Identification, characterization, and statistical analysis of mudstone aggregate clasts, Cretaceous Carlile Formation, Central Alberta, Canada

Accepted Manuscript Identification, characterization, and statistical analysis of mudstone aggregate clasts, Cretaceous Carlile Formation, Central Alberta, Canada Dallin P. Laycock, Per Kent Pedersen, Benjamin C. Montgomery, Ron J. Spencer PII:

S0264-8172(17)30096-X

DOI:

10.1016/j.marpetgeo.2017.03.012

Reference:

JMPG 2850

To appear in:

Marine and Petroleum Geology

Received Date: 6 August 2016 Revised Date:

16 December 2016

Accepted Date: 9 March 2017

Please cite this article as: Laycock, D.P., Pedersen, P.K., Montgomery, B.C., Spencer, R.J., Identification, characterization, and statistical analysis of mudstone aggregate clasts, Cretaceous Carlile Formation, Central Alberta, Canada, Marine and Petroleum Geology (2017), doi: 10.1016/ j.marpetgeo.2017.03.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Identification, characterization, and statistical analysis of mudstone aggregate clasts, Cretaceous Carlile Formation, Central Alberta, Canada

ABSTRACT

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Dallin P. Laycock, Per Kent Pedersen, Benjamin C. Montgomery, Ron J. Spencer

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While clastic mudstones and shale were traditionally interpreted to have been deposited in quiet water settings, recent flume experiments and studies have shown that mud can be transported in and deposited by traction currents as migrating ripples of mud aggregates.Despite these recent advances, mud aggregates have rarely been adequately described in the rock record.

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These mud aggregates and the sedimentary structures they form in mudstone successions are difficult to observe in the rock record due to compaction, which often obliterates the aggregates and flattens bedforms. This paper documents unambiguously identifiable sand sized mudstone aggregates in thin sections and SEM, transported in traction, and deposited in a series of prograding clinothems. These aggregates were sufficiently indurated to locally preserve shelter porosity, significantly improving the hydrocarbon reservoir properties.

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Grain size analysis of the aggregates was performed on thin sections, as well as disaggregated samples measured by a laser diffraction grain size analyzer for comparison. These analyses showed that sand sized aggregates often comprise more than half of the sediment volume. While the clay-rich composition of the Carlile Formation would suggest that it is a mudstone, statistical analysis of these grain size measurements show that it could alternatively be described as a silty sandstone. These findings potentially change how we think about mudstone classification, fine-grained sedimentation, and mudstone dominated petroleum reservoirs.

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Keywords: Mudstone, Shale, Mud, Clasts, Clinoform, Clinothem, Gas, Aggregates

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INTRODUCTION Mudstones (including shale, siltstone, and claystone) comprise nearly two thirds of the

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sedimentary rock record. They form crucial components of petroleum systems, and can act as source, reservoir, and seal. Despite their abundance and economic significance, relatively little attention has been given to understanding the complex sedimentary processes responsible for their deposition. Misconceptions about the depositional processes involved in mudstone

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deposition have long persisted. Geologists commonly interpret mud-rich facies to have been

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deposited through suspension fallout during periods of quiescence. Recent publications have advanced understanding of heterolithic, thin bedded facies, demonstrating that fine-grained mud dominated sediment can be deposited by a variety of processes (Wilson and Schieber, 2014; Wilson and Schieber, 2015; Li et al., 2015). In addition, other studies have shown that mud can be deposited by energetic currents, forming traction current bedforms (Ellwood Terwindt and

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Breusers, 1982; Pasierbiewicz and Kotlarczyk, 1997; Schieber et al., 2007; Schieber and Southard, 2009; Schieber et al., 2010; Schieber, 2011; Plint, 2014). These studies demonstrate that silt and sand sized aggregates of smaller particles can be transported by traction currents.

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Floccules and other varieties of mud-aggregates are widely observed in a variety of modern environments (e.g. Edzwald and O’Melia, 1975; Ellwood, 1979; Eisma, 1986; Syvitski et al.,

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1995; Wolanski et al., 1995; Eisma et al., 1996; Syvitski and Hutton, 1996; Berhane et al., 1997; Hill et al., 1998; Hill et al., 2001; Winterwerp, 2002; Kessarkar et al., 2010). Although such structures are now well documented in modern and laboratory settings, identification in the rock record is rare and often somewhat ambiguous (Cossey and Ehrlich, 1981; Al-Ramadan et al., 2005; Aplin and Macquaker, 2011; Plint et al., 2012). Compaction and diagenesis may obliterate the aggregated mud particles, making them difficult to identify in the rock record.

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This paper presents a unique dataset from the thick marine mudstone of the Upper Cretaceous Carlile Formation, which forms a relatively shallow biogenic gas reservoir. The abundant mudstone aggregates are larger and less compacted compared to previously documented

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examples (e.g. Cossey and Ehrlich, 1981; Al-Ramadan et al., 2005; Aplin and Macquaker, 2011; Plint et al., 2012). The mudstone aggregates are sufficiently indurated to locally preserve

intergranular porosity between the aggregates, which greatly enhances the reservoir properties of

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this clay-dominated reservoir. These findings likely not only have implications for other

mudstone reservoirs and seals but also for fluid migration during burial. In addition, these

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findings potentially call into question how geologists describe and characterize mudstones as a whole. Wireline log characteristics and mineralogy both suggest that the Carlile Formation is a mudstone, while careful petrographic analysis and grain size statistics show that much of the reservoir is better characterized as a muddy sandstone comprised of silt to sand sized mudstone

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GEOLOGIC SETTING

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aggregates.

The Upper Cretaceous Carlile Formation is a clastic, non-calcareous silty mudstone with variable

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fissility and a light to dark grey appearance in core and outcrop. It ranges from 40-100 m thick within the study area. Deposition within the Wildmere study area in eastern Alberta (Figure 1) occurred on a broad shelf on the western margin of the Western Interior Seaway during the Turonian and early Coniacian in water depths less than 100 m (Nielsen et al., 2003; Nielsen et al., 2008; Plint et al., 2009). The Carlile Formation in eastern Alberta represents the distal offshore equivalent of the shoreface sandstones of the Cardium Formation, deposited during the regressive portion of the Greenhorn transgressive-regressive cycle (Figure 2; Nielsen et al., 2003; 3

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Nielsen et al., 2008). Recent studies identify a series of broad clinothems that characterize the internal architecture of the Carlile Formation (Figure 3; also see Laycock, 2014), and suggest shore parallel to oblique transport of sediment from north to south across the study area. Several

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of the clinothems form shallow gas reservoirs, charged with mainly biogenic gas, at depths of approximately 400-500 m.

