Integrated hyperspectral remote sensing, geochemical and isotopic studies for understanding hydrocarbon-induced rock alterations

Marine and Petroleum Geology 35 (2012) 292e308

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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo

Integrated hyperspectral remote sensing, geochemical and isotopic studies for understanding hydrocarbon-induced rock alterations Ana Petrovic a, Shuhab D. Khan a, *, Allison K. Thurmond b a b

Department of Earth & Atmospheric Sciences, University of Houston, United States StatOil, Postboks 7200, NO-5020 Bergen, Norway

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 January 2011 Received in revised form 18 December 2011 Accepted 11 January 2012 Available online 21 January 2012

The main objective of this work was to determine if there are characteristic mineral assemblages and chemical changes in areas affected by hydrocarbon microseepages. For this purpose remote sensing was utilized for mapping surficial rock alterations, and geochemical tools were used to understand the alteration processes. The key area chosen for this type of work were altered and unaltered Wingate Sandstone outcrops in Lisbon Valley, Utah. The Spectral Angle Mapper method was applied on HyMap hyperspectral data to classify the extent of altered and unaltered outcrops, as well as to map the changes in mineral content within the outcrops. The Spectral Feature Fitting method was used to identify lithological changes in the area. Reflectance spectroscopy, thin section studies, major, minor, and trace element analyses, and stable carbon and oxygen studies on both bleached (altered) and unbleached (unaltered) samples were successfully used to delineate areas of similar rock composition and relate changes due to hydrocarbons leaking from underlying petroleum reservoirs. Unbleached Wingate Sandstone samples had higher hematite and feldspar content than bleached Wingate samples, which were characterized by larger amounts of clay, calcite, and pyrite. Some bleached samples also had higher concentrations of elements (U, Mo) characteristic of hydrocarbon-related reducing environments, and were depleted in 13C when compared to the unbleached samples. Based on these results, the following model of chemical reactions is suggested for diagnostic changes within Wingate Sandstone. Hydrocarbon-induced reducing environment caused the transformation of sulfate ion (obtained from groundwater or from oxidation of H2S) to sulfide ion, resulting in the reduction of hematite to pyrite. The released hydrogen ion from this reaction reacted with available feldspars in the rock, leading to precipitation of kaolinite. These conditions favor the reaction between bicarbonate ion and Ca2þ ions that can be obtained from the groundwater, leading to precipitation of calcite in pore spaces left open after the reduction and removal of hematite. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Hydrocarbon-induced rock alteration Hyperspectral remote sensing Carbon isotopes Geochemistry Wingate sandstone Petroleum exploration

1. Introduction Surface expressions of oil and gas seeps have been observed for thousands of years. Some of these indications have led to discoveries of many commercially important petroleum reservoirs. Even where the presence of surface anomalies does not lead to the detection of large oil and/or gas deposits, it can establish the hydrocarbon presence in the area. Over the past 70 years many geophysical and geochemical techniques were developed with a goal to make petroleum detection and evaluation easier and more precise (Campbell, 1994; Dalziel and Donovan, 1980; Horvitz, 1980; etc.). * Corresponding author. E-mail address: [email protected] (S.D. Khan). 0264-8172/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2012.01.004

The goal of this paper is to test the hypothesis that subtle mineral and chemical changes carry a potential for future hydrocarbon exploration because these changes occur during reactions between hydrocarbons and rocks. We wanted to establish if there is a pattern between surficial rock changes, which could be detected using various research tools, and underlying petroleum reservoirs and propose its use as a starting point for hydrocarbon prospecting. For this purpose surficial rock alterations in Lisbon Valley, Utah are identified and mapped using reflectance spectroscopy, hyperspectral, geochemical and isotopic data. The areal extent of red and bleached Wingate Sandstone outcrops is detected and mapped, and mineral maps are created in order to find subtle mineralogical and chemical changes. The potential benefits of such techniques are numerous and could lead to finding petroleum potential in remote and inaccessible areas where field work is a difficult task.

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2. Geological background Lisbon Valley is located in the southeast of Utah (Fig. 1). It is a northwest-trending, doubly plunging anticline. The core of the fold (mainly salt) has been breached by erosion and undergone subsequent collapse (Segal et al., 1986). This whole region of southeastern Utah is underlain by the Paradox Basin, a late Paleozoic intracratonic basin filled with a mixture of carbonate, siliciclastic, and evaporite sedimentary strata (Nuccio and Condon, 1996). The Wingate Sandstone is the focus of this study; it was accumulated as a Lower Jurassic eolian depositional system, composed primarily of dune and sandsheet deposits that come from an erg margin (Segal et al., 1986). It comprises a complex assemblage of facies that reflect the initiation, periodic growth, and eventual destruction of a large Jurassic erg. It is easily recognized as an orangeered, thick-bedded, erosion-resistant, cliff-forming sandstone that is prominently displayed in the southwest-dipping cuesta of the west flank of the Lisbon Valley anticline (Fig. 2). Steep to vertical cliffs display pervasive vertical joints, and

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tangential large scale cross-bedding; many show the effects of spalling of huge blocks of sandstone along vertical joint faces (Segal et al., 1986). The Wingate Sandstone forms the lowermost part of the Glen Canyon Group, which also includes the Kayenta and Navajo Sandstones. The Wingate Sandstone in the Lisbon Valley area is generally up to 90 m thick (Weir and Puffett, 1981), but because of its cliff-forming habit it is mostly inaccessible and difficult to measure precisely. Previous workers in Lisbon Valley (Segal et al., 1984, 1986; Conel and Alley, 1985; Schumacher, 1996; Petrovic, 2006) have discovered and described various surface alterations of the Wingate Sandstone that were assumed to be caused by the influence of hydrocarbons emanating from the underlying petroleum reservoir. Segal et al. (1984, 1986) have observed that Wingate Sandstone is bleached on the southwest flank of Lisbon Valley anticline. In these areas it is gray and displays anomalous mineralogical and weathering patterns, which include anomalous limonite and carbonate concentrations, anomalous kaolinite and other clay mineralizations, and the transformation of the Wingate Sandstone from an erosionally resistant to a more easily

Figure 1. Geologic map of the Lisbon Valley anticline; Wingate Sandstone represented with yellow pattern and symbol Jw. Data downloaded from http://geology.utah.gov/ and then adjusted using ArcGIS. Index map shows location of surficial rock and soil alterations above petroleum reservoirs in the USA (sources: Love, 1957; Eargle and Weeks, 1973; Donovan, 1974; Olmstead, 1975; Goldhaber et al., 1978; Donovan et al., 1979, 1981; Ferguson, 1979a, 1979b; Dalziel and Donovan, 1980; Abrams et al., 1983; Allen and Thomas, 1984; Duchscherer, 1984; Lilburn and Al-Shaieb, 1984; Oehler and Sternberg, 1984; Segal et al., 1984, 1986; Reynolds et al., 1984, 1988, 1990; Conel and Alley, 1985; Lang and Nadeau, 1985; Roeming and Donovan, 1985; Richers et al., 1986; McCoy and Wullstein, 1988; McCoy et al., 1989; Klusman et al., 1992; Reid et al., 1992; Al-Shaieb et al., 1994; Bammel et al., 1994; Campbell, 1994; Schumacher, 1996; Van der Meer et al., 2002; Petrovic, 2006; Petrovic et al., 2008; Khan and Jacobson, 2008). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Figure 2. Wingate Sandstone outcrops in the Lisbon Valley area: a. northern exposure of altered Wingate Sandstone; b. unaltered central valley exposure; c. southernmost altered outcrops.

eroded friable unit (Segal et al., 1984, 1986). These previous investigations used remote sensing analysis of LANDSAT multispectral satellite data, XRD diffraction and thin section analyses. Our previous work (Petrovic, 2006; Petrovic et al., 2008) included the interpretation of ASTER multispectral data, as well as limited stable carbon isotope and major, minor, and trace elements analyses. 3. Methods and data Data used for research of hydrocarbon-related surficial rock alterations in Lisbon Valley, were obtained from detailed field work, spectroscopy, hyperspectral data, thin sections, and a number of geochemical tools, such as major and trace elements analyses and stable carbon and oxygen isotope studies.

