Quartz types in the Upper Pennsylvanian organic‐rich Cline ......the west may also have...
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Quartz types in the Upper Pennsylvanian organic-rich Cline Shale (Wolfcamp D), Midland Basin, Texas: Implications for silica diagenesis, porosity evolution and rock mechanical properties JUNWEN PENG , KITTY L. MILLIKEN and QILONG FU Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, 78713, USA (E-mail: [email protected]) Associate Editor – Catherine Reid ABSTRACT The origin and form of quartz in mudrocks has significant implications for interpretation of depositional environments, diagenetic pathways, mecha- nisms of porosity reduction and rock mechanical-property evolution. Quartz types in the Upper Pennsylvanian Cline Shale, Midland Basin, Tex- as, were examined using a combination of field-emission scanning electron microscopy-based energy-dispersive spectroscopy elemental mapping (to determine mineralogy) and scanning electron microscopy-based cathodolu- minescence imaging (to determine quartz types) with the goal of elucidat- ing a high-resolution imaging protocol at the micrometre scale for shale petrology. Also, the unconfined compressive rock strength of shale samples with contrasting proportions of different quartz types was measured using Equotip Bambino analyses. The results suggest that extrabasinal detrital quartz, which accounts for an average of 26 vol.% of the rock in all ana- lyzed samples, is the dominant form of quartz in the Cline Shale. The intergranular clay-size microquartz, which accounts for an average of 10 vol.% of the rock in all analyzed samples, is the dominant form of authigenic quartz. Dissolved radiolarians and sponge spicules are likely sources of silica for clay-size microquartz and other authigenic quartz showing pale-mauve to dark greyish cathodoluminescence colour. Some authigenic quartz in the form of intragranular pore filling and mollusc skeletal replacement displays bright-reddish cathodoluminescence colour, which may be associated with silica released at a different time in the rock’s diagenetic history, such as during smectite illitization. Porosity reduction in the Cline Shale predominantly resulted from compaction because of extremely low intergranular volume and the general lack of early cementation. Quartz form significantly impacts rock mechanical prop- erties in the Cline Shale: widely distributed intergranular clay-size micro- quartz cement is a major factor controlling rock strength. This correlation also applies to other mudrock successions of various geological ages, tec- tonic histories and lithologies. Keywords Authigenic quartz, Cline Shale, rock strength, shale petrology, silica diagenesis. 1 © 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists Sedimentology (2020) doi: 10.1111/sed.12694
Transcript of Quartz types in the Upper Pennsylvanian organic‐rich Cline ......the west may also have...
Quartz types in the Upper Pennsylvanian organic-rich ClineShale (Wolfcamp D), Midland Basin, Texas: Implications forsilica diagenesis, porosity evolution and rock mechanicalproperties
JUNWEN PENG , KITTY L. MILLIKEN and QILONG FUBureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin,Austin, Texas, 78713, USA (E-mail: [email protected])
Associate Editor – Catherine Reid
ABSTRACT
The origin and form of quartz in mudrocks has significant implications for
interpretation of depositional environments, diagenetic pathways, mecha-
nisms of porosity reduction and rock mechanical-property evolution.
Quartz types in the Upper Pennsylvanian Cline Shale, Midland Basin, Tex-
as, were examined using a combination of field-emission scanning electron
microscopy-based energy-dispersive spectroscopy elemental mapping (to
determine mineralogy) and scanning electron microscopy-based cathodolu-
minescence imaging (to determine quartz types) with the goal of elucidat-
ing a high-resolution imaging protocol at the micrometre scale for shale
petrology. Also, the unconfined compressive rock strength of shale samples
with contrasting proportions of different quartz types was measured using
Equotip Bambino analyses. The results suggest that extrabasinal detrital
quartz, which accounts for an average of 26 vol.% of the rock in all ana-
lyzed samples, is the dominant form of quartz in the Cline Shale. The
intergranular clay-size microquartz, which accounts for an average of
10 vol.% of the rock in all analyzed samples, is the dominant form of
authigenic quartz. Dissolved radiolarians and sponge spicules are likely
sources of silica for clay-size microquartz and other authigenic quartz
showing pale-mauve to dark greyish cathodoluminescence colour. Some
authigenic quartz in the form of intragranular pore filling and mollusc
skeletal replacement displays bright-reddish cathodoluminescence colour,
which may be associated with silica released at a different time in the
rock’s diagenetic history, such as during smectite illitization. Porosity
reduction in the Cline Shale predominantly resulted from compaction
because of extremely low intergranular volume and the general lack of
early cementation. Quartz form significantly impacts rock mechanical prop-
erties in the Cline Shale: widely distributed intergranular clay-size micro-
quartz cement is a major factor controlling rock strength. This correlation
also applies to other mudrock successions of various geological ages, tec-
tonic histories and lithologies.
Keywords Authigenic quartz, Cline Shale, rock strength, shale petrology,silica diagenesis.
1© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists
Sedimentology (2020) doi: 10.1111/sed.12694
INTRODUCTION
Quartz is one of the volumetrically significantminerals in fine-grained sedimentary rocks (Blatt& Schultz, 1976; Dean et al., 1985). Authigenicquartz has been identified in organic-richmudrocks of different ages, such as the Devonian–Mississippian Bakken and Woodford formations(Fishman et al., 2013, 2015), the MississippianBarnett Formation (Milliken et al., 2012), the Per-mian Wolfcamp Formation (Baumgardner et al.,2016), the Jurassic Haynesville-Bossier Formation(Dowey & Taylor, 2017, 2020) and the CretaceousEagle Ford Group (Milliken et al., 2016). Further-more, authigenic quartz occurs in a variety offorms in mudrocks, such as intergranular cementand grain replacement (Milliken et al., 2012,2016). Because quartz has diverse origins andtakes diverse forms in mudrock systems (Milliken& Olson, 2017), bulk quartz content conveys littleinformation on primary depositional environment,potential diagenetic pathways, mechanisms ofporosity reduction and rock mechanical-propertyevolution.Conventionally, brittle behaviour in mudrocks
has been described as a function of the bulk con-tent of so-called ‘brittle minerals’ (such as quartz)(Jarvie et al., 2007; Rickman et al., 2008), whereasthe form of the minerals (i.e. detrital grain versuscement) has rarely been considered. However,Spinelli et al. (2007) and Ishii et al. (2011) suggestthat authigenic quartz cement has significantimplications for diagenetic pathways and rockmechanical properties: the impact of the biogenicopal-A transition to opal-CT (at around 40 to50°C) and finally to intergranular pore-fillingmicrocrystalline quartz cement (at around 65 to80°C) has a well-documented impact on the gener-ation of brittle mechanical properties inmudrocks. It is worth noting that the form ofquartz cannot be assessed by bulk chemical orbulk mineralogical analytical technology [forexample, X-ray diffraction (XRD), X-ray fluores-cence (XRF), etc.]. However, high-resolutionscanning electron microscopy (SEM)-basedcathodoluminescence (CL) imaging is sensitive tosubtle contrasts in trace elemental composition orcrystal defects in minerals at the micrometre scalethat can be used to discriminate different forms ofquartz (Milliken, 2013). Thus, a combination ofSEM-based energy-dispersive spectroscopy (EDS)elemental mapping (to determine mineralogy) andSEM-based CL imaging (to determine quartztypes) offers the potential to assess the nature andform of quartz at micrometre scale in mudrocks.
The Upper Pennsylvanian organic-rich ClineShale (Wolfcamp D) in the Midland Basin, Tex-as, has been recognized as a new hydrocarbon-producing reservoir in recent years (Hamlin &Baumgardner, 2012). However, some operatorsin the Cline have encountered issues thatincrease the difficulty of production; the highclay-mineral content, which yields ductilemechanical behaviour unfavourable for hydrau-lic fracturing (Jacobs, 2013), especially increasesthis difficulty. The main aims of this study areas follows: establish a high-resolution imagingprotocol to discriminate quartz types and textu-ral variation at micrometre scale, specifically tointegrate with rock mechanical-property mea-surements in the organic-rich Cline Shale; andreveal what quartz types indicate about diage-netic pathways in this particular example fromthe Midland Basin. Characterization of quartztypes, textural variation and the unconfinedcompressive rock strength (UCS) of shale sam-ples with contrasting proportions of differentquartz types will help researchers understandsilica diagenesis, the mechanism for porositydecline and rock mechanical behaviour in theCline Shale. Such characterization can also beapplied to inform models for diagenetic path-ways and rock mechanical properties in fine-grained sedimentary systems.
