Origin and Heterogeneity of Pore Sizes in the Mount Simon Sandstone and Eau Claire Formation: Implications for Multiphase GEOSPHERE; V

Research Paper

GEOSPHERE Origin and heterogeneity of pore sizes in the Mount Simon Sandstone and Eau Claire Formation: Implications for multiphase GEOSPHERE; v. 12, no. 4 fluid flow doi:10.1130/GES01245.1 Peter S. Mozley1, Jason E. Heath2, Thomas A. Dewers2, and Stephen J. Bauer2 16 figures; 3 tables 1Department of Earth and Environmental Sciences, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA 2Department of Geomechanics, Sandia National Laboratories, P.O. Box 5800, Mail Stop 0750, Albuquerque, New Mexico 87185-0750, USA

CORRESPONDENCE: peter.mozley@nmt.edu ABSTRACT energy storage (CAES) operations (Medina and Rupp, 2012; Heath et al., 2013). CITATION: Mozley, P.S., Heath, J.E., Dewers, T.A., The Mount Simon Sandstone is overlain by the upper Cambrian Eau Claire For- and Bauer, S.J., 2016, Origin and heterogeneity of pore sizes in the Mount Simon Sandstone and The Mount Simon Sandstone and Eau Claire Formation represent a poten- mation, a generally low-permeability mudstone and regional seal (Neufelder

Eau Claire Formation: Implications for multiphase tial reservoir-caprock system for wastewater disposal, geologic CO2 storage, et al., 2012; Lahann et al., 2014). Of primary importance are reservoir-caprock fluid flow: Geosphere, v. 12, no. 4, p. 1341–1361, and compressed air energy storage (CAES) in the Midwestern United States. properties that govern multiphase flow, because injectivity, sweep efficiency, doi:10.1130/GES01245.1. A primary concern to site performance is heterogeneity in rock properties that and capillary trapping greatly affect site performance. Geologic controls on could lead to nonideal injectivity and distribution of injected fluids (e.g., poor heterogeneity of pore structure and flow properties can be complex and diffi- Received 6 August 2015 Revision received 11 April 2016 sweep efficiency). Using core samples from the Dallas Center domal structure, cult to characterize due to the interplay of textures from primary depositional Accepted 26 May 2016 Iowa, we investigate pore characteristics that govern flow properties ofmajor environments and a variety of potential postdepositional processes, includ- Published online 23 June 2016 lithofacies of these formations. Methods include gas porosimetry and perme ing precipitation-dissolution with a range of textures, mechanical compaction, ametry, mercury intrusion porosimetry, thin section petrography, and X-ray pressure solution, and fracture porosity or mineralization (Hoholick et al., 1984; diffraction. The lithofacies exhibit highly variable intraformational and inter- Bowen et al., 2011). Despite the regional extent and storage potential of the formational distributions of pore throat and body sizes. Based on pore-throat Mount Simon–Eau Claire system, few conventional cores are available that size, there are four distinct sample groups. Micropore-throat–dominated sam- cut the entire thickness of the Mount Simon Sandstone and the Eau Claire– ples are from the Eau Claire Formation, whereas the macropore-dominated, Mount Simon contact (Bowen et al., 2011), and thus detailed studies of the mesopore-dominated, and uniform-dominated samples are from the Mount major lithofacies and their associated multiphase flow properties have been Simon Sandstone. Complex paragenesis governs the high degree of pore heretofore limited. and pore-throat size heterogeneity, due to an interplay of precipitation, non In this study we use mercury intrusion porosimetry (MIP) to quantify pore uniform compaction, and later dissolution of cements. The cement dissolution size distribution and evaluate the impact of porosity heterogeneity on multi- event probably accounts for much of the current porosity in the unit. Mercury phase flow. MIP is a readily available tool for examining capillarity and pore intrusion porosimetry data demonstrate that the heterogeneous nature of structure of porous media. The technique has been applied extensively for the pore networks in the Mount Simon Sandstone results in a greater than assessing sealing capacity of caprock in order to understand hydrocarbon normal opportunity for reservoir capillary trapping of nonwetting fluids, as traps (Almon et al., 2005). It is also applied, but less commonly, to the study

quantified by CO2 and air column heights that vary over three orders of mag- of sandstones and siltstones, including argillaceous sandstones and tight- nitude, which should be taken into account when assessing the potential of gas sandstones. Wardlaw and Cassan (1979) examined 27 samples of coarse-

the reservoir-caprock system for waste disposal (CO2 or produced water) and grained siltstone to medium-grained sandstone from a wide variety of units of resource storage (natural gas and compressed air). Our study quantitatively variable ages and a wide range of permeability; they examined relationships demonstrates the significant impact of millimeter-scale to micron-scale poros- among pore-throat aperture, grain size, pore size, and mercury recovery effi- ity heterogeneity on flow and transport in reservoir sandstones. ciency, which is a proxy for residual trapping. They noted that the presence of carbonate cement increases heterogeneity in pore-throat aperture along INTRODUCTION with a decreased recovery efficiency. Wardlaw and Cassan (1979) also found that high mercury recovery efficiency is found in samples that have high The Cambrian Mount Simon Sandstone, and its stratigraphic equivalents, porosity, small pore to throat size ratios, and small mean particle sizes, the For permission to copy, contact Copyright occur throughout the Midwestern United States, where it is a target injection latter being somewhat counterintuitive. Pittman (1992) used an unpublished Permissions, GSA, or [email protected]. horizon for wastewater disposal, geologic CO2 storage, and compressed air industry data set of 196 sandstone samples for which conventional porosity

GEOSPHERE | Volume 12 | Number 4 Mozley et al. | Porosity heterogeneity in sandstone Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1341/3337069/1341.pdf 1341 by guest on 28 September 2021 Research Paper

and permeability data as well as mercury intrusion data were available; he 94.0° W used these data to develop empirical relationships to allow determination of pore-aperture size parameters. Nelson (2009) provided a summary of a number of MIP studies on sandstone. In this study we build upon this prior IOWA, USA –1960 work by using MIP data to evaluate the potential impact of millimeter-scale Rinehart “A” #1 to micron-scale pore-aperture heterogeneity on multiphase flow in reservoir Keith #1 sandstone. We examined a conventional core from the Dallas Center domal structure

(area of Dallas Center, Iowa) that was obtained for site evaluation of a planned 41.7° N CAES project (Heath et al., 2013; Dewers et al., 2014). This continuously cut core Mortimer #2 includes both a complete section of the Mount Simon Sandstone (~28.3 m) as well as a portion (~36.1 m) of the overlying Eau Claire Formation, including the reservoir-caprock interface. Using gas porosimetry and permeametry, MIP, thin Keith #1 –1980 section petrography, and X-ray diffraction (XRD), we characterize pore types –1960 and rock textures and quantify the range of pore-throat sizes in major litho- –1920 facies of the Mount Simon Sandstone and Eau Claire Formation. These data then allow us to make direct comparisons of pore characteristics of the reser- –1900 voir-caprock system. The large number of MIP samples (n = 30) and detailed Mortimer-1980 #1 petrographic observations provide a unique data set for characterizing the nature and origin of the pore-size heterogeneity within a sandstone reservoir (saline aquifer) and across and into the overlying caprock. Because the petro- –2180 physical characteristics and sealing capacity of the Eau Claire Formation caprock were discussed in detail elsewhere (Neufelder et al., 2012; Lahann et al., –2080 1 mile 2014; Swift et al., 2014), we mainly provide detailed analysis for the Mount Simon Sandstone. 1 kilometer

Figure 1. Structural elevation map of the top of the Mount Simon Sandstone at the Dallas Center GEOLOGIC SETTING domal structure. Contours are in feet below sea level (modified from Heath et al., 2013).

