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Browsing Theses and Dissertations by Author "Amerman, Robert, committee member"

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    Spectral gamma ray characterization of the Elko Formation, Nevada - a case study for a small lacustrine basin
    (Colorado State University. Libraries, 2015) McGowan, Erin M., author; Egenhoff, Sven O., advisor; Schutt, Derek, committee member; Cavdar, Gamze, committee member; Amerman, Robert, committee member
    Handheld gamma ray spectrometry is a cost-effective and time-efficient means of furthering understanding of lake facies and small-scale lake systems. Spectral and total gamma ray data were recorded every foot vertically through a succession in situ at four outcrops along a NNW-to-SSE transect representing the lower to middle Eocene Elko Formation in northeast Nevada, USA. The lacustrine Elko Formation consists of, from oldest to youngest, four major units: 1) a basal conglomerate, 2) an overlying carbonate, 3) a fine-grained organic-rich mudstone with intercalated carbonate mudstones, and 4) volcaniclastics. These units comprise fourteen sedimentological facies identifiable in outcrop. In this study, these fourteen facies have been reduced to eight that are discernable by spectral gamma ray (SGR) signals. Each recorded interval in the Elko Formation succession was assigned to one of these eight facies. These eight facies comprise five siliciclastic (plant-bearing mudstone, clay-dominated mudstone, microbial-mat-bearing mudstone, ash-bearing mudstone, and conglomerate) facies, two carbonate (calcareous mudstone and fossiliferous mudstone-wackestone) facies, and one volcanic tuff facies. In conjunction with SGR, outcrop observation, X-ray diffraction (XRD), thin section observation, and total organic carbon (TOC) analyses allowed a thorough understanding of facies composition and its SGR signal. The primary controls of SGR components [potassium (K), uranium (U), and thorium (Th)] reflect K-bearing volcanic minerals (feldspars and micas), U-enriched organic material, and clay abundance (illite and montmorillonite; potentially derived from volcanic ash), respectively. High radioactivity, with signals above 120 American Petroleum Institute (API) units, was demonstrated for five facies (plant-bearing mudstone, clay-dominated mudstone, microbial-mat-bearing mudstone, ash-bearing mudstone, and volcanic tuff) in contrast to the remaining three facies (calcareous mudstone, fossiliferous mudstone-packstone, and conglomerate) exhibiting low radioactivity of less than or equal to 120 API. Distribution of radioactive minerals across the outcrops was largely found to be not only a function of general lithologic composition, but also the paleogeographic locations of the outcrops within the lake basin, due to the differing contributions of organic debris and volcanic constituents. This distribution of radioactive minerals across each outcrop supports a recent depositional model of north-to-south diachronous deposition of the Elko Formation (Horner, 2015). The microbial-mat-bearing mudstone facies was a traceable, deep-lake sediment throughout the basin by which outcrops could be assigned to more proximal or more distal positions within the paleo-lake by applying Th/U ratios. The proximal outcrop data show a low Th/U ratio (below 2.5), as opposed to the distal outcrop data that have a high Th/U ratio (2.5–4). These data confirm that most likely Th/U ratios reflect the increase in the amount of clay with distance from the proximal outcrops. Thin section microscopy and scanning electron microscope (SEM) analyses allowed for recognition of eight diagenetic cements reflecting a strongly varying diagenetic history in the Elko Formation sedimentary rocks. One calcite, four stages of dolomite, two types of silica, and one zeolite cement were identified. The calcite, dolomite, and silica cements were typically formed sequentially as listed above, whereas the zeolites formed independently. Intraparticle, interparticle, matrix, shelter, and fracture porosity types are also present, each forming post-deposition, except for interparticle and matrix porosity within volcaniclastics. Calcite, dolomite,and silica cements were found succeeding shelter and fracture porosity, whereas the timing of zeolite cement in matrix porosity was unclear. Overall, cement phases and porosity were found to be minimal, and therefore, probably had only a minor influence on the overall gamma ray signal of the Elko Formation sedimentary rocks. SGR characterization of each of the eight facies across a proximal-to-distal transect of the Elko continental-lacustrine sedimentary basin reflects the strong influence that climate and tectonics have on depositional changes in a small-scale lake. Applying gamma ray techniques to the small-scale lake system of the Elko Formation was found to be a useful tool and provides a framework to apply to lacustrine studies as a predictive tool in future exploration.