GEOLOGICAL AND DEPOSITIONALSETTING OF THE CLINE SHALE
The Upper Pennsylvanian Cline Shale is anorganic-rich mudrock in the Midland Basin,Texas (Fig. 1). From the Mississippian to Penn-sylvanian, collision of the North American andGondwana plates caused rapid subsidence of theMidland Basin (Sarg et al., 1999). High-ampli-tude and high-frequency glacioeustatic sea-levelchanges resulted in the deposition of mixed sili-ciclastic–carbonate systems on the shelf (i.e. theCisco and Canyon groups) and the generally sili-ciclastic Cline Shale in the basin (Brown et al.,1990; Wright, 2011). The basinal Cline Shale isbounded by the Eastern Shelf to the east, theCentral Basin Platform to the west, the OzonaArch to the south-west and Horseshoe Atoll tothe north. These four distinct boundariesresulted in relatively restricted environmentalconditions in the Cline Shale (Wright, 2011).The total thickness of the Cline Shale rangesfrom 60 to 130 m (Roush, 2015). The OuachitaOrogenic Belt to the east of the Eastern Shelf,
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
2 J. Peng et al.
Midland Basin, was the major source of silici-clastic sediment for the basin, and the erodedclastics prograded westward across the EasternShelf forming basin-floor fans during sea-levellowstands (Brown et al., 1990). The HorseshoeAtoll to the north and Central Basin Platform tothe west may also have contributed carbonatedebris as hemipelagic plumes or as densityflows down the slope (Hamlin & Baumgardner,2012). However, the carbonate intervals (includ-ing wackestone and calcareous mudrocks) arevolumetrically minor (less than 10 vol.%) andare characterized by thin beds (usually <30 cm),poor reservoir quality [low total organic carbon(TOC), porosity and permeability], low quartzcontent (generally <10 wt.%) and heavy diage-netic overprint. Thus, this study only focuses onsiliciclastic intervals of the Cline Shale.
SAMPLING AND METHODS
The study obtained 140 siliciclastic samples fromfourwells in the Cline Shale (Fig. 1). Polished thin
sections were prepared for all of the samples andobserved using conventional transmitted polar-ized light microscopy. Field-emission SEM usinga FEI Nova NanoSEM 430 (Thermo Fisher Scien-tific, Waltham, MA, USA) was conducted on 32samples (Table 1) chosen from the 140 obtained.A carbon coating approximately 25 nm thick wasapplied to thin sections before SEM observation toreduce charging. Eight samples (labelled inTable 1) were prepared by argon–ion cross-sectionpolishing for pore observation. Argon–ion millingwas conducted using a Leica TIC 3X triple ion gunsystem (Leica Microsystems GmbH, Wetzlar, Ger-many) at 8 kV and 2�8 mA settings for 10 h.Milled surfaces were coated with 6 nm of iridiumusing a sputter coater Leica EM ACE600 to mini-mize surface-charging effects during SEM observa-tion. For these 32 samples chosen for SEMobservation and eight samples chosen for poreobservation, selection criteria were as follows: vol-umetrically significant major and minor litholo-gies, and material as homogeneous as possiblewithin the individual sampling intervals
Fig. 1. Map of the Midland Basinshowing the distribution of theCline Shale, general structuralfeatures, and well locations ofstudied cores. CBP, Central BasinPlatform; DB, Delaware Basin; ES,Eastern Shelf; HA, Horseshoe Atoll;MA, Matador Arch; MB, MidlandBasin; NM, New Mexico; NWS,Northwest Shelf; OA, Ozona Arch;SC, Sheffield Channel; TEX, Texas.Map modified from Wright (2011)and Baumgardner et al. (2016).
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 3
Table
1.
Bulk
compositionoftheClineShale
asdeterm
inedbyX-raydiffraction(X
RD)andpetrographic
measu
rement.IG
Vm,‘intergranularandintra-
granular(primary
pore)volumeformudrocks’
ofMilliken&Olson(2017);COPL,compactionalporosity
loss;CEPL,cementationalporosity
loss;UCS,unc-
onfinedcompressivestrength.Thetotalorganic
carbon(TOC),point-countdata,XRD
andporosity
data
are
compiledfrom
Pengetal.(2020).
Well†
Depth
(m)
Compositional
classification
TOC
(wt.%)
Calculated
Ro(%
)
Gas-filled
porosity
(vol.%)
Quartz
(XRD;
wt.%)
Total
carbonate
(XRD;
wt.%)
Total
clay
(XRD;
wt.%)
Point-
count
detrital
quartz
(vol.%)
Point-
count
quartz-
replaced
allochems
(vol.%)
Point-count
intragranular
quartz
cement
(vol.%)
Point-
count
total
quartz
(vol.%)
Matrix-
dispersed
microquartz
cement
(vol.%)
IGVm
(vol.%)
COPL
(vol.%)
CEPL
(vol.%)
UCS
(HLD)
Model
type‡
12399�5*
Argillaceous
2�33
1�03
—39�0
3�0
46�0
25�8
0�2
0�0
26�0
13�0
NA
NA
NA
537
A
12400�4
Argillaceous
8�32
1�03
1�08
33�0
3�0
43�0
15�2
0�5
0�0
15�7
17�3
21�7
36�2
11�0
548
A
12404�4
Argillaceous
0�93
0�99
—28�0
8�0
52�0
27�9
0�0
0�0
27�9
0�1
NA
NA
NA
500
A
12412�4*
Argillaceous
1�98
1�01
1�36
27�0
15�0
55�6
17�9
0�0
0�0
17�9
9�1
10�3
44�2
5�1
458
A
12419�2
Argillaceous
7�93
1�08
1�86
26�0
7�0
47�9
21�8
1�8
0�0
23�6
2�4
6�8
46�4
1�3
457
A
12419�7
Siliceous
0�84
1�08
1�30
37�0
28�0
39�0
20�0
0�0
0�0
20�0
17�0
17�7
39�3
10�3
572
B
12422�9
Argillaceous
1�64
1�08
1�69
32�6
14�6
58�9
24�5
0�0
0�0
24�5
8�1
9�3
44�9
4�5
551
A
12426�0
Argillaceous
7�38
1�08
2�60
40�0
3�0
42�0
27�2
0�0
0�0
27�2
12�8
17�0
39�7
7�7
557
A
12433�9
Argillaceous
1�58
1�08
1�81
30�0
11�0
51�0
21�7
0�0
0�0
21�7
8�3
9�4
44�8
4�6
552
A
12435�5*
Argillaceous
7�29
1�01
2�33
33�0
4�0
46�0
22�7
0�0
0�0
22�7
10�3
14�4
41�6
6�0
513
A
12436�1
Argillaceous
4�33
1�10
1�09
36�0
4�0
50�0
24�5
0�5
0�0
25�0
11�0
13�4
42�3
6�3
491
A
12440�5
Argillaceous
1�72
1�03
1�00
27�1
6�3
41�0
24�3
0�0
0�0
24�3
2�9
3�9
48�0
1�5
508
A
12444�6
Argillaceous
2�68
0�99
1�09
35�0
4�0
45�0
26�5
0�0
0�0
26�5
8�5
10�0
44�4
4�7
572
A
12446�8
Siliceous
0�13
—0�72
66�0
16�0
9�0
45�8
0�5
0�0
46�3
19�7
19�9
37�6
12�3
806
C
12449�1
Argillaceous
0�97
0�92
1�14
36�0
5�0
57�0
32�1
0�0
0�0
32�1
3�9
4�6
47�6
2�0
547
A
12453�3*
Siliceous
4�35
0�90
1�87
40�0
6�0
24�0
25�9
2�7
0�0
28�6
11�4
13�9
41�9
6�6
617
B
12455�4
Siliceous
5�07
0�94
1�78
40�0
5�0
39�0
24�7
5�2
0�5
30�4
9�6
12�5
42�9
5�5
586
B
12456�2
Siliceous
4�88
1�05
2�60
44�0
7�6
37�0
27�8
5�0
0�0
32�8
11�2
14�1
41�8
6�5
607
B
12457�3
Siliceous
6�61
0�99
3�12
43�0
5�0
32�0
31�5
0�0
0�0
31�5
11�5
15�4
40�9
6�8
614
B
12457�5
Siliceous
5�85
0�99
3�12
51�0
4�0
32�0
25�3
4�0
0�0
29�3
21�7
25�2
33�2
14�5
790
C
12459�5*
Siliceous
7�19
1�01
3�29
51�0
3�0
29�0
35�9
4�2
0�0
40�1
10�9
15�1
41�1
6�4
581
B
12460�3*
Siliceous
5�45
0�98
2�67
48�0
2�0
42�0
35�7
1�0
0�0
36�7
11�3
14�6
41�5
6�6
591
B
12463�7
Siliceous
7�37
1�08
2�04
50�0
3�0
36�4
27�8
0�7
0�0
28�5
21�5
25�5
32�8
14�4
703
C
12467�5*
Argillaceous
3�82
0�96
0�97
33�5
5�1
47�3
24�3
0�5
1�0
25�8
7�8
9�9
44�5
4�3
544
A
12472�8
Argillaceous
0�56
—1�15
27�0
4�0
60�8
21�1
0�0
0�0
21�1
5�9
6�4
46�6
3�1
517
A
12473�0
Argillaceous
0�71
1�41
0�93
25�6
2�8
46�0
20�1
0�0
0�0
20�1
5�5
6�1
46�8
2�9
400
A
12475�3
Argillaceous
0�58
1�50
1�61
24�2
3�6
45�0
18�0
0�0
0�0
18�0
6�2
6�8
46�3
3�3
497
A
12476�0
Argillaceous
0�67
1�17
0�46
21�2
3�2
46�0
19�6
0�0
0�0
19�6
1�6
2�0
49�0
0�8
428
A
12478�8*
Siliceous
5�11
0�92
1�10
48�0
1�0
36�0
31�6
0�0
1�0
32�6
15�4
18�2
38�9
9�4
606
B
23004�0
Argillaceous
2�81
——
36�0
4�0
50�0
24�0
0�0
0�0
24�0
12�0
NA
NA
NA
—A
32933�9
Argillaceous
0�94
——
27�0
23�0
40�0
17�7
0�0
0�0
17�7
9�3
NA
NA
NA
—A
43164�8
Argillaceous
6�88
——
26�0
9�0
41�0
14�3
2�5
0�0
16�8
9�3
NA
NA
NA
—A
Thesymbol‘*’sampleswere
preparedbyargon-ionmillingforpore
observations.