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MUDSTONE CLAST TERMINOLOGY

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For the purposes of this study, the term "floccule" refers exclusively to electrochemically bound mud particles (Sherman, 1953; van Olphen, 1963; Guy, 1969; Tsai et al., 1987; Droppo and Ongley, 1994; Fox et al., 2004). "Pellets" and "coprolites" refer to those mud particles bound through biological fecal processes (e.g. Hattin, 1975; Herbig, 1993; Gazdzicki et al., 2000; Senowbari-Daryan et al., 2007; Bujtor, 2011). The terms "aggregate", “composite”, or

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“mudstone aggregate” are used here as the generic term to describe amalgamated clasts of mud particles without reference to their origin. References to “mudstone aggregates” refer to mudrich composite grains in the rock record, and “mud aggregates” refer to mud-rich composite

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grains in modern environments or flume experiments.

METHODS

PETROGRAPHY

Thin section and SEM observations and data were collected to obtain direct measurements and observations of the aggregates being examined, and provide a control dataset for other measurements for other datasets to be compared against. A total of 221 thin sections were

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prepared from samples of 6 cores within the study area. These were analyzed and photographed with a petrographic microscope to classify sedimentary structures, bioturbation, and

under fluorescent light to analyze visible organic material.

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mineralogical abundances. Thin section photomicrographs from 3-33-049-10W4 were observed

Grain diameters of over 6500 mudstone aggregate clasts and individual grains from 4 different representative thin sections were measured on photomicrographs with the use of the image

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analysis software ImageJ. Grain size measurements were initially taken from any individual grain within the measurement area, excluding any visible composite grains. After the composite

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grains were identified, their interpreted outlines were drawn on the photomicrographs in Adobe Illustrator. Minimum diameters of the composite grains were then measured in the same areas and at the same magnification as individual grain measurements to ensure accurate comparison between these datasets.

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Thin sections were later coated with carbon to allow for imaging and chemical analyses through Scanning Electron Microscopy (SEM) and electron microprobe analyses. Thin sections were initially examined on a JEOL JXA 8200 electron microprobe. Backscatter images and element

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maps were collected for locations of interest. Element maps include Si, Ca, C, Mg, S, Na, Fe, K, and Al. Further analysis of carbon coated thin sections was performed at 20 KV by a Quanta

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FEG 250 field emission SEM using a Bruker Quantax acquisition system. Compaction estimates are based on measurements of elliptical silt-filled Planolites burrows, assumed to have originally been circular in shape. 15 burrows were measured for compaction estimates. Image analysis software ImageJ and Adobe Illustrated were used to measure the amount of eccentricity of the burrows. The percentage of eccentricity between the burrows and a circle was used as an estimate for compaction. 5

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STATISTICS

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Statistical analyses were performed on the grain size data measured from thin sections based on methodologies established by Folk and Ward (1957) and refined by Blott and Pye (2001). These were performed in an effort to quantitatively describe the overall impact that the grain size

characteristics of the sediment might have on depositional processes. Classification of grain size

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utilizes the logarithmic Udden-Wentworth grade scale (Udden, 1914; Wentworth, 1922). Grain

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diameter measurements were converted from metric measurements to the logarithmic phi scale to aid in statistical calculations and graphical representations. Conversion from millimeters to phi was done using the following equation:

Φ = −

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Where d represents the grain diameter in mm. Phi values (ϕ) were then used to calculate descriptive statistical parameters for each sample on aggregate grain diameters, individual constituent grain diameters, and diameters from all grain types together. Mean (  ), standard

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deviation (  ), and skewness ( ) were calculated using the following formulae (respectively):

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 =

 =

 +  +  3

 −   −  + 4 6.6

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( +  − 2 ) ( +  − 2 ) + 2( −  ) 2( −  )

DISAGGREGATION AND PARTICLE SIZE ANALYSIS

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 =

The poorly cemented nature of the Carlile Formation allows for relatively easy sample

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disaggregation. Samples were disaggregated to provide additional insight into changes in grain size characteristics vertically through the Carlile succession. A total of 132 samples of

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approximately 1.5 grams each were collected through the entire Carlile Formation from the 3-3349-10W4 core (average sample spacing of 0.5 m). Each sample was disaggregated by two separate disaggregation methods for particle size analysis. Initially, samples were disaggregated using a slightly modified version of the freeze-thaw technique described by Yang and Aplin (1997), which was used to provide a gentle and gradual form of disaggregation. Samples were

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submerged in 10 mL of distilled water in isolated plastic trays and placed in a freezer for at least five hours to ensure that the water was completely frozen. Disaggregation occurred as water percolated into the pore spaces of the rock and expanded during freezing. After completion of the

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freezing process, samples were removed from the freezer and left at room temperature until the

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ice had melted. Samples were then re-inserted in the freezer for subsequent freeze-thaw cycles. Most samples were completely disaggregated after about 10 freeze-thaw cycles, but certain samples required additional cycles to fully disaggregate due to localized zones of increased calcite cementation. All of the samples were subjected to 50 freeze-thaw cycles. The disaggregated samples were then divided in half. One half of each sample was set aside for later analysis. This set is referred to as the “freeze-thaw samples” below. The other half underwent a more rigorous disaggregation by means of ultrasonic agitation similar to the 7

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methodology described by Robinson (2008). This set is referred to as the “ultrasonically disaggregated samples” below. Samples were agitated in approximately 15 mL of water by a Cole Parmer ultrasonic processor and probe with a 13 mm tip and a maximum output of 750

45 seconds in 3 separate intervals of 15 seconds.

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watts. The probe was inserted several millimeters into the water and agitated at 50% intensity for

Grains within both the freeze-thaw and ultrasonically disaggregated samples were measured

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using a Horiba LA 950 particle size analyzer, which uses laser diffraction to measure particle

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size. Particles from each sample pass across a window and scatter the light emitted by two lasers. The amount of light scattering varies according to the size of the particles. Larger particles scatter light at narrower angles and higher intensity, while smaller particles scatter light at higher angles and lower intensity. Between each sample the instrument was emptied and thoroughly

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rinsed to prevent cross sample contamination.