Regional and site-specific geologic information are presented to facilitate comparison between properties of the Mount Simon Sandstone and the over- and heterolithic sandstone-mudstone (Saeed and Evans, 2012). Bowen et al. lying Eau Claire Formation at the Dallas Center structure (Fig. 1), where the (2011), focusing on the Illinois Basin, stated that depositional environments core was collected, with other locations in the Midwestern United States. For may include shallow-marine, deltaic, fluvial, eolian, and possibly sabkha set- example, Decatur, Illinois, is the site of the Illinois Basin–Decatur Project by tings, with lithofacies including cobble conglomerate, stratified conglomer-

the Midwest Geological Sequestration Consortium, where CO2 is injected at ate, poorly to well-sorted sandstone, and interstratified sandstone and shale; a rate of 1000 t/day in the Mount Simon Sandstone (Finley, 2014). The Cam- and shale. The Mount Simon Sandstone thus exhibits strong heterogeneity brian Mount Simon Sandstone, a major regional aquifer, extends broadly in lithofacies (Bowen et al., 2011); such heterogeneity motivates an under- throughout the Midwestern United States and overlies Precambrian crystal- standing of intraformational variability in flow properties on site-specific to line basement or sedimentary rocks in Iowa, Illinois, Indiana, Ohio, Michi- regional scales. gan, Minnesota, Wisconsin, and Nebraska, reaching thicknesses of >305 m in Overlying the Mount Simon Sandstone for much of its regional extent is eastern Iowa, Illinois, Indiana, and Michigan (Anderson, 1998), and >800 m in the Cambrian Eau Claire Formation. The formation varies, north to south, from north-central Illinois (Medina et al., 2011). In Ohio, several lithofacies (as many siliciclastic dominated in its type area near Eau Claire in western Wisconsin to as eight) have been identified and interpreted as indicating deposition as a mixed siliciclastic-carbonate to carbonate dominated in northeastern Missouri tidally influenced transgressive barrier sequence, with environments including (McKay, 1988; Lahann et al., 2014). McKay’s (1988) descriptions of three (upper, swash and surf zones; mud, sand, and mixed flats; sand flats to tidal channels; middle, and lower) informal units of the Eau Claire Formation in eastern Iowa tidal inlet channels, and bioturbated sand flats (Saeed and Evans, 2012). Lithol- include an upper portion of northeastern Iowa with finely and coarsely inter- ogies include conglomerate to pebbly-to-fine sandstone, siltstone, mudstone, layered feldspathic and glauconitic sandstone and shale that contain inarticu-

late brachiopod shells and trilobite and hyolithid molds. In southeastern Iowa, MATERIALS AND METHODS the sandstone-shale facies grades into a variably glauconitic, dolomitic, feldspathic siltstone-shale facies, with sparse inarticulate brachiopod shells and To understand pore-scale characteristics that control heterogeneity in flow fragments of trilobite molds. The middle Eau Claire Formation, from north- properties of the Mount Simon Sandstone and Eau Claire Formation, we used eastern to southeastern Iowa, grades from siltstone-shale facies to a shale and a combination of gas porosimetry and permeametry, MIP, XRD, and standard dolomite facies. The dolomite facies includes skeletal dolomite packstones petrographic techniques. Samples were taken from the Keith No. 1 well core to grainstones, bioturbated dolomites, dense crystalline dolomite, and dolo- at depths of the major lithologic units of the Mount Simon Sandstone and mite algal thrombolites and stromatolites. The lower Eau Claire is a fine sand- several lithofacies of the Eau Claire Formation (Fig. 2; also see Heath et al., stone and shale facies in eastern Iowa. Lithologies range from laminated to 2013; Dewers et al., 2014). Core plugs included those of vertical and horizontal ripple laminated to bioturbated shaly sandstone and interlayered sandstone orientation for directional gas permeametry and MIP. Gas porosimetry and per- and shale, shale, flat-pebble conglomerate, and dolomite echinoderm pack- meametry analyses were performed on dried samples by methods detailed in stones. In summary, the Eau Claire Formation has significant large-scale facies Heath et al. (2013). A set of samples of mudstones or low-permeability lithol- changes from siliciclastic to carbonate, and includes a large variety of litholo- ogies were analyzed by TerraTek, a Schlumberger company, using their tight gies (Neufelder et al., 2012). rock analysis method to determine pressure- and/or pulse-decay permeability Our study site, the Dallas Center domal structure, Iowa, is near the south- and porosity. Poro-Technology in Sugar Land, Texas (now Micromeritics), per- eastern edge of the Iowa horst of the Mid-Continent Rift System, next to the formed MIP on a Micromeritics AutoPore IV 9500 V1.07 instrument. MIP was Thurman-Redfield structural zone, where domal structures occur due to de- done on 28 cylindrical core plugs, ~1.91 cm long × 1.91 cm diameter, that were formation near the horst and the structural zone (Anderson and McKay, 1989; jacketed with epoxy for directional mercury intrusion (Table 1). The epoxy Heath et al., 2013). The Dallas Center structure is 9.6 km west of the Redfield jacket coated the outside of the cylindrical samples, but not the tops and bot- gas storage field. The Hydrodynamics Group oversaw seismic mapping and toms. Thus, mercury intruded from the top and bottom of the plugs during the coring of the Keith No. 1, Mortimer No. 1, and Mortimer No. 3 wells to con- measurements. Omnidirectional measurements were performed on two sam- firm and update previous structural interpretation of seismic reflection data ples that were too thin for epoxy jacketing (Table 1). Closure pressure was per- collected by Bay Geophysical, Inc. to support assessment of a planned CAES formed using a compressibility method (see Heath et al., 2013, for full details). facility (Heath et al., 2013). We used the core from the Keith No. 1 well for Conversion of intrusion pressure to pore-throat diameter used the Washburn this study. equation (version of the Young-Laplace equation; Washburn, 1921; Dullien, Using electric log and core attributes, Dewers et al. (2014) subdivided the 1992). Permeability calculated from MIP data was based on Swanson’s (1981) Mount Simon Sandstone in the Keith No. 1 well into upper, middle, and lower equation. Samples were analyzed for drainage, but not imbibition. units following the approach of Bowen et al. (2011) and Saeed and Evans Mercury breakthrough pressure, which represents the pressure when a (2012). Dewers et al. (2014) noted gamma ray and density-porosity signatures continuous connected path of mercury has been attained across the sample, strikingly similar to those reported by Saeed and Evans (2012) for a well in was estimated for each MIP sample (using methods after Dewhurst et al., Ohio, and concluded that this probably reflects a similar depositional setting 2002). Conversion from the mercury-air-rock system to the groundwater-air-

of transgressive sheet sands overlying basement highs. Dewers et al. (2014) rock, groundwater-CO2-rock, and gas column heights held by capillary forces indicated that the upper unit in the Keith No. 1 well is probably equivalent to (the sealing capacity) followed procedures of Ingram et al. (1997). Density of the middle unit of Barnes et al. (2009) and Bowen et al. (2011) in the Michigan pure nitrogen as a proxy for injected air for CAES was determined from Span and Illinois Basins, respectively, and lacks the B-cap unit of those authors. The et al. (2000). We used the value of interfacial tension (IFT) for mercury-air of middle and lower units of Keith No. 1 from Dewers et al. (2014) likely corre- 484 mN/m and 140° for contact angle (Shafer and Neasham, 2000; Dewhurst spond to the lower unit of Bowen et al. (2011). et al., 2002). We used an IFT value of 67 mN/m (Wiegand and Franck, 1994) for Heath et al. (2013) and Dewers et al. (2014) described the lithofacies pres- water-air and a contact angle of 0° for the groundwater-air-rock system. Car- ent in the Mount Simon Sandstone in the Keith No. 1 well. The upper unit bon dioxide and groundwater densities were estimated using TOUGH2 soft- consists principally of fine-grained quartzose sandstone that is locally fos- ware (Pruess et al., 2012), with geothermal gradient and groundwater density siliferous and glauconitic. Planar bedding, small-scale trough cross-stratifi- gradients for the sample depths of 0.025 C°/m and 0.009792 MPa/m, respec-

cation, and bioturbation are present locally. The middle unit is heterolithic, tively. Interfacial tension values for CO2-groundwater were estimated follow-

consisting dominantly of subarkosic fine- to coarse-grained sandstone, with ing the algorithm of Heath et al. (2012). The groundwater-CO2-rock contact an- interbeds of mudstone and conglomerate. The lower unit consists mainly of gle range of 17°–70° was based on the general contact angle ranges of mineral quartzose fine- to coarse-grained sandstone, which is locally pebbly. Clasts of phases found in the samples for which contact angle data are available; the the underlying Precambrian red clastic sandstone are present just above the phases included quartz, calcite, feldspar, and mica (Table 2), and the contact nonconformity. angle data was from Iglauer et al. (2015).

Thin section

Figure 2. X-ray diffraction (XRD) data, well logs, laboratory-measured porosity and permeability (air), mercury-air breakthrough pressure, and wire-line log units of Keith No. 1 well. U, M, and L indicate upper, middle, and lower, respectively, following wire-line log units of Dewers et al. (2014). TRA is tight rock analysis, a crushed-rock permeability method. Data points labeled vertical and horizontal indicate orientation of core plugs.