3.1. Field work Field work was conducted to sample affected rocks that were identified using HyMap data, samples were also collected from unaltered outcrops for comparison. Field relationships were observed and digital photographs of outcrops were taken. Handheld Global Positioning System was used to record all field routes and sampling locations, for field orientation and correlation with satellite data. Field work was done during June and November of 2008. Samples were collected from the northern, central, and southern exposures of Wingate Sandstone (Fig. 1). Rock samples from both altered and unaltered outcrops were collected, with outcrops photographed for the verification of field observations (Fig. 2a, b, c). Samples were also collected from formations stratigraphically adjacent to the Wingate Sandstone, to help map the

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area using remote sensing analyses. Local road maps were used for field orientation, as well as printed satellite data and older geologic maps. 3.2. Remote sensing data Two sets of HyMap hyperspectral data were acquired for this work by HyVista Corporation in June of 2008 with spatial resolution of 3.3 m and spectral resolution of 13 nm. The two flights covered parts of central and western part of the Lisbon Valley anticline, in order to analyze both altered and unaltered outcrops. Also, spectroscopy was performed on the rock samples collected in the field in order to compare results with hyperspectral data and as a starting point for HyMap image classifications. Rock samples collected in the study areas were analyzed in the laboratory, where spectral reflectance of rocks was measured using ASD FieldSpec Pro portable spectroradiometer with 10 nm spectral resolution and using contact probe as a light source. Resulting reflectance curves were then used to detect mineral changes within different rocks and for choosing spectral endmembers for further hyperspectral image classifications. Two HyMap data acquired for the Lisbon Valley were already spectrally and radiometrically calibrated. Unique spectral endmembers were identified from rock reflectance data prior to hyperspectral image classifications. Two algorithms were implemented for the classification of these images: Spectral Angle Mapper (SAM), and Spectral Feature Fitting (SFF). SAM is used to identify pixels from the satellite image that have spectral signatures similar to already defined endmembers. This method treats both questioned and known spectra as vectors and calculates the spectral angle between them. This method is insensitive to illumination since the SAM algorithm uses only the vector direction and not the vector length. The result of the SAM classification is an image showing the best match at each pixel. SFF is an absorption-featurebased method for matching image spectra to reference endmembers, which are selected from either the image or a spectral library. Here user specifies a range of wavelengths within which a unique absorption feature exists for chosen targets, which were in this case study hematite, clay, and calcite minerals, and different Wingate Sandstone samples. SFF uses continuum removed pixel spectra, which are compared to continuum reference spectra of known mineralogy (Kruse et al., 1993). For this study, three minerals already identified as the cause of differences between red, unaltered, and bleached, altered Wingate sandstones, were chosen. Bands covering the spectral range of characteristic absorption feature for hematite, kaolinite, and calcite were chosen using their respective spectral reflectance curves obtained from USGS spectral library (Clark et al., 2007). Hematite absorption was narrowed down to the 0.75e1.1 mm range, kaolinite to 2.11e2.25 mm, and calcite to 2.25e2.40 mm.

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measurements of the following international standards: BCR-2 (basalt), SCO-1 (shale), W-2 (diabase), JGB-1 (gabbro), MAG-1 (marine mud), and RGM-1 (rhyolite). ICP-MS and ICP-AES analyses were performed in order to obtain minor and trace concentrations of elements present in both altered and unaltered areas. For this research samples for both analyses were prepared in the same way. Standards (RGM-1 and MAG-1) were also analyzed to generate a calibration curve and the signals from analyzed samples were compared against the calibration curve to determine the concentration. Thirty seven selected sandstone samples were analyzed for stable carbon and oxygen isotope values (d13C and d18O). This provided the way to evaluate whether the surficial rock alterations at these locations are related to and caused by hydrocarbon microseepages. Carbon isotope values were measured in the calcite cement of the sandstones, therefore, samples were chosen on the basis of their reactivity with hydrochloric acid (HCl), and on the basis of the reflectance spectroscopy results that indicated the carbonate content in the rocks. The goal of these analyses was to compare the resulting d13C and d18O isotope values between the areas to see if a difference between altered and unaltered rocks could be detected. Measurements were performed using a computer-controlled stable isotope ratio mass spectrometer (Finnegan MAT Delta S). Both d13C and d18O isotopic signatures are reported relative to the PDB scale. 4. Results 4.1. Reflectance spectroscopy Reflectance spectroscopy measurements were performed on samples that had similar texture and grain size, with the main difference being mineral composition. Three endmembers were selected among Wingate Sandstone samples: red, unbleached sample from the middle of the anticline’s west flank; gray, bleached sample from the SW part; and yellow, bleached sample from the NW of the Lisbon Valley (Fig. 3). All altered and unaltered sandstone samples have quartz and feldspars as their framework grains (Petrovic et al., 2008); the main difference is in their hematite, calcite, and clay (mainly kaolinite) content. The amount and the type of feldspars and quartz could not be measured this way, because both of them have nondescript spectral signatures in the

3.3. Petrography and geochemistry Twelve representative selected sandstone samples were studied in order to obtain detailed mineralogical composition of the sandstones of the Wingate Sandstone outcrops. A 200-point counting method was used to record statistical measurements of mineral occurrences. Thin section results were then compared to each other, as well as to the findings from reflectance spectroscopy. Major, minor, and trace elements analyses were performed on twenty-five samples. The goal of the analyses was to differentiate between altered and unaltered samples. Analyzed samples were selected from altered and unaltered outcrops so that the adequate representation of field settings was achieved. Obtained concentrations were corrected by comparing them with the

Figure 3. Spectral reflectance curves for three selected Wingate Sandstone endmembers. AWF-S: altered Wingate Sandstone from the south of Lisbon Valley; AWF-N: altered Wingate Sandstone from the north of Lisbon Valley; UWF e unaltered Wingate Sandstone from the central part of Lisbon Valley.

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0.35e2.5 mm wavelength range (Bowen et al., 2007). However, hematite, calcite, and clays (kaolinite) have distinctive absorption features and their presence can be observed in sandstone reflectance curves (Clark, 1999). The strength and depth of minerals’ characteristic absorption features on rock reflectance curves shows their relative amounts: the deeper the absorption feature, the more mineral is present in the rock sample (Clark, 1999). Given that sandstone is a mixture of different minerals, the resulting reflectance signal can become a highly non-linear combination of the endmember spectra (Clark, 1999).

