Pore networks in continental and marine mudstones

ABSTRACT

Mudstone pore networks are strong modi-fi ers of sedimentary basin fl uid dynamics and have a critical role in the distribution of hydrocarbons and containment of injected fl uids. Using core samples from continen-tal and marine mudstones, we investigate properties of pore types and networks from a variety of geologic environments, together with estimates of capillary breakthrough pressures by mercury intrusion porosimetry. Analysis and interpretation of quantitative and qualitative three-dimensional (3D) obser-vations, obtained by dual focused ion beam–scanning electron microscopy, suggest seven dominant mudstone pore types distinguished by geometry and connectivity. A dominant planar pore type occurs in all investigated mudstones and generally has high coordina-tion numbers (i.e., number of neighboring connected pores). Connected networks of pores of this type contribute to high mercury capillary pressures due to small pore throats at the junctions of connected pores and likely control most matrix transport in these mudstones . Other pore types are related to authigenic (e.g., replacement or pore-lining precipitation) clay minerals and pyrite nodules; pores in clay packets adjacent to larger, more competent clastic grains; pores in organic phases; and stylolitic and micro-fracture-related pores. Pores within regions of authigenic clay minerals often form small

isolated networks (<3 μm). Pores in stringers of organic phases occur as tubular pores or slit- and/or sheet-like pores. These form short, connected lengths in 3D reconstruc-tions, but appear to form networks no larger than a few microns in size. Sealing effi ciency of the studied mudstones increases with greater distal depositional environments and greater maximum depth of burial.

INTRODUCTION

Mudstones, rocks composed of 50% or more silt- and clay-sized material with grain sizes <62.5 μm (MacQuaker and Adams, 2003), compose the major portion of sedimentary basin fi ll (Blatt, 1982) and thus play an inte-gral role in many geologic processes and sys-tems. These rocks dominate fl uid dynamics of the subsurface, affecting pore pressure distri-bution and local- to regional-scale patterns of groundwater and hydrocarbon transport (Aplin et al., 1999). Mudstones can act as source rock, migration pathway, and caprock for oil and gas, and even as reservoirs for shale gas (Loucks et al., 2009). Properties such as low permeability and high capillary breakthrough pressure make these rocks ideal as barriers (seals or caprocks) to the movement of single or multiphase fl uids (Potter et al., 2005). Hence, they are targets, as bounding formations or caprocks, for hazardous waste storage (Marty et al., 2003; Davy et al., 2009) and subsurface containment of anthropo-genic CO

2 (Intergovernmental Panel on Climate

Change, 2005). Despite their societal and scien-tifi c importance, only limited work has achieved a detailed characterization of pore networks

in mudstones, especially in three dimensions, due to constraints in sampling, preparation, measure ment resolution, and the heterogeneous nature of these rocks (Desbois et al., 2009). Greater understanding of the geometry, topol-ogy, and pore-lining material of pore networks in mudstones will yield insight into their pre-dicted controls on transport and storage of geo-logic and injected fl uids (U.S. Department of Energy, 2007).

Recent studies have demonstrated the util-ity of dual-beam focused ion beam–scanning electron microscopy (FIB-SEM) systems for imaging submicrometer-scale textures and com-positions, including mudstones (Kotula et al., 2003; Kotula and Keenan, 2006; Tomutsa et al., 2007; De Winter et al., 2009; Desbois et al., 2009). These methods combine milling of smooth nanometer-scale surfaces and high-resolution imaging by fi eld emission SEM (Yao, 2007). Successive milling and imaging yield a series of two-dimensional (2D) images, which are stacked and processed to construct a 3D pore network geometric model (Holzer et al., 2004; Tomutsa et al., 2007). Images taken in backscattered electron mode (BSE; De Winter et al., 2009) provide information on mineral and organic matter distributions by variation in mean atomic number (Z). Recent FIB-SEM studies on low-permeability geologic samples focused on defi ning and describing submicrometer pore types, and morphology, capillarity, fractal scal-ing, and fl uids in pores (Tomutsa et al., 2007; Desbois et al., 2008, 2009; De Winter et al., 2009; Loucks et al., 2009).

In this paper we investigate properties of pore geometries and pore networks in mudstones from

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429

Geosphere; April 2011; v. 7; no. 2; p. 429–454; doi: 10.1130/GES00619.1; 17 fi gures; 3 tables; 4 animations.

Pore networks in continental and marine mudstones: Characteristics and controls on sealing behavior

Jason E. Heath1,2,*, Thomas A. Dewers3, Brian J.O.L. McPherson4, Robin Petrusak5, Thomas C. Chidsey, Jr.6, Alex J. Rinehart1, and Peter S. Mozley1

1Department of Earth and Environmental Science, 801 Leroy Place, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA2Department of Geophysics and Atmospheric Sciences, P.O. Box 5800, MS 0750, Sandia National Laboratories, Albuquerque, New Mexico 87815-0750, USA3Department of Geomechanics, Sandia National Laboratories, P.O. Box 5800, MS 0751, Albuquerque, New Mexico 87185-0751, USA4Department of Civil and Environmental Engineering, 122 South Central Campus Drive, 104 CME, University of Utah, Salt Lake City, Utah 84112-0561, USA5Advanced Resources International, Inc., 4501 Fairfax Drive, Suite 910, Arlington, Virginia 22203-1661, USA6Utah Geological Survey, 1594 West North Temple, P.O. Box 146100, Salt Lake City, Utah 84114-6100, USA

Advances in 3D Imaging and Analysis of Geomaterials themed issue

*Present address is only affiliation 2. Email: [email protected].

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Heath et al.

430 Geosphere, April 2011

several depositional environments. Of interest is the relative importance of depositional environ-ment, burial history, and diagenesis as primary controls of pore network properties. We incor-porate high-resolution (i.e., submicrometer) 3D petrography using FIB-SEM techniques (see discussions labeled “Materials and Methods” and “FIB-SEM Imaging and 3D Image Analy-sis”) to identify pore and pore network types, to permit comparison of pore networks from dif-ferent mudstone lithofacies, and to compile pore network statistics (see discussion in Results). Mercury intrusion porosimetry (MIP; see dis-cussions in Materials and Methods) is applied to the same samples to determine pore throat distributions of the connected pore network and breakthrough pressures, and evaluated in light of the 3D pore network information (see discus-sion in Results). Our results suggest that there is a strong association between depositional environment, burial history, and capillary seal-ing behavior of these mudstones, all of which have been proposed as caprocks for subsurface CO

2 storage (see discussion of Pore Networks

and Sealing Quality).

GEOLOGIC SETTINGS

The Southeast and Southwest Regional Car-bon Sequestration Partnerships (SECARB and SWP; Litynski et al., 2008) provided mudstone samples from cores through caprock sealing sequences at recently deployed Phase II CO

2

sequestration sites (Fig. 1A; see Litynski et al., 2008, for descriptions of SECARB, SWP, and the U.S. Department of Energy’s Regional Carbon Sequestration Partnerships program in general). The mudstones cover a range of depositional environments from continental to marine shelf (Table 1; Figs. 1B–1D). Geologic units investigated include, from generally prox-imal to more distal or deeper water depositional environments, the Late Cretaceous Kirtland For-mation, the Late Cretaceous Lower and Middle Tuscaloosa Group, and the Gothic shale of the Pennsylvanian Paradox Formation.

Kirtland Formation

The Late Cretaceous Kirtland Formation of the San Juan Basin, New Mexico and Colo-rado (Fig. 1A), is a regional aquitard and res-ervoir seal (Ayers, 2003). It was deposited by streams fl owing toward the retreating shoreline of the Western Interior Seaway in an alluvial plain with fl oodplain and channel deposi-tional environments, which were landward of the swampy environments of the underly-ing Fruitland Formation (Fig. 1B; Fassett and Hinds, 1971; Fassett , 2009). The Kirtland For-

mation is divided into upper and lower shale (i.e., mudstone rich) members and a middle sandstone-rich member, the Farmington Sand-stone (Bauer, 1917; Fassett and Hinds, 1971; Stone et al., 1983; Molenaar and Baird, 1992). Throughout most of the basin the Kirtland For-mation conformably overlies the coal-bearing Fruitland Formation (Fassett and Hinds, 1971), which contains the world’s most prolifi c coal-bed methane play (Ayers, 2003).

Starting in May 2008, the SWP oversaw drill-ing of CO

2 injection well EPNG Com A Inj 1

(API No. 30–045–34305; lat 36.307735°N, long 107.251278°W; Fig. 1A) with coring of the upper and lower shale members of the Kirtland Formation from depths of 624–631 m and 820–822 m. The SWP injected CO

2 into unmineable

coals seams at depths between ~889 m and 957 m for a 12 month period (i.e., from 30 July 2008 to 12 August 2009). Core samples studied for pore network characteristics herein were obtained from depths of 624.75 and 820.80 m, representing the upper and lower shale mem-bers, respectively (Table 1).

Optical petrography complemented by stan-dard SEM, X-ray diffraction (XRD), total organic carbon (TOC), porosity, and permeabil-ity analyses of several upper Kirtland samples taken along the length of the core indicates sandy to silty argillaceous mudstones (Figs. 2A, 2B; Heath, 2010). Mottled, disorganized, and poorly laminated textures are common. Pedo-genic features include illuviation envelopes that contain well-aligned clays transported into the sediments along root channels or desicca-tion cracks. Sand- and silt-sized grains include quartz, plagioclase feldspar, and volcanic and sedimentary rock fragments (e.g., chert-replaced volcanic clasts and clay rip-up clasts). Some lithofacies are dominated by sand-sized grains, whereas others are mostly silt- and clay-sized material. Clay minerals include smectite, illite, mixed layer smectite/illite, chlorite, and minor kaolinite. Clay expandability ranges from 20% to 23%. Authigenic minerals include minor carbonate cement (usually iron rich) and pyrite.

Macroporosity types were summarized in Heath (2010) and include abundant induced porosity from coring (i.e., pressure-release frac-tures), core handling, and dry-out (~5–20 μm in size); root and organic particle casts left by the decayed organic material (2–20 μm); and matrix-hosted pores between clay fl akes and within cements (<5 μm). Minor intragranu-lar porosity within feldspar clasts also occurs. Porosity and permeability range from 6.5% to 9.9% and 7 × 10–20 to 1 × 10–19 m2, respectively, with total organic carbon ranging from 0.06 to 0.09 wt%. Vitrinite refl ectance values for the Kirtland Formation for the area of interest may

range from 0.8% to 1.3% Rm, with maximum depth of burial of the upper and lower Kirtland Formation being ~1630 and 1830 m, respec-tively (Law, 1992).

Tuscaloosa Group

In the northeastern Gulf of Mexico Basin, the Tuscaloosa Group represents the basal strata of a thick wedge of predominately clastic Late Cretaceous sediments, which thin updip (to the north) and thicken downdip (to the south), representing at least four region-scale marine transgressive-regressive cycles on the Creta-ceous shelf (Mancini and Puckett, 2005). The Tuscaloosa Group contains predominately silici-clastic sediments and represents fl uvial-deltaic and marginal marine to mid-shelf open-marine depositional environments (Mancini et al., 1987; Rosen and Rosen, 2008). In the sub-surface of Alabama and Mississippi, the Tusca-loosa Group contains a lower sandstone unit, a middle unit of predominately marine shale, and an upper sequence of interbedded sandstone and shale. These three Tuscaloosa divisions persist along depositional strike over a large region of the northern Gulf of Mexico Basin. The middle Tuscaloosa Group, informally called the middle Tuscaloosa Marine Shale or Marine Tuscaloosa, ranges from Cenomanian to Turonian in age and represents the maximum marine transgression during deposition of the Tuscaloosa (Liu, 2005). The deltaic and marginal marine sandstones of the lower Tuscaloosa Group (the so-called basal massive sand unit) represents a period of aggra-dational sandstone deposition preceding the maximum marine transgression represented by the middle Tuscaloosa Marine Shale. Thin silt-stone and sandstone beds become thicker and more abundant upward in the Marine Tusca-loosa due to subsequent progradational infi lling (Liu, 2005; Mancini and Puckett, 2005; Pashin et al., 2008).