TABLE 1. SAMPLE IDENTIFICATIONS, DEPTHS, ORIENTATION, AND OTHER INFORMATION FOR MERCURY POROSIMETRY SAMPLES FROM KEITH NO. 1 WELL Sample Sample identiﬁcation Depth number (depth, ft) (m) Plug orientation* FormationComments 1 2837.94V 865.00 VerticalEau Claire 2 2850.84V 868.94 VerticalEau Claire 3 2863.23V 872.71 VerticalEau Claire 4 2879.09V 877.55 VerticalEau Claire 5 2891.68V 881.38 VerticalEau Claire 6 2908.99V 886.66 VerticalEau Claire 7 2913.34V 887.99VerticalEau Claire 8 2918.11V 889.44VerticalEau Claire 9 2918.52V 889.56VerticalEau Claire No epoxy jacket† 10 2919.68V 889.92VerticalEau Claire 11 2919.68V-C 889.92VerticalEau Claire No epoxy jacket†; clay rich 12 2925.92V 891.82 VerticalMount Simon Sandstone 13 2926.30V 891.94 VerticalMount Simon Sandstone 14 2931.10H 893.40 Horizontal Mount Simon Sandstone Perpendicular to fracture§ 15 2931.10V 893.40VerticalMount Simon Sandstone Parallel to fracture§ 16 2932.72H 893.89Horizontal Mount Simon Sandstone 17 2948.91H 898.83 Horizontal Mount Simon Sandstone 18 2951.79V 899.71 VerticalMount Simon Sandstone 19 2963.56H 903.29 Horizontal Mount Simon Sandstone 20 2964.86H 903.69 Horizontal Mount Simon Sandstone 21 2967.20H 904.40 Horizontal Mount Simon Sandstone 22 2967.20V 904.40 VerticalMount Simon Sandstone 23 2968.24H 904.72 Horizontal Mount Simon Sandstone 24 2970.30V 905.35 VerticalMount Simon Sandstone 25 2983.42V 909.35 VerticalMount Simon Sandstone 26 2989.15V 911.09 VerticalMount Simon Sandstone 27 2995.59H 913.06 Horizontal Mount Simon Sandstone 28 2995.59V 913.06 VerticalMount Simon Sandstone 29 3003.71H 915.53 Horizontal Mount Simon Sandstone 30 3003.71V 915.53VerticalMount Simon Sandstone *All samples from core were taken in approximately vertical orientation parallel to the long axis of the core or perpendicular to it in the horizontal orientation. †Sample not jacketed with epoxy underwent omnidirectional mercury intrusion. §The samples with a mineralized fracture were cut with the axis of the plugs being either perpendicular or parallel to the orientation of the subvertical fracture.

The X-ray diffraction samples were taken at 28 depths, near the loca- RESULTS tions of MIP and thin section samples; X-ray diffraction was performed with a Siemens model D500 q-2q powder diffractometer (see Heath et al., Here we describe the mineral composition, porosity, permeability hetero- 2013). Thin section samples were selected after hand-sample observations geneity, and diagenetic history of the Mount Simon Sandstone, with an em- of the core for major lithofacies. Thin sections were impregnated with phasis on events that had the greatest impact on porosity and permeability. epoxy and cut and ground to ~30 mm thickness (see Heath et al., 2013). We then present MIP data, which provide a quantitative assessment of the We performed optical petrography with a Leitz Wetzlar Orthoplan-Pol po- pore-throat size distribution in the Mount Simon Sandstone. We show that larizing microscope and a Lecia DFC 425 digital camera. Intergranular vol- the current range in pore-throat distributions is largely the result of complex umes were determined for selected regions of thin sections using ImageJ diagenetic modification of the original primary porosity of the unit. We also (Ferreira and Rasband, 2012) with two-dimensional binarized images of se- present XRD, porosity, permeability, and MIP results of the Eau Claire Forma- lected regions. Binarized images were created by thresholding and manual tion to allow for comparison; however, detailed paragenetic observation of the area selection. Eau Claire Formation is beyond the scope of this paper.

TABLE 2. X-RAY DIFFRACTION RESULTS Sample identiﬁcation Depth Quartz Microcline Orthoclase Hematite Gypsum Dolomite Calcite Illite Kaolinite Hydroxylapatite Anatase (depth, ft) (m) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) 2836.50 864.57 22.6(5) 20.3(5) 16.3(5)0.1 –12.4(5) 21.5(5)5.7(5)0.5 –0.6 2837.96 865.01 22.5(5) 0.6 22.8(5)– –12.7(5) 34.2(5)6.0(5)0.8 –0.4 2850.89 868.95 19.6(5) 0.2 17.6(5) ––18.8(5)36.1(5) 6.2(5) 1.2(5) –0.3 2860.76 871.96 2.2(5) 0.8 1.3(5)– –25.9(5) 65.1(5)4.7(5)– –– 2863.17 872.69 7.1(5) 0.7 1.8(5) ––86.3(5)1 3.1(5) ––– 2868.27 874.25 5.3(5) 0.1 1.8(5) ––89.3(5)3.5(5)– ––– 2879.09 877.55 13.6(5) 9.2(5) 20.7(5) ––45.2(5)– 8.7(5) 2.6(5) –– 2891.68 881.38 12.7(5) 0.3 8.8(5) ––73.8(5)3.8(5)0.6 ––– 2896.00 882.70 27.2(5) 0.2 19.2(5)0.6 –52.8(5) –– ––– 2909.10 886.69 24.3(5) 0.4 19.6(5)– –47.3(5) –7.3(5)1.1(5)– – 2913.34 887.99 18.4(5) 0.2 1.8(5) ––76.3(5)– –– 3.0(5) 0.3 2918.27 889.49 84.6(5) 0.1 4.2(5) –2.8(5)– –– –8.3(5)– 2919.27 889.79 85.3(5)– 1.3(5)–11.5(5)– –– ––1.9(5) 2925.92 891.82 96.0(5) 0.3 1.2(5) ––––– –2.0(5)0.5 2931.10 893.40 97.7(5) –––1.2(5) –––– 1.1(5) – 2948.91 898.83 96.3(5) ––––––– –3.7(5)– 2951.58 899.64 47.3(5) 19.9(5) 25.9(6) ––––5.5(5)0.8 0.6– 2953.73 900.30 23.6(5) 35.4(5) 30.6(7) ––0.8–7.5(5) 0.8–1.3(5) 2963.56 903.29 86.1(5) 0.6 7.6(5)1.7(5)2.3(3)– –1.5(5)–0.2 2964.81 903.67 62.2(5) 1.4(5) 25.2(5) ––––9.3(5)0.9 –1.0(5) 2967.20 904.40 82.6(5) 0.6 4.7(5) 0.87.9(5)– –2.4(5)0.4 –0.6 2968.24 904.72 72.3(5) 0.6 26.2(5) 0.9–––––– 2970.30 905.35 59.4(5) 0.5 29.4(5) 3.3(5) –––6.5(5)0.9 –– 2983.42 909.35 67.8(5) 0.7 8.6(5) –17.5(5) ––4.9(5) 0.5–– 2989.15911.09 27.2(5) 20.1(5) 27.8(5)16.5(5) –––7.9(5)0.5 –– 2990.53911.51 51.4(5) 1.2(5) 26.6(5)–4.7(5) ––13.9(5)1.0(5)– 1.2(5) 2994.92 912.85 85.4(5) 1.6(5) 6.7(5)–5.5(5) ––0.8 ––– 3003.71 915.53 96.0(5) 0.2 1.8(5)0.4 1.6(5) ––– ––– 3004.55 915.79 90.2(5) 0.1 4.4(5)2.3(5)0.9 ––1.6(5) 0.5–– Note: Numbers in parentheses are standard deviation of the last reported digit. Dashes indicate an amount below the detection limit. Standard deviation for some samples (those without parentheses) could not be determined. Samples above depths of 891.82 m belong to the Eau Claire Formation; samples at 891.82 m or deeper are from the Mount Simon Sandstone.

Variability in Mineral Composition, Porosity, and Permeability and 3). A pronounced transition to more variable permeability occurs at depths >~899.16 m in the Mount Simon Sandstone, and corresponds to a lithofacies Mineral phases of the Eau Claire Formation determined by XRD (Heath change from a quartzarenite to arkose and subarkose sandstones with depth et al., 2013) include quartz, feldspar (microcline and orthoclase), dolomite, cal- (Fig. 2). Unlike the Mount Simon Sandstone in some portions of the Illinois cite, illite, kaolinite, and minor hematite, hydroxyapatite, and anatase (Table 2; Basin (Medina et al., 2011), porosity and permeability display no systematic Fig. 2). Dolomite can be predominant, reaching values as high as 89 wt%. trends with depth and are not correlated with one another (Figs. 2 and 3). The Mount Simon Sandstone mineralogy is dominated by quartz, with lesser amounts of feldspar (microcline and orthoclase) and illite, and typically minor Paragenesis of the Mount Simon Sandstone hematite, gypsum, hydroxyapatite and anatase; however, hematite and gypsum can attain values as high as 16 wt% and 17 wt%, respectively, at certain We detail porosity-altering diagenetic changes that have affected the depths (Table 2; Fig. 2). Permeability within the Mount Simon Sandstone of the Mount Simon Sandstone, as well as evidence supporting our inferred para sampled lithofacies of the Keith No. 1 core varies over five orders of magni- genetic sequence (Fig. 4). This work is based upon analysis of 16 thin sections, tude, testifying to dramatic heterogeneity in pore size and connectivity (Figs. 2 representing samples from the upper, middle, and lower units (Fig. 2). This is

a general paragenetic sequence for the entire unit; individual samples do not contain evidence for all the alterations. Prior work on the paragenesis of these phases in the Illinois Basin is also noted, in part to establish whether the Mount Simon Sandstone at the Dallas Center structure has an alteration history simi lar to elsewhere. Figure 3. Porosity versus permeability (air) for Keith No. 1 well. TRA is tight rock analysis, a crushed-rock permea Quartz Overgrowths bility method. Data points labeled vertical and horizontal indicate orientation of core plugs. Vert.—vertical; Quartz overgrowths occur in all samples from the Mount Simon Sand- Horiz.—horizontal. stone portion of the core. The presence of quartz overgrowths within barite and gypsum indicates that the overgrowths predate these phases (Fig. 5A, 5B). Some overgrowths (and gypsum) fill pores in which the adjacent grains have sutured contacts (Fig. 5C). This suggests that some quartz and gypsum may have formed after pressure solution of quartz. In the Illinois Basin, the origin of quartz overgrowths has received considerable attention (Duffin et al.,

Mount Simon Paragenesis

Early Late

Fe-Ti oxide grain diss.