There are three intervals in acquired reflectance spectra where main changes between Wingate Sandstone endmembers could be observed (Fig. 3): 0.4e0.75 mm (due to change in iron content), around 2.2 mm (kaolinite), and around 2.35 mm (calcite). Spectra of red samples (UWF) are characterized by the steep drop at around 0.60 mm toward the beginning of the visible range (at around 0.4 mm), and then they flatten out. This drop is caused by chargetransfer absorption in iron-bearing minerals and gives iron-rich beds their red color (Fig. 3). Yellow, bleached samples (AWF-N) from the NW exposure of Wingate Sandstone show a similar drop

Figure 4. (A) Spectral Angle Mapper classification results of altered Wingate Sandstone from southern outcrops in Lisbon Valley, presented as a mosaic. Wingate Sandstone is outlined in green color, classification result in white. (B) Spectral Angle Mapper classification results for unaltered Wingate Sandstone from the central part of the Lisbon Valley, presented as a mosaic. Wingate Sandstone is outlined in green color, classification result in red. (C) Spectral Angle Mapper classification result for altered Wingate Sandstone from the north part of Lisbon Valley. Wingate Sandstone is outlined in green color, classification result in coral orange. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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towards the visible part of the spectra, but that drop is at around 0.55 mm and it does not flatten out. Gray and yellow Wingate Sandstone samples also show stronger absorption features for kaolinite and calcite than red samples. In analyzed samples kaolinite is characterized by absorption doublet at around 2.2 mm, which is very easily recognized in all endmembers, but the doublet is much deeper in altered gray and yellow samples. This suggests that bleached samples contain higher amount of kaolinite than unbleached ones. Calcite has the strongest absorption features outside the visible, short and near-infrared part of the spectra, but it also shows

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noticeable absorption at around 2.35 mm (Clark, 1999). Gray, bleached samples from the SW part of the Lisbon Valley anticline show the deepest calcite absorption feature among the samples. Yellow bleached rocks show small absorption, while red, unbleached rocks show no characteristic absorption for calcite. This suggests that the gray rocks have the highest concentrations of calcite, yellow rocks have some calcite present, and red rocks have very little or none. There are also two strong absorption features recognized in samples, one at 1.4 mm and the other at 1.9 mm; however they weren’t considered to be useful for mineral identification because

Figure 5. (A) Hematite mineral map for Wingate Sandstone and greater Lisbon Valley area, presented as a mosaic. Hematite is classified in red color. Wingate Sandstone is outlined in light gray color. (B) Kaolinite mineral maps for Wingate Sandstone and greater Lisbon Valley area, presented as a mosaic. Kaolinite is classified in white color. Wingate Sandstone is outlined in green color. (C) Calcite mineral maps for Wingate Sandstone and greater Lisbon Valley area, presented as a mosaic. Calcite is classified in cyan color. Wingate Sandstone is outlined in green color. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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they are caused by the presence of water and hydroxyl (OHe) ion. Hydroxyl ion is responsible for the 1.4 mm absorption, and the combination of water (HeOeH) and hydroxyl ion causes the absorption at 1.9 mm (Clark, 1999). Finally, the spectral curves of bleached and unbleached rocks, unbleached rocks have generally lower reflectance values than gray rocks; due to its darker color e they are less reflecting and more absorbing. 4.2. HyMap hyperspectral data Spectral reflectance curves of three selected Wingate Sandstone endmembers from Figure 3 were used to map Lisbon Valley areas so that surficial exposure and relationships between mineralogically different (altered and unaltered) Wingate Sandstone outcrops could be described. All three Wingate Sandstone endmembers were used for classifying the two HyMap data covering the broader Lisbon Valley area. The results (mapped in white color in Fig. 4A) show the dominance of altered rocks at the southwest part of the Lisbon Valley and their absence or very minor occurrence in the middle part of the anticline. Red, unbleached rocks are concentrated only in the central section of Wingate Sandstone exposure in the anticline (mapped in red color (in the web version) in Fig. 4B), which matches the observations from the field. The same results were found from classification using reflectance values from the rock samples collected at the north altered part of the Wingate Sandstone. They are concentrated in the northern part of the anticline (mapped in orange color (in the web version) in Fig. 4C), with some areas at the south mapped as well. This is also due to the great compositional and textural similarity between northern and southern altered zones. The areal extent of Wingate Sandstone outcrops is 12.24 km2. Unaltered outcrops cover 3.52 km2, altered outcrops at the northwest flank cover 6.02 km2, and altered outcrops at the southern part of the valley’s west flank cover 2.55 km2. There are some areas outside the Wingate mapped as unaltered Wingate Sandstone as well; however this is due to the compositional similarities of adjacent sandstone formations. In order to create mineral maps, the specific mineral spectra were obtained from the USGS spectral library, and then compared with the reflectance spectra from HyMap data. Since reflectance spectroscopy analyses revealed that the main mineralogical difference between altered and unaltered Wingate Sandstone samples was in the relative amount of hematite, calcite, and kaolinite, those three minerals were chosen as classification targets. Hematite mineral maps for all images correlate with the spectroscopy results, showing lower hematite concentration in the SW and NW part of the anticline, and higher concentration in the central areas (Fig. 5A). Figure 5A shows how hematite content changes going from the SW part, where Wingate Sandstone is bleached and very little hematite is present, towards the central part of the valley, where more hematite is present (classified in red color (in the web version) in the Fig. 5A) and rocks are not bleached. Kaolinite mineral maps also correlate with spectroscopy results. Since kaolinite has rather narrow absorption feature at 2.2 mm in the 0.4e2.5 mm wavelength range (Clark, 1999), the classification was focused only on the bands covering the 2.15e2.30 mm range. It can easily be observed that kaolinite is abundant in the SW and NW part of the Wingate Sandstone exposure in Lisbon Valley, but is not that plentiful in the central part (Fig. 5B). The change of kaolinite content is most noticeable on the HyMap image of the SW part of the Lisbon Valley, where kaolinite is wide spread in the southernmost Wingate Sandstone outcrop (as represented with white color on classification data in Fig. 5B); however going NW from this location kaolinite is almost completely gone.

Similar results were gained when the SAM technique was performed to classify calcite mineral distribution throughout Wingate Sandstone exposure. However, in the spectral range in which this work was focused, calcite has a very narrow absorption feature (around 2.35 mm) so a spectral subset for HyMap data was created to focus on that range at 2.25e2.40 mm. Results for both the classifications of HyMap image point to the same calcite distribution pattern: it is more abundant in bleached Wingate Sandstone than in unbleached rocks due to the fact that it mostly occurs as a cement (Petrovic et al., 2008) as can be observed in Figure 5C. Unaltered Wingate Sandstone outcrops in central Lisbon Valley contain less amounts of calcite. Hematite, kaolinite, and calcite, the three minerals already identified as the cause of differences between red (unaltered) and bleached (altered) Wingate sandstones, were also chosen as a starting point for the Spectral Feature Fitting technique. Bands covering the spectral range of characteristic absorption feature for hematite, kaolinite, and calcite were chosen using their respective spectral reflectance curves obtained from USGS spectral library. Hematite absorption was narrowed down to the 0.75e1.1 mm range, kaolinite to 2.11e2.25 mm, and calcite to 2.25e2.40 mm. Results for mineral’s SFF are shown in Figure 6, with RGB combination being SFF results for hematiteecalciteekaolinite. The presence and relative amount of minerals in question are displayed by the type and intensity of the dominant color. For example, it can be observed by the lack of red hues that hematite is absent or present in minor amounts at the SW and NW part of the Lisbon Valley. Going towards the central part of Wingate Sandstone exposure in Lisbon Valley red and pink hues start to dominate, showing that the presence of hematite is becoming more dominant. These are the areas of

Figure 6. Spectral Feature Fitting results for Wingate Sandstone and greater Lisbon Valley area, presented as a mosaic. RGB combination: R-SFF hematite, G-SFF calcite, BSFF kaolinite. Wingate Sandstone is outlined in green color. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Mineral distribution in analyzed Wingate Sandstone samples, based on the 200-point counting and normalized to 100%. Sample Numbers

Sample Type

Quartz [%]

Feldspars [%]

Clay [%]

Rock Fragments [%]

Other [%]