MINERALOGY AND GEOCHEMISTRY

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X-Ray Diffraction (XRD) was performed at the University of Calgary on 20 samples from well 7-12-48-10W4, which were ground to a powder using a mortar and pestle, and analyzed using a

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Rigaku Multiflex X-Ray Diffraction system. Mineralogical abundances were calculated using MDI Jade software and the CDD PDF-2 powder diffraction database. X-Ray Fluorescence (XRF) analyses were performed to augment the XRD data and stratigraphy, as well as provide insights on depositional processes. All samples were analyzed using an Innovex Instruments model X5000 bench top XRF. 569 samples were analyzed from the 3-3349-10W4 well at 10 cm spacing, and were measured by placing the slabbed piece of core directly 8

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on the XRF sensor for approximately 6 minutes. Each sample was run over approximately 6 minutes on the instrument. The instrument is set up to run in two “modes.” The first mode mainly measures major elements using a low energy X-Ray beam at 10 Kev with a 90 second

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count time plus a high energy beam at 50 Kev with a 30 second count time. The second mode uses 3 levels of X-ray beams: 15 Kev with a 45 second count time, 35 Kev with a 45 second count time and 50 Kev with a 45 second count time. Different elements are analyzed at each

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energy level and some at more than one. A total of 40 elements (mostly trace elements) are

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analyzed in this “mode”.

Rock Eval pyrolysis was performed on samples collected approximately every 1 m through the entire Carlile Formation in the 3-33-49-10W4 well, totaling 64 samples, to quantify the amount and composition of organic matter. Rock Eval pyrolysis was performed using the Rock Eval II Plus TOC module. Rock–Eval analysis is a two-step process, involving pyrolysis in an inert

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atmosphere (nitrogen) and combustion in an oxic atmosphere (air). The Rock Eval pyrolysis method consists of programmed temperature heating (in a pyrolysis oven) of a small sample (~75 mg) in an inert atmosphere (helium) to quantitatively and selectively determine (1) the free

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hydrocarbons contained in the sample and (2) the hydrocarbon that are volatilized during the

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cracking of the unextractable organic matter in the sample (kerogen). During the first step the pyrolysis oven temperature program is kept isothermally at 300°C for 3 minutes and the free hydrocarbons (such as short chain lipids and other small volatile compounds) are volatilized and measured as the S1 peak as detected by a flame ionization detector (FID), which senses the hydrocarbons generated. As the temperature is increased gradually from 300° to 650°C (at 25°C/min) the very heavy hydrocarbon compounds (>C40) are volatilized and nonvolatile organic matter is cracked. Hydrocarbons released are measured as the S2 peak. S1 and S2 are

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determined by a FID. The temperature at which S2 reaches its maximum depends on the nature and maturity of the kerogen and is referred to as Tmax. The pyrolysis step heats the sample at 300 °C to release the S1 fraction (mg HC/g), releasing volatile compounds. This stage is

(mg HC/g).

DATA

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MUDSTONE AGGREGATES - DESCRIPTION

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followed by a temperature increase of 25 °C/min until 650 °C, which releases the S2 fraction

Thin section photomicrographs display numerous silt and sand size clay-mineral rich composite grains that are referred to in this paper as mudstone aggregates. These composite grains are mostly composed of clay-minerals, but also include varying amounts of silt and very-fine size

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grains of quartz, micritic calcite, shell fragments, siderite, and organic matter (Figure 4). They are most easily observed where outlined by silt sized quartz grains providing visual contrast, outlining the mudstone aggregates (Figure 4). In thin section, mudstone aggregates are difficult

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to recognize in the absence of silt outlines, as many have a similar color to that of the encasing matrix mud. In the absence of silt outlines, they can occasionally be observed in SEM element

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maps, as they occasionally have slightly different compositions than the matrix (Figure 5). Despite compaction during burial, most of the mudstone aggregates observed in thin section are rounded to sub-rounded, displaying less compaction than Planolites burrows, which show approximately 50-70% compaction. Aggregate competency appears to have been sufficient to provide pressure shelters, which occasionally preserve primary porosity during compaction (Figure 4f).

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Furthermore, even in samples where the mudstone aggregates are clearly outlined by quartz silt grains (Figure 6a, and 6d), they are only faintly observable in SEM backscatter images as the density of the surrounding mudstone matrix and the mudstone aggregates is similar (Figure 6b).

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However, the mudstone aggregates often have different compositions from the surrounding

(Figure 5d).

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SEDIMENTOLOGY AND STRATIGRAPHY

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matrix, which make the aggregates visible in the element maps, e.g. Al (Figure 6c) and Mg

Sedimentary structures are abundant throughout the cores. While larger sedimentary structures are visible with the naked eye (Figure 7a), smaller sedimentary structures are better observed in thin section (Figure 7b, 7c). Interlaminated lenticular current ripples and planar laminae are the

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most abundant bedforms, while normally graded laminae and soft sediment deformation are rare (Figure 7). Mudstone aggregates are interlaminated with other silt and sand sized grains of varying mineralogy (Figure 4d), and are observed within ripples, planar laminae, normally

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graded beds, as well as within massive and bioturbated intervals. Other secondary sedimentary features such as scours and burrows (Figure 8) are also commonly visible in both core and thin

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section.

Core descriptions of 6 cores within the Carlile Formation are shown in Figure 3, showing the location of sedimentary structures within the succession. The depositional dip oriented West to East core cross-section show the succession is comprised of clinothems separated into 6 parasequences. Each parasequence generally coarsens vertically (with grain sizes rarely exceeding silty sandstone), and is often capped by winnowed surfaces (Figure 8).

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ORGANIC PETROGRAPHY Thin sections observed in fluorescent light show a yellow-green fluorescence of organic algal

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material, showing it is thermally-immature, consistent with Rock Eval data, reflecting the shallow maximum burial of the platform area of eastern Alberta (Figure 9A). Tasmanite material occurs embedded within an inorganic matrix of mud and silt-sized silica grains (Figure 9A). Rounded acritarch microfossils (Figure 9B) and aggregates consisting of abundant organic

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components (Figure 9C, bottom panel) also occur within this interval of the Carlile Formation.

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The organic-rich fluorescing aggregates occur together with abundant non-fluorescing inorganic mud aggregates and silt-sized quartz grains (Figure 4).