In 2009, Southeast Regional Carbon Seques-tration Partnership (SEACARB) conducted a CO

2 injection test (Litynski et al., 2008) in

the basal lower Tuscaloosa Group sandstones at the Plant Daniel site in Jackson County, Mississippi (Fig. 1C). Both the midddle Tus-caloosa Marine Shale and an interbedded mud-stone unit between two thick lower Tuscaloosa Group sandstones are considered potential seals for the CO

2 injected into these sand-

stones. Core samples for characterization of pore networks were obtained from the Missis-sippi Power Company No. 1 observation well (MPC #11–1; API No. 23–059–20023–00; lat 30.536902°N, long 88.558073°W). Depth intervals cored include 2409.7–2418.3 m in the middle Tuscaloosa Marine Shale, and 2600.2–


Pore networks in mudstones

Geosphere, April 2011 431

A

Gothicshale

KirtlandFormation

Tuscaloosa Group

SWP

SECARB

lowerKirtland

upperKirtland

FruitlandB

Gothic shale

C

D

swampsea level

marineshale

marinesandstone

continentaldeposits

fluvialsandstone

coal

sea level

middle Tuscaloosa Groupor "Marine Tuscaloosa"

lower Tuscaloosa Group

sea level

blackshale

gypsum sand-stone

carbonateshalite potash

marineshale

marginal marineenvironments

Figure 1. (A) Map of regions covered by the U.S. Department of Energy’s Southeast and Southwest Regional Carbon Seques-tration Partnerships (SECARB and SWP), with locations of wells of the core samples. (B) Depositional systems of the Kirtland Formation shown at a time when aggradation of the Fruitland Formation was occurring (modifi ed from Fassett and Hinds, 1971). Vertical exaggeration is ~60×. The upper Kirtland Formation and lower Kirtland Formation are more proximal than the swampy systems of the Fruitland Formation. (C) Tuscaloosa Group depositional environments (modifi ed from Liu, 2005). The lower Tuscaloosa Group was deposited as a transgressive systems tract. Marine Tuscaloosa deposition occurred during the maximum fl ooding event of the latest Cenomanian to early Turonian and represents the second Late Cretaceous oceanic anoxic event. (D) Cross section showing the transgressive systems tract that deposited the Gothic shale (at the Aneth Unit) or black shale facies over shelfal carbonates (modifi ed from Goldhammer et al., 1994). Water depth was estimated as >35 m.


Heath et al.


2618.7 m in the interbedded mudstone unit in the lower Tuscaloosa Group. The lower Tus-caloosa Group core includes the bottom part of the upper massive sand unit from 2600.2 to 2611.8 m; interbedded and bioturbated sand-stone and silt- and clay-bearing mudstones from 2611.8 to 2615.8 m; and predominately clay-rich mudstone with silt-rich mudstone laminations from 2615.8 to 2618.7 m. Pore network samples for this study are from depths 2415.7 m, 2417.6 m, 2618.2 m, and 2618.5 m. These sample depths correspond with those of the Marine Tuscaloosa and the interbedded mudstones in the lower Tuscaloosa Group.

A thin-section image and photomicrograph for the lower Tuscaloosa Group (Figs. 2E, 2F) from core depth of 2615.05 m show an interbed-ded sand-, silt-, and clay-rich mudstone, with individual layers that thin and pinch out. Sand- and silt-sized grains include angular quartz, micas, and possible feldspar. Opaque regions are authigenic pyrite and organics. Bioturba-tion (burrows) is visible at the thin-section scale (Fig. 2E). XRD from a similar depth (2613.6 m) indicates that clays are chlorite, kaolinite, and illite, and nonclay minerals are predominately quartz with lesser amounts of plagioclase feld-spar and minor K-feldspar (U.S. Department of Energy, 2008).

A thin-section scan and photomicrograph of the Marine Tuscaloosa (Figs. 2G, 2H) from a core depth of 2416.27 m also show inter-bedding of sand-, silt-, and clay-rich layers. Clays do not show alignment under crossed polarizers as strong as that in the Lower Tus-caloosa Formation thin section. Sand- and silt-sized grains include quartz and lesser amount of micas and feldspars. XRD on a sample from 2417.6 shows that clays are chlorite, kaolinite, and illite with ~5 wt% calcite (U.S. Depart-ment of Energy, 2008).

Petrophysical and other properties for the Tuscaloosa samples as determined by SEACARB were summarized in U.S. Depart-ment of Energy (2008). Porosity and Klinken-berg-corrected permeability at 17 MPa net confi ning stress for the lower Tuscaloosa Group from a depth of 2618.2 m are 9.5% and ~7 × 10–17 m2, respectively. The TOC at depth 2617.7 m is 1.0 wt%. Marine Tuscalo-osa porosity and permeability at 17 MPa net confi ning stress at depth 2415.7 m are 2.2% and ~1 × 10–19 m2, respectively. The TOC for depths of 2415.4 m and 2417.2 m are 0.65 and 0.73 wt%, respectively. Calculated vitrinite refl ectance values (%Ro) of the Marine Tus-caloosa core are 0.51 and 0.47 for depths of 2415.4 and 2417.2 m, respectively. Lower Tuscaloosa Group core samples, at depths of 2616.6 and 2617.7 m, have calculated vitrinite refl ectance values of 0.69 and 0.65, respectively. Production index and maturity (based on the maximum temperature during Rock-Eval pyrolysis) indicate that the Marine Tuscaloosa and lower Tuscaloosa Group core samples are at low maturity (i.e., low level of conversion of kerogen). The core contains mixed type II-III kerogen. Current core depths represent the maximum depth of burial of the core samples (Mancini et al., 1999).

Gothic Shale

The Pennsylvanian Gothic shale, composing the basal unit of the Ismay zone of the Paradox Formation, occurs within the Blanding sub-basin of the Paradox Basin in southeastern Utah and southwestern Colorado (Fig. 1D), overly-ing the Desert Creek hydrocarbon-producing zone of the Greater Aneth oil fi eld (Peterson, 1992). Goldhammer et al. (1994) described Gothic shale lithology as a sapropelic dolomite

and dolomitic shale to silty carbonate mud-stone, deposited as a transgressive systems tract (TST) during fourth-order sea-level rise. At the Aneth Unit, the Gothic shale ranges from 1.5 to 8.2 m thick and is separated by an unconfor-mity (sequence boundary) from the underlying mottled carbonates of a mound buildup com-plex (Chidsey et al., 2009). The Gothic shale is Goldhammer et al.’s (1994) black laminated mudstone lithofacies that represents the TST, possibly representing a deeper water subtidal zone with deposition in quiescent, reducing bot-tom conditions as much as ~30 m deep.

The Gothic shale is the reservoir caprock at the Aneth Unit (Chidsey et al., 2009). As part of Phase II evaluation of CO

2 injection into the

Desert Creek zone for enhanced oil recovery, the SWP obtained Gothic core from the Aneth Unit H-117 well (API No. 43–037–30153; lat 37.3093°N, long 109.3035°W), which was drilled in 1974. The SWP performed a suite of tests on the core to evaluate sealing integ-rity (Chidsey et al., 2009). The cored interval examined by the SWP included a complete 5.09 m section of Gothic mudstone from depths of 1638.5–1643.8 m. The core sample studied herein for pore network and petrographic prop-erties came from a depth of 1643.1 m (Chidsey et al., 2009).

Figures 2I and 2J show a thin-section scan and photomicrograph of the Gothic shale sample examined in this study, showing a fi ne-grained mudstone with faint lamination and well-developed fi ssility. Textural components include minor silt-sized quartz, calcite, dolo-mite, and mica in a dominant clay matrix, with common authigenic pyrite. Compacted white nodules (Fig. 2J) are composed predominately of chert. Fossils include conodonts, brachio-pods, and condalarids. Besides induced porosity, macroporosity and microporosity are diffi cult to

TABLE 1. MUDSTONE CORE SAMPLE IDENTIFICATION AND GENERAL INFORMATION

Sample identifi cationDepth

(m) Geologic unit Site of interest State Depositional environment and dominant rock typesupper Kirtland 2049.7A 624.75 upper shale member of

Kirtland FormationPump Canyon

ECBM/CO2*

NM Multiple environments: fl oodplains, point bar deposits, possible channel fi lls, reducing environments including swamps and oxbow lakes; sandy to silty argillaceous mudstones MN57.426B7.9402dnaltriKreppu

lower Kirtland 2692.9A 820.80 lower shale member of Kirtland Formation

Pump Canyon ECBM/CO2*

NM Coastal swamp, river, fl oodplain, and lake deposits; silty to argillaceous mudstonesMN08.028B9.2962dnaltriKrewol

Marine Tuscaloosa 7925.5 2415.69 middle Tuscaloosa Group or Marine Tuscaloosa

Plant Daniel CO2

injection†

MS Represents maximum marine transgression during deposition of Tuscaloosa Group; inner shelf environments but deeper water than lower Tuscaloosa; sandy to silty to argillaceous mudstonesSM46.71429.1397asoolacsuTeniraM

lower Tuscaloosa 8590 2618.23 lower Tuscaloosa Group

Plant Daniel CO2

injection†

MS Trangressive systems tract deposition; inner shelf but shallower water than Marine Tuscaloosa; sandstones with interbedded sandy to silty to argillaceous mudstonesSM15.81629.0958asoolacsuTrewol

Gothic shale 5390.8A 1643.12Gothic shale Aneth Unit EOR§

UT Trangressive systems tract deposition; deeper subtidal in quiet, reducing bottom conditions; calcareous to phosphatic, silty to argillaceous mudstonesTU21.3461B8.0935elahscihtoG

Note: Samples were obtained from core from the Southeast and Southwest Regional Carbon Sequestration Partnerships at Phase II CO2 sequestration sites (Litynski et al. 2008). NM—New Mexico; MS—Mississippi; UT—Utah.

*Pump Canyon Site is a project that involved enhanced coalbed methane (ECBM) recovery and CO2 injection into unmineable coal seams.†Plant Daniel Site was a CO2 injection project into saline aquifers.§The Aneth Unit is part of the Greater Aneth oil fi eld in Utah, where CO2 is injected underground for enhanced oil recovery (EOR).




identify . Thus, the Gothic appears less porous and more homogeneous than the Kirtland or Tuscaloosa samples.

Porosity, pressure-decay permeability, and TOC measurements taken from four depths from the core range from 2.7% to 4.3%, 1.3 × 10–19 to 1.4 × 10–19 m2, and 2.2–4.4 wt%, respectively (Chidsey et al., 2009). Vitrinite refl ectance values (calculated %Ro) range from 0.83 to 0.96. Production index and matu-rity (based on the maximum temperature dur-ing Rock-Eval II pyrolysis) indicate that the

kerogen within the Gothic shale core is mature, containing type II and mixed type II-III kero-gen. The maximum depth of burial is ~3050 m (Nuccio and Condon, 1996).

MATERIALS AND METHODS

To facilitate interpretation of geologic controls on pore network properties and capillarity, FIB-SEM techniques were used to image pores, con-trasting mineralogy, organic phases, and geologic structures. Representative samples from each

core were characterized fi rst by FIB-SEM and then with MIP. The MIP was used to allow com-parison of interpreted MIP pore size distribu-tions to the direct imaging of the pore networks , as well as to obtain breakthrough pressures in an assessment of sealing quality.