Carbonate fluorapatite

Pyrite

Hematite

Euhedral Ti (rutile?)

400 Ma (Duffin et al., 1989) Feldspar overgrowths

Quartz overgrowths 40–124 °C (Pollington et al., 2011) ????? 100–130 °C, saline (Fishman, 1997) Fracturing <260 Ma (Fishman, 1997) Barite ????? Gypsum/anhydrite

Compaction, pressure solution

Gypsum/anhydrite dissolution

Feldspar dissolution

MTV deposits ~270 Ma (Pannalal et al., 2004)

Figure 4. Relative timing of diagenetic alterations influencing the Mount Simon Sandstone. Solid lines indicate the period during which an event occurred; dashed indicate a range of time over which the event may have occurred. Relatively early events are plotted toward the left, and late events are plotted toward the right; question marks indicate possible second stage of precipitation. Constraints on the age of authigenic feldspar overgrowths and temperature of precipitation of quartz overgrowths are indicated from the cited studies in the Illinois Basin. MVT—Mississippi Valley type; diss.—dissolution.

A B g g Figure 5. Thin section photomicrographs documenting paragenetic relationships in the Mount Simon Sandstone. (A) Fracture-filling gypsum (g—pink due to red rhodamine f dyed epoxy in microporosity) and plumose barite (b—high b relief) cements. Because barite is surrounded by gypsum, it likely predates it. The presence of quartz overgrowths (arrows) within both barite and gypsum indicates that the overgrowths predate these phases. Sample 2931.10 (893.4 m), plane polarized light (PPL). (B) Termination of the quartz overgrowth (arrow) against feldspar overgrowth (f) indicates that the feldspar overgrowth formed first. Disso g 0.5 mm 0.1 mm lution of the feldspar overgrowth likely occurred after gypsum (g) precipitation, otherwise the secondary pores in the overgrowth would be filled with gypsum. Sample D 3009.55 (917.3 m), PPL. (C) Sutured (s) and long (l) contacts C between quartz grains adjacent to a pore filled with hema- s tite (opaque), quartz overgrowths (arrows), and gypsum (g) cements. Quartz overgrowths are absent where the hema l tite grain coating is thick, suggesting that the hematite g inhibited quartz cementation. Because the sutured grain g contact intersects a quartz overgrowth, overgrowth precipi f tation may have postdated significant pressure solution. Sample 3009.55 (917.3 m), PPL. (D) The presence of feldspar overgrowths (f) completely surrounded by gypsum (g) suggests that they predate gypsum precipitation. Sample 2953.73 (900.3 m), PPL. g

0.1 mm 0.1 mm

1989; Fishman, 1997; Chen et al., 2001; Pollington et al., 2011; Bowen et al., that some overgrowths formed prior to gypsum (Fig. 5D). The paragenesis, 2011). Most previous workers concluded that the quartz overgrowths formed geochemistry, and environment of formation of the feldspar overgrowths have at elevated temperatures (Fishman, 1997; 100–130 °C; Pollington et al., 2011; received considerable attention in studies of the Mount Simon Sandstone in 40–124 °C). Bowen et al. (2011) noted that in the Illinois Basin overgrowths are the Illinois Basin (Fishman, 1997; Duffin et al., 1989; Chen et al., 2001). Radio- present in both relatively shallow and deep samples, with no depth-related metric dating of the feldspar indicates that it may have begun to precipitate pattern to their occurrence; they speculated that either the conditions suitable in the Late Ordovician and continued into the Devonian (Duffin et al. 1989; Liu for quartz cementation existed in shallow as well as deep settings (e.g., hydro- et al., 2003). thermal fluids at shallow depths) or that overgrowths formed at greater depths and subsequently underwent uplift. Fracturing

Feldspar Overgrowths Natural fractures are relatively common in the Mount Simon Sand- stone at Dallas Center: they are present at numerous depths in the core Feldspar overgrowths are very common at all stratigraphic levels in the and occur in 25% of our thin sections. Most of the fractures are filled by Mount Simon Sandstone. Because quartz overgrowths terminate against feld- gypsum, anhydrite, and barite cements. In some cases the cements that spar overgrowths, the feldspar overgrowths most likely formed first (Fig. 5B). fill the fractures are confined to the fractures and do not penetrate the sur- The presence of feldspar overgrowths in gypsum-cemented regions indicates rounding sandstone (Fig. 6A), despite the porous nature of the sandstone.

A B

g g b Figure 6. Thin section photomicrographs documenting paragenetic relationships in the Mount Simon Sandstone. B (A) Opening-mode fracture filled with gypsum (g) and barite (b). These cements are confined to the fractures and do g not extend into the surrounding porous sandstone. This suggests that a more soluble cement (halite?) may have been present in the surrounding sandstone at the time that gypsum and barite precipitated. Sample 2931.10 (893.4 m), plane polarized light (PPL). (B) Region with sutured (arrows) 1 mm 0.5 mm and long grain contacts, which provide evidence for significant compaction. Sample 3009.55 (917.3 m), PPL. (C) Varia ble stylotization of quartz grain margins as a function of C proximity to phyllosilicates (mica and illitic clay). Quartz D grain at center has smooth margins except where it is in contact with phyllosilicate band (arrows). Sample 2990.53 (911.5 m), crossed-polarized light. (D) Highly altered skeletal feldspar grain. Brown (hematite inclusion rich) portions are probably remnants of the original detrital feldspar, whereas clear areas may be authigenic feldspar (arrows). This suggests that some authigenic feldspar may have precipitated after detrital feldspar dissolution. Sample 3003.71 (915.5 m), PPL.

0.5 mm 0.1 mm

This suggests that a soluble cement, such as halite or another evaporite cant compaction has occurred. Additional evidence for compaction includes mineral, was present filling pores in the sandstone at the time of fracture common long, concavo-convex, and sutured grain contacts as well as stylo filling and was subsequently dissolved. Fractures were also described in the lites (Figs. 5C, 6B, and 6C). Makowitz and Milliken (2003) studied the compac- Mount Simon Sandstone of the Illinois Basin by Chentnik (2012). In addition tion of the Mount Simon Sandstone in the Illinois Basin and concluded that it to throughgoing fractures (i.e., fractures are not confined to single grains), accounts for the majority of porosity loss in the unit. intragranular fractures are also common in the Mount Simon Sandstone The degree of compaction is exceptionally heterogeneous, both among (Makowitz and Milliken, 2003). and within samples (e.g., Figs. 7–9). Heterogeneity in IGVs can also been seen in the data for the Mount Simon Sandstone from the Illinois Basin provided by Compaction and Pressure Solution Makowitz and Milliken (2003). Such heterogeneity is relatively uncommon in sandstones (McBride, 2012). Possible explanations for the highly variable IGVs The Mount Simon Sandstone has undergone variable amounts of me- include the following. chanical and chemical compaction. The intergranular volume of sandstones 1. Differences in the degree of sorting can cause variation in primary IGV, provides a measure of the degree of compaction that the rock has undergone with more poorly sorted regions having lower IGVs. Although some of the (e.g., Paxton et al., 2002). Freshly deposited sand typically has intergranular heterogeneity may be explained to some degree by this mechanism (Fig. 7), volumes (IGVs) of >40% (Atkins and McBride, 1992). The highest IGVs that we significant variations in IGV occur in regions that are texturally similar (Figs. 8 have noted in the Mount Simon Sandstone are ~30%, indicating that signifi- and 9). Thus, this explanation cannot explain most of the observed heterogeneity.