MINERALOGY AND GEOCHEMISTRY

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XRD data shows that the Carlile Formation contains on average approximately 66% clay minerals, with varying amounts of quartz, calcite, K-feldspar, and plagioclase forming the bulk of the remaining material (Figure 10). XRF data in the 3-33-49-10W4 well shows how elemental

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trends change vertically through the section (Figure 11). Trends within the Molybdenum profile

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roughly match the TOC profile.

PARTICLE SIZE ANALYSIS – THIN SECTION Individual constituent mineral grains measured in thin sections range from 0.002 mm (clay size) to 0.132 mm (fine sand) with silt size grains representing a large majority of these grains (99.25%). These results are likely slightly coarser than the actual overall average grain sizes, as

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the fine-grained muddy matrix is not accurately represented in these data because of difficulties in distinguishing grain boundaries and measuring grain diameters within the muddy matrix. Most of the measurements are of silt size quartz grains, with minor amounts of feldspar, shell and bone

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fragments, and mica flakes. On average these grains account for approximately 15-30% of the bulk composition, with most of the remaining material being clay-minerals, further supported by XRD data (Figure 10) which shows similar amounts of clay-minerals. Constituent mineral grain

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size distributions (excluding mudstone aggregates) are near symmetrical (skewness between 0.100 and -0.020) and moderately well sorted to moderately sorted (standard deviations between

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0.625 and 0.790) in the majority of examined samples (Table 1). Collectively these data show the contrast in grain size between the mudstone aggregates and the constituent mineral grains that compose them.

Mudstone aggregate grain sizes range from 0.014 mm (fine silt) to 0.611 mm (coarse sand) with

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sand size aggregates representing 67% of all the measured aggregates and silt size aggregates representing 33%. Grain sizes of the mudstone aggregates from all of the samples have a mean that is 2.33 φ lower than the individual constituent mineral grains. Distributions of measured

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mudstone aggregates show they are all moderately well sorted (standard deviations between

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0.605 and 0.698), with varying skewness ranging from a strong coarse skewness to a fine skewness (skewness between -0.310 and 0.144; see Table 1). Grain size distribution significantly changes when the measurements of the mudstone aggregates are included with the measurements of the individual constituent mineral grains (Figure 12). A net bimodal distribution with a strong coarse skewness is observed when measurements from both the constituent grains and aggregates are included. Grain size data including the mudstone aggregates show the deposits are more accurately described as a muddy sandstone with a strong 13

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coarse skewness instead of a mudstone or shale (summary statistics are shown in Table 1 with an example displayed in a histogram in Figure 12). As a whole, these optically measured grain sizes provide data for comparison with the grain sizes measured from disaggregated samples described

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hereafter.

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PARTICLE SIZE ANALYSIS – SAMPLE DISAGGREGATION

Data obtained from ultrasonic disaggregation display less sand with higher amounts of clay size

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grains than the freeze-thaw dataset. Generally, the amount of silt displayed by the two datasets is similar (Figure 13). Vertically cyclical variations in silt content visible in the ultrasonic dataset resemble the coarsening upwards trends observable in the freeze-thaw dataset. In general, grain size distributions measured from ultrasonically disaggregated samples closely

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corresponds to grain size distributions of mineral constituent grains (excluding mudstone aggregates) as analyzed from thin sections of the same sample. Likewise, grain size distributions of samples disaggregated by freeze-thaw cycles closely resemble grain size distributions of the

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mudstone aggregates as collected from thin sections of the same sample.

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Samples disaggregated by freeze-thaw cycles generally displayed coarser distributions than those disaggregated by ultrasonic agitation, as freeze-thaw cycles disaggregate the samples more gently than the more rigorous ultrasonic agitation. Despite these contrasts, similar trends can be observed in the results from both methodologies. Both disintegration methodologies display several coarsening upward cycles throughout the Carlile Formation (Figure 13).

DISCUSSION 14

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PARTICLE SIZE ANALYSIS – DISAGGREGATION Disaggregation of the mudstones into the aggregates and their constituent particles makes it

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possible to characterize the mudstone dominated strata of the Carlile Formation, interpret the relative abundance of the mudstone aggregates, and provides a context to understand the

observed geochemical variations. The ultrasonically disaggregated dataset show a strong

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correlation between the amount of clay size grains and clay minerals throughout the Carlile

Formation. Clay size grains comprise 28.7% to 81.5% of the grains analyzed in each sample,

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with an average of 59.04% (Figures 14 and 15), consistent with XRD data which suggests similar percentages of clay minerals per rock volume (Figure 10).

The freeze-thaw dataset provides an estimate of the size of the mudstone aggregates and other sedimentary particles at the time of deposition, and indicate a higher percentage of sand size

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grains than expected, based on the clay-dominated mineralogy. Sand sized grains comprise 0% to 94.41% of the grains analyzed, with an average abundance of 40%. This is consistent with thin section observations, which show variable amounts of interlaminated mud and sand within

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sedimentary structures, indicative of co-mingled bedload transport.

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The Carlile Formation is comprised of a series of basinward prograding clinothems (Figure 3; Laycock 2014). Clinoform surfaces bounding clinothems correspond to significant shifts in grain sizes in the disintegration datasets. These include changes found at 478.5 m, 463 m, 458 m, and 455 m of the 3-33-49-10W4 core, which correspond to the tops of Parasequence 1, Parasequence 2, Parasequence 3, and Parasequence 4 respectively. Core observations show that the clinothems between these surfaces display vertical increases in quartz content (Figure 3). Similarly,

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coarsening upward cycles are observable in the grain size data from both disaggregation methodologies (Figure 13).

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Breakdown of mudstone aggregates during ultrasonic agitation did not drastically affect the abundance of silt sized grains detected between the two methodologies, as these are primarily quartz grains visible in thin sections within and between the mudstone aggregates. These silt grains also appear to adhere to the surface of the aggregates, which likely armored them during

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transport (Figure 3a).

INTEGRATION OF PARTICLE SIZE ANALYSES

Visual grain size analysis (as described in the Methods section of this paper) was performed on thin sections of the same depths as selected samples to confirm the adequacy of the experiment.

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Diameters of both aggregates and the constituent grains were measured in thin sections and compared to the results obtained by the particle size disaggregation experiment as summarized in

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Table 1, Figure 13, and Figure 14.