Sample Preparation

We prepared 10 mudstone samples, 2 from each of the study units, from plugs (2.5 cm diameter by <1.5 cm long). The samples were

B

IA C

JD

E

F

G

H

Figure 2. Scans of thin sections and photomicrographs of continental and marine mudstones. Thin sections (standard size of 24 × 46 mm) for a particular formation are located above corresponding photomicrographs. Images and petrographic descriptions for C, D, and J are from TerraTek (Chidsey et al., 2010; Heath, 2010). Thin-section samples were impregnated with a fl uorescent red dye epoxy resin. Fractures (i.e., magenta lines through the rock) are considered induced features due to desiccation of clays, unloading, and sample preparation. (A) Thin section for upper Kirtland Formation from core depth of ~625.60 m. The upper two-thirds is dominated by sand- and silt-sized grains. The lower third is a silt-bearing, argillaceous mudstone. Large opaque grain is pyrite. Younging direction is upward. (B) Plane light photo micro-graph from the lower third of A. Diagonal parallel lines are artifacts from thin-section polishing. The clay matrix has yellow birefringent colors in places and shows strong parallelism under crossed polarizers when the stage is rotated. The parallelism, however, is not across the entire area, but concentrated in a zigzag pattern. Clasts include chert, volcanic rock fragments, quartz, feldspar, and sedimentary rock fragments. Opaque grains are pyrite and organics. Scale bar is 0.5 mm. (C) Thin section for lower Kirtland Formation from core depth of 820.60 m. The left half of the section was stained. White areas are preparation artifacts that removed patches of the rock. (D) Plane light photomicrograph of unstained area of overlying thin section. Silt and minor sand grains are quartz and feldspars. The irregular, off-whitish , relatively large minerals in the lower left of the image are ferroan calcite. Clays are illite/smectite with kaolinite and chlorite and minor smectite. Scale bar is 0.5 mm. (E) Thin section for Lower Tuscaloosa Formation from core depth of 2615.05 m. Younging direction is to the left. Thin section shows abundant silt- and sand-sized grains. Bioturbation as burrows is visible at the thin-section scale. (F) Plane light photomicrograph of E showing silt or fi ne sand layers with clay-rich layers above and below. Larger grains include angular quartz, probable feldspar, and micas. Authigenic minerals include pyrite. Many dark patches are visible in the thin section, which may be organics or pyrite. Pyrite framboids in clusters are visible at 40× magnifi cation. Scale bar is 0.9 mm. (G) Thin section for Marine Tuscaloosa from core depth of 2416.27 m. The upper surface marked by the notch indicates the younging direction and is at a natural shear fracture that was visible in hand sample. The left side of the thin section may show deformation associated with the fracture. (H) Plane light photomicro-graph of G. Sand- and silt-sized grains include quartz and micaceous fl akes. Clay-rich areas do not show strong parallelism under crossed polarizers. Clay- and silt and/or sand-rich regions are abundant throughout the thin section and have undulatory or nonplanar boundaries. Scale bar is 0.43 mm. (I) Thin section for Gothic shale from core depth of 1642.6 m. Younging direction is to the left. (J) Plane light photo-micro graph taken of thin section from core depth of 1643.1 m. Sample is a phosphatic argillaceous mudstone. Flattened, amalgamated pellets (lighter brown) are phosphatic in composition. Compacted siliceous forms (white) are composed of chert. The lighter brown matrix color and abundance of siliceous fossils, as well as phosphatic pellets, suggest a siliceous matrix cement component. Scale bar is 0.5 mm.


Heath et al.


taken from the same or similar rock types as those of the thin sections shown in Figure 2. Plug orientation was approximately parallel to the axis of the core and roughly perpendicular to bedding. Prior to microanalysis, the top and bottom of the plugs were lightly sanded with fi ne-grit paper to achieve a nominally fl at sur-face. Due to potential fl uid sensitivity of these clay- and organic-rich samples, polishing using a saline solution or oil was not performed to avoid imbibition of fl uids, defl occulation, con-tamination, or other alteration of pore structure, organics, and other solid phases. Plug cleaning was also not performed. Signifi cant penetration of drilling fl uids into the plugs was unlikely due to low permeability of the rocks and appearance of fresh surfaces.

FIB-SEM Microscopy

Samples were gold-palladium coated and painted with silver dag to provide a current path and mitigate specimen charging. Samples were placed in mounts or vises and electrically grounded. Serial sectioning and imaging by FIB-SEM was performed with FEI Company’s Helios 600 Nanolab DualBeam instrument and semiautomated Slice and View software. To promote smooth, cross-sectioned faces and avoid curtaining artifacts (i.e., uneven vertical striations) during milling (Hayles et al., 2007), and to provide a fi ducial reference for image alignment, a layer of platinum was deposited over the area of sample to be milled using the FIB deposition system (Yao, 2007). During milling, the Ga+ ion beam was normal to the sample surface, and the electron beam was at an angle of 52° from the ion beam. Accelera-tion voltage for Ga+ was 30 kV, and the beam current was 2.8 nA. A rough-cut trough was made to provide an area for deposition of milled material and to reveal the vertical cross section. Vertical cross-sectioned surfaces per-pendicular to bedding and the upper surface of the sample were milled by the ion beam at 25 nm spacing. The Slice and View software facilitated cross sectioning with automation for beam shifts and auto-focusing; however, manual adjustments were made during milling to ensure acquisition of high-quality electron micrographs. Backscattered electron (BSE) imaging mode with a through-the-lens detec-tor was chosen for obtaining mineralogical information in terms of differences in mean atomic number, Z, which provided strong con-trast in mineralogy, organics, and pores for most samples. The acceleration voltage and beam current were 1 kV and 1.4 nA, respec-tively. The BSE micrograph fi eld of view was 16.00 μm × 13.81 μm.

Image Analysis

Digital reconstruction and quantifi cation of 3D pore networks and organic distribution for selected samples required several steps. The com-puter software ImageJ 1.42q (Rasband, 1997–2010; Ferreira and Rasband, 2010) was employed to align, crop, and segment stacks of Slice and View images into regions of pore or nonpore or organics or nonorganics. The ImageJ plugin TransformJ was used to interpolate between segmented images (Meijering, 2008), result-ing in a cubic voxel length size of 15.6 nm. The cropped regions consisted of 299 × 299 × 299 voxels, with a total cubic data volume with side length sizes of 4.66 μm. TransformJ interpolation produced gray-scale information, which necessi-tated further segmentation for proper delineation of pore or nonpore or organics or nonorganics. We performed 3D fl oodfi ll image rendering with calculation of pore volume, organic volume, and porosity using ScanIP by Simpleware Ltd. Seg-mented images of pore or nonpore regions were input into the 3DMA-Rock code (Lindquist, 1999) for further quantifi cation of the pore networks.

The 3DMA-Rock algorithms performed the following steps in sequence (Lindquist, 1999; Lindquist and Venkatarangan, 1999; Lindquist et al., 2005; Neethirajan et al., 2008; Udawatta et al., 2008): medial axis construction of pore pathways; throat fi nding and pore partitioning; throat and pore characterization including pore volume, coordination number, and throat area; and geometrical construction of the pore and throat network. The medial axis construction produces a centrally located skeletonization of the pore space, which preserves topology and geometry of the pore network. Voxel paths con-sisting of fi ve voxels or fewer were excluded from the fi nal medial axis construction. Throats are minimal cross-sectional surface areas located on the medial axis, and the throat surfaces may be nonplanar. The coordination number of a pore gives the number of neighboring pores. The 3DMA analysis enabled compilation of pore sta-tistics in the form of pore volume and pore throat radii frequency distributions and other topologic properties (see Results discussion).

The distribution of throat and pore body sizes, connectivity of the pore network, and surface area and shapes of pores have strong control on drainage and imbibition processes and fl uid fl ow properties such as relative permea bility (Neethirajan et al., 2008).

MIP

MIP was performed on a Micromeritics AutoPore IV 9500 Series porosimeter. The inner diameter of the penetrometer bulb (Sigal,

2009) was 2.54 cm, which necessitated light sanding of the 2.54 cm plugs to a smaller size for placement into the bulb. Sanding removed the gold-palladium coating and silver dag previ-ously applied to the samples. Prior to analysis, samples were dried at 100 °C and photographed. To investigate capillary and transport properties in the direction perpendicular to bedding, some plugs were jacketed with epoxy for directional intrusion. Some plugs were too thin for proper jacketing and directional intrusion.

Closure pressure corrections (Sneider et al., 1997) followed a compressibility method that estimated the pressure and corresponding vol-ume of mercury when the mercury fi rst pen-etrated the pore network (Colombo and Carli, 1981; Almon et al., 2008). The closure volume was subtracted from the incremental volumes of mercury intruded into the penetrometer bulb. Breakthrough capillary pressure (also called bubbling pressure or sealing pressure) is the pressure at which a continuous fi lament of mercury extends across a MIP sample, or equivalently, the pressure when the nonwet-ting phase fi rst appears on the outlet side of a sample plug (Katz and Thompson, 1987; Dullien , 1992; Dewhurst et al., 2002). For this study, it was estimated by identifying the point on the cumulative mercury saturation versus pressure curve at which it had maximum infl ec-tion upward (Dewhurst et al., 2002; Daniel and Kaldi, 2008).

RESULTS

FIB-SEM Imaging and 3D Image Analysis

The 3D pore network reconstructions of 101.5 μm3 regions from each mudstone are pre-sented in Figures 3–10, arranged in order from proximal to distal from sediment source and deepening water depth of deposition. Shown for Kirtland Formation, Tuscaloosa Group, and Gothic shale samples are a single 1024 × 884 pixel 2D BSE image, usually taken from the back of an image stack, an extracted 299 × 299 segmented image to distinguish pores (in black), a 3D reconstruction of pores, and medial axis and pore throat 3D visualizations. Organic-rich samples have additional images to show 3D distributions of organic phases as layers and possible pore-fi lling entities. Medial axes, rep-resenting the skeletonization of pore networks, are shown using a rainbow color scale wherein red corresponds with low burn numbers, and blue and violet are higher burn numbers. Burn numbers represent the maximum norm distance in number of voxels from the medial axis to the grain boundary where a pore sharing a grain boundary has a number of one (Neethirajan




et al., 2008). The cubic voxel size for all 3D images is 15.6 nm.

To allow viewing of the FIB-SEM images used to construct 3D pore network models, Ani-mations 1–4 present sets of serial sections for the upper Kirtland Formation, the lower Tus-caloosa Group, Marine Tuscaloosa, and Gothic shale, respectively. These present a variety of distinct 3D pore types for the different geologic environments. A pore type classifi cation scheme is introduced herein, and shown in Figure 11. Pore statistics for all analyzed digital samples are given in Figures 12 and 13.

A common artifact visible in most of the FIB-SEM images is induced pores near the upper surface (i.e., top of micrographs) of the samples where mechanical cutting and rough polishing were performed. Pore shapes at that location can be much different and nonrepresentative of those deeper in the cross section. Specimen charging, visible as bright white spots in the images, is an occasional imaging artifact and was especially prevalent on organics. It can be differentiated from high Z phases like pyrite by a dark halo around the margins of the bright areas.

Figure 3 presents the pore network of the upper Kirtland Formation, from a sample 624.75 m (below ground surface, bgs). The lack of lamination or planar clay fabric, mottled texture, and the more or less random distribu-tion of pore and clay orientations (observable from the larger, higher Z clay packets that are

likely chlorite) in the BSE image (Fig. 3A) may refl ect the compacted analogues of deposited clay packets or fl occules, as observed experi-mentally by Schieber and Southard (2009). The disorganized texture may refl ect pedogenic processes of aggregation formation and weath-ering before deeper burial, such as by shrink-

Animation 1. Video fi le of a set of 325 images of focused ion beam serial sections taken in backscattered electron mode at 25 nm spac-ing for sample upper Kirtland 2049.7B. The horizontal fi eld of view is 16.00 μm. Up direction is not defi nitely known, but it is either toward the top or bottom of the images. Original TIFF images (1024 × 884 pixels) were compressed using JPEG algo-rithm at 100 quality when making the video fi le. Video fi les were further compressed by resampling images to 400 × 345 pixels and compressing using the Cinepak algorithm at 75 quality. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00619.S1 or the full-text article on www.gsapubs.org to view Animation 1.

Animation 2. Video fi le of a set of 278 images of focused ion beam serial sections taken in backscattered electron mode at 25 nm spacing for sample lower Tuscaloosa 8590. The horizontal fi eld of view is 16.00 μm. The large zone of charging (bright area) is probably at the location of organics. Origi-nal TIFF images (1024 × 884 pixels) were compressed using JPEG algorithm at 100 quality when making the video fi le. Video fi les were further compressed by resam-pling images to 400 × 345 pixels and com-pressing using the Cinepak algorithm at 75 quality. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00619.S2 or the full-text article on www.gsapubs.org to view Animation 2.