LV11 LV14 LV21 LV22 LV25 LV26 LV39 LV5 LV28 LV29 LV32 LV33

Altered Wingate Formation-North (AWF-N) Altered Wingate Formation-North (AWF-N) Altered Wingate Formation-North (AWF-N) Altered Wingate Formation-North (AWF-N) Unaltered Wingate Formation-Central (UWF) Unaltered Wingate Formation-Central (UWF Unaltered Wingate Formation-Central (UWF) Altered Wingate Formation-South (AWF-S) Altered Wingate Formation-South (AWF-S) Altered Wingate Formation-South (AWF-S) Altered Wingate Formation-South (AWF-S) Altered Wingate Formation-South (AWF-S)

60.4 60.0 59.6 59.4 60.0 60.1 60.4 54.0 59.0 61.5 57.2 59.4

21.5 25.9 22.9 27.6 30.5 25.9 25.7 20.7 19.4 23.3 23.8 18.0

13.4 10.1 10.0 9.7 4.6 8.4 9.0 15.0 15.1 12.4 8.8 13.0

2.0 0.7 3.1 2.0 2.6 3.5 4.2 3.9 4.3 0 2.7 2.1

2.7 3.3 4.4 1.3 3.3 2.1 0.7 6.5 2.2 2.8 7.5 7.5

unbleached, unaltered Wingate Sandstone. The opposite can be said for kaolinite, this mineral is present in the significant amounts in the SW and NW Lisbon Valley areas where bleached Wingate sandstones are located. Also, it can be noticed that kaolinite is more abundant in SW than in the NW part of the Wingate outcrops. Green colors (in the web version) in Figure 6 delineate the presence of calcite, which is not observed commonly in Wingate Sandstone outcrops, especially in unaltered ones. However, the presence of calcite could be observed in the other, limestone-rich formations, such as the Hermosa Group (in the very center of an anticline) and alluvium. 4.3. Petrography Thin sections of twelve Wingate Sandstone samples were studied: four from the northern valley outcrops, three from the central part, and five from the southern valley outcrops. The results show that mineral composition of the Wingate Sandstone varies within the study area, reflecting changes in the relative abundances of kaolinite, hematite, pyrite, calcite, and altered feldspar grains, as well as in overall porosity (Table 1). All analyzed Wingate Sandstone samples fall into the arkose and subarkose categories, with 54%e62% of the framework grains made of quartz and 18%e31% of feldspar. These sandstones are fine- to medium-grained, mostly well rounded, and show good sorting. The amount of feldspar is greater in the Wingate Sandstone outcrops at the central part of the valley (26%e31% of all grains present in the rock), and smaller in the samples from the northern (22%e28%) and southern areas (18%e24%). Percentage of feldspars is inversely proportional to the percentage of clay (kaolinite) minerals. The least amount of clay is present in the samples from the central part of the valley (5%e9% of all grains present in the rock), and greater amounts are present in

the samples from the northern part (10%e13%) and in the samples from the southern Wingate Sandstone exposure (9%e15%). The shape and appearance of clay minerals, along with the amount, suggest that they formed as a result of feldspar alteration. This argument is supported by the fact that a significant number of feldspars showed some degree of alteration, and that more altered feldspars were observed in the altered and bleached Wingate Sandstone samples from the southern and northern part of the valley, than in the unaltered, red Wingate Sandstone samples from the central part of the valley (Fig. 7). The porosity of all observed rock samples is between 8% and 16%. These values are similar to or somewhat lower than the porosities measured in previous studies (Jamison and Stearns, 1982; Petrovic, 2006). Porosities are slightly lower in samples from the bleached areas than in samples from the unaltered parts of the Wingate Sandstone in Lisbon Valley, this may be because of accumulation of alteration byproducts and secondary cementation in bleached areas. All analyzed samples contain hematite cement, calcite cement, or both. The unbleached red Wingate Sandstone samples from the central part of the Lisbon Valley contain abundant hematite cement (71%e92% of all cement present), with significantly less calcite cement. Alternatively, in the bleached rock samples, where hematite occurs sparsely and only in the form of micro-nodules, calcite cement is dominant. The bleached rock samples from the northern part of the valley contain between 60% and 87% of calcite cement, and samples from the southern part contain between 78% and 88%. Interestingly, the bleached samples from the southern and northern part of the anticline contain relatively more pyrite than unbleached samples from the central part of the valley, which contain little if any pyrite. Areas with abundant pyrite also contain much less hematite than areas with very little pyrite.

Figure 7. Relationship between feldspar and clay distribution in Wingate Sandstone in Lisbon Valley, Utah; WS e Wingate Sandstone; c e average, s e standard deviation.

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Table 2 Major and trace element concentrations in the Wingate Sandstone samples. LOI ¼ Loss in Ignition. Sample Type

Unaltered (Central)

Unaltered (Central)

Unaltered (Central)

Unaltered (Central)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Sample No Longitude Latitude SiO2 wt% Al2O3 wt% CaO wt% Ca (ppm) Fe2O3 wt% Fe (ppm) MgO wt% Mn (ppm) Ti (ppm) Na (ppm) K (ppm) LOI (%) Li (ppm) Be (ppm) Sc (ppm) V (ppm) Cr (ppm) Co (ppm) Ni (ppm) Cu (ppm) Zn (ppm) Ga (ppm) Rb (ppm) Sr (ppm) Y (ppm) Zr (ppm) Nb (ppm) Mo (ppm) Cs (ppm) Ba (ppm) La (ppm) Ce (ppm) Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Dy (ppm) Ho (ppm) Er (ppm) Yb (ppm) Lu (ppm) Pb (ppm) Th (ppm) U (ppm)

LV26 109.270 38.190 88 1.7 0.1 744 0.7 4651 1.8 229 1799 872 17,650 1.9 12 36 0.3 20 0 4 5 5 50 0.5 25 129 5 362 15 0.9 7 154 31 22 19 15 10 8 5 5 5 5 6 6 5 62 105

LV39 109.177 38.124 80 2.2 0.1 622 0.7 4619 1.4 146 1085 800 14,800 2 6 18 0.2 7 0 1 3 3 23 0.3 20 69 3 281 6 0.9 5 145 17 12 10 8 5 5 3 3 2 2 3 3 3 37 52

LV66 109.278 38.187 82 3.4 0.6 4396 1.4 9963 1.8 2810 2063 611 23,190 3.2 14 32 0.7 19 4 5 12 10 53 0.6 29 174 6 405 17 0.4 8 167 37 25 23 19 13 9 6 7 6 6 8 10 6 77 109

LV70 109.278 38.187 83 2 0.3 1787 1.3 9096 1.4 222 1734 301 19,524 1.3 12 24 0.3 15 5 4 12 14 41 0.6 28 132 5 412 14 0.6 7 170 30 21 19 15 10 8 5 5 5 5 6 8 4 57 55

LV28 109.168 38.130 85 1.5 0.5 3534 0.4 2566 1.4 223 1529 1763 24,397 1.6 7 24 0.3 14 0 3 5 9 46 0.5 26 67 4 335 10 0.9 7 162 25 17 16 13 9 7 4 5 4 4 4 5 5 49 117

LV31 109.168 38.131 89 3.6 4.5 31,864 0.9 5926 1.5 331 1928 3600 19,724 5.7 10 27 0.6 22 0 4 5 9 60 0.6 27 88 7 489 15 3.3 8 157 39 27 24 20 13 9 7 7 6 6 9 11 5 76 190

LV33 109.169 38.129 85 1.4 0.6 4254 0.1 898 1.4 213 1434 2485 19,935 1.5 7 29 0.3 24 0 2 3 7 37 0.5 27 77 4 289 9 0.8 7 167 25 17 16 13 8 7 4 4 4 4 4 5 5 50 102