Grain size measurements from thin section show similar distributions to those obtained by the

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disaggregation experiments on samples from the same depths (Figure 14). Grain size distributions of samples disaggregated by ultrasonic agitation are similar to distributions of the constituent grains that compose the aggregates (Figure 14). In addition, grain size distributions of samples disaggregated by freeze-thaw are similar to distributions of the aggregates. Slight discrepancies between the distributions from the particle size analyzer and thin section measurements are attributed to three main factors: First, thin sections do not intersect the equator of every grain, biasing these data to provide slightly smaller grain diameters. This is indicated by 16

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the fact that the results from the particle size analyzer are overall slightly larger than those measured from thin section. Second, the volume of the samples analyzed by the particle size analyzer is larger than the area analyzed in the thin sections, which could yield slightly different

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results. This is especially prevalent within heterolithic intervals, as the average grain size varies significantly over several millimetres as seen in Figure 7. Third, measurements of the finest grain sizes within the muddy matrix in thin section is somewhat difficult, as the compacted clay

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minerals in the matrix do not display discernible grain size boundaries that can be measured. As

thin section grain size measurements.

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a result, the grain size measurements of disaggregated samples show higher clay content than the

Results from these experiments and analyses show that the Carlile Formation contains abundant mudstone aggregates. Mudstone aggregates comprise more than half of the sediment volume in certain intervals, while in other intervals they are completely absent. Their presence within

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sedimentary structures shows they were transported by traction currents as approximate hydrodynamic equivalents of the silt and very fine sand they are interlaminated with (Figure 4). The mineralogy and appearance of the Carlile Formation suggests that it is a “mudstone”, but

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with such a large portion of the rock volume composed of sand sized mudstone aggregates, the

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classification of “mudstone” is not representative of the depositional processes responsible for the Carlile Formation. These results suggest that much of the Carlile Formation more closely resembles a sandstone in its grain size and sedimentologic behavior than that of mudstone. Statistical analysis would describe these sediments as a muddy sandstone with a strong coarse skewness (Figure 13). Movement of aggregates in traction controlled the development of facies, clinothem architecture, and geochemical changes between stratigraphic units.

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Further differentiating the Carlile Formation from other mudstones is the local shelter porosity preserved between mudstone aggregates (Figure 4f), which contributes to the available storage space for hydrocarbons. This porosity becomes even more significant considering the thermal

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immaturity of the organic material (Figure 9) and subsequent lack of organic porosity, which

CLINOTHEMS AND GEOCHEMICAL VARIATION

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forms important storage networks for many other fine-grained reservoirs.

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Abrupt geochemical changes within the Carlile Formation are related to clinothem development, and are connected to changes in the mudstone aggregates. Differences in composition between some of the mudstone aggregates and the surrounding mudstone matrix suggest they originated some distance away from the area of deposition (Figure 6). They may have been sourced from

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the seafloor in a different part of the basin with a dissimilar substrate composition, and subsequently transported by subaqueous geostrophic currents (Slingerland, 1996; Korus and Fielding, 2015). Compositional changes are often observed in the overall composition of the

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sediment between and within parasequences, and reflect changes in sedimentary processes.

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Vertical variation in the abundance and composition of the mudstone aggregates are reflected by changes in elemental composition of both major elements and trace elements. Figure 11 displays the compositional variation revealed by XRF analysis. Much of the vertical variations are subtle, but several intervals show systematic compositional changes suggesting changes in sediment source or depositional processes. The most striking example is seen at the top of Parasequence 1, which displays a sharp increase in Ca associated with the shell hash accumulated along the surface. This surface also marks sharp decreases in the relative abundance of Si, K, and Zr, and 18

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corresponds to a sharp grain size changes seen in Figure 13. XRF trends also correspond to other facies changes and parasequence boundaries, such as the top of Parasequence 4, which shows spikes in Ca, as well as the trace elements Mn and Ti. These correspond to the subtle grain size

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change at the top of Parasequence 4. Compositional variations that correspond with changes in the proportion of mudstone aggregates are interpreted to be primarily the result of changes in the composition of the mudstone aggregates transported to the study area. Similar changes can also

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be observed in the relative abundance of organic material (Figure 11).

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Compositional changes do not always correspond to grain size changes, and vice versa. Above the top of Parasequence 1, there is a gradual increase in Zr and decrease in K to 469 m (Figure 11), followed by abrupt changes in both and a return to values close to those at the base of the interval. These variations are interpreted as an abrupt change in sediment source at 479 m, followed by a gradual change in source, and then another change in source at 469 m, which

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corresponds to a subtle facies change within the overall larger parasequence. However, these changes are not significantly reflected in changes in the grain size (Figure 13). Not all of these changes correspond to grain size changes, but aid in understanding subtle environmental changes

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associated with the prograding clinothems.

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The most dramatic change in mudstone aggregate abundance is observed above the top of Parasequence 1 (Figure 13), which corresponds to a decrease in TOC (Figure 11). Environmental changes are indicated by the TOC and redox sensitive trace elements like Mo and Mn (Figure 11). As previously discussed, some mudstone aggregates contain organic material. Progradation of the clinothem and associated changes in basin circulation greatly affect oxygenation and could transport aggregates of varying organic content, which would control the amount of organic material preservation. Figure 11 shows that Mo decreases vertically though Parasequence 1, 19

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corresponding to a decrease in TOC over the same interval. This suggests vertically increasing oxygenation through the section, thus decreasing the preservation potential of organic material. Low TOC levels above this surface are likely the result of organic-poor mudstone aggregates

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transported after the sediment source change at that surface as suggested by Figure 13, or increased oxygenation as suggested by suppressed Mo values, or a combination of both processes.

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Variations such as these are observed throughout the clinothems of the Carlile Formation,

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suggesting multiple shifts in sediment source. This is corroborated by grain size data from disintegrated samples, which show variations in the proportions of aggregates, silt, sand, and clay size grains, associated with many of these changes in sediment source.