Animation 3. Video fi le of a set of 340 images of focused ion beam serial sections taken in backscattered electron mode at 25 nm spac-ing for sample Marine Tuscaloosa 7925.5. The horizontal fi eld of view is 16.00 μm. Original TIFF images (1024 × 884 pixels) were compressed using JPEG algorithm at 100 quality when making the video fi le. Video fi les were further compressed by resampling images to 400 × 345 pixels and compressing using the Cinepak algorithm at 75 quality. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00619.S3 or the full-text article on www.gsapubs.org to view Animation 3.

Animation 4. Video fi le of a set of 325 images of focused ion beam serial sections taken in backscattered electron mode at 25 nm spac-ing for sample Gothic shale 5390.8A. The horizontal fi eld of view is 16.00 μm. The viewer should be careful in distinguishing pores from organics. Organics have a low gray-scale value. Pores are discerned by the blackest gray-scale values and careful obser-vation of the grain-nonpore margins. Origi-nal TIFF images (1024 × 884 pixels) were compressed using JPEG algorithm at 100 quality when making the video fi le. Video fi les were further compressed by re sampling images to 400 × 345 pixels and compressing using the Cinepak algorithm at 75 quality. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00619.S4 or the full-text article on www.gsapubs.org to view Animation 4.


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swell of clays. Pores are dominantly slit shaped, as shown by the image of the segmented pores (in black; Fig. 3B). Larger pores are subparal-lel to surrounding clay fabric and discernable by subtle changes in gray level, roughly per-pendicular to the vertical axis. Voxel counts of the segmented 3D image (Fig. 3C), with pores shown in red, indicate a porosity of 1.04%, and 3DMA-Rock analysis indicates that 34% of pores are connected. Figures 3D and 3E show representations of pore network medial axes (rainbow colored) and pore throat shapes (in gray). Even through some of the imaged pores appear quite large (Fig. 3C), the pore throats are small. Throat size has implications for interpre-tation of MIP data presented herein.

An example from a carbonate-rich zone with quartz clasts (lower gray-scale value) in the lower Kirtland Formation is shown in Figure 4A. Although there are a number of induced fractures in the upper portion of the image, the planar pores observed in the BSE image in Figure 4A may actually be associated with

a microstylolitic structure [this is interpreted from scanning transmission electron micros-copy (STEM)–energy dispersive spectroscopy (EDS) analysis presented in Heath, 2010]. The interconnected porosity can be observed in the 3D representation shown in Figures 4C and 4D, with larger pore throat radii than in the clay matrix of Figure 3. Total porosity by voxel count is 0.72%, with 28% of that being connected.

A polygonal crack pore structure is evident in an example from the lower Tuscaloosa Group at 2618.2 m (bgs), shown in Figure 5. This may represent boundaries between depositional clay packets, but more likely represents a wetting-drying structure not unlike hexagonal fractures in mud. A 3D fl oodfi ll image (shown in blue) shows a large degree of connectivity across the sampled region. Largely due to this structure, this example has 2.64% porosity with 74% con-nectivity. The crack pore may be induced by coring and/or sample preparation, and may not be an in situ feature.

A sampled region in Figure 6, from the lower Tuscaloosa Group, shows a nominally horizontal microfracture along with crescent-shaped pores aligned with a possible authigenic clay fabric. It is not known if the porosity was preexistent to the clays, providing access to mineralizing fl uids and concomitant authigenic precipitation, or if the porosity results from a phase change, perhaps as more dense chlorite replaced a less dense clay. Crystal habit of the clay minerals, perhaps rosettes, controls the non–slit-like pore shapes. Chlorite rosettes, as seen in traditional SEM photomicrographs by other researchers (Welton, 1984), have triangular, trapezoidal, and rectangular pore shapes between the chlorite mineral plates similar to those that are visible in Animation 2.

Figure 7 shows interlamination between organics (lower Z and thus darker gray) and clay matrix (lighter gray) from an example from the Marine Tuscaloosa at 2415.7 m (bgs). Organic layer thickness can exceed 2 μm. The bound-ary between the clays and organics is wavy and

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Figure 3. Results of focused ion beam (FIB) milling and imag-ing of sample upper Kirtland 2047.9B and three-dimensional (3D) pore network reconstruc-tion. (A) Backscattered elec-tron (BSE) image of FIB serial section. Horizontal fi eld of view is 16.00 μm. Outlined box indi-cates cubic region of 3D pore network reconstruction, which has side lengths of 4.66 μm. (B) Segmentation of BSE image into pore (black) and nonpore (white). (C) 3D fl oodfi ll ren-dering of pores. Voxel size is 15.6 nm. (D) 3D plot of medial axis. (E) 3D plot of pore throat surfaces.




convolute . Some clays appear isolated within the organics in the 2D images. The organic phase has been confi rmed as dominantly car-bon with chloride and sulfur by EDS analysis (Heath, 2010). Because the organics are located as an intact layer, they are likely syngenetic with surrounding clays. There are tiny pores, barely discernable at the resolution of our method, seen in the segmented image in Figure 7B. The 3D reconstruction (Fig. 7D) shows that some of these are aligned to form large tubes. Although representing very little porosity (0.58%), con-nectivity on the 3 μm scale is high at 62%, and these pathways may represent primary migra-tion pathways for petroleum generated within them. Connectivity on larger length scales may be poorer, but this may be an artifact of our imaging resolution.

A clastic portion of the same image data set for the Marine Tuscaloosa is shown in Figure 8. Pores from this portion appear to have a slit-like pore morphology similar to that in the upper Kirtland; although the porosity is less (0.47%),

a higher percentage is connected (52%). Elon-gate phases (higher Z and brighter gray scale) indicate possible detrital muscovite, which is consistent with thin-section observations. XRD data indicate that clay phases may include chlo-rite, illite, and kaolinite (U.S. Department of Energy, 2008).

Figures 9A and 10A show BSE images of the Gothic shale at 1643.1 m (bgs) with 3D repre-sentations of the distribution of organic matter at 15.19% and 5.59% by volume, respectively (Figs. 9B, 9C, 10B, and 10C), and porosity at 0.42% and 1.33% by volume (Figs. 9E and 10E), occurring largely adjacent to clastic quartz and feldspar grains. All imaged porosity appears to be located within or adjacent to organic mat-ter (see Animation 4). Organic content is high, although not concentrated in layers as in the Marine Tuscaloosa sample. While some pores may be relatively large (shown in blue in Fig. 9E), pore throats, shown in Figure 9G, are small, and the pore bodies are not well connected (42%). Pore morphology in these organics is

slit and/or sheet like, as opposed to the circular and/or tubular pores of the Marine Tuscaloosa. The large bright circular structure to the lower right of Figure 9A is a pyrite framboid that con-tains pores. The orientation of clays and clasts around the framboid indicate pyrite formation prior to signifi cant compaction.

Summary of Pore Types

Based on 2D observations of morphology and size of a FIB serial section from a Boom Clay sample, Desbois et al. (2009) identifi ed three mudstone pore types, which they termed pore types I, II, and III: the type I pores encom-pass elongate pores in similarly oriented clay sheets and are <100 nm in the elongate direc-tion. Type II includes crescent-shaped pores in folded clays and are 100–1000 nm in size. Type III pores are large, jagged pores associated with margins of larger, more competent grains and are typically >1000 nm (Desbois et al., 2009). Ideally, one would like to use our 3D

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Figure 4. Results of focused ion beam (FIB) milling and imag-ing of sample lower Kirtland 2692.9A and three-dimensional (3D) pore network reconstruc-tion. (A) Backscattered elec-tron (BSE) image of FIB serial section. Horizontal fi eld of view is 16.00 μm. Outlined box indi-cates cubic region of 3D pore network reconstruction, which has side lengths of 4.66 μm. (B) Segmentation of BSE image into pore (black) and nonpore (white). (C) 3D fl oodfi ll ren-dering of pores. Voxel size is 15.6 nm. (D) 3D plot of medial axis. (E) 3D plot of pore throat surfaces.


Heath et al.


observations of pores and pore networks along with those of Desbois et al. (2009) to propose a universal classifi cation scheme for mudstone pore networks. Such a classifi cation scheme could be based on characteristics of pores in mudstone microfacies such as morphology; sizes and size distributions of pores in net-works; connectedness and/or topology of pores; size relationship of pore throats to pore bodies; roughness of pore body walls; geometrical rela-tionships between the pores and surrounding grains; and the characteristic of being induced or present in situ. Clearly, such an ambitious scheme is beyond the scope of this paper.

For our purposes here, distinct types of pores and 3D pore networks can be distinguished and placed into at least seven descriptive groups (Fig. 11), including the Desbois et al. (2009) pore types I–III, and adding pore types IV–VII (Fig. 11). Type IV, pores in organics, includes two subtypes, IVa and IVb. Type IVa refers to circular and/or tubular pores in organics in the Marine Tuscaloosa (Figs. 7 and 11; Anima-

tion 3), whereas IVb refers to slit-like pores in organics in the Gothic shale (Figs. 9 and 11; Animation 4). Type V refers to microstylolitic or other diagenesis-related pores, as based on lower Kirtland 2692.9A in Figures 4 and 11. The pores in Figure 4 in particular have a jag-ged, dentate morphology that appears more natural than induced, and this is brought out more in STEM–EDS imaging analysis on this sample discussed in Heath (2010). Type VI is for microfractures, which may or may not be induced (Figs. 5 and 6). Type VII designates pores in pyrite framboids, illustrated by the Gothic shale in Figure 9.

Type I pores are abundant and dominant in the upper Kirtland Formation. Some of the larger pores exceed 1000 nm in their axial directions and have a length perpendicular to their axis of <~130 nm. A range of smaller sizes has similar shapes. The Tuscaloosa Group mudstone samples also show an abundance of type I pores, although less commonly than the Kirtland. The Gothic shale has the most infre-

quent occurrence of type I pores. Desbois et al. (2009) described the orientation of type I pores as being similar to nearby clay sheets, and thus compaction-derived fabric, which is verifi able in our FIB-SEM images as clay sheets based on differences in gray scale of parallel sheets (see Figs. 3A–3E). However, some FIB-SEM images show elongate pores where the clay fabric is not clearly visible. In addition, some elongate pores contain apparent bridging material inside the pores, not noted by Desbois et al. (2009), that is approximately perpendicular or slanted relative to the margins of the pores. Some pores have sharp tips, whereas others have terminations that are curved, and thus the pores can appear as narrow ellipses. In the lower Tuscaloosa Group (Fig. 6) many of these pores are curved and follow the undulate texture of the surrounding sheet clays (see Animation 2).

Type I pores are interconnected through small pore throats at the tips of the pores, and the small throat radii undoubtedly contrib-ute to high capillary pressures (see following

Figure 5. Results of focused ion beam (FIB) milling and imag-ing of sample lower Tuscaloosa 8590 and three-dimensional (3D) pore network reconstruc-tion of region with polygonal pores (area 1). (A) Backscat-tered electron (BSE) image of FIB serial section. Horizontal fi eld of view is 16.00 μm. Out-lined box indicates cubic region of 3D pore network reconstruc-tion, which has side lengths of 4.66 μm. (B) Segmentation of BSE image into pore (black) and nonpore (white). (C) 3D fl oodfi ll rendering of connected (blue) and unconnected (red) pores. Voxel size is 15.6 nm. (D) 3D plot of medial axis. (E) 3D plot of pore throat sur-faces.

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Connected (blue) andunconnected (red) pores




discussion of MIP). In addition, due to the sheet-like or fracture-like geometries, these pores are likely enlarged in size relative to those occurring in situ, due to unloading and/or clay mineral shrinkage. The orientation of most of the observed type I pores, being subperpendicu-lar to overburden and thus maximum principal compressive stress, could contribute to a stress-sensitive permeability of mudstones with abun-dant type I pore networks.

Type II pores of Desbois et al. (2009) are crescent shaped in folded clays. We extend their defi nition to include pores associated with clays that are in predominately nonparallel orienta-tions. Thus, type II can include pores associ-ated with authigenic pore-lining or pore-fi lling clay minerals, pores with recrystallized miner-als, compacted detrital fl occules (if preserved), or clays in nonhorizontal orientations that have been deformed. The lower Tuscaloosa Group contains prime examples of this pore type (Figs. 6 and 11; Animation 2). The nonparallel orien-tations of groups of clay crystals probably indi-

cate that they are authigenic and formed after primary deposition. Thus, the pores may be associated with chlorite precipitation that may be a pore-fi lling or replacement feature. For example, replacement textures could entail con-version of less dense hydrous smectite to denser chlorite plus secondary pores.

