of the exposed site. The skeletal remains occurred within a single unit of poorly sorted but generally upward-fining quartz grit with a basal mudflake conglomerate. The bones are not abraded, but have suffered from severe compaction damage and recent freeze-thaw action. The sediment represents a mass flow accumulation deposited during the waning stages of a flash flood. The large number of individuals of a single taxon suggests that part of a herd of titanotheres was killed in a single mass mortality event. The cause of death is not known, but it is likely that the accumulation represents a small fraction of a large herd that was crossing a flooded river. The carcasses drifted downstream to be left as a carcass jam on the flood plain as the floodwaters withdrew. It follows that the assemblage represents a close approach to a single taxon slice from a terrestrial biocoenosis, and allows interpretations to be made about the autecology (social behavior, age composition and community structure) of titanotheres. Decomposition of the integument and other tissues of partially buried carcasses may have been a factor contributing to the “pockety” nature of the sediment.

LaBarge Project: Availability of CO2 for Tertiary Projects

Abstract Exxon Co., USA is constructing a large gas processing facility northeast of Kemmerer on the Lincoln-Sweetwater County line in southwest Wyoming. The plant will process raw gas from three Exxon operated units northwest of LaBarge, in Sublette County. The LaBarge Field is being developed primarily to recover and sell methane; however, the field is also a significant source of carbon dioxide, sulfur and helium. As a source of CO, the LaBarge Field will provide oil producers in the Rocky Mountain area an opportunity to increase production in their mature fields through EOR projects. A 649-mile pipeline is planned to transport up to 650 Mcfd of CO, to users throughout Wyoming and Willisiton Basin. Exxon's CO, marketing survey indicates potential CO, demand of 4.4 trillion cubic feet in the planned marketing area for enhanced oil recovery. About 25% of the CO2 demand identified is already contracted. LaBarge CO2 will allow Northern Rocky Mountain 'operators to potentially recover an additional one billion barrels of oil reserves. Introduction Exxon's gas plant, scheduled for startup this year, is designed to process 480 million cubic feet per day of raw gas yielding 114 Mcfd ( m = million, G = billion, T = trillion) of methane, and 250 mcfd of CO2. The initial phase of the plant is currently being built and a future expansion is under evaluation. Expansion timing will depend on the economic climate and commitments for plant products. while methane is the primary product being developed in the LaBarge Project, the field will also produce marketable quantities of CO2, sulfer, and helium. The CO2 is being offered for sale to operators in Colorado, Wyoming, North Dakota, Montana, and Canada as an injectant for their EOR projects. Background The well field consists of three Exxon operated units covering over 63 square miles (see Fig. 1). These units are located about 10 miles northwest of the town of LaBarge in Sublette County. Exxon owns an average of 95% (Jan.'86, leased committed acreage) of the Lake Ridge, Fogarty Creek, and Graphite Units. Production is from the Madison, a 900 foot thick carbonate reservoir ranging in depth from 14,000 to 18,000 feet. Structurally the Madison Reservoir is located on a large NW-SE trending anticline. The boundaries have not yet been accurately defined but do extend beyond the three Exxon operated units. The National Petroleum Council estimated that the LaBarge-Big Piney area contains 20–25 Tcf of CO2 reserves alone (EOR, p.33, 1984). P. 249^