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ORIGIN OF SEMI-INDURATED MUDSTONE AGGREGATES This paper focuses on the identification and sedimentological implications of these mudstone aggregates, however, a short discussion about their possible origins assists in understanding the

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various processes occurring within the basin. There are several possibilities for the origins of

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these aggregates: 1) Intra-basinal rip-ups 2) Extra-basinal rock fragments 3) Micro coprolites. An interpretation as Intra-basinal rip-ups relies upon the ability to explain the lack of compaction of the aggregates, see discussion by Plint et al. (2012) and Schieber (2013). Early cementation or rapid dewatering could be possibilities to support this interpretation. Extra-basinal rock fragments is another possibility that explains the lack of aggregate compaction. However, it seems unlikely that sand-size eroded mudstone clasts would be preferentially eroded, transported, and deposited to such a degree without a larger proportion of similarly sized clasts 20

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of other compositions. In addition, the organic petrography shows that the organic material within the clasts is immature, which would not be expected of eroded clasts. The mudstone aggregates may also have a fecal origin. Adhesion of particles by extracellular polysaccharides

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exuded by microbial organisms attached to grain surfaces could explain their competency and lack of compaction. Pryor (1975) demonstrates that crustaceans in nearshore environments are capable of producing large amounts of sand-sized mud aggregates, with these organisms able to

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annually pelletize 12 metric tons of material per kilometer, and deposit sediment as thick as 4.5 mm per year. Similar processes are well documented in other varieties of marine aggregates

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(Paerl, 1973; Paerl, 1975; Alldredge and Silver, 1988; Bura et al., 1998; Aplin and Macquaker, 2011; Zeller et al., 2015), but the internal grain alignments and geometries of pellets in these examples do not resemble the mudstone aggregates observed in the Carlile Formation. Each potential origin has different implications for provenance, sediment dispersal mechanisms,

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and diagenetic processes. Observations suggest that varying amounts of mudstone aggregates are consistent with each of the hypotheses, and as such it is interpreted that the mudstone aggregates originated from multiple sources. Given the vast number of aggregates present in the section,

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limited thin section availability, and the difficulty of identifying the aggregate origin of

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individual grains, a reliable statistical correlation of aggregate origin to stratigraphic interval could not be obtained.

CONCLUSION

Mud aggregates are described within a variety of depositional settings, and are interpreted to proceed from a wide range of processes including electrochemically bound floccules, rip-up

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clasts, fecal pellets, and other varieties of composite grains. These mud aggregates are difficult to recognize in the rock record, as the aggregates tend to be obliterated during compaction. This study presents a unique dataset where mudstone aggregates are abundant and detectable through

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thin section analyses and grain size analysis of disaggregated rock samples. These results

demonstrate that aggregates form a significant portion of the overall rock volume, forming over

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half of the total sediment volume in some intervals.

While the Carlile Formation is dominated by clay minerals, the grain sizes of the sediment

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during deposition are not indicative of this mineralogy. The Carlile Formation is best described mineralogically a mudstone. However, classification of “mudstone” does not accurately describe original characteristics of the sediment during deposition, and implies common inaccurate assumptions about depositional environments. Given the observed sedimentary structures, complex clinothem architectures, grain size statistics, and large volume of sand size mudstone

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aggregate grains, the Carlile Formation more closely resembled a muddy sandstone or sandy mudstone during its deposition from a sedimentary process point of view.

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Recognition of these mudstone aggregates broadens our understanding of mudstone depositional systems, as movement of aggregates in traction controlled the development of facies, clinothem

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architecture, and geochemical changes between stratigraphic units. Data from the Carlile Formation provide an excellent example of the error in unconditionally interpreting mudstone strata as suspension deposits, and demonstrate the wide variety of hydrodynamic processes that can transport mud through a basin. Misunderstanding of these processes will undoubtedly influence the probability of successful hydrocarbon recovery from “tight” reservoir rocks such as these. In addition, these have been shown to preserve occasional shelter porosity between clasts, enhancing porosity networks. 22

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Acknowledgements

Figure Captions:

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We would like to thank all those who helped fund the research, including ConocoPhillips, ExxonMobil, Lundin, and Nexen. We would also like to thank Perpetual Energy for contributing so much data to the project. We would also like to thank Dr. Juergen Schieber for providing great discussions about the data, and for helping us with the ion-milled SEM work.

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Table 1: Grain size statistics reported as phi (ϕ) units. Confidence interval 95%. A) Grain size statistics summary from well 04-01-49-11W4 at 496.88 m. B) Grain size statistics summary from well 3-33-4910W4 at 469.81 m. C) Grain size statistics summary from well 3-33-49-10W4 at 447.66 m. D) Grain size statistics summary from well 3-33-49-10W4 at 465.96 m.

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Figure 1: Index map showing the location of the study area within Alberta, and the location of the cores within the study area.

Figure 2: Stratigraphy of the study area. Adapted from Nielsen et al., 2003; Tu et al. (2007).

Figure 3: East-West cross section showing core logs and associated facies following the line A-A’ shown in Figure 1.

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Figure 4: A) Uninterpreted thin section photomicrograph showing silt lined mudstone aggregates. From well 04-01-49-11W4 at 496.88 m. B) Interpreted photomicrograph from Figure 4a. Mudstone aggregates outlined in yellow. C) Interpreted thin section photomicrograph showing mudstone aggregates floating within a silty matrix. Mudstone aggregates outlined in yellow. From well 07-12-48-10W4 at 519.11 m. D) Un-interpreted Photomicrograph with visible ripple lamination as outlined by quartz silt laminations. The silt sized quartz grains also provide outlines for the aggregates. From well 04-01-49-11W4 at 496.88 m. E) Interpreted photomicrograph from the area shown in Figure 4d with mudstone aggregates indicated with yellow outlines. Black box indicates the area shown in Figure 4f. F) Magnification of the area outlined by the black box in 4e. Inter-aggregate porosity indicated by blue epoxy between the quartz grains in the spaces between aggregates.

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Figure 5: A) SEM backscatter image of a thin section from 07-12-48-10W4 at 507.49 m. Mudstone aggregates can faintly be seen as slightly darker spots, but are more clearly seen in the Mg element map. All images in this figure are of this same location. B) Si element map. Quartz grains are orange-yellow, feldspars are uniform green, and clays speckled green to blue. Non-silicates are black. C) K element map. K feldspar grains have blocky shapes and are orange to green. Micas are elongate green grains. Illitic clays are blue. D) Mg element map. The dull blue areas indicate Mg is present is small amounts associated with the illitic clays (likely part of the smectitic component in mixed layer illite-smectite). Silt sized mudstone aggregates with higher Mg content are best seen in this element map, are shown with brighter blues, and are indicated by yellow arrows.