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    ItemOpen Access
    Tertiary lake sedimentation in the Elko Formation, Nevada -- the evolution of a small lake system in an extensional setting
    (Colorado State University. Libraries, 2015) Horner, William H., author; Egenhoff, Sven O., advisor; Harry, Dennis, committee member; von Fischer, Joe, committee member; Amerman, Robert, committee member
    The Lower to Middle Eocene Elko Formation of northeastern Nevada consists of basal coarse-grained siliciclastics and carbonates which are overlain by an organic-rich succession consisting of fine-grained siliciclastics, in places with fine-grained carbonates, and fine- to coarse-grained volcaniclastics at the top. Based on lithological and sedimentological characteristics in four documented localities arranged along a north-south transect, the succession shows fourteen facies, which are grouped into five facies associations (FAs): Siliciclastic mudstones and conglomerates (FA1); Massive coal-rich mudstones (FA2); Microbial-mat-bearing mudstones and carbonates (FA3); Microbial-mat-bearing mudstones and volcaniclastics (FA4); Carbonates and volcaniclastics (FA5). The succession is interpreted to reflect deposition in a broad continental-lacustrine setting. FA1 rocks record sedimentation in the most proximal environment, consisting of alluvial-fluvial sedimentation. FA2 rocks reflect deposition in a marginal low-energy swamp environment, while FA3 rocks denote "open-water" lacustrine sedimentation in a limnetic setting that was highly sensitive to lake-level fluctuations. FA4 rocks record the onset of extrabasinal airfall tuff in the limnetic portion of the lake, and FA5 rocks record volcaniclastic sedimentation outpacing subsidence in the lake, ultimately "filling" up available accommodation space and ending lacustrine sedimentation. The studied succession is subdivided into four vaguely chronostratigraphic intervals referred to as Stratigraphic Intervals 1 to 4, which record a lake system with significant lateral changes in accommodation space and resulting facies patterns in a north to south progression through time. Based on two recent 40Ar/39Ar dates and four previous radiometric age dates, the northern outcrop, which is significantly older than the central and southern ones, records initial subsidence and the onset of lake sedimentation (Stratigraphic Interval 1). Subsidence varied over time causing the lacustrine depocenter and limnetic depozone to progressively shift southwards (Stratigraphic Intervals 2 through 4). Black shale source rocks in the measured sections therefore occur along the entire north-south transect of the studied lacustrine system, yet they represent rocks of different ages not correlatable throughout the Elko Formation. Coeval volcanism, which led to increased volcaniclastic sediment supply, followed black shale deposition and contributed to the north-south "filling in" of the lake system, ultimately culminating with the end of lacustrine sedimentation around 37.5 Ma. The Elko Formation black shales have high source rock potential as an unconventional resource play, as their organic content consists almost entirely of Type-I (oil prone) kerogen. Contrary to deep-water, thermally-stratified anoxic-lake source rock models, long considered to be the only environments in which significant organic-matter preservation may have occurred, this study provides evidence for black shale deposition in the Elko lake to have occurred within a "shallow," mostly oxic environment in the photic zone. Further, this research indicates that depositional environments in lacustrine settings may be scale-dependent. The Elko Formation is not merely a scaled-down version of large-lake systems, such as that in which the Green River Formation formed, but a unique type of system with its own set of controls. With increased industry attention being placed on this potential lacustrine petroleum system, this study provides a new source rock model, as well as a temporal and spatial framework to be used as a predictive tool for the identification of rich source rock intervals in the Elko Basin.