LV35 109.167 38.129 94 1.9 1.8 12,733 0.8 52,917 1.5 300 2098 4648 20,878 2.8 7 26 0.4 34 1 4 4 7 54 0.5 27 95 6 333 17 2.2 7 163 34 24 21 18 12 8 6 6 6 5 6 8 6 72 137

LV4 109.201 38.128 84 1.6 1.1 8122 0.2 1241 1.5 212 1381 36,401 22,947 2 8 27 0.3 11 0 3 4 5 41 0.5 27 102 5 402 10 2.7 7 164 24 16 15 13 9 7 4 5 4 4 5 7 4 46 81

Sample Type

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (North)

Sample No Longitude Latitude SiO2 wt% Al2O3 wt% CaO wt% Ca (ppm) Fe2O3 wt% Fe (ppm) MgO wt% Mn (ppm) Ti (ppm) Na (ppm) K (ppm) LOI (%) Li (ppm) Be (ppm) Sc (ppm) V (ppm) Cr (ppm) Co (ppm) Ni (ppm) Cu (ppm)

LV47 109.155 38.134 83 1 0.3 2199 0.8 5275 1.4 172 1403 1141 18,005 0.3 9 18 0.3 14 4 3 11 23

LV48 109.155 38.134 86 0.5 0.2 1590 0.5 3257 1.4 196 1274 418 17,296 0.9 8 14 0.2 13 2 3 8 12

LV49 109.155 38.134 88 0.5 0.3 1906 0.5 3699 1.4 240 12,034 9586 13,819 0.9 8 18 0.2 9 3 3 8 13

LV51 109.156 38.133 88 1.9 0.1 989 0.7 4862 1.4 259 1793 1656 17,541 1.5 10 53 0.4 17 4 14 25 35

LV52 109.156 38.133 94 0.6 0.2 1059 0.3 2255 1.4 183 13,916 1290 17,851 1.9 7 20 0.3 13 4 4 11 16

LV57 109.157 38.131 87 0.1 0.2 1269 0.7 4729 1.4 147 1223 1061 15,808 0.9 6 16 0.2 9 3 3 9 13

LV6 109.201 38.128 79 2.8 3 21,498 0.8 5238 2 407 2100 4651 22,956 4.5 10 33 0.6 21 1 5 7 7

LV61 109.163 38.124 85 0 1.2 8423 1.1 7821 1.5 234 1681 656 13,794 2.4 7 13 0.3 20 6 6 16 17

LV82 109.298 38.256 84 2.7 0.1 1032 2.9 20,064 1.4 435 1744 890 9713 2.2 13 17 0.5 28 2 6 12 12

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Table 2 (continued ) Sample Type

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (South)

Altered (North)

Zn (ppm) Ga (ppm) Rb (ppm) Sr (ppm) Y (ppm) Zr (ppm) Nb (ppm) Mo (ppm) Cs (ppm) Ba (ppm) La (ppm) Ce (ppm) Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Dy (ppm) Ho (ppm) Er (ppm) Yb (ppm) Lu (ppm) Pb (ppm) Th (ppm) U (ppm)

49 0.5 24 57 3 280 10 3.8 6 151 21 16 10 8 5 4 2 3 3 3 3 4 4 45 86

36 0.4 22 51 3 169 8 2.2 6 144 19 14 10 7 4 4 2 3 2 2 3 4 3 47 88

37 0.4 23 52 2 223 7 2.7 6 147 20 14 11 9 6 5 3 3 2 2 3 3 2 38 85

165 0.5 26 67 8 319 13 4.6 6 158 31 30 25 22 19 14 9 10 9 8 8 10 3 54 139

48 0.5 22 60 4 406 10 3.9 6 146 23 17 14 11 8 6 3 4 4 4 4 5 3 51 110

43 0.3 23 49 3 295 7 3.4 6 134 19 14 11 8 5 4 2 3 2 2 3 4 6 40 81

71 0.6 26 133 7 377 18 2.5 7 160 43 30 27 22 15 10 7 7 6 6 8 10 11 83 133

43 0.3 17 44 5 319 11 5.1 6 138 32 26 20 16 11 7 5 6 5 5 6 7 3 80 92

60 0.6 15 41 6 325 14 6.8 4 100 35 24 22 19 12 9 6 6 6 6 7 8 8 64 132

Sample Type

Altered (North)

Altered (North)

Altered (North)

Sample No Longitude Latitude SiO2 wt% Al2O3 wt% CaO wt% Ca (ppm) Fe2O3 wt% Fe (ppm) MgO wt% Mn (ppm) Ti (ppm) Na (ppm) K (ppm) LOI (%) Li (ppm) Be (ppm) Sc (ppm) V (ppm) Cr (ppm) Co (ppm) Ni (ppm) Cu (ppm) Zn (ppm) Ga (ppm) Rb (ppm) Sr (ppm) Y (ppm) Zr (ppm) Nb (ppm) Mo (ppm) Cs (ppm) Ba (ppm) La (ppm) Ce (ppm) Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Dy (ppm) Ho (ppm) Er (ppm) Yb (ppm) Lu (ppm) Pb (ppm) Th (ppm) U (ppm)

LV83 109.298 38.256 82 1.7 0.3 1850 0.6 3929 1.3 178 1684 977 8213 1.7 12 13 0.4 14 3 3 7 11 27 0.5 16 38 4 314 12 3.6 4 100 26 18 16 14 10 7 4 5 4 4 5 6 10 52 92

LV84 109.30 38.260 88 0.4 0.3 1865 0.8 5572 1.3 180 1273 615 10,072 1.7 12 12 0.3 10 2 3 7 10 35 0.4 14 33 4 373 6 4.5 4 174 21 15 14 12 8 6 4 4 4 4 4 5 5 43 91

LV85 109.301 38.261 86 5.5 0.2 1045 1.2 8452 1.4 608 3517 1003 14,246 2.7 15 19 0.7 34 3 7 15 13 75 0.8 18 55 12 365 34 3.4 6 128 68 52 43 35 23 13 11 13 11 11 12 15 6 151 204

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4.4. Major and trace elements geochemistry Major, minor, and trace element concentrations for Wingate Sandstone are presented in Table 2. Four samples from unaltered Wingate Sandstone outcrops were analyzed and compared to seventeen samples from altered outcrops from both north (four) and south (thirteen) portions of the Lisbon Valley. The statistical comparison (t-test) between the two average elemental concentrations from altered and unaltered Wingate Sandstone samples are presented in Table 4. Most of the major and trace elements do not have significantly different concentration among the altered and unaltered samples. Only Sr and Mo show statistically significant differences between the two populations of geochemical analyses. The discussion on major and trace elements compares relative mean concentrations between the two populations. Thin section studies suggests that the Wingate Sandstone is a formation mostly composed of quartz, feldspar, and a significant amount of clay (kaolinite), and is cemented either by hematite or by calcite. Unaltered, red samples contain more feldspar, less clay, and are hematite cemented, unlike bleached samples, that contain less feldspar, more clay, and are calcite cemented. Using major and minor element geochemical analyses more than 85% of the composition of Wingate Sandstone samples can be accounted for by a combination of SiO2, Al2O3, CaO, Fe2O3, and MgO (Table 2). Silica is the most dominant phase, as can be expected from siliciclastic rocks. Several minor and trace elements, listed below, have different mean concentrations in altered and unaltered Wingate Sandstone

Table 4 Statistical evaluations of major, minor, and trace elements for analyzed Wingate Sandstone samples. c e average; s e standard deviation; L e confidence limits; t e comparison calculations between the two averages; UW e unaltered Wingate Sandstone; AW e altered Wingate Sandstone. Element