—,noavailable
data.NA,thevaluecould
notbecalculatedbecause
no
porosity
data
wasavailable.†T
henumeral1denoteswellHorw
ood2151-H
;2denoteswellGreerO.L.2;3denoteswellPowellE.L.1;4denoteswellGlass
G.W
.3-B.‡‘Modeltype’refers
tothetexturalandcompositionalclassificationtypessh
ownin
Fig.16.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
4 J. Peng et al.
(<50 mm) so the different types of subsample anal-yses could be correlated.The SEM observations include secondary elec-
tron (SE) imaging, backscattered electron (BSE)imaging, X-ray elemental mapping by EDS(using twin Bruker 30 mm2 detectors; BrukerCorporation, Billerica, MA, USA), and CL imag-ing (using a Gatan ChromaCL detector; Gatan,Pleasanton CA, USA). The EDS imaging wasperformed at 10 to 15 kV accelerating voltage,5�0 spot size, 30 lm aperture, working distanceof about 9 to 10 mm, and about 530 sec scan-ning time. A consistent colour model wasapplied to the major elements of mudrock to dis-criminate minerals on EDS images: potassium(K) (yellow; K-feldspar, muscovite and illite),silicon (Si) (red; quartz), sodium (Na) (aqua;Na-feldspar), magnesium (Mg) (fuchsia; dolomiteand chlorite), calcium (Ca) (blue; calcite), iron(Fe) (yellow; pyrite and ankerite) and carbon (C)[orange; organic matter (OM)]. The EDS mapspresented in this study are mixed with SE mapsto improve edge contrast. The CL images wereperformed at 15 kV accelerating voltage, 5�0 spotsize, 40 lm aperture, working distance of about12 to 13 mm, and 20 min scanning time. The CLmaps are also mixed with the SE signal toimprove edge sharpness.Petrographic point-counting was conducted
on EDS maps using the image-analysis programJMICROVISION (Roduit, 2008). The analysis counted200 random points on one EDS image with mag-nification of about 25009, at which silt-size(<8φ) particles can be confidently identified andcounted. Two images were counted on eachsample in order to obtain a total 200 9 200 lm2
area, which is the representative elementary areafor mudrocks as suggested by Yoon & Dewers(2013) and Kelly et al. (2016). Anomalouslylarge allochems (usually extra-large, robust mol-luscan skeletal material and agglutinatedforaminiferans) may heavily influence the point-counting result, so areas having anomalouslylarge allochems were intentionally avoided. It isworth noting that clay-size particles are gener-ally hard to identify and distinguish under ca25009 magnification, thus non-clay minerals ofclay size (for example, poorly resolved clay-sizefeldspar or quartz) were likely counted as clayminerals in the matrix (as discussed below).Unconfined compressive strength (UCS) is one
of the most fundamental parameters used tocharacterize well bore stability, reservoir geome-chanics, and natural and hydraulic fractures(Zahm & Enderlin, 2010). Although traditional
laboratory-derived analyses (for example, uniax-ial strain analyses) provide the most straightfor-ward and reliable measurement of UCS, thisexpensive and time-consuming method is rarelyapplied in industry. Furthermore, the challengeof obtaining samples of appropriate size andcondition for direct uniaxial strain analysis isanother obstacle to widespread industrial adop-tion of this method, especially in the case ofmudrocks. In contrast to the traditional labora-tory-derived measurements, the Equotip Bam-bino [a micro-rebound hammer (MRH); ProceqSA, Schwerzenbach, Switzerland] provides afine-scale (theoretically, each test point is only3 mm wide), cost-saving and non-destructivemethod to estimate rock strength (Verwaal &Mulder, 1993; Aoki & Matsukura, 2008; Zahm &Enderlin, 2010; Xu & Sonnenberg, 2016).Although Equotip Bambino measurements maybe influenced by rock volume and core condi-tions (Brooks et al., 2016), recent applications ofEquotip Bambino analysis to the Aoshima For-mation (Aoki & Matsukura, 2008), Eagle FordGroup (Zahm & Enderlin, 2010), Bakken Forma-tion (Xu & Sonnenberg, 2016) and Cretaceouscarbonates in South Texas (Brooks et al., 2016)have shown that this instrument gives repeat-able values within acceptably precise variancesaround reported UCS values. Furthermore, Xu &Sonnenberg (2016) find a good correlation(R2 = 0�74) between MRH-derived UCS and tra-ditional laboratory-derived rock unconfinedcompressive strength in the Bakken Formation.In this study, the MRH-derived UCS was mea-
sured with the Equotip Bambino held in a verti-cal position and striking downward upon theflat surface of core slab. The core slab wasplaced in a polystyrene-foam cradle on a firmand stable table. To avoid the ‘volume effect’(i.e. smaller sample volumes yield significantunderestimation of the UCS), measurementswere conducted on samples with volume largerthan 200 cm3, as suggested by Brooks et al.(2016). For each sample from the same location,10 measurements were conducted. Measuredsamples were selected to be as homogeneous aspossible (for example, they exhibit no notablebioturbation). The highest and lowest valueswere removed from data sets before the calcula-tion of an average value. Finally, the calculatedaverage value was regarded as the MRH-derivedUCS value for the sample and reported as Leebhardness [HLD, hardness according to the Leebprinciple with a spherical MRH of tungsten car-bide–cobalt with a radius of 1�5 mm and a
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 5
weight of 5�45 g (Leeb, 1978)] (Mennicke, 2009).The Leeb strength is calculated based on theLeeb principle: the hardness value is derivedfrom the energy loss of a defined impact body(for example, the defined MRH) after striking arock sample, calculated as the Leeb quotient:rebound velocity/impact velocity 9 1000 (Leeb,1978). The MRH rebounds faster from harderrock samples than it does from softer ones,resulting in a greater Leeb strength value. Beforemeasurement, the Equotip Bambino used in thisstudy was calibrated to laboratory steel and con-crete measurements with a certain UCS value(750 HLD in this study) as suggested by Zahm &Enderlin (2010).X-ray diffraction, TOC and Gas Research Insti-
tute (GRI) crushed-rock porosity data collectionfor these same samples was conducted by Fire-wheel Energy, LLC of Salt Lake City, Utah, andcompiled from Peng et al. (in press) (Table 1).The XRD analysis was conducted using a BrukerD8 ADVANCE powder X-ray diffractometer usingcopper-spectrum radiation (40 kV, 100 mA) froma long line focus tube, a graphite monochromatorin the diffracted beamline, and a vacuum deviceto minimize absorption of the X-rays by air. Min-erals were identified by Bruker AXS DIFFRAC.EVA software that compared data used in thisstudy to reference mineral patterns archived inthe Powder Diffraction Files of the InternationalCentre for Diffraction Data. The TOC analysis wasconducted on a LECO CS-200 analyzer (LECO, St.Joseph, MI, USA). Before analysis, 200 mg of eachsample powder was treated with 10 vol.%hydrochloric acid at 60°C to remove carbonateminerals then washed with distilled water toremove residual hydrochloric acid. The sampleswere then dried for 24 h at 50°C before analysis.Porosity was measured using the GRI method(Luffel et al., 1996) for a crushed sample with amesh size of 20 to 50. Grain volume was calcu-lated based on system volume, initial pressureand equilibrium pressure using Boyle’s law. TheAs-received gas-filled porosity can be calculatedas one � (grain volume/bulk volume). CalculatedRo was provided by Firewheel Energy, LLC of SaltLake City, Utah, based on Rock-Eval Tmax data:Ro = 0�018 9 Tmax � 7�16 (Jarvie et al., 2001).
BULK MINERAL COMPOSITION ANDLITHOFACIES
Lithology nomenclature of mudrocks used inthis study was modified from compositional
approaches proposed by Lazar et al. (2015).Mudrocks dominated by quartz or clay mineralsin this study are called siliceous or argillaceous,respectively (Table 1). Quartz and clay contentin argillaceous and siliceous mudrocks weredetermined on the basis of XRD analysis. Quartzcontent in argillaceous mudrocks ranges from 21to 40 wt.%, with a mean content of 31 wt.%.Siliceous mudrock quartz content ranges from37 to 66 wt.%, with a mean content of 47 wt.%.Total clay content in argillaceous mudrocksranges from 41 to 61 wt.%, with a mean contentof 49 wt.%. Total clay content in siliceousmudrocks ranges from 9 to 42 wt.%, with amean content of 32 wt.% (Table 1). In general,grain assemblages in the Cline Shale are domi-nated by extrabasinally derived grains, includingdetrital quartz, feldspar, lithic fragments, micaand K-rich clay. Extrabasinal grains contributeapproximately 87�5 vol.% of total grains on aver-age, with individual sample contributionsranging from 69�5 to 98�5 vol.% according topoint-count. On the basis of the compositionalclassification for grain assemblages in fine-grained sedimentary rocks proposed by Milliken(2014), the Cline Shale belongs to the tarl (ter-rigenous–argillaceous mudrocks; relative contentof terrigenous grains >95 vol.%) or biosiliceoustarl (75 vol.% < relative content of terrigenousgrains < 95 vol.%) group.