Wind Effects on Needles of Timberline Conifers: Seasonal Influence on Mortality

In the Rocky Mountains, USA, year—round wind exposure has been correlated with widespread needle dehydration and death in winter for alpine timberline conifers. Needle death may be due solely to the drying effects of winter wind or may also result from predisposition to winter injury associated with suboptimal summer growth conditions. Our experiments showed that in the lower timerbline ecotone in southwest Wyoming, needle mortality was primarily due to winter wind and cuticle abrasion; death was frequent only in wind—exposed needles of flagged trees and surface needles of krummholz mats. At higher elevations where only krummholz mats and a few flagged trees exist, mortality averaged ≥75% for needles unprotected by snow, regardless of wind exposure. Snow covered needles had low mortality throughout the timberline ecotone. Winter death of naturally wind—exposed needles of Picea engelmannii occurred at ≤—4.5 MPa water potential (≤60% relative water content). Cuticular resistance of wind—exposed needles declined from 100—250 ks/m in autumn to ≤30 ks/m by midwinter. Experimentally reversing the windward—leeward orientation of small, flagged trees in early winter resulted in lower xylem pressure potentials and needle viabilities for newly wind—exposed (originally leeward) compared to newly sheltered (originally windward) shoots. Also, sheltering exposed branches from winter wind on flagged trees in the lower timberline ecotone (3200 m) increased mean overwinter needle survival from near 0 to ≥50% in both Picea engelmannii and Abies lasiocarpa. Scanning electron micrographs of dehydrate wind—exposed needles collected in March at this site showed little cuticular surface wax, probably because of windborne ice crystal abrasion. However, winter dehydration and death in both experimentally and naturally wind—sheltered needles at higher elevation may have been due to inadequate needle maturation during summer, which could act to exclude flagged trees from the upper timberline ecotone.

Style of Deformation in Productive Fairway of Absaroka Plate: Southwest Wyoming and North-Central Utah: ABSTRACT

Deformation of the Absaroka plate is characterized by two subparallel trends which result from ramp anticlines. Each thick competent unit in the stratigraphic section (Paleozoic carbonates, Thaynes Formation and Nugget/Twin Creek formations) forms hanging-wall ramp anticlines and productive structures. The western trend (anticline) is associated with truncation of the Paleozoic section against the Absaroka thrust, and the eastern trend (en echelon anticlines) with truncation of the Triassic/Lower Jurassic section. Variations in the style of folding are different on the two trends. This variation results from the vastly different mechanical properties of their stratigraphic sections; 4,500 ft (1,370 m) of nearly continuous, competent Paleozoic carbonate and quartzite produce a single ramp anticline 50 mi (80 km) long. Imbricate faults at the base of this anticline (western trend) cause oscillations of closure along strike and minor strike offsets. Figure The eastern trend is dominated by folding rather than internal faulting. The Mesozoic section, which immediately overlies the thrust in the eastern trend, consists of alternating competent and incompetent units, each ranging from 750 to 2,500 ft (230 to 760 m) thick. Two competent mechanical units which form the ramp anticlines are the Thaynes Formation and the Nugget Sandstone/Twein Creek Limestone unit. Ramp anticlines develop from a single ramp through both competent units (producing one anticline) as well as stair-stepped ramps separated by a tread or glide plane in the incompetent unit between the Thaynes and Nugget formations (producing two anticlines). Additional folding may also result, apparently from simple compression which produces three anticlines stacked en echelon in he transport direction. These genetically related, stacked en echelon along an elongate zone of deformation are termed herein folds and are not associated with imbricate faulting. Fold length along the eastern Mesozoic trend ranges from 1 to 6 mi (2 to 10 km). In contrast, oscillations in the 50-mi (80-km) long western anticline range from 5 to 15 mi (8 to 24 km). A clear relationship becomes apparent between both length and style of ramp anticlines and the thickness of the stratigraphic section traversed by the ramp. End_of_Article - Last_Page 1361------------

K/Ar Dating of Illitic Clays in Jurassic Nugget Sandstone and Timing of Petroleum Migration in Wyoming Overthrust Belt: ABSTRACT