Ca

Fe

Cu

Zn

Sr

Mo

Ba

U

Median (UWS) c (UW) s (UW) L ¼ 50% (UW) L ¼ 95% (UW) Median (AW) c (AW) s (AW) L ¼ 50% (AW) L ¼ 95% (AW) t-test, P (T <¼ t) two-tail

1265 1887 1752 692 2786 1906 6190 8586 1437 4352 0.07

6873 7082 2848 1125 4528 4862 5357 4304 720 2182 0.75

7 8 5 2 8 12 13 7 1 4 0.16

45 42 13 5 21 46 54 31 5 16 0.29

130 126 43 17 69 57 65 26 4 13 0.05

1 1 w0 w0 w0 3 3 1 w0 1 0.00

160 159 12 5 18 147 141 27 4 13 0.15

95 98 26 10 42 102 115 37 6 19 0.11

samples. These include uranium (U), molybdenum (Mo), strontium (Sr), barium (Ba), copper (Cu), and zinc (Zn), as can be observed from Table 2. Uranium levels (Fig. 8A) in unaltered samples range from 52 ppm to 109 ppm, whereas in altered rocks values range from 81 ppm to 204 ppm. Although altered sandstones have higher average U concentrations, only seven out of 17 analyzed bleached samples have U levels higher than the average in unbleached samples. Molybdenum (Mo) average concentration in bleached samples is 3.3 ppm, and in unbleached, unaltered samples it is 0.7 ppm. Standard deviations are lower as well: 0.2 ppm in unaltered and 1.5 ppm in altered sandstones. Also, in all altered

Table 3 Measured stable carbon and oxygen isotope values in selected Wingate Sandstone samples, with a description of sampling area. Sample Numbers

Longitude

Latitude

Samples Description

d13C (PDB)

d18O (PDB)

d18O (SMOW)

LV3 LV4 LV5 LV6 LV11 LV12 LV14 LV15 LV16 LV18 LV19 LV20 LV22 LV26 LV27 LV28 LV29 LV31 LV33 LV34 LV35 LV39 LV48 LV49 LV51 LV52 LV59 LV61 LV66 LV67 LV70 LV79 LV81 LV82 LV83 LV84 LV85

109.201 109.201 109.201 109.201 109.298 109.298 109.296 109.293 109.287 109.284 109.279 109.279 109.261 109.269 109.273 109.168 109.168 109.168 109.169 109.167 109.167 109.177 109.155 109.155 109.156 109.156 109.160 109.163 109.278 109.278 109.278 109.298 109.298 109.298 109.298 109.298 109.298

38.128 38.128 38.128 38.128 38.263 38.263 38.255 38.243 38.241 38.243 38.231 38.231 38.209 38.186 38.186 38.130 38.131 38.131 38.129 38.130 38.129 38.124 38.134 38.134 38.133 38.133 38.129 38.124 38.187 38.187 38.187 38.257 38.257 38.256 38.256 38.256 38.256

Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Weathering on Wingate surface Wingate (altered, north) Wingate (altered, north) Wingate (altered, north) Wingate (altered, north) Wingate (altered, north) Wingate (altered, north) Wingate (altered, north) Wingate (unaltered, center) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (unaltered, center) Altered Wingate (south) Altered Wingate (south) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (altered, south) Wingate (unaltered, center) Wingate (unaltered, center) Wingate (unaltered, paler/whiter) Wingate (altered, north) Wingate (altered, north)- redder Wingate (altered, north)- redder Wingate (altered, north)- whiter (3e4 m above LV82) Wingate (altered, north)- yellow (right above LV82) White altered Wingate (few m below sample LV82)

4.09 4.69 4.67 5.01 3.09 4.2 3.9 0.2 5.17 3.07 5.89 3.66 0.2 3.47 4.27 4.81 0.67 5.69 4.11 3.84 8.05 3.67 13.21 5.73 22.28 11.74 1.09 5.06 3.55 5.57 7.02 3.43 3.69 5.01 12.43 8.05 14.45

1.84 0.33 4.37 1.28 1.95 2.88 8.92 3.97 2.29 0.94 1.1 1.52 0.34 6.22 3.13 5.81 0.61 1.99 5.31 3.2 9.88 6.49 2.13 0.88 7.58 3.09 1.16 1.32 7.72 0.16 5.97 0.37 6.42 2.209 2.08 4.09 4.79

29.02 30.57 35.42 32.23 32.92 27.94 21.72 34.99 28.54 29.95 29.77 32.47 30.56 37.32 34.14 24.92 31.54 28.85 25.43 27.61 20.72 24.21 28.72 30 23.09 27.73 32.1 29.55 38.87 30.75 24.76 31.29 37.53 28.63 28.76 26.69 25.97

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Figure 8. Trace element concentrations in Wingate Sandstone, (A) Uranium (U) distribution in altered and unaltered Wingate Sandstone samples, (B) Molybdenum distribution in altered and unaltered Wingate Sandstone samples, (C) Strontium distribution in altered and unaltered Wingate Sandstone samples, (D) Barium distribution in altered and unaltered Wingate Sandstone samples, (E) Zinc distribution in altered and unaltered Wingate Sandstone samples, (F) Copper distribution in altered and unaltered Wingate Sandstone samples. c e average, s e standard deviation.

(bleached) samples but one, Mo concentration levels are higher than in the unbleached sample (Fig. 8B). Contrary to U and Mo, strontium (Sr) and barium (Ba) show higher mean concentrations in unaltered than altered sandstones (Fig. 8C and D) Zinc (Zn) and copper (Cu) show somewhat higher average concentrations in altered Wingate sandstones (Fig. 8E). However, only six out of seventeen altered sandstones have higher zinc content than is found in unaltered samples. A similar situation was detected with Cu (Fig. 8F).

4.5. Stable isotope geochemistry Selected rock samples from both bleached and unbleached Wingate Sandstone outcrops from Lisbon Valley, Utah, were analyzed for stable carbon (13C) and oxygen (18O) isotopes in order to determine and understand the origin of surficial alterations. The Wingate Sandstone d13C values lie between 0.2& and 22.3& relative to the PDB (Table 3). The results show that the distribution of altered versus unaltered outcrops can be outlined using the

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rocks’ isotopic signatures. Samples taken from the central, unbleached section of Wingate Sandstone in Lisbon Valley generally show the highest d13C, ranging from 0.2& to 7.01& relative to the PDB. Samples from the northern, bleached Wingate Sandstone outcrops in the valley show d13C values in a range from 0.2& to 14.45& (PDB), with two of the sandstones showing depletion of 13C with values lower than 10& (PDB). Two of analyzed samples from the southern bleached Wingate Sandstone outcrops are the most depleted in 13C (lower than 11&), with the values ranging from 0.66& to 22.28& (PDB). Measured d18O values for all analyzed samples range from þ7.72& to 9.88& (PDB), with unbleached samples having values from þ7.72& to 6.5& (PDB), bleached samples from the northern part of the valley range between þ4.37& and 8.92& (PDB), and bleached samples from the southern part of the valley having oxygen values between þ3.13& and 9.88& (PDB) (Table 3; Fig. 9). The d18O values measured relative to the PDB generally follow the change in the measured d13C values. When plotted, most of the results show clustering in one large group but there are few values that fall outside of this group (outlined with an orange circle (in the web version) in Fig. 9 and represented as orange dots (in the web version) in Fig. 10). These samples contain the lowest d13C isotopic values (between 11.74& and 22.28& relative to the PDB) and are also located the closest to the Lisbon Valley fault, which is a documented conduit for gas seeps from the subsurface (Shipton et al., 2004). 5. Discussion Vertical migration is the mechanism used to explain the hydrocarbon macro- and microseepages to the surface. This process causes the surficial geochemical manifestations by which petroleum accumulations could be detected using various shallow geological methods (Tedesco, 1995). The cross-sectional shape of the leaking pattern has conventionally been called a ‘chimney effect’. Most chimneys are nearly vertical, although hydrodynamic laws suggest that groundwater movement and dipping strata could deflect the ascending hydrocarbons. However, the upward movement of gases and buoyancy are so great that hydrocarbon transport to the surface (Tedesco, 1995; Van der Meer et al., 2002). Matthews (1996) concluded that diffusion of gases, can pass