The Desbois et al. (2009) type III represents pores associated with margins of relatively large, competent grains, which we assume are detrital, nonclay clasts (e.g., detrital quartz). Our samples do not have clear examples of this pore type; however, the Gothic shale samples appear to have compaction shadows, or rather struc-tures associated with deformation of relatively small material around larger clasts with possible preexisting pores that are fi lled with organ-ics (Figs. 9–11; Animation 4). Organic phases may have fl owed into type III pores during deep burial and compaction of the Gothic shale.

Type IVa pores in Marine Tuscaloosa organ-ics are generally circular to bulbous to ellipsoi-dal in two dimensions (Figs. 7 and 11), whereas

type IVb pores in organics in Gothic shale are slit like (Fig. 11). Networks of type IVa pores can be arranged linearly and appear tubular. The generally circular pore cross sections indicate in situ formation that postdates most compaction, probably due to interfacial tension and mini-mization of energy between gases and liquids during conversion of kerogen to hydrocarbons (Loucks et al., 2009). The slit pores in organics in the Gothic shale (Fig. 11) can be located near margins of organic particles while also cutting through the interior of the particles away from the margins. The orientation of several slit pores is similar to the general subhorizontal fabric of the rock. Formation of this pore type is unclear, but probably related to maturation of the organic material within the Gothic shale.

Types V and VI represent subplanar stylolitic and microfracture-related features. These can have generally planar, slit-like morphology and high connectivity across the 3D reconstructions (Figs. 4, 6, and 11). These pore types may be induced due to pressure release during coring

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Figure 6. Results of focused ion beam (FIB) milling and imaging of sample lower Tuscaloosa 8590 and three-dimensional (3D) pore network reconstruction of region with crescent-shaped pores (area 2). (A) Backscat-tered electron (BSE) image of FIB serial section. Horizontal fi eld of view is 16.00 μm. Out-lined box indicates cubic region of 3D pore network reconstruc-tion, which has side lengths of 4.66 μm. (B) Segmentation of BSE image into pore (black) and nonpore (white). (C) 3D fl oodfi ll rendering of pores. Voxel size is 15.6 nm. (D) 3D plot of medial axis. (E) 3D plot of pore throat surfaces.


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and desiccation of swelling clays. Their large size could contribute signifi cantly to lower cap-illary pressures if well connected throughout an MIP plug sample.

Type VII pores occur within pyrite fram-boids (Fig. 9; Animation 4). Framboids are generally isolated in both the Marine Tusca-loosa and Gothic shale; thus, this probably does not represent a dominant, connected pore type. Pores are irregular due to the packing of the individual pyrite crystals and precipitation around the crystals.

Pore Network Statistics

Figure 12 presents relative frequency histo-grams of pore throat areas (ta; Fig. 12A), pore body volumes (pv; Fig. 12B), and ratios of the

equivalent circular radii of the pore throat areas and pore body volumes (rt/rb; Fig. 12C), com-piled from the 3DMA-Rock analysis. The histo-grams are organized from left to right in order of proximal to more distal, deeper water (i.e., lower energy) depositional environments. The abscissa used a log scale to facilitate examina-tion of the large range in values. Bin sizes are the same for a particular row. The count num-ber (e.g., n = 96) on each histogram refl ects dif-ferences in pore structure as the same sample volume (i.e., 101.5 μm3) is used for all the pore network reconstructions. The histograms do not include pore throats and bodies that were unresolved (i.e., below the resolution) by the FIB-SEM imaging: BSE imaging in this study resolved features (i.e., pores) as large or larger than the 15.6 nm pixel size.

The histograms of the pore throat areas (Fig. 12A) generally exhibit positive skewness (i.e., the tail of the distribution is longer on the right side than the left) except for Marine Tusca-loosa sample 7925.5 organic, which focused on an organic-rich region (see Fig. 7), and both Gothic shale samples. The organic material in the Gothic Shale contains the majority of the pores; thus, the pore throats in the organics have different distributions, which may favor uniform or possibly log normal distributions (see sample 5390.8A in Fig. 12A). The samples with domi-nant pores in organics also have relatively lower count numbers, except for inorganic-focused sample in the Marine Tuscaloosa (MT 7925.5 inorganic). The histograms of pore body volume (Fig. 12B) have generally log normal distribu-tions, except for the samples that have pores

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Figure 7. Results of focused ion beam (FIB) milling and imaging of sample Marine Tuscaloosa 7925.5 and three-dimensional (3D) pore network and organic reconstruction of organic-rich region. (A) Backscattered electron (BSE) image of FIB serial section. Horizontal fi eld of view is 16.00 μm. Outlined box indicates cubic region of 3D pore network and organics reconstruction, which has side lengths of 4.66 μm. (B) Segmen-tation of BSE image into pore (black) and nonpore (white). (C) Segmentation of BSE image into organics (black) and non-organics (white). (D) 3D fl ood-fi ll rendering of pores. Voxel size is 15.6 nm. (E) 3D plot of medial axis. (F) 3D plot of pore throat surfaces. (G) 3D fl ood fi ll rendering of organic phase.




mainly in the organics (Fig. 12B, MT 7925.5 organic and GS [Gothic Shale] 5390.8B). The histograms for the ratios of the equivalent cir-cular radii of pore throat areas and pore body volumes (Fig. 12C) have generally log nor-mal distributions for the Kirtland samples, the in organic sample of the Marine Tuscaloosa, and the Gothic shale sample 5390.8A (with some skewness) and uniform distributions for the lower Tuscaloosa Group samples. Two of the samples with pores mainly in the organ-ics, the Marine Tuscaloosa 7925.5 organic and Gothic shale 5390.8B, have low count numbers and distributions that differ from those of the other samples. The circular radii histograms (Fig. 12C) indicate the degree of departure of the pore network from a cylindrical tube model (i.e., values <1 are more strongly dominated by bulges and constrictions than tubes of uniform diameter; see Dullien, 1992).

To investigate the relation between pore size and connectivity, Figure 13 presents plots of

joint relative frequency and marginal relative frequency histograms of coordination number and pore body volume. Joint relative histograms give the number of occurrences of different pore body volumes associated with a particular coor-dination number, normalized by the total count number for a particular sample. Each row in the joint plot can be thought of as the distribution of pore body volumes for a given coordination number. The relative frequency of occurrence of a given range of pore body volumes at a given coordination number is indicated by shading from light (low frequency) to dark (high fre-quency). The two marginal histograms are, respectively, the summation of the relative fre-quencies of the rows and columns in the joint relative histogram, which give the univariate relative frequency histograms of pore body vol-ume and coordination number. The pore body volume marginal histograms are equivalent to those of Figure 12A, although they are plotted slightly differently (i.e., bins sizes are different

and the range is limited to the data) for simplic-ity in presentation.

For sample upper Kirtland 2047.9B (Fig. 13A), two clusters of high frequency (relatively dark areas) occur for values of log pore volume of ~6.2 nm3, at coordination numbers of 0 and 2. This shows that the most frequently occurring pores exist both as isolated pores and as con-nected networks. The highest frequency in the lower Kirtland sample (Fig. 13C) is similar, occurring at a slightly larger pore volume and coordination number of 2. This suggests that conductivity in the clay matrix of the upper Kirt-land and carbonate-rich portions of the lower Kirtland may not be controlled by the smallest pore bodies resolvable by the FIB-SEM imag-ing, but rather that the imaging method can be useful for study of transport properties in this mudstone. A similar conclusion can be reached for the lower Tuscaloosa Group samples (Figs. 13C, 13D), although the most frequent pore volumes are slightly smaller and coordination

Figure 8. Results of focused ion beam (FIB) milling and imaging of sample Marine Tuscaloosa 7925.5 and three-dimensional (3D) pore network reconstruction of organic-poor region. (A) Backscattered elec-tron (BSE) image of FIB serial section. Horizontal fi eld of view is 16.00 μm. Outlined box indi-cates cubic region of 3D pore network reconstruction, which has side lengths of 4.66 μm. (B) Segmentation of BSE image into pore (black) and nonpore (white). (C) 3D floodfill ren-dering of pores. Voxel size is 15.6 nm. (D) 3D plot of medial axis. (E) 3D plot of pore throat surfaces.

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numbers differ (2–3 for area 1 in Fig. 13C and 1 for area 2, Fig. 13D). The Marine Tuscaloosa inorganic (clay matrix) sample (Fig. 13F) shows no connectivity for the most frequent pore vol-ume, although the number of pores sampled in the interrogated mudstone volume was small compared to the lower Tuscaloosa Group and Kirtland samples.

A different picture emerges for the organic pores of the Marine Tuscaloosa (Fig. 13E) and Gothic samples (Figs. 13G, 13H). The Marine Tuscaloosa pore volumes, showing a uniform distribution, lack any connectivity at the scale of the interrogated volume. Gothic sample 5390.8A, with a slightly larger pore volume at the highest frequency, also shows little connec-tivity. Gothic sample 5390.8B displays frequent relatively larger pore volumes with no connec-

tivity, but also a few smaller pore volumes with higher connectivity, with coordination numbers of 1 and 2.

For the upper Kirtland sample, connectivity may be governed by the touching of tips of the elongate type I pores that are similarly oriented (Figs. 3C, 3D). The lower Kirtland, and area 1 of the lower Tuscaloosa Group (Figs. 13B, 13C) have dominant fracture or stylolitic-like pores, types V and VI, which have peak coordination numbers of 2 and 3. The pores in organics for the Marine Tuscaloosa 7925.5 sample (Fig. 13E) are dominated by unconnected pores. The up-to-the-right diagonal trend on the joint plot of the Gothic 5390.8A sample probably indi-cates a scaling relationship between connectiv-ity and pore body size, but this is not explored further here.

MIP

Graphs of cumulative mercury saturation ver-sus pressure or Washburn pore radii for the 10 samples, corrected for closure pressure, have a range of shapes that cover approximately two orders of magnitude of pressure (Figs. 14 and 15), refl ecting a broad distribution of pore throat sizes as based on the cylindrical “bundle of tubes” model (Diamond, 2000). As seen in Fig-ure 12, however, the pore-throat-to-body ratio indicates that pores are not cylindrical tubes (for most samples), so that applying the bundle of tubes model is a simplifi cation at best.

Upper Kirtland curves differ the most from the other samples in terms of shape and range of pressure values. There is a broad shoulder of lower pressures possibly due to lacunar pores

B

0.415% porosity

A

E

F

C

D

G

organic phase vol.fractio

and poren 15.19%

Figure 9. Results of focused ion beam (FIB) milling and imag-ing of sample Gothic shale 5390. and three-dimensional (3D) pore network and organic reconstruction. (A) Backscat-tered electron (BSE) image of FIB serial section. Horizontal fi eld of view is 16.00 μm. Out-lined box indicates cubic region of 3D pore network and organics reconstruction, which has side lengths of 4.66 μm. White, circu-lar region is a pyrite framboid. (B) Segmentation of BSE image into organics and pores (black) and nonorganics and nonpores (white). (C) 3D fl ood fi ll render-ing of both the organic phase and pores in red. Voxel size is 15.6 nm. (D) Segmentation of BSE image into pore (black) and nonpore (white). (E) 3D fl oodfi ll rendering of pores in cyan. (F) 3D plot of medial axis. (G) 3D plot of pore throat sur-faces.




associated with silt- to sand-sized grains (see Fig. 2; Fies, 1992). The Gothic shale curves have the highest pressure values for the corre-sponding mercury saturations and the highest breakthrough pressure (these are compiled for all samples in Table 2), whereas upper Kirtland samples have the lowest values (see discussion of Pore Networks and Sealing Quality).

Figure 15 compiles MIP data for the suite of mudstone samples. Linear relationships in log-log space exist for portions of the cumula-tive volumetric pore-density distributions (Figs. 15B, 15D, 15F), suggesting power law scaling of pore throat sizes, in possible agreement with relationships in the FIB histograms (Fig. 12). Incremental volumetric pore density distribu-tion curves (the slope of the cumulative mercury intrusion curves; Figs. 15B, 15D, 15F) are uni-modal for the Kirtland, Gothic, and one Marine

Tuscaloosa sample. The other Marine Tusca-loosa and both the lower Tuscaloosa Group samples show bimodality. All modes occur at <10 nm (Figs. 15B, 15E, 15H).