EXTRABASINAL DETRITAL QUARTZ
Extrabasinal detrital quartz is the most commontype of quartz in the Cline Shale and was foundin all samples examined in this study. Detritalquartz is characterized by a subangular–angularshape and variable (reddish to light-blue) butoverall brighter CL colour (Fig. 2) than authi-genic quartz (as discussed below). A variety offabrics of detrital quartz, including zoning, inter-nal fractures and overgrowth were also observed(Fig. 2). The overgrowths on these detrital quartzgrains display three main CL colour varieties:dark greyish luminescing (Fig. 2A), light blueluminescing (Fig. 2D) and a layered fabric ofdark-luminescing quartz followed by brighterreddish or light-blue quartz (Fig. 2C).Some quartz overgrowths are considered to
pre-date transportation and deposition for thefollowing reasons: overgrowth rims generallylack euhedral terminations (for example, Fig. 2Aand B), a sequence of CL colours displayed bythe overgrowths varies from grain to grain in a
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
6 J. Peng et al.
specific sample (for example, Fig. 2C), and over-growths are larger than the largest possible inter-granular pore spaces (around 1 to 3 lm) inmudrocks at any stage of the burial history (forexample, Fig. 2B and C).
INTRABASINAL BIOSILICEOUSALLOCHEMS
Agglutinated foraminiferans were very commonlyobserved in the Cline Shale (Fig. 3). Their long
axes are usually parallel to bedding, and in somecases, they take the form of mats with many indi-vidual agglutinated tests very closely stacked. Inmost cases, compaction has collapsed the for-merly hollow spheroidal or tubular bodies ofthese foraminifera. The CL images reveal thatthese agglutinated foraminiferans are composedprimarily of quartz-cemented, silt-size detritalquartz and albite (Fig. 3B).Sponge spicules are the dominant form of
silica that were confidently identified in theprimary intrabasinal biosiliceous allochem
Fig. 2. Secondary-electron/cathodoluminescence (CL) images showing range of CL colours, intensities and fabricsof silt-size to sand-size detrital quartz, suggesting an extrabasinal origin. (A) Detrital silt-size quartz (Qtz) exhibitsreddish CL colour. Quartz overgrowth (arrows) shows dark greyish CL colour; from Horwood 2151-H, 2446�8 mdepth. (B) Detrital silt-size quartz exhibits reddish CL colour. Quartz overgrowth (arrows) shows dark greyish CLcolour; from Horwood 2151-H, 2467�5 m depth. (C) Detrital silt-size quartz exhibits reddish or light-blue CLcolour. Quartz overgrowth (arrows) shows a layered fabric of dark-luminescing quartz followed by brighter reddishor light-blue quartz. The upper-right quartz fragment exhibiting light-blue CL colour may have come from aquartz-cemented sandstone; from Horwood 2151-H, 2446�8 m depth. (D) Detrital sand-size quartz exhibits light-blue CL colour. Yellow arrows indicate overgrowth zoned with blue CL colour. Ankerite cement (white arrows)fills most of the intergranular space; from Horwood 2151-H, 2446�8 m depth. (E) Detrital silt-size quartz withlight-blue CL colour; from Horwood 2151-H, 2463�7 m depth. (F) Detrital silt-size quartz with light-blue CL colourand an inherited quartz-filled fracture (arrow); from Horwood 2151-H, 2467�5 m depth. (G) Detrital quartz withlight-blue CL colour and inherited quartz-filled fractures (white arrows). Quartz overgrowth (yellow arrows)exhibits dark greyish CL colour; from Horwood 2151-H, 2453�3 depth. (H) Detrital quartz with zoning fabric; fromHorwood 2151-H, 2463�7 m depth. (I) Detrital quartz with zoning fabric; aggregate grain is some sort of quartzlithic fragment; from Horwood 2151-H, 2467�5 m depth.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 7
assemblages (Fig. 4). The CL colour of quartz-replaced sponge spicules [originally opal-A(Scholle & Ulmer-Scholle, 2003)] is pale mauveto dark greyish (Fig. 4B and C) and is generallyof low intensity compared with the CL colour ofextrabasinal detrital quartz.Poorly preserved radiolarians were also
observed in the Cline Shale (Fig. 5). Radiolarians[originally opal-A (Scholle & Ulmer-Scholle,2003)] are extensively replaced and infilled bycalcite, dolomite, ankerite, kaolinite or pyrite inthe Cline Shale (Fig. 5A). In this study, a smallnumber of probable quartz-replaced radiolarians(Fig. 5B) were identified. The CL colour of quartz-replaced radiolarians is pale mauve to dark
greyish (Fig. 5C) and is generally of low CL inten-sity compared with that of extrabasinal detritalquartz. Although the internal structure of the fos-sil fragment shown in Fig. 5B and C is indistin-guishable, the size, discrete oval shape, presenceof a spine and CL characteristics suggest that thisfossil is likely a radiolarian (replaced by quartz).
AUTHIGENIC QUARTZ
Grain replacement
In addition to replaced radiolarians and spongespicules (Figs 4 and 5), this study identified some
Fig. 3. Images of agglutinatedforaminifera from Horwood 2151-H,2463�7 m depth. (A) Energy-dispersive spectroscopy mapshowing that the agglutinatedforaminifera test (yellow dashedline) is composed mostly of silt-sizequartz (red) and albite (aqua). BSE,backscattered electron imaging. (B)Cathodoluminescence (CL) image ofarea within blue rectangle in (A).The foraminifer test is composed ofsilt-size detrital quartz (arrows) thatdisplays the same range of CLcolour and intensity as the detritalsilt quartz in Fig. 2. These detritalsilt particles within the test arecemented with dark-luminescingquartz.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
8 J. Peng et al.
carbonate allochems (for example, molluscs, fora-minifera and crinoids) also partly replaced byquartz (Fig. 6). The CL colour of this replacementquartz ranges from dark greyish to reddish(Fig. 6B and D). This reddish luminescence vari-ety is somewhat different from the pale mauve todark greyish CL colour of quartz-replaced radio-larians and sponge spicules in Figs 4 and 5.
Cement
Quartz also takes the form of cement in theCline Shale, observed in both primary
intragranular pores (Fig. 7) and intergranularpores (Fig. 8). The largest quartz crystalsobserved were those that fill anomalously large(diameter >200 lm) primary intragranular spaceswithin allochems (Fig. 7A and B). Quartzcement observed in intergranular pores includedclay-size microquartz cement dispersed through-out the rock matrix (Fig. 8), cement within theagglutinated foraminifera tests (Fig. 3) andminor cement on detrital quartz grain surfaces(Fig. 2). The clay-size microquartz cement dis-persed throughout the rock matrix is the mostabundant authigenic quartz in the Cline Shale,
Fig. 4. Images of sponge spicules;from Horwood 2151-H, 2457�5 mdepth. Image (A) is an energy-dispersive spectroscopy (EDS) map.S, sponge spicules; BSE,backscattered electron imaging.Images (B) and (C) arecathodoluminescence (CL) imagescorresponding to dashed-line boxeson the EDS map. Sponge spiculesshow pale-mauve to dark greyishCL colour and low intensitycompared with extrabasinal detritalquartz in Fig. 2.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 9
with a mean of 10�3 vol.%, as discussed in thenext section. Microquartz cement exhibits pale-mauve to dark greyish CL colour (Fig. 8) verysimilar to the CL variation seen in quartz-replaced sponge spicules and radiolarians, asshown in Figs 4 and 5.
BULK QUARTZ CONTENT
The XRD-derived bulk quartz content of theobserved samples ranges from 21�2 to 66�0 wt.%,with a mean of 37�0 wt.%. This value includes allextrabasinal quartz and intrabasinal quartz ofclay-size and silt-size. However, point-count data(Table 1) revealed that the total quartz content(i.e. the sum of point-count detrital quartz, point-count quartz-replaced allochems and point-countintragranular pore-filling quartz cement fromTable 1) is lower than the XRD-derived quartzcontent for the same sample set. The total point-count-derived quartz content ranges from 15�7 to46�3 vol.%, with a mean of 26�7 vol.%. Althoughthe XRD data unit is weight percent and thepoint-count unit is volume percent, quartzdensity (2�648 g cm�3) is similar enough to thebulk density of the whole rock (2�563 g cm�3)(Milliken et al., 2012) to reasonably justify theinformative value of comparing quartz contentusing these two methods. Bulk XRD quartzcontent and total point-count quartz content aredifferent because petrographic data only includesilt-size quartz (both detrital and authigenic), assuggested previously. Poorly resolved clay-sizemicroquartz is inadequately identified at the mag-nification at which the point-count was con-ducted (about 25009). Higher-magnificationobservations (60009 to 80009) showed that theareas rich in clay-size sediments contain authi-genic clay-size microquartz dispersed throughoutthe rock matrix (Fig. 8). In this study, the differ-ence between total quartz determined by XRDand total quartz determined by point-count wasassumed as an approximate estimation of theauthigenic clay-size microquartz cement content.Thus, the calculated authigenic clay-size micro-quartz cement content ranges from 0�1 to21�7 vol.%, with a mean of 10�3 vol.% (Table 1).In order to test the reliability of this approximatecalculation, a direct measurement of actual clay-size microquartz cement volume following themethod proposed by Milliken & Olson (2017) wasconducted on a few high-magnification CL imagesof clay size fraction-rich areas from five samples.As shown in Table 2, the calculated clay-size
Fig. 5. Images of radiolarians. (A) An energy-dispersivespectroscopy (EDS) map showing a grain, which is prob-ably a radiolarian (R) replaced by ankerite. BSE,backscattered electron imaging. Note the intragranularpore filled by kaolinite (arrow); from Horwood 2151-H,2457�5 m depth. (B) EDS map showing a possible radio-larian (replaced by quartz) that was compacted and sep-arated from its spine (arrow); from Horwood 2151-H,2460�3 m depth. (C) Cathodoluminescence (CL) imageof dashed-line area in (B). Probable radiolarian showspale-mauve to dark greyish CL colour and low intensitycompared with extrabasinal detrital quartz in Fig. 2. Aspine (arrow) also shows dark greyish CL colour.Because of diagenesis (mainly replacement and com-paction), radiolarians are poorly preserved in the ClineShale. Although the internal structure of the allochemas shown in (B) and (C) is indistinguishable, the size,oval outline, presence of spine and CL characteristicssuggest this allochem is likely a radiolarian (replaced byquartz). Fig. 5A is modified from Peng et al. (2020).