10%, cU, P 3%, vfL, W

15%, vfL, W

30%, mL, M 16%, vfU, W

18%, cL/cU, P 20%, vfL, W

0.5 cm

0.5 cm Figure 8. Thin section and binarized images of regions illustrating variation in intergranular volume (IGV) in sandstone layer in a mixed sandstone-mudstone sample. IGV in analyzed regions 30%, fU, W/M varies from 3% to 20%. The variation is not controlled by primary texture, as grain size and sorting are similar for the four regions. The most likely causes of the variation in this sample are either the presence of a cement (later partially removed by dissolution) that filled pores and prevented Figure 7. Thin section and binarized images of regions illustrating variation in intergranular vol- compaction where present, or variable pressure solution as a function of phyllosilicate distribu- ume (IGV) within the sample. IGV in analyzed regions varies from 10% to 30%. Coarser regions tion (low IGV regions have more phyllosilicates in this sample). Sample 2953.73 (900.3 m). Abbre- have lower IGV than finer regions. This is unlikely to be due to greater pressure solution in viations: vfU—very fine upper, vfL—very fine lower, W—well sorted. coarse-grained layers, as pressure solution is typically greater for finer sands (Houseknecht, 1984, 1988), so it is due to preferential cementation of finer sands by a later dissolved cement, or perhaps the poorer sorting of the coarse-grained layers. Sample 3009.55 (917.3 m). Abbre- viations: cU—coarse upper, cL—coarse lower, mL—medium lower, fU—fine upper, P—poorly lationship between IGV and grain size, lower IGVs are observed in coarser sorted, M—moderately sorted, W—well sorted. regions (Fig. 7). 3. Preferential pressure solution in association with phyllosilicates. Be- 2. Grain size control on chemical compaction. Grain size is thought to cause phyllosilicates facilitate pressure dissolution (e.g., Heald, 1955; Dewers strongly influence the amount of pressure solution and resultant chemical and Ortoleva, 1991; Bjørkum, 1996; Oelkers et al., 1996; Kristiansen et al., 2011), compaction undergone by sandstones, with finer grained regions undergoing more phyllosilicate-rich regions can become more compacted due to greater greater amounts of solution (Houseknecht, 1984, 1988). However, this expla- pressure solution. Some of the IGV heterogeneity in the Mount Simon Sand- nation does not apply to the Mount Simon Sandstone, as regions of similar stone likely results from such a process (Figs. 6C and 8); however, many of the grain size have very different IGVs (Figs. 8 and 9), and where there is a re- compacted areas do not contain evidence for pressure solution and do not

0.5 cm <1%, vfU, P

Figure 9. Thin section and binarized images <1%, fL, P of regions illustrating variation in intergranular volume (IGV). IGV in analyzed regions varies from 1% to 9%. The variation is not controlled by primary texture, as grain size and sorting are similar for the four regions. A likely cause of the variation in this sample is the presence of a cement (later partially removed by dissolution) that filled pores and prevented compaction where present. However, because the low IGV regions are relatively feldspar rich compared to the high IGV regions, some of the IGV difference may be a function of undetected feldspar overgrowths (feldspar overgrowths can be very difficult to de- 7%, fU, P 9%, fU, P tect in the Mount Simon Sandstone with standard petrography for fine-grained material). Sample 2990.53 (911.5 m). Abbre viations: fU—fine upper, fL—fine lower, vfU—very fine upper, P—poorly sorted.

have greater amounts of phyllosilicates than nearby less-compacted regions, to facilitate pressure solution (e.g., Bjørkum, 1996; Kristiansen et al., 2011). Al- so this cannot be the only explanation. though phyllosilicates appear to have facilitated some of the pressure solution 4. Variable cementation can produce heterogeneous IGVs (e.g., McBride, in the Mount Simon Sandstone, many sutured contacts are free of phyllosili- 2012) if cementation is spatially heterogeneous and occurs prior to significant cates, so this mechanism cannot explain all of the pressure solution features. compaction. In the case of the Mount Simon Sandstone, patches of intergran- This suggests that the samples were either (1) buried to considerably greater ular evaporite cements are common in areas with high IGV, making this the depths in the past, or (2) subjected to anomalously high temperature fluids most likely cause of most of the heterogeneity. (which would facilitate chemical compaction). The compaction textures exhibited by portions of the samples seem in- The Mount Simon Sandstone in the Dallas Center area appears to have compatible with their present, relatively shallow burial depths. In particular, undergone both greater burial depths than the present day and anomalously features such as sutured grain contacts and stylolites are often associated with high temperatures. Hegarty et al. (2007) examined the thermal history of considerably greater burial depths. For example, Renard et al. (1999), based Pennsylvanian through Precambrian units in a nearby well (~64 km west upon their integrated model, noted that significant pressure solution should of Dallas Center) using apatite fission track and vitrinite reflectance data and not normally occur at depths <2 km because quartz kinetics are too slow at concluded that three thermal events affected the area. The first occurred from low temperatures. Alternatively, some workers concluded that such features 300 to 200 Ma, resulting from an elevated geothermal gradient (~35 °C/km can form at shallow depths if phyllosilicates are present at the grain contacts versus 16 °C/km for the present gradient). The other events, which occurred at

70–50 Ma and 35–10 Ma under the present-day geothermal gradient, resulted suggesting that a more soluble cement may have been present, filling inter- from higher temperatures related to 3100 m and 2100 m of paleoburial, respec- granular porosity. Carbonate cements are often reported to have undergone tively. R. Anderson (2012, personal commun.) and others also concluded that extensive subsurface dissolution in sandstones; however, there is no evidence the region was subjected to greater burial depths in the past (~760 m greater for former carbonate cements in the Mount Simon Sandstone at Dallas Center. depths), though by considerably less than that calculated by Hegarty et al. Given the presence of remnants of gypsum cement and direct evidence of (2007). In addition, the area may have been covered by as much as 3 km of gypsum dissolution (i.e., microporosity in gypsum), the most likely cement glacial ice between ~2.5 Ma and 13 ka (R. Anderson, 2012, personal commun.). phases to have undergone dissolution are gypsum and anhydrite. However, Data from the Illinois Basin indicate that the Mount Simon Sandstone also other more soluble phases such as halite may also have been affected, as is underwent temperatures considerably greater than present due to greater suggested by the gypsum-filled fractures in porous sandstone. than present-day burial depths and flow of deep basinal fluids. As noted here, Cement dissolution has also been reported in the Illinois Basin. Hoholick Bowen et al. (2011) indicated that some of the quartz overgrowths occur at et al. (1984) concluded that all of the porosity in the unit is secondary (with unexpectedly shallow depths and suggested that this might be the result of the exception of small amounts of primary porosity near outcrops), resulting greater burial in the past; they noted that coal maturation studies (Damberger, mainly from subequal amounts of cement and grain dissolution. Fishman 1971) suggest that as much as 1.5 km of additional sediment filled the Illinois (1997) concluded that most of the current porosity in the Mount Simon Sand- Basin at its maximum burial. In addition, Hoholick et al. (1984) estimated that stone resulted from the dissolution of cements, perhaps ferroan dolomite. the Illinois Basin underwent 914–1524 m of additional burial based upon work by Altschaeffl and Harrison (1959), and Grathoff et al. (2001) concluded that Feldspar Dissolution authigenic illite in Ordovician and Cambrian shale partings formed ca. 300 Ma from hot deep basinal brines brought to shallower depth by gravity-driven Skeletal feldspar grains indicate that dissolution of feldspar has occurred flow. Rowan et al. (2002) used vitrinite reflectance data and fluid inclusions (Fig. 6D). The timing of feldspar dissolution is somewhat difficult to interpret. to conclude that that the rocks underwent a combination of additional burial Because dissolution voids in feldspar overgrowths are not filled by adjacent of as much as 1.2 km and elevated temperatures due to Permian magmatic gypsum and quartz overgrowths (Fig. 5B), the dissolution must postdate these activity and advective heat transport due to fluid flow. Thus, although there phases, as well as the overgrowths. However, possible authigenic feldspar in- may be disagreement concerning the absolute amount of burial, it is clear that side skeletal feldspar grains (Fig. 6D) suggests that some feldspar precipitation the Dallas Center area underwent both considerably greater burial depths than may have occurred after feldspar dissolution. the present day and a prolonged period during which the geothermal gradi- Oversized pores are common in the Mount Simon Sandstone and are ent was almost double the present gradient. The combination of these factors most likely the result of grain dissolution. In some cases the grains may have explains the anomalously compacted nature of the Mount Simon Sandstone been unstable heavy minerals, as discussed here. However, because dissolu- at this locality. tion of such grains typically results in nearby precipitation of Fe-bearing and sometimes Ti-bearing authigenic minerals, most of these oversized pores that Gypsum and Anhydrite contain no such authigenic phases probably resulted from feldspar dissolution. Dissolution of feldspar grains has been reported for the Mount Simon Gypsum and to a lesser extent anhydrite are present as intergranular pore Sandstone of the Illinois Basin by most previous workers (Hoholick et al., 1984; and fracture filling cements. The gypsum commonly has microporosity, giv- Fishman, 1997; Bowen et al., 2011). ing it a pinkish color in plane-polarized light (Figs. 5 and 6A). While gypsum and anhydrite are common in the Mount Simon Sandstone at Dallas Center, Pore System Characterization Fishman (1997) noted that these minerals are rather rare in the Illinois Basin. However, where present, the paragenesis of the gypsum and anhydrite in the The size distribution of pore throats resulting from the complex para two areas is apparently similar (Fishman, 1997). genetic history of the Mount Simon Sandstone at this locality is highly varia ble. Using the pore-throat distributions as derived from the MIP analysis Cement Dissolution (summarized in Fig. 10), most samples fit into one of three main groups based upon the dominant pore-throat size. These three groups are: mesopore-throat As discussed here, multiple lines of evidence suggest that cement disso- to macropore-throat–dominated (4–40 µm diameter), micropore-throat domi lution occurred, including (1) unusually large heterogeneity in IGVs, (2) the nated (<0.07 µm), and an intermediate group (0.15–1.2 µm). Although the presence of microporosity in gypsum and irregular patches of gypsum and intermediate group can be classified as micropore-throat–dominated (cf. anhydrite in high IGV regions, (3) solution pits in some feldspar overgrowths, Nelson, 2009; Figs. 10–13), for this investigation it is referred to as interme- and (4) the observation that gypsum cements are often confined to fractures, diate. A fourth group is characterized by no dominant pore-throat size, but

Figure 10. Plot of incremental mercury

) saturation versus pore-throat diameter for samples from the Eau Claire Formation and Mount Simon Sandstone. Samples can be subdivided into four groups based on pore-size characteristics: macropore dominated (red), micropore dominated (black), intermediate (blue), and evenly distributed (magenta). One sample exhibits an unusual bimodal distribution (green), with populations of both micropores and

Incremental Hg Saturation (% macropores. All of the micropore-throat– dominated samples are from the Eau Claire Formation, all of the macropore-dominated samples are from the Mount Simon Sandstone, and intermediate and evenly distributed types occur in both units.