Figure 9. d13C versus d18O (both relative to the PDB) plot for the Wingate Sandstone samples from the Lisbon Valley, Utah. Most of the samples have stable isotope values clustering in a group with d13C values between 0 and 10& and d18O values between þ10 and 10& (within yellow circle), but a few samples from bleached outcrops have somewhat lower d13C signatures (within orange circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

through seemingly impenetrable barriers and thus is the most common form of migration that produces microseepages. The Wingate Sandstone has a relatively uniform composition and characteristics throughout its exposure on the Colorado Plateau, except at isolated locations such as Lisbon Valley, where it is locally bleached, has an anomalous mineralogical content, and is easily mechanically weathered in areas overlying the petroleum reservoir (Segal et al., 1986). The remote sensing and thin section studies of the Wingate Sandstone were used to determine the mineral composition of all sampled outcrops and to observe changes between bleached and unbleached samples. The remote sensing analyses suggest that there are three different areas within the Wingate Sandstone in Lisbon Valley: unaltered outcrops at the central part of the valley, and altered outcrops at the north and the south part of the valley. Remote sensing was also used to identify and quantify different minerals and alterations. Thin section studies confirmed remote sensing results with respect to the distribution of hematite, kaolinite, and calcite. Thin section studies also revealed that the amount of feldspar grains becomes smaller when going from unbleached outcrops towards the bleached ones. The opposite is happening to kaolinite: it is more abundant in bleached (altered) than in unbleached (unaltered) sandstones. The shape and appearance of clay minerals suggest that their presence is a direct result of feldspar alteration, especially because significant numbers of feldspar grains showed some degree of alteration, and that more altered feldspars were observed in the altered and bleached Wingate Sandstone samples from the southern and northern part of the valley, than in the unaltered, red Wingate Sandstone samples from the central part of the valley. A similar observation can be made for the relationship between hematite and calcite cement content: in sandstones where hematite cement is dominant (unbleached samples), calcite is present in very minor amounts, and vice versa. In bleached samples where calcite was the dominant cementing agent significant amounts of pyrite were recognized. This redistribution of iron from hematite to pyrite is another sign of changes and processes during bleaching of Wingate Sandstone outcrops. In order for red, hematite-rich beds to get bleached, redox environments need to be established that will cause immobile Fe3þ ion to be reduced to more mobile Fe2þ and either leached out of the system or converted into pyrite, siderite, or other bivalent iron-bearing minerals. This can be done in several ways: by introducing leaking hydrocarbons, organic acids, methane, or hydrogen sulfide, all of which are reducing and/or acidic agents. These reducing and acidic agents also help in altering the feldspars and precipitation of clay minerals (Chan et al., 2000), which can explain the loss of feldspars and their alteration to clays in bleached Wingate Sandstone outcrops. This finding, along with the porosity and cement content increasing in bleached samples suggest the possibility of some reducing fluid that removed most of hematite cement and reduced some of it to pyrite, opening the way to subsequent calcite cementation. The major, minor, and trace element geochemical analyses revealed several facts about bleached Wingate Sandstone areas and their differences from the unaltered outcrops. Average uranium content is higher in altered than in unaltered Wingate Sandstone samples. The whole Lisbon Valley area has been a major uranium producer for decades, with ore in both Permian Cutler Formation and Triassic Chinle Formation (Denis, 1982). These uranium deposits belong to the so-called roll-front type of uranium accumulations (Finch, 1996), which are tabular uranium accumulations resulting from a reaction between a geochemically reducing rock and an oxygenated U-bearing fluid (Goldhaber et al., 1983). Sulfidebearing solutions from underlying hydrocarbon accumulations leaking along faults were important in the preservation of these

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Figure 10. ASTER multispectral image (bands 3e2e1 displayed as RedeGreeneBlue) with the areal distribution of d13C (PDB) isotope values for the selected samples from the Wingate Sandstone and neighboring formations in Lisbon Valley, Utah. Wingate Sandstone outcrops are outlined in pink color. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

deposits because reducing conditions were maintained within the host rocks (Reynolds et al., 1982). The biggest uranium concentrations in Lisbon Valley area are located in the upper parts of Cutler and Chinle Formations, in areas directly beneath bleached Wingate Sandstone outcrops (Segal et al., 1986). This suggests not just that the higher uranium concentrations in bleached sandstones are caused by the interaction with leaking hydrocarbons, but it also connects bleaching of Wingate Sandstone outcrops with the same hydrocarbons. Still, attention must be paid to the fact that both altered and unaltered Wingate sandstones showed significant variability in uranium content within similarly affected outcrops. The occurrence of molybdenum is very common as a follow up to

uranium in the same roll-front type of deposits (Goldhaber et al., 1983), which can account for Mo having higher concentrations in altered than in unaltered Wingate sandstones. Also, the standard deviation and average of Mo in analyzed samples do not vary as much as they did for some other elements. Strontium preferentially accumulates in feldspars, and its higher content in unaltered Wingate Sandstone samples confirms the thin section results of feldspars being significantly more abundant in unbleached than in bleached rocks. Barium is an element that accumulates in feldspars, but also in calcite. Its relatively uniform distribution within Wingate Sandstone and just slight drop of concentrations in bleached rocks can

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be accounted for by the drop of feldspar and rise in calcite content in the same samples. Statistical evaluations of Ba concentration measurements reveal that its concentration probably wouldn’t change much even if the larger sample population was analyzed. Zinc and copper are common trace elements in the mineral pyrite. Their higher concentrations in analyzed bleached Wingate Sandstone samples can be explained with the formation of pyrite after the reduction of hematite. Higher presence of pyrite when compared to hematite in bleached samples was observed during thin section analyses. Still, the relatively large standard deviations of average concentrations for both elements suggest that more samples need to be analyzed before final conclusion about concentration disparity of these two elements between altered and unaltered samples could be made. However, it has to be pointed here that statistical assessment of the geochemical results for the altered and unaltered Wingate sandstone samples demonstrate that only Sr and Mo results exhibit statistically significant differences between the two sample populations. Stable carbon and oxygen signatures of bleached and unbleached Wingate Sandstone samples revealed that there were at least two fluids affecting the bleached outcrops. It was found that altered outcrops, the ones in the southern part of the anticline, were influenced with a fluid strongly depleted in 13C, especially the samples taken from outcrops closest to the Lisbon Valley fault, promoting the fault as a conduit for fluids that bleached Wingate Sandstone outcrops. All unbleached samples and some of the bleached ones were characterized by an isotopic signature characteristic of groundwater values. Combination of two different signature types in samples is consistent with the mixing of two different sources, one being groundwater/freshwater, and the other being much more depleted in 13C. Wingate Sandstone is part of the so-called N-aquifer, along with younger Jurassic Navajo and Entrada formations, which is a relatively deep groundwater aquifer not just in the Lisbon Valley (Avery, 1986), but also throughout greater Utah and Arizona area (Zhu, 2000). In Lisbon Valley it deepens to the north of the valley and becomes shallower in the south, establishing a south-to-north gradient (Hahn and Thornson, 2005). Zhu (2000) sampled calcite from sandstones containing the N-aquifer throughout Arizona (mostly Navajo Formation and Wingate Sandstone), in areas not overlying petroleum reservoirs, and determined that their d13C values range from 8.3 to þ1.8&, relative to the PDB. The carbon signature of all unaltered and some of the altered samples corresponds to the values from Zhu (2000). Carbon in calcite that cemented sandstones analyzed during this research could have come from several sources: freshwater, atmosphere, marine environment/saline brines, or hydrocarbon sources. Calcite, with carbon derived from the atmosphere, freshwater, or the marine environment, most commonly has a carbon isotopic value of about 10 to þ5& relative to the PDB standard (Schumacher,1996). The carbon isotopic composition of most crude oils ranges from about 20 to 32&, whereas that of methane can be range from 30 to 90&. Calcite formed from oxidized petroleum incorporates carbon from the organic source which typically has an isotopic composition more negative than 20&. Depending on the proportion of oxidized hydrocarbon incorporated, the isotopic composition of the resultant carbonate can range from 10 to 60& (Schumacher, 1996). The underlying Mississippian petroleum reservoirs in Lisbon Valley area do contain a gas phase with volatile hydrocarbons and hydrogen sulfide (Parker, 1968). The 13C depleted hydrocarbons indeed present the most likely possibility for the other fluid that influenced bleached Wingate Sandstone outcrops. The proximity of the fault, as a leaking pathway, and its relationship to bleached outcrops and lowest d13C signatures support this concept. It has also been noticed that d13C isotopic signature in Wingate Sandstone outcrops changes