Comparison of MIP and FIB-SEM Pore Properties

As MIP and FIB data sampled different num-bers of pore bodies, a direct comparison of the two data sets in terms of pore volumes is not possible. MIP data do not give direct informa-tion on pore body sizes (Meyer and Klobes, 1999). Thus, we present the two data sets in a manner more conducive to investigate scaling relationships, via a different type of pore size distribution (PSD) plot. Figure 16 presents MIP data with y and x axes given by the following, respectively:

Hg-ii

Hg-total

sorted V

V

Δ∑ (1)

Hg-isorted VΔ , (2)

where ΔVHg-i is the sorted ith incremental mer-cury intrusion volume for a pressure step; the incremental volumes were sorted from largest to smallest volumes; VHg-total is the total intruded mercury volume. The new PSD plots cumulative sorted incremental volumes per total volume, a nonstandard form of saturation, by the corre-sponding incremental volumes. Thus, only pore volume information is presented in the plot with-out explicit reference to entry throats. FIB data are plotted by similarly sorting the FIB-derived pore body volumes from largest to smallest,

Figure 10. Results of focused ion beam (FIB) milling and imaging of sample Gothic shale 5390.8B and three-dimensional (3D) pore network and organic reconstruction. (A) Backscat-tered electron (BSE) image of FIB serial section. Horizon-tal fi eld of view is 16.00 μm. Outlined box indicates cubic region of 3D pore network and organics reconstruction, which has side lengths of 4.66 μm. White, euhedral shapes near the left-center region of the image are pyrite minerals. (B) Seg-mentation of BSE image into organics and pores (black) and non organics and nonpores (white). (C) 3D fl ood fi ll render-ing of both the organic phase and pores in red. Voxel size is 15.6 nm. (D) Segmentation of BSE image into pore (black) and nonpore (white). (E) 3D fl oodfi ll rendering of pores in cyan. (F) 3D plot of medial axis. (G) 3D plot of pore throat sur-faces.

B

porosity1.33%

A

E

F

C

D

G

organic phase vol.fractio

and poren 5.59%


Heath et al.


Typ

e I

Elo

ngat

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sim

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orie

nted

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–0.7

–0.7

–0.7

–0.7

–0.7

–0.7

–0.7

–0.7

–1.4

–1.4

–1.4

–1.4

–1.4

–1.4

–1.4

–1.4

uppe

rm

embe

r20

49.7

B

low

erm

embe

r26

92.9

A

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erT

usca

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erT

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are

a 2

Mar

ine

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osa

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ic

Mar

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Tus

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osa

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For

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Tus

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osa

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5390

.8A

5390

.8B

Relative Frequency

log 1

0[p

v (nm

3 )]

46.

258.

50

0.21

0.42

n=78

46.

258.

5

n =

33

46.

258.

5

n =

67

46.

258.

5

n=36

46.

258.

5

n=15

46.

258.

5

n =

32

46.

258.

5

n =

18

Relative Frequency

log 1

0(r t/r

b)

46.

258.

5

n=88

Relative Frequency

00

0.350.7

n =

49

0

n =

107

0

n =

43

0

n =

53

0

n =

10

0

n =

8

0

n =

85

0

n =

3

log 1

0[t a (

nm2 )]

2.2

4.3

6.4

0

0.14

0.28

0.42

n =

96

2.2

4.3

6.4

n =

55

2.2

4.3

6.4

n =

135

2.2

4.3

6.4

n =

113

2.2

4.3

6.4

n =

27

2.2

4.3

6.4

n =

9

2.2

4.3

6.4

n =

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2.2

4.3

6.4

n =

28

A B C

Fig

ure

12. M

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f re

lati

ve f

requ

ency

his

togr

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of p

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(ta)

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.


Heath et al.


dividing by the total volume of all the FIB-derived pore bodies, and then plotting cumula-tive pore body volumes versus the corresponding sorted pore body volume. The y and x axes for the new FIB PSD are given by, respectively:

v-ii

v-total

sorted p

p

∑ (3)

v-isorted p , (4)

where pv-i is the ith sorted pore body volume, and pv-total is the summation of all the pore body volumes .

Closure pressure corrections probably do not affect the comparison of the MIP and FIB data in terms of the slope of the PSD curves. The same closure volume of mercury was subtracted from all ΔVHg-i values of equation 1; thus, the slope of the MIP PSD curve would not change. Closure corrections affect the calculation of the largest pore throats of the pore throat size dis-tributions (given in MIP discussion). Induced pore bodies (i.e., not found at depth under in situ conditions) include features such as cracks and/or fractures due to drying out of the samples or unloading. Both MIP and FIB estimates can include these induced pores, but we have made no attempt to remove possible induced pores from the FIB-SEM pore body data.

The slopes of the linear portions of the MIP and FIB PSD curves for sample upper Kirt-land 2049.7B are similar, –0.359 and –0.394, respectively. This suggests that the MIP and FIB sampled similar pore bodies, and that they both exhibit power law scaling. The scaling probably relates to the slit- and/or sheet-like type I pores that dominate the FIB sample. The lower Kirt-land 2692.9A has different slopes for the MIP and FIB data, suggesting that the FIB did not sample a representative pore type (Fig. 16A). Correspondence between FIB and MIP slopes for the Tuscaloosa Group is low, probably due to the low number of pores sampled in the FIB data sets. The Gothic shale MIP PSDs have unique shapes, probably resulting from the extremely high capillary pressures, which may be due to pore structure alteration during the MIP test. In summary, the small volumes imaged by FIB-SEM were suffi cient to capture representa-tive pore statistics as seen by MIP analysis only in the case of the upper Kirtland.

PORE NETWORKS AND SEALING QUALITY

Mudstone textural properties (i.e., pore and grain size, shape, connectivity) can be subdi-vided into four categories of infl uence: type of

framework (i.e., framework supported or matrix supported); degree of diagenesis (e.g., pores are fi lled with cement, secondary, or created by dissolution); sorting (e.g., well sorted or poorly sorted, relative abundance of silt and clay); and compaction (e.g., pore volume loss through compaction processes) (Katsube and Willam-

son, 1994). Textural parameters that vary as a function of these categories are typically macro-scopically measured (i.e., effective porosity, permeability, tortuosity, and specifi c surface area) (Katsube and Willamson, 1994). From our 3D imaging of a variety of mudstone types, it is evident these parameters are macroscopic

Kirtland FormationUK 2049.7B LK 2692.9A

LT 8590 area 2LT 8590 area 1Tuscaloosa Group

A B

C D

4.4 6.2 8.0

log10

[pv (nm3)]

Rel

ativ

eF

requ

ency

4.5 6.1 7.7log

10[p

v (nm3)]

Rel

ativ

eF

requ

ency

4.3 5.9 7.6

log10

[pv (nm3)]

Rel

ativ

eF

requ

ency

4.4 5.9 7.5

log10

[pv (nm3)]

Rel

ativ

e F

requ

ency

0

0.21

0.42

0 0.21 0.42

0

7

Coo

r. N

o.

RelativeFrequency

0 0.1 0.2

JointRelative Frequency

n = 78

0

0.21

0.42

0 0.21 0.42

6

Coo

r. N

o.

RelativeFrequency

0 0.1 0.2

Joint Relative Frequency

n = 33

0

0.21

0.42

0 0.21 0.420

11

Coo

r. N

o.

RelativeFrequency

0 0.1 0.2Joint

Relative Frequency

n = 67

0

0.21

0.42

0 0.21 0.420

7

Coo

r. N

o.

RelativeFrequency

0 0.1 0.2Joint

Relative Frequency

n = 88

0

Figure 13 (on this and following page). A–D. Joint relative frequency histograms of pore body volume and coordination number (Coor. No.), with marginal relative frequency histo-grams for the Kirtland and Tuscaloosa samples. The series of joint plots are presented in order from proximal to distal in terms of depositional environment. Binning for the coor dination number is such that the number of bins is one plus the maximum number of coordi nation for a particular sample. This results in bins containing integer values. The fi rst horizontal bin, starting from the bottom, is for a frequency of zero. The second horizontal bin is for frequency of two, and so on. The last bin is for the frequency of the maximum coordination number. For example, UK 2049.7B had a maximum coordination number of seven, and thus it has eight bins. Plotting of values on the ordinate is such that the maximum value of the ordinate corresponds to the maximum coordination number, which is for the top horizontal bin.




manifestations of the collection of pore body types (described in Summary of Pore Types) and the connecting pore throat radii such as imaged in Figures 3–10.

Although a full comparison of imaged pore types and macroscopic properties is beyond the scope of this paper (due to the complexity of pore types and the large degree of heterogene-ity of the rocks at all scales), it is valuable to examine depositional controls (i.e., framework and sorting) and diagenetic controls (cementa-tion, compaction, and dissolution) on the pore networks as well as MIP-estimated sealing effi -ciency. This information is directly applicable to subsurface CO

2 storage, as an understanding of

the sealing processes could enable proper design of a CO

2 sequestration project with regard to

performance of a proposed mudstone seal. In addition, it is applicable to other studies on multi phase fl ow in low permeability rock, such as efforts in recovering hydrocarbons from shale gas reservoirs. For our purposes, we equate seal-ing effi ciency with MIP breakthrough pressures (see discussion of MIP in Results).

Framework and sorting correlate with depo-sitional environment. Poorly sorted mudstones with a high framework-supporting silt content exist proximal to sediment sources and would be expected to have pore types and resulting macroscopic properties much different from more well-sorted clay-rich mudstones occurring more distal to sediment sources. In Figure 17, we place our mudstone samples into the shale lithofacies depositional succession devised by Schieber (1999); the lithofacies designations, as well as those discussed by Almon et al. (2005), are given in Table 3. Dominant pore types as imaged by FIB-SEM are also listed in Table 3.

The mudstone-rich portions of the Kirtland Formation, interpreted as a fl oodplain, over-bank, and crevasse-splay lithofacies, refl ect deposition in shallow water depths (~1–2 m), proximal to sediment source. With abun-dant mottled texture and pedogenic features, it is equivalent to the red-gray mudstones of Schieber (1999). It represents the most proxi-mal to source and most poorly sorted of the mudstones investigated. Note that some of the clay fabric is suggestive of compacted ana-logues of the clay fl occule textures discussed by Schieber et al. (2007); consequently, some cau-tion is required when comparing depositional paleoenvironments of muddy sediments. The high silt content is likely responsible for the low breakthrough pressures (Tables 2 and 3), which are likely due to possible lacunar pores occur-ring in silt-clay mixtures. The higher break-through pressures are probably a macroscopic manifestation of abundant networks of type I pores observable in Figure 3.

Figure 13 (continued). E–H. Joint with marginal relative frequency histograms for the Tusca-loosa Group and Gothic shale.

Tuscaloosa GroupMT 7925.5 organic MT 7925.5 inorganic

GS 5390.8BGS 5390.8AGothic shale

E F

G H

5.7 6.4 7.0

log10

[pv (nm3)]

Rel

ativ

e F

requ

ency

4.6 6.2 7.7

log10

[pv (nm3)]

Rel

ativ

e F

requ

ency

4.7 6.0 7.3

log10

[pv (nm3)]

Rel

ativ

e F

requ

ency

5.1 6.4 7.6

log10

[pv (nm3)]

Rel

ativ

eF

requ

ency

0

0.21

0.42

0 0.21 0.42 0.7

0

3

Coo

r. N

o.

RelativeFrequency

0 0.1 0.2


n = 36

0

0.21

0.42

0 0.21 0.42 0.7

0

4

Coo

r. N

o.

RelativeFrequency

0 0.1 0.2


n = 15

0

0.21

0.42

0 0.21 0.42

6

Coo

r. N

o.

RelativeFrequency

0 0.1 0.2


n = 32

0

0.21

0.42

0 0.21 0.42

0

4C

oor.

No.