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Figure 6: A) Thin section photomicrograph from well 3-33-49-10W4 at 465.96 m showing mudstone aggregates within a silty matrix. Blue dyed epoxy visible within fractures and occasional interparticle porosity. All images in this figure are of this same location. B) SEM backscatter image. Similar densities make it difficult to differentiate mudstone aggregates from the matrix. C) Al element map. Clay rich mudstone aggregates with higher amounts of Al differentiable from silty matrix surrounding the aggregates. D) Si element map. Bright spots represent quartz grains, which outline the dim spots corresponding to the mudstone aggregates.

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Figure 7: A) Core photo showing an interval with abundant interlaminated silt ripples. From well 8-1150-9W4 at 423 m. B) Thin section photomicrograph from 3-33-49-10W4 at 490.66 m showing thin planar laminations. C) Photomosaic of thin section images showing a ripple composed of interlaminated clay and silt grains. D) Core photo displaying minor soft sediment deformation. From well 04-01-49-11W4 at 478 m. E) Thin section photomicrograph showing thin normally graded laminae. From well 3-33-4910W4 at 496.81 m

Figure 8: A) Tops of core pieces showing an accumulation of winnowed shell fragments. From well 333-49-10W4 at 448.5 m. B) Core photo showing a winnowed shell hash demarcated with a black arrow and a scoured surface demarcated with a white arrow. Above this surface the mud has a slightly darker color. From well 3-33-49-10W4 at 383.0 m. C) Photograph of a pebble lag at the top of Parasequence 2. From well 11-07-49-11W4 at 474.7 m D) Core photo with orange arrows point out two small Planolites burrows. From well 08-11-50-9W4 at 405.8 m.

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Figure 10: XRD data from 7-12-48-10W4 within the Carlile Formation.

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Figure 9: Photomicrographs exemplifying organic matter variation and inorganic aggregates in the Carlile Formation at 4-1-49-11W4 at 496.66m. All images were captured under oil immersion and UV light. A) Fluorescent tasmanite material surrounded by silt-sized quartz grains. B) fluorescent acritarch microfossil. C) Aggregate containing horizontally-oriented yellow-green fluorescing organic fragments. D) Uninterpreted thin section photomicrograph showing inorganic aggregates of variable morphology. E) Interpreted aggregates outlined in yellow dashed lines. F) Uninterpreted thin section photomicrograph showing inorganic aggregates of variable morphology. G) Interpreted aggregates outlined in yellow dashed lines.

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Figure 11: A) XRF data from 3-33-49-10W4 showing selected elemental abundances of elements commonly associated with lithological variation within the Carlile Formation. Red dotted lines correspond to parasequences identified in Figure 3. Note that chemical abundances often display sharp changes across stratigraphic boundaries.

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Figure 12: Grain size histogram of thin section measurements from 04-01-49-11W4 at 496.88 m, showing a bimodal grain size distribution, with one of the modes representing mostly sand sized mudstone aggregates and the other representing mostly silt size quartz grains. Cumulative percentages demarcated by the red line.

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Figure 13: A) Grain size data from freeze-thaw disaggregated samples from 3-33-49-10W4. Y-axis displays core depths, and X-axis displays percent abundance of grain diameters. Red lines correspond to parasequences identified in Figure 3, and correlate to changes in grain size trends. B) Grain size data from ultrasonically disaggregated samples from 3-33-49-10W4. Y-axis displays core depths, and X-axis displays percent abundance of grain diameters. Red lines correspond to parasequences identified in Figure 3, and correlate to changes in grain size trends. The shift in grain sizes from Figure 13a to Figure 13b represent the mudstone aggregates which were disaggregated by ultrasonic agitation. Bold black line represents the boundary between sand and mud.

Figure 14: A) Histogram from 3-33-49-10W4 at 465.96 m, comparing results from thin section grain size measurements (blue bars) and disaggregated sample grain size measurements (freeze-thaw results shown with the orange line and ultrasonic shown with the blue line). This is the same sample that is shown in figures 4e, 4f, and 4g. B) Histogram from 3-33-49-10W4 at 447.60 m, comparing results from thin section grain size measurements (blue bars) and disaggregated sample grain size measurements (freezethaw results shown with the orange line and ultrasonic shown with the blue line). In both figures 14a and 14b the freeze-thaw grain sizes closely align with the measured aggregates in thin section, and the ultrasonic grain sizes closely align with the grain size measurements from thin section of individual constituent grains when aggregates are ignored. These results are consistent with the other samples compared, with minor variations as described in the text. Disaggregated samples consistently indicate

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more clay size grains than can be measured in thin section due to the difficulty in measuring grain sizes in the matrix.

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B) 3-33-49-10W4 - 469.81 m

C) 3-33-49-10W4 - 447.66 m

D) 3-33-49-10W4 - 465.96 m

Combined Grain Sizes Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

4.84 5.32 5.97 1.44 -0.49 0.94 7.97 1604

Combined Grain Sizes Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

Combined Grain Sizes Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

Combined Grain Sizes Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

Aggregates Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

3.06 3.09 3.51 0.63 -0.31 0.94 5.72 552

Aggregates Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

5.78 5.8 5.97 0.63 0.02 2.36 7.97 1052

Constituent Grains Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

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Aggregates Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

6.12 6.16 6.64 0.79 -0.03 3.8 8.97 1103

Constituent Grains Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

M AN U

3.16 3.13 3.17 0.6 0.14 1.62 4.88 454

TE D

EP

AC C

Constituent Grains Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

5.26 5.72 6.64 1.54 -0.49 1.62 8.97 1557

RI PT

A) 04-01-49-11W4 - 496.88 m

5.33 5.38 6.51 1.05 -0.13 2.34 7.97 2115

4.64 5.16 5.8 1.47 -0.74 0.71 8.38 1668

4.3 4.27 4.44 0.62 0.06 2.34 6.16 800

Aggregates Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

2.56 2.59 3.56 0.7 -0.2 0.71 4.04 481

5.95 5.97 6.51 0.7 -0.02 3.57 7.97 1315

Constituent Grains Mean Meidan Mode Standard Deviation Skewness Minimum Maximum Sample Size

5.49 5.51 5.8 0.63 -0.1 2.92 8.38 1187

ACCEPTED MANUSCRIPT

R12

R11

R10

R9

R8

R7W4

T50

T49

T48

AC C

EP

T51

TE D

M AN U

SC

RI PT

Study Area

A

8-11

3-33

11-7

A’

9-16 4-01 0

T47

7-12

0

Kilometres 5

10

5

Miles

15

10

Coniacian

EP

Carlile Fm.

AC C

Turionian

TE D

89.8

93.9

Cenomanian 100.5

2WS Fm.