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
10 J. Peng et al.
microquartz cement content is similar to thedirectly petrographically measured microquartzcement content from high-resolution CL images;the results of the two methods deviate from eachother by <20%. Thus, the calculated microquartzcement content is a reasonable estimation of theactual authigenic microquartz cement content. Asimilar contrast between XRD-derived totalquartz and point-count-derived total quartz isalso observed in the Mississippian Barnett For-mation (Bunting & Breyer, 2012), CretaceousMowry Shale (Milliken & Olson, 2017) and EagleFord Group (Milliken et al., 2016).
UNCONFINED COMPRESSIVE STRENGTH
A large variation of MRH-derived UCS (400 to806 HLD) was detected for the Cline Shale(Table 1). Examination of MRH-derived UCSfrom the Cline Shale shows a close relation-ship with mineral composition and texturalvariation (Fig. 9). The MRH-derived UCS forargillaceous mudrocks with silt-size particlesfloating in ductile clay matrix (Type A rocksin Fig. 16; discussed below) is lowest and
ranges from 400 to 572 HLD, with a mean of510 HLD. The MRH-derived UCS for siliceousmudrocks with a mixed four-component com-position (silt-size to clay-size particles fromintrabasinal and extrabasinal origins, Type Brocks in Fig. 16; discussed below) is interme-diate and ranges from 572 to 617 HLD, with amean of 597 HLD. The MRH-derived UCS forsiliceous mudrocks with abundant microquartz(Type C rocks in Fig. 16; discussed below) ishighest and ranges from 703 to 806 HLD, witha mean of 766 HLD.
DISCUSSION
Silica sources and precipitation timing
As described previously, authigenic quartz inthe Upper Pennsylvanian Cline Shale, MidlandBasin, Texas, takes several forms: overgrowth ondetrital quartz, allochem replacement, intragran-ular pore-filling cement and intergranular clay-size microquartz cement dispersed throughoutthe rock matrix. Previous studies have suggestedseveral common sources for authigenic quartz in
Fig. 6. Example of carbonateallochems replaced by quartz in theCline Shale. (A) An energy-dispersive spectroscopy (EDS) mapshowing a mollusc skeletal fragmentreplaced by quartz (red arrows) andalbite (yellow arrow); from Horwood2151-H, 2435�5 m depth. BSE,backscattered electron imaging. (B)Cathodoluminescence (CL) image ofsame area in (A). Reddish CL colourof quartz (red arrows) contrasts withthe dark greyish CL colour ofpossible sponge spicules andradiolarians in Figs 4 and 5. (C) EDSmap showing mollusc skeletalfragment replaced by quartz (redarrows) and albite (yellow arrow);from Horwood 2151-H, 2440�5 mdepth. (D) CL image of dashed-linebox in (C). The CL colour of thisquartz replacement (red arrows) isdark greyish to reddish, contrastingwith the dark greyish CL colour ofpossible sponge spicules andradiolarians in Figs 4 and 5.Figure 6A is modified from Penget al. (2020).
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 11
Fig. 7. Quartz cement inintragranular pores. (A) An energy-dispersive spectroscopy (EDS)mapshowing euhedral quartz cement (red)and euhedral ankerite cement (blue)in a possible bioclasticmould; fromHorwood 2151-H, 2433�9 mdepth.BSE, backscattered electron imaging.(B) Cathodoluminescence (CL) imageof the dashed-line box in (A). Reddishto brownish CL colour of quartzcontrasts with the dark greyish CLcolour of the silicified biosiliceousallochems but appears similar to thequartz replacement in Fig. 6B. (C) EDSmap showing chambers of a bioclastfilledwith euhedral quartz cement(red); fromHorwood 2151-H,2433�9 mdepth. (D) CL image of thesame area in (C). Quartz in darkgreyish CL colour is similar to thesilicified biosiliceous allochems inFigs 4 and 5.
Fig. 8. Cathodoluminescence (CL)images of matrix-dispersed, clay-size microquartz cement. Quartzcrystals are more rigid and havebetter-polished surfaces than softerclay matrix. Other rigid grainspresent include calcite (ca), albite(ab) and pyrite (py). Yellow circleshighlight the clay-size microquartzcement with pale-mauve to darkgreyish CL colour dispersed in thematrix. Note possible spongespicules (yellow arrows). (A)Horwood 2151-H, 2460�3 m depth;(B) Horwood 2151-H, 2463�7 mdepth; (C) Horwood 2151-H,2459�5 m depth; (D) Horwood 2151-H, 2457�5 m depth.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
12 J. Peng et al.
mudrocks: biosiliceous allochems [i.e. opal-A(Fishman et al., 2013; Milliken & Olson, 2017)],smectite illitization (Hower et al., 1976; Van deKamp, 2008; Thyberg et al., 2010), volcanic glass(Rubey, 1929; White et al., 2011) and pressuresolution of detrital quartz (Evans, 1990).Volcanic glass is unstable and usually dis-
solves during early diagenetic stages (Rubey,1929). Dissolution of volcanic glass releases acomplex assortment of cations, including Si, K,Na and aluminum (Al), that normally reprecipi-tate as authigenic polymineralic assemblages,including kaolinite, chlorite and zeolite (Rubey,1929). In the Cline Shale, no petrographic evi-dence of typical characteristics of authigenic sil-ica arising from volcanic glass, such as a largeamount of authigenic clay minerals, amorphoussilica in contact with volcanic glass, or amor-phous silica coating on clay-size particles(Rubey, 1929; White et al., 2011) were observed.In contrast, clay minerals identified from SEMobservation are mainly composed of detritalK-mica and detrital illite. This study petrograph-ically identified no volcanic glass in the ClineShale. Regionally, no volcanic ashes have beenidentified in the Upper Pennsylvanian succes-sion of the southern Midland Basin (Roush,2015; Zheng, 2016). Thus, alteration of volcanicglass is an unlikely silica source for authigenicquartz in the Cline Shale. Furthermore, nosutured or penetrative contacts suggestive ofpressure solution between detrital quartz grainswere observed in the Cline Shale. In most cases,quartz occurs in the Cline Shale as isolatedangular silt particles floating in a matrix com-posed dominantly of clay-size clay minerals.This study suggests the dissolution of biogenic
silica as the primary cause of clay-size micro-quartz cementation (Fig. 8) in the Cline Shaleand is supported by the observed poorly pre-served biosiliceous allochems (Figs 4 and 5).Abundant sponge spicules and radiolarians wereobserved in the Cline Shale. Most of these bio-siliceous allochems (originally opal-A) havebeen replaced by quartz, calcite, pyrite andalbite. Some poorly preserved sponge-spiculefragments (Fig. 8) suggest that the original opa-line skeletons experienced dissolution and silicarelease. The poor preservation state of radiolari-ans, most having been replaced by carbonateminerals (Fig. 5A), further indicate that the orig-inal opaline skeletons of these fossils were sub-jected to extensive early diagenesis, whichmobilized silica for reprecipitation. The volu-metric abundance of the clay-size authigenicT
able
2.
Comparisonofclay-sizemicroquartzcontentestim
atedbypetrographic
point-countmeasu
rementandapproxim
ate
calculation.Petrologicalmea-
surementofclay-sizeauthigenic
microquartzcontentthroughoutthematrix
followsMilliken&
Olson(2017).Theupperandlower95%
confidencebounds
ofpoint-countmeasu
rementare
calculatedfollowingHowarth(1998).