with the color of a rock. In a northernmost part of the formation’s exposure in Lisbon Valley outcrops, previously described as bleached, are not uniformly colored, and all three variations of Wingate Sandstone are present (Fig. 11). Red colored, unbleached layers have the highest isotopic signature, and gray colored, bleached layers are the most depleted in 13C. Values for the yellow bleached rocks are in between the two. This observation suggests that the fluid that bleached the rocks also lowered the d13C signature in the affected rocks. The other formations cropping out in Lisbon Valley, Utah, namely Hermosa Group, Cutler Formation, Chinle Formation, Kayenta Formation, and Entrada Sandstone, showed no sign of bleaching or any other similar type of changes at the surface. The first three formations are older, stratigraphically lower than the Wingate Sandstone, and overlie the Paradox salt. All three are closer to the Lisbon Valley fault but are also much more impermeable than Wingate Sandstone, which helps to explain why only Wingate Sandstone was altered on such big scale. The reducing fluids rose upward along the Lisbon Valley fault and flowed laterally to first available permeable strata. To summarize, the proximity of altered, bleached Wingate Sandstone outcrops to the Lisbon Valley fault, reduction of hematite to pyrite, advanced alteration of feldspars to clays, higher concentrations of Mo, and depletion of 13C in the bleached outcrops closest to the fault suggest that the fault was a major pathway for fluids that changed the outcrops. Geochemical signature in the bleached rocks revealed the mixing of two fluids in the same outcrops. The fluids have to be reducing, in order for iron to be removed and reduced from hematite to pyrite, and for U and Mo to be immobile and concentrated in one area. The low d13C values in some bleached samples and the extent of altered Wingate Sandstone outcrops in the surface that correspond to the outline of the petroleum reservoirs underneath point to the leaking hydrocarbons and/or hydrogen sulfide. The combination of field work data and laboratory analyses (remote sensing and geochemical) showed that a connection between surficial alterations and underlying petroleum reservoirs can be established. Remote sensing techniques can map changes within rock formations, whereas geochemical analyses can reveal the true cause of alterations. Therefore, the combined use of aforementioned techniques is suggested for hydrocarbon prospecting in unexplored areas of the world.

Figure 11. Example of an outcrop from a northern exposure of Wingate Sandstone in Lisbon Valley, Utah, where a contact of three regionally typical color styles of the same formation occur. d13C signature changes from highest in the red, unbleached rocks, to being the lowest in gray, bleached rocks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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6. Conclusions This study utilized remote sensing methodologies and geochemical tools to understand the hydrocarbon-induced rock alteration process. The main area of interest was Lisbon Valley in southeast Utah, where previous researchers discovered surficial rock changes in the Wingate Sandstone outcrops above petroleum reservoirs and assumed they were caused by leaking hydrocarbons. Remote sensing techniques (Spectral Angle Mapper and Spectral Feature Fitting) were successfully applied on HyMap hyperspectral data. The Spectral Angle Mapper was used to classify the extent of altered and unaltered outcrops, as well as to map the changes in mineral content within the outcrops. Reflectance spectroscopy, petrography, and major, minor and trace element analyses, as well as stable carbon and oxygen studies on both bleached and unbleached samples were used to successfully relate changes to hydrocarbons leaking from underlying petroleum reservoirs. Unbleached, red Wingate Sandstone samples had higher hematite and feldspar content than bleached Wingate Sandstone samples, which were characterized with larger amounts of clay and calcite, as well as the presence of pyrite. Some bleached samples also had higher concentrations of elements (U, Mo) characteristic of hydrocarbon-related reducing environments, and were depleted in 13 C when compared to the unbleached samples. Based on these results, the following model of reactions is proposed to explain the bleaching and mineralogical and geochemical changes within Wingate Sandstone. The hydrocarboninduced reducing environment caused the transformation of sulfate ion (obtained from groundwater or from oxidation of H2S) to sulfide ion, resulting in the reduction of hematite to pyrite. The released hydrogen ion from this reaction reacted with available feldspars in the rock, leading to precipitation of kaolinite. These conditions favor the reaction between bicarbonate ion and Ca2þ ions that can be obtained from the groundwater, leading to precipitation of calcite in pore spaces left open after the reduction and removal of hematite. Our data supports the hypothesis that subtle mineral and chemical changes carry a potential for future hydrocarbon exploration. This model and the techniques could be used in other parts of the world with similar surficial manifestations to explain the effects of leaking hydrocarbons. Acknowledgments We are thankful to Dr. Aziz Ozyavas for his help in the field. Dr. William Dupre is thanked for his help in the field and his valuable suggestions. This manuscript improved considerably from Dr. Henry Chafetz, suggestions and criticisms. We also thank Mike Darnell for his guidance in carbon isotope analyses and Dr. Yongjun Gao and Yingqian Xiong for their help in geochemical analyses. Dr. Brenda Bowen and an anonymous reviewer are thanked for their detailed and very thorough reviews. This work was funded by Statoil. References Abrams, M.J., Goetz, A.F., Lang, H., 1983. New techniques for clay mineral identification by remote sensing. AAPG Bulletin 67 (3), 410. Allen, R.F., Thomas, R.G., 1984. The uranium potential of diagenetically altered sandstone of the Permian Rush Springs Formation, Cement district, southwest Oklahoma. Economic Geology 79, 284e296. Al-Shaieb, Z., Cairns, J., Puckette, J., 1994. Hydrocarbon-induced diagenetic aureoles: indicators of deeper leaky reservoirs. Association of Petroleum Geochemical Explorationists Bulletin 10, 24e48. Avery, C., 1986. Bedrock Aquifers of Eastern San Juan County, Utah State of Utah Department of Natural Resources Technical Publication No. 86. Bammel, B.H., Chamberlain, C.P., Birnie, R.W., 1994. Stable isotope evidence of vertical hydrocarbon microseepage, Little Buffalo Basin oil field. Association of Petroleum Geochemical Explorationists Bulletin 10, 1e23.

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