Belle Fourche Mbr. Fish Scales Fm.

Greenhorn T.R. Cycle Niobrara T.R. Cycle

Niobrara Fm.

M AN U

86.3

Mowry T.R. Cycle

Santonian

RI PT

Milk River Fm.

Upper Colorado

83.6

SC

Belly River Fm.

Campanian

Montana

Stage Sea-Level Curve Formation Group ACCEPTED MANUSCRIPT Age (Ma) Low High

9.7 km

04-01-49-11W4

16.6 km

3-33-49-10W4

21.3 km

07-12-48-10W4

9.8 km

RI PT

8.1 km

11-07-49-11W4

08-11-50-9W4

A

Facies Trends

Top Carlile

Core Descriptions

Mudstone Muddy siltstone Sandy Siltstone Erosional Lag Ash Bed

Parasequence 6

Parasequence 4 v

v

EP AC C

Mud Glauconite Rich Concretion Shell Hash Ripples Planar Laminations Bioturbation Systems Tract Boundary

A’ Top Carlile Parasequence 5 xxxx

Parasequence 3 xxxx

Clay VF Sand Med Sand Pebbles/Shells

Parasequence 1

Sand

Clay VF Sand Med Sand Pebbles/Shells

Clay VF Sand Med Sand Pebbles/Shells

Clay VF Sand Med Sand Pebbles/Shells

Parasequence 2

v

TE D

Parasequence 3

M AN U

SC

5m

09-16-49-8W4

Parasequence 2

v v v

Parasequence 1 v v v

v v v v v v

2WS

GR

Res

GR

Res

GR Res

GR Res

GR

Res

GR Res

2WS

Clay VF Sand Med Sand Pebbles/Shells

Clay VF Sand Med Sand Pebbles/Shells

B

D

E

ACCEPTED MANUSCRIPT

C

AC C

EP

TE D

M AN U

SC

RI PT

A

F

100 μm

B)

Si

100 μm

D)

Mg

100 μm

AC C

EP

TE D

A) Backscatter

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

C)

K

100 μm

Thin Section

B)

500 μm

Backscatter

500 μm

AC C

EP

TE D

A)

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

C)

Al

500 μm

D)

Si

500 μm

B

M AN U

A

AC C

EP

TE D

C

D

SC

RI PT

ACCEPTED MANUSCRIPT

E

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

A

AC C

EP

TE D

B

C

D

ACCEPTED MANUSCRIPT

D

SC

500 µm

M AN U

B

E

250 µm

G

AC C

EP

100 µm

TE D

250 µm

C

50 µm

F

RI PT

A

250 µm

250 µm

AC C

Quartz

K-spar

500 m

Plagioclase

XRD Calcite

Chlorite

Kaolinite

Illite

Mixs I/S Parasequence 6

Parasequence 4

505 m

510 m

Parasequence 3

515 m Parasequence 2 0%

40% 0%

6% 0%

10% 0%

30% 0%

8% 0%

40% 0%

40% 0%

40%

Ca (wt %) 0

40 0

Ti (ppm) Mo (ppm) K (wt %) Zr (ppm) Mn (ppm) ACCEPTED MANUSCRIPT 1.5 30 400 0 6000 0 50 2500 0

TOC (wt %) 2 4 6

RI PT

445 m

455 m v

v

SC

v

M AN U

465 m

EP

485 m

v

v

v

v

v

v

v

v

v

v

495 m

Parasequence 3

Parasequence 2

Parasequence 1

AC C

v

Parasequence 4

TE D

475 m

Parasequence 6

v

Clay Silt VF Sand Fine Sand Shells/Pebbles

Selected Major Elements

Selected Trace Elements

Selected Redox Sensetive Elements

ACCEPTED MANUSCRIPT

400 400

100%

350 350

RI PT M AN U

40%

EP

100 100

5050

0

Frequency

TE D

150 150

Other Grains

20%

AC C

Frequency

Frequency

Aggregates

200 200

60%

0

0.5

Coarse Sand

1

1.5

Medium Sand

2

2.5

Fine Sand

3

3.5

Very Fine Sand

4

4.5 5 5.5 6 Grain Size (phi ф) Coarse Silt

Medium Silt

6.5

Fine Silt

7

7.5

Very Fine Silt

8

8.5

9

9.5 10

Clay

Cumulative %

250 250

0

80%

SC

300 300

0%

0

20

40

60

80

ACCEPTED MANUSCRIPT

A

100

Parasequence 6

450

Parasequence 4 460

RI PT

Parasequence 3

470

490

500

0

20

EP

450

AC C

460

470

480

490

40

TE D

B

M AN U

SC

480

60

Parasequence 2

Parasequence 1

2WS 80

100

Parasequence 6 Parasequence 4 Parasequence 3

Parasequence 2

Clay Silt Very Fine Sand Fine Sand

Parasequence 1

Medium Sand

500

Coarse Sand

2WS

400

350

Other Grains

300

Aggregates

6%

250

200

4%

RI PT

Frequency

150

100

SC

Frequency (Thin Section Measurements)

8%

ACCEPTED MANUSCRIPT

0

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

M AN U

50

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

More

Freeze Thaw Anundance % Ultrasonic Anundance %

A

2%

0%

Grain SizeBin(phi ф) Medium Sand

Fine Sand

Very Fine Sand

Coarse Silt

400

350

Very Fine Silt

12%

Other Grains

10%

8%

EP

300

Clay

250

AC C

Frequency (Thin Section Measurements)

Fine Silt

TE D

B

Medium Silt

200

6%

Aggregates 150

4%

100 2% 50

0%

0 0

0.5

Coarse Sand

1

1.5

Medium Sand

2

2.5

Fine Sand

3

3.5

Very Fine Sand

4

4.5

5

5.5

6

Grain Size (phi ф) Coarse Silt

Medium Silt

6.5

Fine Silt

7

7.5

Very Fine Silt

8

8.5

9

Clay

9.5

10

Freeze Thaw Anundance % Ultrasonic Anundance %

Coarse Sand

ACCEPTED MANUSCRIPT

Highlights

•

AC C

EP

TE D

M AN U

SC

•

Clay-rich mudstone contains sand sized mud aggregates visible in thin section. Grain size analysis suggests this unit could be classified as a muddy sandstone. Aggregate clasts sufficiently indurated to locally preserve intergranular porosity.

RI PT

•