Authigenic illite is a prominent pore-fill in the Nugget Sandstone, the main reservoir rock of most fields in the southwest Wyoming Overthrust belt. Illite, a good K/Ar clock, was dated from several well samples, all from the Absaroka thrust sheet. This includes a producing well in the Clear Creek field where seven samples traverse the gas, oil, and water zones. The ages of the Clear Creek suite are virtually concordant at 110 ± 2 m.y. Assuming hydrocarbon emplacement would have arrested authigenesis in the oil and gas zones, the similarity of ages from the hydrocarbon zones with the water zone indicates hydrocarbon emplacement was post 110 m.y. ago (middle Cretaceous). Ages obtained from the other Absaroka sheet Nugget samples fall in the narrow range of 102 to 120 .y.b.p. This indicates illite authigenesis was a relatively short-lived event for the Nugget in the Absaroka sheet. The Wyoming-Idaho-Utah overthrust belt involves several thrust sheets each of which was emplaced over its foreland sequentially from west to east over a time spanning tens of millions of years. We attribute the mid-Cretaceous illite growth in the Absaroka sheet to burial conditions established when that part of the Nugget was thrust upon by the Crawford sheet. The burial was accomplished both tectonically and by synorogenically derived sediment. If true, our illite dates imply a somewhat older age for the Crawford sheet than previously interpreted. We attribute the post-illite hydrocarbon emplacement in the Absaroka Nugget as a result of thrusting of the Absaroka sheet on top of its foreland containing middle Cretaceous petroleum source-bed shales. These beds were thermally matured wh n buried by the emplacement of the Absaroka sheet and its derived sediment in the Late Cretaceous. End_of_Article - Last_Page 414------------

Whitney Canyon Sour Gas Well Completion Techniques

Kilstrom, Kevin J., Amoco Production Co. Summary Sour gas wells require special considerations for successful production. Corrosion, safety considerations, prolific gas rates, the casing program, and hydrate problems affect the final completion design for wells in the Whitney Canyon sour gas field. A completion technique that addresses all these constraints yet maintains a degree of simplicity and therefore presents less risk is being used in Whitney Canyon well completions. Additionally, problems that do not exist in Whitney Canyon, such as salt or sulfur deposition, can be handled with this same basic system. Introduction The economics associated with exploration of hydrocarbons in the U.S. have improved during the past 8 years, resulting in more deep drilling and frequent discovery of sour gas. One such discovery is the Whitney Canyon field located in the Overthrust Belt of southwest Wyoming, about 15 miles (24 km) north-northeast of Evanston (Fig. 1). The field was discovered in 1977 with the drilling of the Amoco/Chevron/Gulf Working Interest Unit Well No. 1, which reached a total depth of 10,691 ft (3258.6 m) in the Permian Phosphoria formation. Sour gas was recovered in the drilling fluid, and subsequent failure of tubular goods resulted in plugging back the well and completing the Triassic Thaynes formation in the 9,178- to 9,266-ft (2797.5- to 2824.3-m) interval for 4.3 MMcf/D (121.8 × 10(3) m3/d) of sweet gas and 96 B/D (15.3 m3/d) of condensate. The field is on the Absaroka thrust plate, which has resulted in placement of Ordovician Big Horn above Cretaceous rock in this vicinity (Fig. 2). The Ryckman Creek field, which produces oil and gas from the Jurassic Nugget and sweet gas from the Thaynes formation, lies 5 to 6 miles (8.0 to 9.7 km) to the east. The Carter Creek field has been shown to be a continuation of the Whitney Canyon field to the north (Fig. 3). A second well was spudded to test the deeper formations in the Whitney Canyon play and reached a total depth of 16,393 ft (4996.6 m) in the subthrust Cretaceous. Testing in this well verified substantial quantities of sour gas in the Ordovician Big Horn and Mississippian Mission Canyon carbonates and indicated sour gas in the Pennsylvanian Weber sandstone. Subsequent field development has indicated commercial quantities of gas in four of the Paleozoic Era formations. These include the Devonian Darby, Mission Canyon, Mississippian Lodgepole, and Big Horn formations. Limited quantities of sweet gas have been tested in the subthrust Cretaceous, and noncommercial sour gas production was indicated from Triassic Dinwoody and Permian Phosphoria. The approximately 4,500-ft (1372-m) thick Paleozoic interval lies at a depth range of 10,000 to 16,000 ft (4877 to 3048 m) in the Whitney Canyon area and exhibits normal pressure gradients of 0.45 to 0.50 psi/ft (3.1 to 3.4 kPa/m) and a maximum temperature of 250 degrees F (121 degrees C). Drilling has taken place for 13 miles (21 km) along the Whitney Canyon/Carter Creek trend, indicating a sizeable productive area (Fig. 3). The reserves of sour gas are trapped in the folding associated with the thrust faulting in the area and are held in primary and secondary porosity (matrix porosity in the 5 to 10% range). Permeability is generally low [matrix permeability less than 0.1 md (9.869 × 10(-5) mum2)] and production is highly dependent on extensive natural fracturing. The gas composition in the Whitney Canyon field varies from formation to formation, with all of the Paleozoics indicating H2S content from 1 to 15 mol%. The Mississippian Age Mission Canyon formation has indicated the largest reserves in the field and one 30-ft (9.1-m) interval in the Champlin 457 A Amoco No. 1 well tested at 33 MMcf/D (934.5 × 10(3) m3/d) (Table 1). The Mission Canyon formation is a 1,000-ft (304.8-m) thick carbonate section with a gas composition of approximately 15% H2S, 5% CO2, and 70% methane, and the balance is ethane and heavier hydrocarbons. To develop a successful completion design for the Whitney Canyon field, we first defined all factors that would be detrimental to production. JPT P. 40^