Pore Diameter (μm)

rather a relatively uniform distribution of sizes. One sample is bimodal, with ented jacketed core plugs (Fig. 15). For all but one of these horizontal-vertical weak modes in both micropore and macropore ranges. All of the micropore- sample pairs, the MIP measured porosity (Table 3) is slightly greater for the throat–dominated samples are from the Eau Claire Formation, whereas all horizontally oriented samples than the vertically oriented samples. Because of the macropore- and mesopore-dominated samples are from the Mount textural and lithologic heterogeneity is greatest in the vertical direction in Simon Sandstone. The intermediate micropore throat and evenly distributed most of the samples, this difference in accessible pore volume may result from types are present in both units. three-dimensional effects in which zones of porosity are enclosed by very The micropore-throat–dominated samples can be further subdivided into small-pore-throat domains. This interpretation is strengthened if we consider three broad categories: those characterized by a single dominant pore-throat the sample exhibiting the greatest difference in horizontal and vertical pore- size; those with a bimodal pore-throat size; and those characterized by a broad throat size distribution. For three of the four paired samples the pore-throat range of pore-throat sizes (Fig. 11). Although many of these samples contain size distribution did not differ significantly between horizontal and vertical abundant macroporosity, it does not appear in the mercury porosimetry data; analyses; however, for sample 2967.20 (904 m), the horizontally-oriented plug which indicates that it is not effective porosity (i.e., poorly interconnected). shows a distinct 9 µm pore-throat diameter peak, whereas the vertically ori- This can be clearly seen in thin section, where macroporosity occurs in poorly ented plug shows a relatively evenly distributed pore-size distribution with no interconnected subdomains (Fig. 14). The variability in pore-throat size distri- clear peaks. Sample 2967.20 (904.4 m) is the most lithologically heterogeneous bution reflects the considerable textural heterogeneity of the samples, includ- of the four samples, containing conglomerate, sandstone, and mudstone. In ing heterogeneities imparted by depositional texture, as well as variable distri- addition, zones of relatively large macropores are bounded above and below bution of pore-filling cements. by thin mudstone laminae. Such zones would have been accessible to mer- The mesopore-throat to macropore-throat samples fall into two main dis- cury injected horizontally, but are less likely to have been accessed by vertical tributions: those that are characterized by a single dominant pore-throat size, injection due to the bounding mudstone. One of the four samples, 2931.10 and those that have a broader distribution of sizes with several modes (Fig. (893.4 m), exhibits no difference in porosity between the horizontal and vertical 12). There is a clear primary textural influence on the two populations; the analyses. This sample has the least textural heterogeneity of the four samples. former type occurs in lithologically and texturally homogeneous samples, However, it is cut by a gypsum-filled fracture that apparently did not influence and the latter type occurs in texturally and lithologically heterogeneous sam- the measurements. This is likely due to the fact that the gypsum fracture fill ples. For a subset of four of the mesopore- to macropore-dominated samples, only traverses ~60% of the sample, allowing ready access of mercury to both mercury intrusion data were collected for both horizontally and vertically ori- sides of the fracture. In conclusion, the results for paired directional samples

2837.94 2891.68 )

Figure 11. Plot of incremental mercury saturation versus pore-throat diameter for samples in the micropore-throat–dominated 2913.34 group. Scans of thin sections from selected samples are shown, in which porosity is highlighted by red-dyed epoxy. Incremental Hg Saturation (%

Pore Diameter (μm)

2948.91 2931.10 14.6%, 360 md H V 13.8%, 262 md 2968.24 H 16.6%, 67.2 md H )

2925.92 Figure 12. Plot of incremental mercury saturation versus pore-throat diameter for samples in the mesopore- to macropore-throat– 14.6%, 350 md 2967.20 dominated group. Scans of thin sections 2963.56 3003.71 from selected samples are shown, in which porosity is highlighted by red-dyed epoxy. Measured porosity (%), calculated permea bility (md; calculations used equation by 12.4%, 36 md

Incremental Hg Saturation (% Swanson, 1981), and orientation of mea- V surement (H = horizontal, V = vertical) are 9.98%, indicated for each sample.

6.92 md 8.57%, 10.3 md H H H 16.3%, 53.4 md V 7.66%, 9.12 md

Pore Diameter (μm)

2989.15 ) 2970.30 2983.42 2967.20 Figure 13. Plot of incremental mercury 2879.09 saturation versus pore-throat diameter for samples in the intermediate (blue), evenly distributed (magenta), and bimodal (green) groups. Scans of thin sections from the samples are shown, in which porosity is highlighted by red-dyed epoxy. Note heterolithic nature of the samples. Incremental Hg Saturation (%

Pore Diameter (μm)

indicate that the MIP-measured porosity for many, perhaps most, of the vertically-oriented measurements underestimate the total sample porosity. In addition, given the clear textural control on the pore-throat results, and the high degree of lithologic and textural heterogeneity of many of the samples, a portion of the vertically measured samples that exhibit no clear pore-throat peak probably would if measured horizontally. Samples exhibiting intermediate, evenly distributed, and bimodal distributions in pore-throat diameters have one thing in common: they are all texturally and lithologically heterogeneous, with the greatest heterogeneity in the vertical direction (Fig. 13). All of these samples were measured in the vertical direction; given the apparent influence of sample orientation discussed here, it is likely that the measured values were influenced by this vertical heterogeneity, and that if measured horizontally, the results may have been considerably different. Data for one of the samples, 2967.20 (904.4 m), support this contention. This sample was measured both vertically and horizontally. The horizontal measurement produced a pore-throat size distribution in the mesopore- to macro pore-throat category, whereas the vertical measurement placed it in the evenly 1 mm distributed category. The sample exhibiting a bimodal pore-throat distribution (2983.42; 909.3 m) appears to have been dramatically influenced by this vertical heterogeneity. It consists of interlayered sandstone and mudstone (Fig. 13). Figure 14. Thin section photomicrograph showing distribution of Two populations of pore sizes are evident, with a major peak in the micropore porosity (red-dyed epoxy) in a dolomitized glauconitic limestone. throat range and a weaker peak in the mesopore throat range. The sample is Zones of macroporosity are separated by regions in which porosity has been destroyed by quartz and dolomite cements, greatly reduc- heterolithic, mainly consisting of sandstone (~90%) with a layer of mudstone at ing the effective pore-throat size of the sample. Eau Claire Forma- its base. Although sandstone makes up the majority of the sample, it appears tion, 2913.34 (1192.8 m), PPL.

2931.10 V 14.6%, 360 md

) H 14.6%, 2967.20 350 md Figure 15. Plot of incremental mercury saturation versus pore-throat diameter for samples in the macropore-throat–domi- 8.57%, nated group for which horizontal and ver- V = 13.2 %, 112 md 10.3 md 3003.71 tical measurements are available. Scans of thin sections from selected samples are H = 14.3%, 132 md H shown, in which porosity is highlighted by red-dyed epoxy. Measured porosity (%),

Incremental Hg Saturation (% calculated permeability (md), and orienta- H tion of measurement (H = horizontal, V = 9.89%, 6.93 md vertical) are shown.