Depth
(m)
Area
(lm
2)
Petrologicalpoint-countmeasu
rement(vol.%
)Upper95%
confidence
boundof
point-count
measu
rement
(vol.%
)
Lower95%
confidence
boundof
point-count
measu
rement
(vol.%
)
Calculated
microquartz
cementin
whole
rock
(vol.%
)Deviations
(%)
Grains
Matrix
1(m
icroquartz
cement)
Matrix
2(claysand
pores)
Microquartz
cementin
matrix
Matrix
inwhole
rock
Microquartz
cementin
whole
rock
2457�5
4209
25
17
58
22�0
86�6
19�1
22�6
15�9
21�7
13�9
2459�5
4092
29
10
61
14�1
89�8
12�6
15�7
10�0
10�9
13�8
2460�3
2600
15
13
72
15�3
84�6
12�9
16�0
10�2
11�3
12�7
2463�7
4077
21
24
55
30�4
87�2
26�5
30�3
22�9
21� 5
18�8
2478�8
1029
20
15
65
18�8
87�4
16�4
19�8
13�4
15�4
6�0
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 13
microquartz cement in some specific samples isas high as 21�7 vol.%, which also suggests theprecipitation of quartz at an earlier stage in theburial history, when porosity was at least ashigh as the volume of the pore-filling micro-quartz. The present poor preservation of theseformerly opaline grains made full assessment oftheir primary abundance difficult, which madeestimation of the initial volumes of now-dis-solved biogenic silica impossible.All samples in this study have reached a
thermal maturity (about 1�0% Ro; Table 1) atwhich some degree of smectite illitizationthrough the precipitation of mixed-layer illite–smectite may have occurred (Hower et al.,1976). The XRD data from previous researchdocument the presence of illite–smectite mixedlayers for the same samples used in this study(Peng et al., in press). However, the precise
mechanism and reaction condition for smectiteillitization remains poorly understood. The vol-ume of released silica from smectite illitizationcan vary tremendously in different potentialreaction scenarios. Milliken (2019) proposesfour possible smectite illitization reaction sce-narios based on different elemental balances(i.e. the balance of major interlayer cation K,relatively immobile Al, Si and the 1 : 1 molarbalance between smectite and illite) and con-cludes that illitization in Gulf of Mexico Oligo-cene mudrocks contributes a maximum ofaround 13 vol.% authigenic quartz withinmudrocks, an amount that far exceeds the vol-ume of authigenic quartz observed in Gulf ofMexico Oligocene mudrocks. Reaction scenar-ios on the basis of elements K and Al balanceare unlikely to occur for the Cline Shalebecause of the immense loss in the total clayvolume; only 0�18 mol and 0�67 mol illitewould be generated for every mole of smectitedissolved based on K and Al balance, respec-tively (Milliken, 2019). The volume loss in theclay fraction would reach about 82% and 33%for reaction scenarios based on K and Al bal-ance, respectively, these reaction formulationsare untenable and inconsistent with petro-graphic observations made in the clay-richCline Shale. In addition, Si balance would notrelease or reprecipitate silica. Thus, the 1 : 1molar balance assumption proposed by Mil-liken (2019) was assumed in this study. Basedon the end-member compositions and densitiesgiven for smectite (�Srodo�n & McCarty, 2008)and illite (�Srodo�n et al., 2009), each mole ofsmectite reacted releases 0�47 mol silica,which, if precipitated in its entirety as quartz,would yield a volume of quartz equivalent toaround 6 vol.% of the final authigenic illitevolume, relying upon import and export toaccomplish charge and elemental balance (Mil-liken, 2019). Here, the maximum case of thefinal authigenic illite volume is assumed: allillite in the sample is authigenic and comesfrom smectite illitization. Thus, the volume ofauthigenic quartz arising from smectite illitiza-tion can be calculated as: (illite vol-ume + mixed-layer clay volume) 9 6%. Basedon this calculation, the total estimated quartzvolume from smectite illitization would rangefrom 0�4 to 3�1 vol.%, with a mean of2�2 vol.%. Compared with the microquartz vol-ume [average 10�2 vol.% (Table 1)] observed inthe Cline Shale samples, the amount of silicareleased by illitization of smectite is clearly
Fig. 9. Unconfined compressive strength determinedby micro-rebound hammer (MRH-derived UCS) forthe Cline Shale, measured in Leeb rebound hardness(HLD). Each box depicts the 10th and 90th percentileof MRH-derived UCS data, and the central line indi-cates the mean value. Types A, B and C represent tex-tural and compositional models of the Cline Shale asshown in Fig. 16. Type A represents argillaceousmudrocks with slit-size particles ‘floating’ in the duc-tile clay matrix. Type B represents siliceous mudrockswith a four-component mixing system: silt-size toclay-size particles of both intrabasinal and extrabasi-nal origin. Type C represents siliceous mudrocks withabundant biosiliceous allochems and clay-size micro-quartz.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
14 J. Peng et al.
too small to have supplied all of the observedauthigenic quartz, even under this maximum-reaction assumption. The second phase ofmore-brightly luminescing reddish authigenicquartz that overgrows the first phase of dark-luminescing authigenic quartz (Fig. 10), fillsintragranular pores (Fig. 7B) and replaces mol-lusc skeletons (Fig. 6), is suggestive of a laterphase of quartz precipitation that contrasted insome way, perhaps by temperature, source orfluid composition, with that of the pale-mauveto dark greyish luminescing biosiliceous allo-chems (Figs 4 and 5) and the earlier clay-sizemicroquartz cement (Fig. 8). It is possible thatthis later phase of quartz precipitation may berelated to illitization. The general absence ofthis later generation of authigenic quartz inmicrocrystals dispersed throughout the rockmatrix (i.e. in intergranular pores) suggests thatmost pores in the matrix were already closedby compaction and earlier cementation at thatlater stage of diagenesis. Similar phenomenaare also observed in the Cretaceous MowryShale and Mississippian Barnett Shale; brightlyluminescing reddish authigenic quartz is onlyobserved in intragranular pores, replaced mol-lusc skeletal fragments, or as overgrowths onthe dark-luminescing authigenic quartz (see fig.9 in Milliken et al., 2012; fig. 2 in Milliken,2013; and fig. 9B to D in Milliken & Olson,2017); whereas dark-luminescing authigenicmicroquartz is commonly observed throughoutrock matrix (see fig. 9 in Milliken et al., 2012;fig. 5D in Milliken, 2014; and fig. 10 in Mil-liken & Olson, 2017).
Mechanisms for porosity loss
The typical depth profile for porosity loss causedby compaction and cementation in rigid-grainsandstone reservoirs was established by Paxtonet al. (2002). Despite compaction being the pre-dominant porosity-reduction mechanism duringsandstone burial (Lundegard, 1992), precipitationof cement following compaction is another impor-tant cause of porosity loss (McBride, 1989; Paxtonet al., 2002). The operation of different mecha-nisms of porosity loss in mudrocks remains poorlyunderstood. Mechanisms for porosity reduction inshale are seldom distinguished as effects of com-paction (pore volume loss caused by rearrange-ment of grains) or cementation (pore filling byaqueous precipitates). Two parameters, com-pactional porosity loss (COPL) (Ehrenberg, 1989)and cementational porosity loss (CEPL)
(Ehrenberg, 1989; Lundegard, 1992), were used inthis study to evaluate porosity reduction. Equa-tions for COPL and CEPL in mudrocks are as fol-lows (Milliken & Olson, 2017):
COPL ¼ Pi � ½ð100� PiÞ � IGVm�=ð100� IGVmÞ ð1ÞCEPL ¼ ðPi � COPLÞ � ðCm=IGVmÞ ð2Þ
where IGVm is the ‘intergranular and intragranu-lar [primary pore] volume for mudrocks’ pro-posed by Milliken & Olson (2017); Pi is theinitial or primary porosity; Cm is the volume ofcement in (intergranular and intragranular) pri-mary pores of the mudrock.From petrographic observations, OM-hosted
pores are the dominant pore types (Fig. 11) in theCline Shale. Quantification of different types of
Fig. 10. Matrix-dispersed clay-size microquartzcement is shown in (A) an energy-dispersive spec-troscopy map and (B) cathodoluminescence images,respectively (highlighted by yellow oval). BSE,backscattered electron imaging. Brightly luminescingreddish quartz – yellow arrows in (B) – overgrows thedark-luminescing authigenic quartz (f), which is plau-sibly identified as a fossil fragment; from Horwood2151-H, 2460�3 m depth.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 15
pores is outside of the scope of this study, butvisual estimates suggest that at least 80 vol.% ofthe present-day pores are OM-hosted. Organicmatter (OM)-hosted pores are present both inmesh-like, particulate migrated bitumen and indiscrete kerogen particles. Application of COPLand CEPL equations to the Cline Shale used thefollowing criteria or assumptions:
1 A 50% Pi value was used, because initialporosity around 80% observed near the sedi-ment–water interface (Velde, 1996) is reducedrapidly to around 50% within 20 m of burial(Milliken et al., 2017).2 The cement volume of each sample is equal
to the volume of clay-size microquartz cement,as shown in Table 1.3 Remaining primary porosity (i.e. the sum of
intergranular and intragranular porosity) is equalto 20% of the total porosity on the basis of aconservative visual estimate from the petro-graphic observation.4 Pore-filling secondary OM is equal to 50% of
the TOC on the basis of a very conservativevisual estimate of the volumetric ratio of dis-crete kerogen to migrated mesh-like bitumenseen in the ion-milled samples.