Physical Evidence for Saline Cycles of Deposition in Eocene Lake Gosiute in Southwest Wyoming: ABSTRACT

The Wilkins Peak Member, the saline unit of the Green River Formation in southwest Wyoming, is more than 985 ft (300 m) thick and contains more than 35 beds of trona or trona with halite. The trona and halite were deposited in the deepest part of the basin of Lake Gosiute, during arid periods of the Eocene Epoch, by the periodic evaporation and contraction of the lake waters. Alternating with the arid periods were more humid periods, when the lake expanded and less saline sediments were deposited across and beyond the previously deposited salt beds. The water-level fluctuations resulted in a concentric pattern of End_Page 1698------------------------------ lithofacies within the Wilkins Peak Member. The innermost of the cyclic lithofacies, which includes bedded evaporites, occupies the depositional center of the ancient lake basin and consists of repetitious ascending sequences of (1) dark-brown oil shale, (2) white to brown trona and halite, and (3) gray or green dolomitic mudstone. A second lithofacies that encircles the first, but does not contain bedded evaporites, shows in vertical section cyclic sequences of (1) gray siltstone, (2) dark-brown oil shale, (3) tan or light-brown oil shale, and (4) gray or green dolomitic mudstone. The oil shale and siltstone in these cycles were deposited as lake-bottom muds and along ephemeral shorelines during periods of lake expansion and water-freshening; the trona and halite and gray and green mudstone were deposited in salt pans and on m d flats during periods of lake contraction and water-salting. A third lithofacies is present at the outermost margins of the lake basin, but the cyclic sequences there, consisting mostly of gray and green mudstone and some thin interbedded tan and brown oil shale, are largely obscured by irregularly interbedded tan algal limestone, oolites, and thin beds of dolomite and sandstone. Floodplain deposits of interbedded red and gray sandstone and mudstone are present nearly everywhere between the outer edges of the lake basin and surrounding mountains. The number of major contractions of Lake Gosiute during deposition of the Wilkins Peak Member can be determined by counting the number of oil shale beds involved in cyclic sequences near the depocenter of the basin. Seventy-three oil shale beds, and hence 73 saline cycles, were counted in a hole cored in the Blacks Fork area by the U.S. Energy Research and Development Administration. Histograms of oil shale beds in drill holes and outcrops in other parts of the basin support 70 to 75 as the probable total number of saline cycles. The time intervals of the saline cycles can be roughly estimated by using potassium-argon dating methods for biotites in tuff, and by counting oil-shale varves. From these data, the shortest cycle in which salines were deposited is believed to have lasted less than 1,500 years and the longest cycle more than 100,000 years. End_of_Article - Last_Page 1699------------