V 8.04%, 2.98 md V 7.66%, 9.12 md

Pore Diameter (μm)

that the thin mudstone layer formed a continuous barrier in the jacketed sam- (see Figs. 2 and 16; Table 3). The Eau Claire Formation in general shows high ple, controlling mercury intrusion and providing a skewed analysis of overall variability in sealing capacity (and capillary pressure behavior), reflecting a pore-throat size distribution. range in pore size distribution, structure, and connectivity. This is consistent with the findings of Lahann et al. (2014) in the Illinois Basin. Estimated pore- Sealing Capacity throat size diameters at breakthrough range from 0.014 to 0.936 mm, equivalent to a range in mercury-air breakthrough pressure over 109.2–1.6 MPa. The Mount Sealing capacity is the column (vertical) height of a non-wetting phase Simon Sandstone exhibits generally more uniform capillary pressure behavior

(e.g., supercritical CO2 or compressed air) that can be held by the capillarity of as expressed as air and CO2 column heights; however, in the middle unit much a water-saturated (water wetting) rock. Variability in sealing capacity as esti- higher values can occur that reflect interbedded mudstones (Fig. 16). Mount mated from MIP data (see Materials and Methods section) for a reservoir rock Simon Sandstone pore-throat diameters range from 0.024 to 37.327 mm, with a or caprock, respectively, can indicate a potential for local (intraformational) corresponding mercury-air breakthrough pressure of 60.98–0.04 MPa. This large trapping and/or change in local multiphase flow conditions, or a range in qual- change in breakthrough pressure and hence sealing capacity indicates that the

ity of sealing behavior of a caprock. Sealing capacity can also be compared interbedded mudstones may act as an upward barrier to the injected air or CO2. to the hydrostatic and lithostatic pressure gradients at a given site in order to Compressed air energy storage requires formation of an initial large bubble or estimate pressures or non-wetting phase column heights that would lead to zone of injected air that efficiently displaces ambient groundwater to irreducible

capillary breakthrough of a caprock, or pressures that may cause fracturing of saturations (Heath et al., 2013). Similarly, CO2 storage requires sweep efficien- the caprock (Heath et al., 2012). Our goal is to use sealing capacity to investi- cies that allow for suitable storage volumes. At this site, the range in sealing

gate heterogeneity of the reservoir and caprock lithofacies; placing the data in capacity reflects that sweep efficiencies may be poor, so that air or CO2 focused terms of hydrostatic pressure gradients, lithostatic gradients, and a measure flow may occur underneath the interbedded mudstones (the zones of high col- of the pressure that would lead to failure and/or fracturing of the caprock is umn heights within the Mount Simon Sandstone; Fig. 16; Heath et al., 2013).

beyond the scope of this study. The range in groundwater-CO2-rock contact angles of 13°–70° used in this The Eau Claire Formation has the highest breakthrough pressures and seal- analysis (see Materials and Methods section) can result in a large difference in

ing capacity (in terms of the columns heights of air or CO2 that can be held by sealing capacity (e.g., ~3054 m difference in CO2 column height at the depth

capillary forces) near its contact with the subjacent Mount Simon Sandstone of 8890 m). This range in groundwater-CO2-rock contact angles was used due

TABLE 3. BREAKTHROUGH PRESSURES, PORE-THROAT DIAMETERS, AND POROSITY BASED ON MERCURY INTRUSION POROSIMETRY Sample identiﬁcation Depth Breakthrough pressure, mercury-air Breakthrough pore-throat diameter Porosity (depth, ft) (m) Plug orientation* (MPa) (µm) (%) 2837.94V 865.00 Vertical51.61 0.0294.71 2850.84V 868.94 Vertical51.61 0.0293.90 2863.23V 872.71 Vertical28.84 0.0514.44 2879.09V 877.55 Vertical1.570.936 10.36 2891.68V 881.38 Vertical28.84 0.0513.91 2908.99V 886.66 Vertical22.47 0.0669.75 2913.34V 887.99 Vertical22.45 0.0662.10 2918.11V 889.44 Vertical20.64 0.07110.80 2918.52V† 889.56 Vertical109.200.014 3.71 2919.68V 889.92 Vertical2.370.621 19.50 2919.68V-C† 889.92 Vertical100.390.015 3.68 2925.92V 891.82 Vertical0.1311.40112.40 2926.30V 891.94 Vertical0.1014.63817.69 2931.10H§ 893.40 Horizontal0.0531.23514.60 2931.10V§ 893.40 Vertical0.0528.55314.60 2932.72H 893.89 Horizontal0.0818.29315.54 2948.91H 898.83 Horizontal0.0531.23513.80 2951.79V 899.71 Vertical0.1015.31017.80 2963.56H 903.29 Horizontal0.0818.293 16.30 2964.86H 903.69 Horizontal0.0437.327 9.09 2967.20H 904.40 Horizontal0.1112.813 9.89 2967.20V 904.40 Vertical0.0720.001 8.04 2968.24H 904.72 Horizontal0.1113.470 16.60 2970.30V 905.35 Vertical1.311.122 10.31 2983.42V 909.35 Vertical60.98 0.0246.55 2989.15V 911.09 Vertical3.290.447 13.18 2995.59H 913.06 Horizontal 0.06 26.137 14.34 2995.59V 913.06 Vertical0.0626.137 13.20 3003.71H 915.53 Horizontal 0.08 18.294 8.57 3003.71V 915.53 Vertical0.0917.293 7.66 *All samples from the core were taken in approximately vertical orientation parallel to the long axis of the core or perpendicular to it in the horizontal orientation. †Sample not jacketed with epoxy that underwent omnidirectional mercury intrusion. §The samples with a mineralized fracture were cut with the axis of the plugs being either perpendicular or parallel to the orientation of the subvertical fracture.

to the multiple mineral phases in rock and the contact angles associated with has less influence on the sealing capacity differences between the air and CO2

those phases. Estimates of CO2 sealing capacity can thus be highly uncertain systems as compared to the density differences between the air and CO2 for due to uncertainty in contact angle values and what minerals line pore throats. the estimated in situ depths and temperatures.

However, even with the high uncertainty, the estimated CO2 column heights are much greater than the thickness of the Mount Simon Sandstone at the DISCUSSION Dallas Center structure for the interbedded mudstone at a depth of 909 m (Fig. Regional Mount Simon Sandstone Paragenesis and 16). The difference in sealing capacity between the air and CO systems is pre- 2 Anomalous Evaporite Cements dominately due to the density difference. At the depths mentioned above, CO2 has an average density for the samples of ~820 kg/m3, whereas nitrogen has an As noted herein for individual alterations, the major diagenetic events estimated density of 90 kg/m3. The buoyancy force is thus much greater for air and their timing are remarkably similar in both this study and in the previ-

for these depths. CO2 is a dense state due to its presence as a supercritical fluid ous studies that examined samples from throughout the Midwest. The sim- at these depths. Therefore, uncertainty in interfacial tension and contact angle ilarity is striking given the large regional extent of the unit, and the widely

Figure 16. Plot of CO2 and air (nitrogen) column heights held by capillarity (i.e., sealing capacity) versus depth. The Mount Simon Sandstone–Eau Claire Formation contact and Mount Simon Sandstone–red clastic unit contact are indicated with horizontal solid lines; horizontal dashed lines (in B) indicate subunits within the Mount Simon Sandstone. The legend indicates nitrogen

or CO2 and a contact angle for the groundwater-air–CO2-rock system. (A) CO2 and air column heights for all mercury porosimetry samples. (B) Subset of part A to highlight data at depths within the Mount Simon Sandstone.

variable burial depths of the samples examined; this similarity was also noted Evolution of Porosity in the Mount Simon Sandstone: by Fishman (1997). Although the overall paragenesis of the Mount Simon Timing of Porosity Development Sandstone is similar throughout the region, there are a few significant differences. The most striking difference is the relatively large amount of gypsum Primary porosity in the Mount Simon in the Dallas Center area was greatly and anhydrite found in the Dallas Center area versus elsewhere. There are reduced by mechanical and chemical compaction, as well as precipitation of two possible explanations for this difference: (1) significant gypsum and an- authigenic minerals, most notably gypsum, anhydrite, feldspar, and quartz. We hydrite precipitation was restricted to certain areas, such as Dallas Center, due agree with Fishman (1997), who concluded that most of the present porosity to a heterogeneous distribution of saline brines, perhaps resulting from locally originated though cement dissolution. Our data suggest that the dissolved ce- focused flow of deeper basinal fluids; and (2) gypsum and anhydrite were more ment consisted of evaporites, such as gypsum, anhydrite, and perhaps halite. abundant throughout the region in the past and were removed in most places The timing of evaporite dissolution, and the probable creation of most of by dissolution. The strong evidence for significant cement dissolution in the the current porosity, can be constrained relative to other diagenetic events, Mount Simon Sandstone favors the latter explanation. If this interpretation is some of which have been dated either directly or indirectly. Based upon thin correct, then something about the geologic setting (e.g., structural position) of section observations, evaporite dissolution is a late-stage event that postdates the Dallas Center area must have protected the Mount Simon Sandstone from all diagenetic alterations in the Mount Simon Sandstone, with the possible the circulation of the more dilute (evaporite dissolving) fluids, allowing more exception of feldspar dissolution (Fig. 4). Determining the absolute timing of of the evaporite cements to be preserved in this region. the dissolution event is not possible with the current data set; however, the

relative timing of dissolution can be constrained. There is clear petrographic is that there is a greater than normal opportunity for capillary trapping (see evidence that the evaporite cements postdate feldspar overgrowths, which, Holtz, 2002; Saadatpoor et al., 2010). The variation in capillary behavior as discussed above, precipitated prior to ca. 400 Ma (Early Devonian). Thus, (expressed through breakthrough pressures and sealing capacity column the dissolution event must have occurred subsequent to 400 Ma. Because the heights; see Table 3; Figs. 2 and 16) of the middle Mount Simon Sandstone dissolution event must postdate significant compaction, based upon the dis- may lead to local regions of higher than average (relative to the rest of the