The calculated COPL, CEPL and IGVm is givenin Table 1. Despite the occurrence of clay-sizemicroquartz cement, which accounts for around10 vol.% of analyzed Cline Shale rock on aver-age, cementation volume is too low to representa major cause of porosity decline. Decrease inoverall porosity is compaction-dominated in allsamples, as suggested by Fig. 12. Except in somecertain siliceous mudrock samples that containabundant authigenic clay-size microquartzcement (discussed in the next section), cementssufficiently distributed throughout the intergran-ular pores to serve as grain binders were not
observed. In the absence of significant cementa-tion, the reduction of total porosity from near-sea floor values (within 20 m of burial) in therange of 50 to 80 vol.% (Velde, 1996; Millikenet al., 2017) to the observed low values in theCline Shale (0�46 to 3�29 vol.%) must have beencaused by compaction. Compaction-dominatedporosity loss is also observed in the morecement-rich (cement averages around 41 vol.%of rock volume) Mowry Shale samples (Milliken& Olson, 2017).The IGVm of the Cline Shale ranges from 2�0 to
25�5 vol.% (Table 1). The sample from 2463�7 mdepth in the Horwood 2151-H well exhibits thehighest potential porosity (25�5 vol.%) at the timeof initial cementation. Samples with less cementmay have had similar porosity at that time butlost it through compaction in the absence ofgreater cementation. It is worth noting that IGVm
gives minimum values for the porosity at the timeof cementation; porosity may have been higher insamples where the cements only partiallyoccluded pores, which is likely the case in theCline Shale. Porosity data for modern marinemud sediment (with no cement) from the OceanDrilling Project and the Deep Sea Drilling Projectsuggest that ca 25% porosity is consistent withburial depths around 1�5 km (Velde, 1996). Thus,precipitation of the clay-size authigenic micro-quartz cement in the sample (2463�7 m depth;Fig. 8B) from the Horwood 2151-H well can beassigned to a relatively early stage (<1�5 km burialdepth) of the diagenetic history. The present-daygeothermal gradient varies between 20°C and35°C per kilometre in the Midland Basin, ReaganCounty, Texas (Birch & Clark, 1945); therefore,the illitization of smectite (which occurs at about80°C) will not have occurred above burial depthsabout 2�0 to 2�5 km, depending on geothermalgradient in a given location within the county.This evidence further supports the conclusion
Fig. 11. Backscattered electronimages of pores associated withorganic matter (OM). (A) Imageshowing residual, intergranularmicropores and nanopores withinOM. Note euhedral quartz crystals(Qtz); from Horwood 2151-H,2459�5 m depth. (B) Nanoporeswithin OM showing continuousmesh-like distribution; fromHorwood 2151-H, 2459�5 m depth.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
16 J. Peng et al.
that clay-size microquartz cement with pale-mauve to dark greyish CL colour (Fig. 8) did notresult from smectite illitization.
Textural and compositional models for theCline Shale
Quartz and other non-clay minerals occur inmultiple forms (for example, clay-size to silt-sizeextrabasinal detrital grains, silt-size biosiliceousallochems, and clay-size microcrystalline quartz)in specific mudrock samples. Therefore, bulkmineral-content data (such as XRD results) areinadequate parameters for characterization ofrock texture. As mentioned in the Introduction,detailed petrographic observations are the onlyway to characterize components and texture infine-grained sedimentary rocks.In argillaceous mudrocks, detrital clay miner-
als are abundant but biosiliceous allochems arerare (Fig. 13). Cement content is very low inargillaceous mudrocks (0�1 to 13 vol.%, mean7 vol.%) compared with siliceous mudrocks (9�6to 21�7 vol.%, mean 14 vol.%). Almost allquartz and feldspar in the argillaceous facies isextrabasinally derived. The XRD analysisrevealed that rigid-mineral content (the sum ofquartz, feldspar and calcite contents) rangesfrom 37�4 to 55�0 wt.%, with a mean of
47�6 wt.% for argillaceous mudrocks. Theserigid particles are isolated grains floating in aductile clay matrix (Fig. 13). Contacts betweensurfaces of rigid grains and clay matrix are theinterface between two components with highlycontrasting mechanical behaviours. Compactionstress can be accommodated and absorbed byparticle rearrangement or by localized displace-ments at grain contacts (Milliken et al., 2018).Thus, these quartz grains floating in the claymatrix will not necessarily produce brittlemechanical behaviour or nucleate natural orinduced fractures. A schematic textural andcompositional model for these clay-rich andcement-poor argillaceous mudrocks is given inFig. 16A (Type A model).By contrast, biosiliceous allochems are com-
mon in siliceous mudrocks. A ‘four-componentmixing system’ (proposed by Milliken et al.,2012) arises from two grain-size populations (siltversus clay), each of which includes composi-tional variants related to extrabasinal versusintrabasinal derivation of the particles observedin most siliceous mudrocks (Fig. 14; Type B inTable 1). The rigid-mineral content determinedusing XRD ranges from 51�0 to 72�0 wt.%, witha mean of 58�2 wt.%. Matrix-dispersed micro-crystalline quartz cement was observed betweensilt-size particles as shown in Fig. 14. The ana-lyzed samples exhibit rigid-mineral percentagesclose to or within the percentage range thatenables formation of a touching framework ofrigid grains (about 60 to 74 vol.%) as suggestedby Paxton et al. (2002). Clay-size microquartzcement can form force chains connecting to silt-size detrital quartz (or other rigid grains; seeFig. 14). This network of rigid grains could theo-retically increase rock brittleness by creatingforce chains that pass through rigid-grain con-tacts (Daniels & Hayman, 2008) and by enhanc-ing the preservation of intergranular pores forcement precipitation within packing flaws belowthose contacts (Schneider et al., 2011). A sche-matic textural and compositional model forthese siliceous mudrocks with a four-componentmixing system is given in Fig. 16B (Type Bmodel).In certain types (although relatively rare) of
Cline Shale siliceous mudrocks, biosiliceousallochems are very common, and intergranularclay-size microquartz cement is volumetricallyabundant (Fig. 15; Type C in Table 1). TheirXRD-determined rigid-mineral content rangesfrom 61 to 91 wt.%, with a mean of 72�3 wt.%.Abundant intergranular microcrystalline quartz
Fig. 12. Cross-plot of compactional (COPL) versuscementational (CEPL) porosity decline in the ClineShale. Compaction is the dominant cause of porositydecline in the Cline Shale.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 17
cement was observed between silt-size spongespicules, albite and calcite (Fig. 15). Generally,detrital clay matrix was rarely observed in thesesamples (Fig. 15). Rigid particles and crystals(both silt-size and clay-size) generally show agrain to grain contact and intergrowth zoning(Fig. 15). Widely distributed clay-size micro-quartz cement acts as a binder of both clay-sizeand silt-size rigid particles (Fig. 15), signifi-cantly increasing rock brittleness. A schematictextural and compositional model for micro-quartz cement-rich siliceous mudrocks is givenin Fig. 16C (Type C model).
Implications for mechanical behaviour
The MRH-derived UCS is a fundamental para-meter of rock strength and brittleness, and isessential for predicting both natural and stimu-lated fracture susceptibility (Zahm & Enderlin,2010). As shown in Fig. 9, samples rich in inter-granular microquartz cement tend to showgreater rock strength, whereas clay-rich sampleswith minor amounts of cement show relativelyductile behaviour. Figure 17 suggests that fivesamples with similar bulk quartz content fromthe Horwood 2151-H well exhibit totally
Fig. 13. Representative argillaceousmudrock (Type A in Fig. 16); fromHorwood 2151-H, 2478�8 m depth.(A) An energy-dispersivespectroscopy map showing silt-sizegrains mainly composed of quartz(red) with minor amounts of albite(aqua) and calcite (blue). Silt-sizegrains appear to float in the claymatrix (yellow). Clay minerals(yellow) are 46 wt.% as measuredby X-ray diffraction. BSE,backscattered electron imaging. (B)and (C) are cathodoluminescence(CL) images corresponding tolabelled dashed-line boxes in (A).Detrital clay-size to silt-size quartzwith reddish to brownish CL colour(arrows) ‘float’ in the clay matrix(highlighted by rectangular boxes).
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
18 J. Peng et al.
different MRH-derived UCS. For these sampleswith similar bulk quartz content, MRH-derivedUCS exhibits a well-correlated direct relation-ship (R2 = 0�84) with volume percentage of clay-size microquartz cement. Specifically, the
samples from 2457�5 m and 2459�5 m depth inthe Horwood 2151-H well contain the same bulkquartz content (51 wt.% by XRD), but the formercontains much more microquartz cement (21�7vol.%) compared with the latter (10�9 vol.%). As
Fig. 14. (A) An energy-dispersive spectroscopy map showing representative siliceous mudrocks with a four-com-ponent mixing system (silt-size to clay-size particles of both intrabasinal and extrabasinal origin; Type B inFig. 16); from Horwood 2151-H, 2459�5 m depth. Quartz exhibits a red colour and accounts for 51 wt.% of thesample as measured by X-ray diffraction. Other silt-size grains mainly include albite (aqua), calcite (blue) and pyr-ite (grey). BSE, backscattered electron imaging. (B) to (F) Cathodoluminescence images corresponding to labelleddashed-line boxes in (A). Dark-luminescing, clay-size microquartz (highlighted by yellow ovals) is dispersedapproximately evenly throughout the matrix and is composed of equant microcrystals (1 to 3 lm). Silt-size spongespicules (s), calcite particles (ca), mica (m) and albite (ab) are labelled.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 19
a result, the MRH-derived UCS of the samplefrom 2457�5 m is 790 HLD, much higher thanMRH-derived 581 HLD UCS of the sample from2459�5 m. The form of quartz significantlyimpacts rock properties in the Cline Shale:widely distributed intergranular clay-size
microquartz cement is a major factor controllingrock strength. This conclusion from the ClineShale is consistent with the study of RockyMountain foreland basin Cretaceous siliceousMowry Shale (Milliken & Olson, 2017), SouthTexas passive shelf Cenomanian–Turonian
Fig. 15. (A) An energy-dispersive spectroscopy map showing representative siliceous mudrocks rich in micro-quartz cement and biosiliceous allochems (Type C in Fig. 16); from Horwood 2151-H, 2457�3 m depth. Quartzexhibits a red colour, and accounts for 51 wt.% of the sample as measured by X-ray diffraction. Other silt-sizegrains mainly include albite (aqua) and calcite (blue). BSE, backscattered electron imaging. (B) to (F) Cathodolumi-nescence images corresponding to labelled dashed-line boxes in (A). Dark-luminescing, clay-size microquartz(highlighted by yellow ovals) is distributed evenly throughout the matrix and is composed of equant microcrystals(1 to 3 lm). Sponge spicules (s), calcite particles (ca) and albite (ab) are labelled.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