RelativeFrequency

0 0.1 0.2


n = 18

0

100

UK 2049.7A

GS 5390.8BGS 5390.8AMT 7931.9MT 7925.5LT 8590.9LT 8590LK 2692.9BLK 2692.9AUK 2049.7B

80

60

40

20

Hg Injection Pressure (MPa)

ΣΔS

HG (

%)

0

10–2 10–1 100 101 102

Figure 14. Cumulative mercury satu-ration (ΣΔΣΔSHg) versus intrusion pres-sure for all mudstone samples. Curves were corrected for closure pressure. UK, LK, LT, MT, and GS stand for upper Kirtland, lower Kirtland, lower Tuscaloosa Group, Marine Tusca-loosa, and Gothic shale, respectively. Curve shapes indicate differences in pore structure and sealing properties.


Heath et al.


Kirtland Formation

A

B

10–2 10–1 100 101 102

Hg Injection Pressure (Mpa)

0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Lower Kirtland 2692.9A

10–2 10–1 100 101 102


0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Lower Kirtland 2692.9B ft

100 101 102 103

rt (nm)

0

0.04

0.08

0.12

0.16

0.2

0.24

ΔSH

g/Δ

r(n

m–1

)

LK 2692.9A

LK 2692.9B

10–2 10–1 100 101 102


0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Upper Kirtland 2049.7A

10–2 10–1 100 101 102


0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Upper Kirtland 2049.7B

100 101 102 103

rt (nm)

0

0.04

0.08

0.12

0.16

0.2

0.24

ΔSH

g/Δ

r(n

m–1

)

UK 2049.7A

UK 2049.7B

100 101 102 103

rt (nm)

10–1

100

101

102

ΣΔS

Hg

UK 2049.7A

UK 2049.7B

100 101 102 103

rt (nm)

10–1

100

101

102

ΣΔS

Hg

LK 2692.9A

LK 2692.9B

Tuscaloosa Group

C

D

10–2 10–1 100 101 102


0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Lower Tuscaloosa 8590

10–2 10–1 100 101 102


0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Lower Tuscaloosa 8590.9

100 101 102 103

rt (nm)

0

0.04

0.08

0.12

0.16

0.2

0.24

ΔSH

g/Δ

r(n

m–1

)

LT 8590

LT 8590.9

10–2 10–1 100 101 102


0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Marine Tuscaloosa 7925.5

10–2 10–1 100 101 102


0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Marine Tuscaloosa 7931.9

100 101 102 103

rt (nm)

0

0.04

0.08

0.12

0.16

0.2

0.24

ΔSH

g/Δ

r(n

m–1

)

MT 7925.5

MT 7931.9

100 101 102 103

rt (nm)

10–1

100

101

102

ΣΔS

Hg

MT 7925.5

MT 7931.9

100 101 102 103

rt (nm)

10–1

100

101

102

ΣΔS

Hg

LT 8590

LT 8590.9

lower lower

Figure 15 (on this and following page). Mercury intrusion porosimetry (MIP) saturation curves and cumulative and incremental volumet-ric pore density distribution (VPD) curves, organized by formations and samples. (A, C, and E) Cumulative mercury saturation (ΣΔΣΔSHg) versus injection pressure for the Kirtland Formation, Tuscaloosa Group, and the Gothic shale, respectively. Raw (before closure) and clo-sure-pressure–corrected (after closure) data are given. (B, D, and F) Cumulative and incremental volumetric pore density distributions, where rt represents the radius from the bundle of tubes (see text) model. The y axis of the cumulative VPD was plotted on a logarithmic scale for evaluation of possible power law distributions. The incremental VPD was plotted on a linear scale for the y axis to emphasize the modal peaks.




The lower Tuscaloosa Group, interpreted as a bioturbated shallow shelf deposit, represents deposition in ~10 m of water (Billingsley, 1980). With abundant burrowed features and lamina-tions, it is probably equivalent to the moderately bioturbated graded mudstone lithofacies of Schieber (1999). Breakthrough pressures for the lower Tuscaloosa Group are between the two breakthrough pressure values for the Kirtland, refl ecting connectivity via a network of type I and type IV pores. The existence of type IV (microfracture) pore types probably enhances connectivity over the poorly connected network of type I pores. The formation of some type II pores (Fig. 6A) may be due to the development of authigenic chlorite.

The more shelfal Marine Tuscaloosa Forma-tion refl ects a deeper water environment, being deposited during the transgressive-regressive cycle (T-R K5) maximum regression during the Late Cretaceous (Mancini and Puckett, 2005). Being well laminated and organic rich, the Marine Tuscaloosa samples are probably equivalent to the laminated mudstone lithofacies of Schieber (1999) or microfacies 2 (organic laminated shales) of Almon et al. (2005). The relative lack of a connected network of type I pores probably is responsible for the high break-through MIP pressures.

The Gothic shale is an open-marine, euxinic , massive, black, organic-rich mudstone, which

was probably deposited below wave base in water at least tens of meters deep (Fig. 16; Goldhammer et al., 1994). It contains the high-est TOC of the mudstone types we examined. As such, it is the closest to belonging to the classic black shale lithofacies, equivalent to the carbonaceous mudstone lithofacies of Schieber (1999) or microfacies 1 (massive organic shale) of Almon et al. (2008). The dominant pore types are largely contained in organics (type IVb), which are dominantly located in the compac-tion shadows between larger quartz, feldspar, or other clasts or grains (Fig. 9). The small amount of porosity and very poor connectivity account for the highest breakthrough pressures of the studied mudstones (Table 3).

The maturity of organic matter may be responsible for the differences in pore types of the organic phases in the high TOC mudstones (type IVa in the Marine Tuscaloosa and type IVb in the Gothic shale). The Marine Tusca-loosa organics are at a low level of conversion, whereas the Gothic shale organics are mature. Pore type IV may factor in the generation and migration of petroleum in these rock types (Loucks et al., 2009). The Gothic shale appears to have undergone the greatest amount of com-pactional pore volume loss. That, and the seem-ing lack of type I pores, is responsible the high breakthrough pressure values for the Gothic shale (Table 3).

In summary, pore type I and a lack of well-connected pore types V and IV result in high breakthrough pressures for lower Kirtland, lower Tuscaloosa Group, and Marine Tusca-loosa samples. High breakthrough pressures for the Gothic shale are due to organics fi lling what otherwise might have been larger, connected pores. Pore type II, based on the evidence of its localized nature in lower Tuscaloosa Group samples, may be involved in the high break-through pressures as mercury intrudes from surrounding slit pores in the clay fabric. Based on matrix pore types imaged herein, we suggest that more distal mudstones make better sealing lithologies (i.e., caprocks) as a result of their higher breakthrough pressures.

CONCLUSIONS

The 3D visualization and quantitative analysis of small (101.5 μm3) samples of fi ve mudstones from continental to deep-marine depositional settings revealed seven distinct pore types. The most dominant pore type is a slit- and/or sheet-like pore in clay-rich mudstones, designated type I following Desbois et al. (2009). It is elon-gate with sharp to curved tips and is located with parallel clay sheets. The associated coordination numbers of typically 1–4 are governed by the sheet-like geometry. Small pore throat connec-tions between these pores limit conductivity and contribute to high (mercury) breakthrough pres-sures and concomitant high sealing capacity. These pore sizes scale over approximately an order of magnitude, possibly as a power law, as illustrated by MIP and FIB data (Fig. 15). Pore type I is expected in all argillaceous mudstones and may be predominant in mudstones with abundant expandable clays, as in the Kirtland Formation. The extent to which pore type I mor-phology is infl uenced by coring and concomi-tant depressurizing and possible dehydration is unclear; however, these pores may indicate the possibility for pore formation or enlargement if desiccation and/or dewatering in the subsurface were to occur (e.g., during injection of dry CO

2).

Other pore types, such as types II (non-slit-like pores in clays) and III (jagged pores in compaction shadows), are associated with authigenic clay minerals and deformation (com-paction shadows). They probably contribute to increased mercury breakthrough pressure if they result in an overall reduction in pore throat sizes (chlorite as a pore-fi lling mineral). Secondary (diagenetic) microfractures and stylolitic fea-tures (pore types V and VI) may not be present in situ; however, as imaged by the FIB and TEM (Heath, 2010), they may still refl ect preexisting features in the subsurface that may respond to changes in stress, as a perturbation due to CO

2

Gothic Shale

E

F

10–2 10–1 100 101 102


0

20

40

60

80

100ΣΔ

SH

g

After closure

Before closure

Gothic 5390.8A

10–2 10–1 100 101 102


0

20

40

60

80

100

ΣΔS

Hg

After closure

Before closure

Gothic 5390.8B

100 101 102 103

rt (nm)

0

0.09

0.18

0.27

0.36

0.45

0.54ΔS

Hg/Δ

r(n

m–1

)GS 5390.9A

GS 5390.9B

100 101 102 103

rt (nm)

10–1

100

101

102

ΣΔS

Hg

GS 5390.8A

GS 5390.8B

Figure 15 (continued).


Heath et al.


1017

1018

1019

1020

Sor

ted

ΔVH

g(n

m3 )

10–1

100

Σ(SortedΔVHg)/VHg-total

LT85

90

LT85

90.9

Slo

pe1

=–1

.07

R2

=0.

904

Slo

pe1

=–0

.968

R2

=0.

925

105

106

107

108

Sor

ted

p v(n

m3 )

10–1

100

Σ(Sortedpv)/pv-total

FIB

LT85

90ar

ea1

FIB

LT85

90ar

ea2

Slo

pe

1=

–0.5

96R

2=

0.91

9

Slo

pe1

=–0

.375

R2

=0.

962

1017

1018

1019

1020

Sor

ted

ΔVH

g(n

m3 )

10–1

100


MT

7925

.5

MT

7931

.9

Slo

pe1

=–1

.40

R2

=0.

867

Slo

pe2

=–0

.817

R2

=0.

942

105

106

107

108

Sor

ted

p v(n

m3 )

10–

1

100


FIB

MT

7925

.5o

FIB

MT

7925

.5i

Slo

pe

1=

–1.6

1R

2=

0.94

3

Slo

pe

2=

–0.3

95R

2=

0.82

2

Tu

sca

loo

saG

rou

p

105

106

107

108

Sor

ted

p v(n

m3 )

10–1

100


FIB

UK

2049

.7B

Slo

pe

=–0

.394

R2

=0.

951

105

106

107

108

Sor

ted

p v(n

m3)

10–1

100


FIB

LK26

92.9

AS

lope

=–0

.439

R2

=0.

991

1017

1018

1019

1020

Sor

ted

ΔVH

g(n

m3 )

10–1

100


LK26

92.9

A

LK26

92.9

B

Slo

pe1

=–0

.220

R2

=0.

850

Slo

pe2

=–0

.514

R2

=0.

963

1017

1018

1019

1020

Sor

ted

ΔVH

g(n

m3 )

10–1

100

Σ(SortedΔVHg)/VHg-totalU

K20

49.7

A

UK

2049

.7B

Slo

pe

1=

–0.3

59R

2=

0.96

4

Slo

pe2

=–0

.309

R2

=0.

940

Go

thic

Sh

ale

Kir

tla

nd

Fo

rma

tio

nA

1017

1018

1019

1020

Sor

ted

ΔVH

g(n

m3 )

10–1

100


GS

5390

.8A

GS

5390

.8B

105

106

107

108

Sor

ted

p v(n

m3)

10–

1

100


FIB

GS

5390

.8A

FIB

GS

5390

.8B

Slo

pe1

=–1

.09

R2

=0.

880

Slo

pe

2=

–0.4

13R

2=

0.92

8

B CF

igur

e 16

. Sor

ted

cum

ulat

ive

volu

met

ric

dist

ribu

tion

s (S

VP

D)

base

d on

mer

cury

in

trus

ion

poro

sim

etry

(M

IP)

and

focu

sed

ion

beam

(F

IB)

pore

net

wor

k m

odel

s.