cussion of overcompaction and thermal history, we can further conservatively formation) capillary trapping of air or CO2 (or the non-wetting phase). The

constrain the timing to younger than 300 Ma (Pennsylvanian). amount of a non-wetting fluid (e.g., air or CO2) in a water-wet system that is The possibility of widespread evaporite dissolution in the Mount Simon held in place by buoyancy due to capillary forces is affected by local varia Sandstone and other units in the Illinois Basin was discussed by Bethke (1986). tions in capillary breakthrough pressure (Saadatpoor et al., 2010). When In considering the origin of Mississippi Valley Type (MVT) ore bodies in the the capillary breakthrough pressure is locally larger than the average of the Upper Mississippi Valley mineral district, Bethke (1986) noted, based upon underlying reservoir rocks (of the same formation), additional capillary trap- mass balance considerations, that meteoric waters that entered aquifers on ping can occur. Saadatpoor et al. (2010) noted that the volume stored by this the Pascola arch might have become saline by dissolving evaporate as the trapping mechanism can be much larger than the average amount residually water moved northward to the Wisconsin arch. Thus, it is possible that the min- trapped for a particular rock type. For the site in question, capillary trapping eralizing fluids that formed the district in part gained salinity from evaporite heterogeneity could possibly lead to the nonuniform formation of a bubble dissolution in the Mount Simon Sandstone. If this was the case, based upon for CAES and poorer sweep efficiency during cycling of air into and out of

radiometric dating of ore mineralization (Pannalal et al., 2004), the dissolu- the formation (see Heath et al., 2013). For CO2 storage, local capillary trap- tion event may have occurred ca. 270 Ma (Permian). Rowan and Goldhaber ping may be desirable as a mechanism that improves the overall security of (1995) concluded that the mineralizing fluids flowed through the Mount Simon confinement by promoting residual-saturation trapping within the storage Sandstone over a period of ~200 k.y., providing a minimum duration of any reservoir for CO storage (see Saadatpoor et al., 2010). However, variations

dissolution event. in capillary breakthrough pressure may also affect CO2 injectivity and lead An additional possibility for widespread dissolution of evaporite cements to poor sweep in storage reservoirs that have interbedded lithofacies with in the Mount Simon Sandstone is that it occurred at least in part in response highly variable capillary entry and/or breakthrough pressures, such as mud- to enhanced recharge of meteoric water related to the presence of Pleistocene stones interbedded with sandstones. Future modeling of the site with the ice sheets. Person et al. (2007) and McIntosh et al. (2002) provided evidence measured variation in capillary pressure behavior could quantify the pre-

indicating that groundwater flow in the Illinois and other midcontinent sedi- dicted site performance of CAES and CO2 storage. Our results quantitatively mentary basins was greatly affected by changes in hydraulic head associated demonstrate the significant impact millimeter-scale to micron-scale porosity with ice sheet topography. McIntosh et al. (2002) concluded that the south to heterogeneity can have on the migration of multiphase fluids in reservoir north tectonically controlled flow system of the Illinois Basin was reversed to a sandstone. Such heterogeneity should be taken into account when attempt- north to south flow system during the Pleistocene; they documented recharge ing to model multiphase flow in such reservoirs. of meteoric waters associated with this mechanism to depths of at least 1 km. Thus, in addition to dissolution that may have occurred in association with tectonically driven groundwater flow and MVT ore mineralization, dissolution CONCLUSIONS likely also occurred more recently, driven by Pleistocene ice sheet hydrology. Consequently, the ice sheets, in addition to possibly contributing to the locally 1. The mineralogy, texture, and paragenesis of the Mount Simon Sand- overcompacted nature of the Mount Simon Sandstone, may have provided a stone at Dallas Center are very similar to those of the Mount Simon Sandstone source of dilute water and elevated hydraulic head necessary to drive further elsewhere in the Midwestern United States, with one exception, relatively cement dissolution and porosity creation at depth. However, this mechanism large amounts of evaporite cements (gypsum, anhydrite, and barite). cannot account for dissolution at depths >~1 km (McIntosh et al., 2012), so it 2. Most of the porosity in the Mount Simon Sandstone at this locality is the cannot be the sole mechanism. result of cement and grain dissolution. The majority of the porosity formed through the dissolution of highly soluble cements, such as gypsum and an- Implications of Pore-Throat Size Heterogeneity for Fluid Flow hydrite, as well as some framework grains. This is probably the case for the Mount Simon Sandstone in other areas as well, including the Illinois Basin. The pore and pore-throat size distribution of the Mount Simon Sandstone This dissolution event most likely occurred either in the Permian, in associ- samples examined at this locality are often highly heterogeneous, a reflec- ation with MVT ore body development, or possibly in the Pleistocene as a tion of (1) variable IGV due to cement dissolution and (2) primary textural result of deep circulation of meteoric water associated with variable ice-sheet heterogeneity at the thin section scale. The net result of this heterogeneity geometry.

3. The pore-size distribution in the Mount Simon Sandstone at Dallas Cen- Bethke, C.M., 1986, Hydrologic constraints on the genesis of the Upper Mississippi Valley min- ter, Iowa, is unusually heterogeneous at small scales due to the combined ef- eral district from Illinois basin brines: Economic Geology and the Bulletin of the Society of Economic Geologists, v. 81, p. 233–249, doi:10.2113/gsecongeo.81.2.233. fects of primary textural heterogeneity, compaction, and cement dissolution. Bjørkum, P.A., 1996, How important is pressure in causing dissolution of quartz in sandstones?: This heterogeneity is particularly evident when comparing vertical and hori- Journal of Sedimentary Research, v. 66, p. 147–154, doi:10.1306 /D42682DE -2B26-11D7 zontal mercury porosimetry data sets for the same samples, which are in some -8648000102C1865D. Bowen, B.B., Ochoa, R.I., Wilkens, N.D., Brophy, J., Lovell, T.R., Fischietto, N., Medina, C.R., and cases dramatically different. 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ACKNOWLEDGMENTS Dewers, T., Newell, P., Broome, S., Heath, J., and Bauer, S., 2014, Geomechanical behavior of The U.S. Department of Energy (DOE) Storage System Program, the Iowa Stored Energy Plant Cambrian Mount Simon Sandstone reservoir lithofacies, Iowa Shelf, USA: International Agency, and the DOE National Energy Technology Laboratory (NETL; grant DEFE0004844) funded Journal of Greenhouse Gas Control, v. 21, p. 33–48, doi:10 .1016 /j .ijggc.2013.11 .010. this work. Dewers and Heath were supported in part by the Center for Frontiers of Subsurface En- Dewhurst, D.N., Jones, R.M., and Raven, M.D., 2002, Microstructural and petrophysical charac- ergy Security, an Energy Frontier Research Center funded by the DOE, Office of Science, Office of terization of Muderong Shale: Application to top seal risking: Petroleum Geoscience, v. 8, Basic Energy Sciences, award DE-SC0001114. Mozley was funded by the NETL portion of the proj- p. 371–383, doi:10.1144/petgeo.8.4.371. ect, which was managed and administered by the New Mexico Institute of Mining and Technology Duffin, M.E., Lee, M., Klein, G., and Hay, R.L., 1989, Potassic diagenesis of Cambrian sandstones and funded by DOE-NETL and cost-sharing partners. and Precambrian granitic basement in UPH-3 Deep Hole, Upper Mississippi Valley, U.S.A.: We thank Raymond Anderson and Robert McKay of the Iowa Geological and Water Survey for Journal of Sedimentary Petrology, v. 59, p. 848–861, doi:10 .1306 /212F908E -2B24 -11D7 discussions about regional lithofacies variation of the Mount Simon Sandstone and the Eau -8648000102C1865D. Claire Formation. Mark Rodriguez of Sandia National Laboratories performed the X-ray diffrac- Dullien, F.A.L., 1992, Porous Media—Fluid Transport and Structure (second edition): London, UK, tion. John Neasham of Poro-Technology performed mercury porosimetry (Poro-Technology has Academic Press, Inc., 574 p. subsequently been acquired by Micromeritics). Sandia National Laboratories is a multiprogram Ferreira, T., and Rasband, W., 2012, ImageJ User Guide—IJ1.46r: http://imagej .nih.gov/ij /docs laboratory managed and operated by the Sandia Corporation, a wholly owned subsidiary of Lock- /guide/user-guide.pdf, 198 p. heed Martin Corporation, for the DOE’s National Nuclear Security Administration under contract Finley, R.J., 2014, An overview of the Illinois Basin—Decatur project: Greenhouse Gases Science DE-AC04–94AL85000. The manuscript benefited greatly from the comments and suggestions of and Technology, v. 4, p. 571–579, doi:10 .1002 /ghg.1433. Shanaka de Silva (editor) and reviewers Brenda Bowen and John A. 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