20 J. Peng et al.
calcareous Eagle Ford Shale (Milliken et al.,2016) and siliceous samples in the East Euro-pean craton Lower Silurian mudrock successionof Poland and Lithuania (Milliken et al., 2018).Thus, the aforementioned conclusion seems tobe widely applicable for mudrock successionsvariable in geological age, tectonic backgroundand lithology. It is worth noting that other ‘brit-tle minerals’ (for example, albite, carbonate min-erals, etc.) are also found as authigenic phasesin many other mudrock successions, such as theUpper Jurassic mudrock succession, EnglishChannel, France (Tribovillard et al., 2018) andUpper Cretaceous Eagle Ford Formation, SouthTexas, United States (Milliken et al., 2016).These authigenic ‘brittle minerals’ could also bea factor that influences rock mechanic behaviourif they are the dominant authigenic mineralsthat are found distributed throughout the matrixand binding silt-size particles.The current study compared MRH-derived
UCS of the Cline Shale with other fine-grainedsedimentary rocks or unconventional shale
reservoirs in North America (Fig. 18). Thestudy results suggest that Type A rocks exhibitmean MRH-derived UCS similar to that ofPronghorn Member argillaceous shale in Willis-ton Basin, North Dakota, but exhibit widervariation. The mean MRH-derived UCS ofType B rocks in the Cline Shale is betweenthat of lower Bakken siliceous shale and mid-dle Bakken silt-bearing or sand-bearing dolomi-tic wackestone in the Williston Basin, NorthDakota (Fig. 18). The range of MRH-derivedUCS of Type B rocks is relatively narrow.MRH-derived UCS of Type C rocks is higherthan that of Eagle Ford and Boquillas calcare-ous shale and similar to Upper Pennsylvanianshallow-water Strawn wackestone, MidlandBasin, Texas (Fig. 18).It is worth noting that SEM and SEM-based
CL petrographic analyses are time-consumingand impractical to perform with the largequantity of samples required in industrialexploration. Some major and trace elements,such as nickel, are recognized as a proxy for
Fig. 16. Schematic textural andcompositional models and theircontrols on mechanical propertiesin the Cline Shale. (A) Type A,argillaceous mudrocks with clay-size and silt-size rigid mineralparticles (‘brittle minerals’, whichmainly comprise quartz with minoramounts of feldspar and calcite)‘floating’ in a matrix composeddominantly of ductile clay-size clayminerals. (B) Type B, siliceousmudrocks with a four-componentmixing system (silt-size to clay-sizeparticles of both intrabasinal andextrabasinal origin). The clay-sizemicroquartz cement binds silt-sizeparticles, which causes furtherrigidification of the framework. (C)Type C, siliceous mudrocks withbiosiliceous allochems andabundant clay-size microquartzcement dispersed throughout thematrix. Pervasive presence ofcement throughout the entire rockmatrix leads to a substantialincrease in rock strength. CL,cathodoluminescence.
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
Quartz types in the organic-rich Cline Shale 21
palaeoproductivity (Tribovillard et al., 2006)and exhibit a well-correlated, direct relation-ship with biosiliceous allochems and reservoirproperties (for example, TOC, porosity, perme-ability and brittleness) in the Cline Shale(Peng et al., 2020). Portable and field-
deployable EDS XRF-based elemental analysisfrom cores, outcrops or cuttings in the field isa rapid and cost-effective way to discriminatefavourable unconventional-play sweet spots forthe Cline Shale and other shale systems inindustrial unconventional exploration.
Fig. 17. Cross-plot of unconfinedcompressive strength by micro-rebound hammer (MRH-derivedUCS), measured in Leeb reboundhardness (HLD), versus volume ofclay-size microquartz cement. MRH-derived UCS correlates well withmicroquartz cement volume.
Fig. 18. Unconfined compressive strength by micro-rebound hammer (MRH-derived UCS), measured in Leebrebound hardness (HLD), for various mudrocks or hydrocarbon-producing reservoirs in North America. Each boxdepicts the 10th and 90th percentile of MRH-derived UCS data, and the central line indicates the mean value.Types A, B and C represent textural and compositional models of the Cline Shale as shown in Fig. 16. MRH-derived UCS data sets for Eagle Ford siliceous shale and Eagle Ford calcareous shale are compiled from Zahm &Enderlin (2010). MRH-derived UCS data sets of Pronghorn Member argillaceous shale, lower Bakken siliceousshale, and middle Bakken silt-bearing or sand-bearing dolomitic wackestone are compiled from Xu & Sonnenberg(2016). MRH-derived UCS data set for Boquillas calcareous shale are compiled from Brooks et al. (2016).
© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists, Sedimentology
22 J. Peng et al.
In the future, the team that produced thisstudy will focus on establishing relationshipsbetween different textural and compositionalmodels (for example, microquartz-cemented ver-sus uncemented rocks) and basin stratigraphy.This approach has great potential for enhancingprediction of rock mechanical properties atlarger scales.
CONCLUSIONS
1 Quartz observed in the Cline Shale, MidlandBasin, Texas includes extrabasinal detritalquartz, overgrowth on detrital quartz, cementwithin agglutinated foraminifera tests, allochemreplacements, intragranular pore-filling cementand intergranular clay-size microquartz. Extra-basinal detrital quartz is the dominant form ofquartz in the Cline Shale; it averages 26 vol.%of the whole rock. Matrix-dispersed clay-sizemicroquartz cement is the dominant form ofauthigenic quartz in the Cline Shale; it averagesaround 10 vol.% of the whole rock.2 The source of silica for authigenic quartz
was most likely biosiliceous allochems (spongespicules and radiolarians) that dissolved duringearly diagenesis. Smectite illitization may alsohave released some silica, which may have origi-nated the authigenic quartz with the brighterreddish cathodoluminescence (CL) colour.3 Porosity reduction in the Cline Shale is dom-
inated by compaction because of extremely lowIGVm [‘intergranular and intragranular (primarypore) volume for mudrocks] and the lack ofearly cementation.4 Three textural and compositional types of
rock are distinguished in the Cline Shale.Argillaceous mudrocks, Type A, exhibit thelowest micro-rebound hammer (MRH)-derivedunconfined compressive rock strength (UCS),which ranges from 400 HLD (hardness accordingto the Leeb principle) to 572 HLD, with a meanof 510 HLD. Siliceous mudrocks with a four-component mixing system, Type B, exhibit anintermediate MRH-derived UCS, which rangesfrom 572 HLD to 617 HLD, with a mean of 597HLD. Siliceous mudrocks with abundant micro-quartz, Type C, exhibit the highest MRH-derivedUCS, which ranges from 703 HLD to 806 HLD,with a mean of 766 HLD. The rock strength ofType B and Type C rocks is comparable to thatof most hydrocarbon-producing shale reservoirsin North America (for example, Eagle FordGroup and Bakken Formation).
5 The form of quartz significantly impacts rockmechanical properties in the Cline Shale: widelydistributed intergranular clay-size microquartzcement is a major factor controlling rockstrength. The occurrence of biosiliceous allo-chems in a sample is favourable for precipitationof intergranular clay-size microquartz cementdispersed throughout the matrix, which is thenmore susceptible to brittle rock failure. By con-trast, samples rich in clay minerals contain onlyrare quartz cement, and the detrital silt-size par-ticles (quartz and minor feldspar and calcite)float in the clay matrix, exhibiting ductilemechanical behaviour.
ACKNOWLEDGEMENTS
This study was financially supported by the Stateof Texas Advanced Resource Recovery (STARR)program at the Bureau of Economic Geology (theBureau) and by the China Scholarship Council(Grant No. 201606440062). The author alsoreceived minor financial support from a Geologi-cal Society of America (GSA) Graduate StudentResearch Grant (Grant No. 9244823). FireWheelEnergy, LLC, of Houston, Texas, is thanked forkindly donating the core and laboratory data fromthe Horwood 2151-H well. The authors thankPatrick Smith for assistance during SEM samplepreparation at the Bureau imaging laboratory,Josh Lambert for assistance during MRH-derivedUCS analysis, Robert Reed for providing severalthin sections and Hanyue Zheng for providingseveral SEM images of pores. The manuscriptbenefited from internal review by Dr Robert Reed.The manuscript was edited by publications edi-tor, Travis Hobbs, at the Bureau media group forlanguage polishing. Comments and suggestionsfrom Prof. Kevin Taylor and two anonymousreviewers greatly improved the clarity and logicflow of this manuscript. Publication is authorizedby the Director of the Bureau of Economic Geol-ogy, Jackson School of Geosciences, The Univer-sity of Texas at Austin.
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Manuscript received 8 May 2019; revision accepted 25November 2019
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