The

gre

en a

nd b

lue

lines

rep

rese

nt M

IP d

ata,

whe

reas

red

lin

es r

epre

sent

FIB

da

ta. T

he e

quat

ions

use

d fo

r th

e SV

PD

s ar

e pl

otte

d by

the

ordi

nate

s. F

or m

ercu

ry,

the

volu

mes

of

mer

cury

intr

uded

per

pre

ssur

e st

ep (

ΔVH

g) w

ere

sort

ed f

rom

larg

-es

t to

sm

alle

st, a

nd t

hen

sum

med

incr

emen

tally

fro

m la

rges

t to

sm

alle

st. V

Hg-

tota

l is

the

tota

l vol

ume

of m

ercu

ry in

trud

ed a

t th

e en

d of

a M

IP t

est.

The

por

e bo

dy v

ol-

umes

(p v

) as

det

erm

ined

fro

m F

IB-b

ased

thr

ee-d

imen

sion

al p

ore

netw

ork

mod

els

wer

e si

mila

rly

sort

ed f

rom

lar

gest

to

smal

lest

, an

d th

en s

umm

ed s

tart

ing

wit

h th

e la

rges

t po

re b

ody.

Pv-

tota

l is

the

tota

l vo

lum

e of

all

the

pore

bod

y vo

lum

es f

or

a si

ngle

sam

ple.

(A

) L

ower

and

upp

er K

irtl

and

data

. (B

) Tu

scal

oosa

Gro

up d

ata.

(C

) G

othi

c sh

ale

data

.




Facies RM

Facies SMFacies BM

Facies GM

Facies LM

Facies CM

very minor bioturbation

sea level

overbank/flood deposits;

soil formation

minor bioturbatio

n

moderate bioturbation

gravity-driven

suspensions

stormwave base

Gothic shale

Marine TuscaloosaGroup

lower TuscaloosaGroup

upper Kirtland Fm

lower Kirtland Fm

mercury breakthrough

pressure (MPa)

storm-induced offshore currents

109100

6656

6629

5647

11

proximal

distal

Figure 17. Shale succession facies distributions (modifi ed from Schieber, 1999) and schematic placement of mudstone types studied in this paper with breakthrough pressures from mercury intrusion porosimetry (see Table 3). The Schieber (1999) microfacies are: RM—red-gray coastal plain mudstones (Kirtland Formation); SM—sandy nearshore mudstones; BM—bioturbated offshore mudstones; GM—moder-ately bioturbated gray mudstones (probably equivalent to the lower Tuscaloosa Group); LM—laminated organic rich mudstones (Marine Tuscaloosa); CM—carbonaceous mudstones or so-called black shales (e.g., the Gothic shale). The bars in gray and black represent two separate mercury intrusion porosity measurements for samples from approximately the same depth.

TABLE 2. CLOSURE AND BREAKTHROUGH PRESSURES AND PORE RADII

maebnoidesucoFyrtemisoropnoisurtniyrucreM

Sample identifi cationDirectionalintrusion

Closurepressure

(MPa)

Closurepore-throat

radius(μm)

Breakthroughpressure

(MPa)

Breakthroughpore-throat

radius(μm)

Secondbreakthrough

pressure*(MPa)

Secondbreakthrough

pore-throat radius*(µm)

Largest pore-throat radius in pore network

(μm) 110.0672.66282.1375.0483.1135.0A7.9402dnaltriKreppu

upper Kirtland 2049.7B x 0.959 0.767 1.221 0.602 92.450 0.008 0.19276164.0310.0411.65620.0248.82xA9.2962dnaltriKrewol

610.0494.74420.0175.03B9.2962dnaltriKrewol7871.0110.0962.66810.0422.04x0958asoolacsuTrewol

lower Tuscaloosa 8590.9 x 17.492 0.042 28.843 0.026Marine Tuscaloosa 7931.9 56.107 0.013 66.272 0.011

3170.0310.0421.65710.0827.345.5297asoolacsuTeniraM †, 0.0645§

9901.0700.0884.001110.0172.66A8.0935elahscihtoGGothic shale 5390.8B 72.026 0.010 109.208 0.007 0.1607

Note: x indicates that directional mercury intrusion was performed on a jacketed plug. Omni-directional mercury intrusion was performed on samples not marked with an x.*The bimodal pore structure of the upper Kirtland samples has an initial breakthrough into large pores (lacunar) and a secondary breakthrough into smaller pores.†Organic area.§Inorganic area.


Heath et al.


injection or hydraulic fracturing for shale gas production is applied to the system. Type VII, the pores in pyrite framboids, do not have strong connectivity to other pore types outside of the framboids.

Pore types IVa and IVb are examples of pores in organic phases. It is intriguing that many mudstones could have transport path-ways that are both water (clay-lined pores) and oil (organics lined) wetting. These pores in our studied samples, however, are small and gener-ally unconnected for the Marine Tuscaloosa and Gothic shale mudstones examined here. In addi-tion, these limited pore networks do not have the scaling relationships of the pores in clay-rich portions of mudstones.

Depositional environment and burial his-tory are both strong geologic controls on pore network properties that bear on the ability of mudstones to serve as sealing lithologies for subsurface CO

2 storage. More distal, organic-

rich mudstone lithofacies may make the best seals, barring any deleterious effect associated with CO

2-organic interactions. Understanding

how pore network topology and geometry vary as a function of depositional environment would provide an important starting point for predict-ing sealing behavior of mudstones. On the basis of our data set, we cannot separate depositional environment, burial history, and diagenesis independently on the basis of their effects on sealing effi ciency, although a clear infl uence of there on pore types and connectivity has been demonstrated Unraveling geologic controls on sealing effi ciency of mudstones is a relevant topic for future research, given that a priori per-formance of mudstone caprocks for carbon stor-age may be needed in portions of basins lacking in subsurface characterization.

ACKNOWLEDGMENTS

Focused ion beam–scanning electron micros-copy (FIB-SEM) imaging and subsequent three-dimensional image analysis was funded by the U.S. Department of Energy (DOE) Offi ce of Basic Energy Sciences , Division of Chemical Sciences, Geosci-ences, and Biosciences. Rock samples were provided by the Southwest (SWP) and Southeast Regional Car-bon Sequestration Partnerships, which are managed by the DOE National Energy Technology Labora-tory. The SWP funded mercury intrusion porosimetry (MIP) analyses. We also thank the Southeast Regional Carbon Partnership and Richard Esposito of the Southern Company for making the Tuscaloosa cores from the Mississippi Power Company #1 available for viewing and core description, as well as for providing rock samples and access to core data.

We thank Joseph Michael and Michael Rye of the Materials Characterization Department at Sandia National Laboratories for performing FIB serial sec-tioning and image acquisition. John Neasham of Poro-Technology performed MIP measurements. TerraTek, a Schlumberger company, aided in petrographic work on Kirtland Formation and Gothic shale thin sections. TA

BLE

3. S

UM

MA

RY

OF

PE

TR

OP

HY

SIC

AL,

BU

RIA

L H

IST

OR

Y, F

AC

IES

TY

PE

S, A

ND

PO

RE

NE

TW

OR

K P

RO

PE

RT

IES

FO

R T

HE

SU

ITE

OF

MU

DS

TO

NE

S.

Sam

ple

ID*

Cor

ede

pth

(m)

Max

imum

buria

lde

pth

(m)

Wat

er

dept

h of

depo

sitio

n(m

)P

oros

ity

(%)

Per

mea

bilit

y###

(×10

–20

m2 )

TO

C(w

t%)

Mer

cury

brea

kthr

ough

pres

sure

(MP

a)

Mer

cury

brea

kthr

ough

pore

-thr

oat

radi

us(μ

m)

Dom

inat

e F

IBpo

re ty

pes

Hig

hest

freq

uenc

y(n

on-z

ero)

coor

dina

tion

num

ber*

***

Fac

ies

desi

gnat

ion†

Alm

on e

t al.

(200

5)S

chie

ber

(199

9)U

K 2

049.

7A62

4.7

1633

1 or

25

RM

6.3–

9.9§

7–10

§,††

†0.

06–0

.13§

11.

282

N.A

.N

.A.

UK

204

9.7B

10.

602

I1

and

2LK

269

2.9A

820.

818

291

or 2

5 (o

r 4?

)R

M o

r S

M?

6.5–

6.7#

8#0.

06–0

.27#

560.

013

V2

and

3LK

269

2.9B

470.

016

N.A

.N

.A.

LT 8

590

2618

.526

18te

ns3

GM

8.

5–9.

0**

0.00

4–0.

5##, 8

0–68

10**

1.06

–1.2

3†††

660.

011

I, II,

and

IV2

and

3; 1

LT 8

590.

926

18.2

2619

290.

026

N.A

.N

.A.

MT

792

5.5

2415

.724

16te

ns1

or 2

CM

to L

M2.

2–9.

9††0.

8–1*

**, 1

0–38

0††0.

56–0

.73§§

§56

0.01

3I a

nd IV

a2;

1M

T 7

931.

924

17.6

2418

660.

011

N.A

.N

.A.

GS

539

0.8A

1643

.130

48se

vera

l ten

s to

35?

1C

M2.

7–4.

3§§13

–14§§

2.2–

4.4§§

100

0.00

7IV

b (a

nd I,

III,

and

IVb)

3G

S 5

390.

8B10

90.

007

IVb

(and

I, II

I, an

d IV

b)1

Not

e: N

.A.:

not a

pplic

able

; TO

C: t

otal

org

anic

car

bon.

*Ide

ntifi

catio

ns: U

K, L

K, L

T, M

T, a

nd G

S s

tand

for

uppe

r an

d lo

wer

Kirt

land

, low

er a

nd M

arin

e Tu

scal

oosa

Gro

ups,

and

Got

hic

shal

e, r

espe

ctiv

ely.

† Sam

ples

are

mat

ched

to m

udst

one

faci

es d

efi n

ed b

y A

lmon

et a

l. (2

005)

and

Sch

iebe

r (1

999)

.§ M

easu

red

on fi

ve U

K c

ore

sam

ples

ove

r th

e de

pth

rang

e of

624

.3 to

630

.3 m

.# M

easu

red

on tw

o LK

cor

e sa

mpl

es fr

om d

epth

s 82

0.6

and

822.

1 m

.**

Mea

sure

d on

thre

e LT

sam

ples

ove

r th

e de

pth

rang

e of

261

6.5

to 2

618.

2 m

.††

Mea

sure

d on

six

MT

sam

ples

ove

r th

e de

pth

rang

e of

241

3.6

to 2

418.

4 m

.§§

Mea

sure

d on

four

G s

ampl

es o

ver

the

dept

h ra

nge

of 1

639.

6 to

164

3.1

m.

##M

easu

red

on th

ree

LT s

ampl

es o

ver

the

dept

h ra

nge

of 2

615.

7 to

261

7.7

m.

***M

easu

red

on th

ree

MT

sam

ples

ove

r th

e de

pth

rang

e of

241

2.2

to 2

417.

2 m

.††

† Mea

sure

d on

LT

sam

ples

ove

r th

e de

pth

rang

e of

261

5.7

to 2

617.

7 m

.§§

§ Mea

sure

d on

MT

sam

ples

ove

r th

e de

pth

rang

e of

241

2.2

to 2

417.

2 m

.##

# The

fi rs

t set

of n

umbe

rs fo

r th

e LT

and

MT

sam

ples

are

Klin

kenb

erg-

corr

ecte

d pe

rmea

bilit

y. A

ll ot

her

perm

eabi

lity

mea

sure

men

ts fo

r al

l sam

ples

are

pre

ssur

e-de

cay

perm

eabi

litie

s.**

**If

mor

e th

an o

ne fo

cuse

d io

n be

am (

FIB

) po

re n

etw

ork

mod

el w

as c

onst

ruct

ed, p

eak

coor

dina

tion

num

bers

for

the

two

netw

orks

are

sep

arat

ed b

y a

sem

icol

on.




Brent Lindquist provided assistance with installa-tion and use of the 3DMA-Rock software. We thank Fabian Duque-Botero, William Dawson, and William Almon of the Seal and Trap Team at Chevron Energy Technology Company, who shared concepts of per-forming caprock analysis in a sequence stratigraphic framework. Dr. Duque-Botero provided assistance with interpretation of MIP data. We thank Susan Delap Heath for drafting expertise.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corpo-ration, a wholly owned subsidiary of Lockheed Mar-tin Company, for the U.s. Department of Energy’s National Nuclear Security Administration under con-tract DE-AC04-94AL85000.

REFERENCES CITED

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Pore networks in continental and marine mudstones

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