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ABSTRACT

The Devonian Woodford Shale and Cretaceous Mowry Shale consist of relatively deep (below storm wave base) intracratonic basin deposits commonly referred to as “shales” because of their dark gray to nearly black color, very fine-grained nature, pelagic fossils such as radiolarians, and common amorphous marine kerogen. These shales typically contain less than 30% detrital clay by weight and more than 50% quartz (locally up to 80%). The quartz is a mix of biogenic grains, mainly radiolarians, and authigenic silica along with some detrital quartz silt of extrabasinal origin. The authigenic silica is dominantly microcrystalline (< 1 micron) and forms a major component of the matrix in these formations, but the rocks also contain authigenic pyrite, commonly as framboids, minor carbonates including magnesite, and quartz overgrowths, but together these authigenic minerals form less than 10% of the rock.

Authigenic quartz in the Woodford and Mowry samples commonly takes the form of silica nanospheres, a type of microquartz less than a half micron in diameter. Textures of this microquartz are best observed directly with a high-resolution electron microscope. In many Woodford and Mowry samples, the silica nanospheres, which tend to be associated with organic matter, form more than 50% of the rock. The large volume of the authigenic quartz, together with “floating” detrital components and the close association with pyrite framboids, indicates that the silica nanospheres formed very early, perhaps in association with microbial activity on or in the seafloor sediments. These early silica nanospheres, which are only weakly luminescent, helped create a lithified sediment during or soon after deposition.

Where the silicification process ceased prior to complete silica cementation, the early silica nanospheres are associated with up to 15% interparticle microporosity. This gives the Woodford and Mowry good potential reservoir quality, at least locally. The authigenic silica nanospheres also enhance the mechanical properties and brittleness of these siliceous mudrocks to a degree much greater than the presence of the detrital quartz particles alone.

INTRODUCTION

Mudrock reservoirs vary widely in composition. Some such as the Permian Wolfcamp and Spraberry in the Permian Basin are rich in detrital clays and siliciclastic silt, others are rich in carbonate grains (e.g., Haynesville, Niobrara, and Eagle Ford formations), still others contain abundant silica grains of biogenic origin (e.g., radiolarians or diatoms), and a few are rich in amorphous kerogen (e.g., the Bakken black shales in the Williston Basin; Meissner, 1978). Here we look at two examples of silica-rich mudrocks to draw attention to the fact that diagenetic silica can also be a significant component of certain mudrock reservoirs. This study focuses first on the 365-million-year-old Upper Devonian Woodford Shale in the Permian Basin of West Texas, and then on the lithologically and texturally similar 100-million-year-old mid-Cretaceous Mowry Shale in the Powder River Basin of Wyoming.

By comparing and contrasting these basin-centered siliceous shales separated by about 900 mi (1400 km) in distance (Figures 1, 2) and 250 million years in time, we show that these formations contain both a component of detrital quartz silt grains and clays of extrabasinal origin, and biogenic silica mainly in the form of radiolarians with authigenic pyrite and enough organic matter (> 2 wt. %) to give them fair to good hydrocarbon source potential. Of special interest in these two formations is the abundance of authigenic microquartz in submicron-size particles that take the form of rounded nanospheres. These silica nanospheres form a significant part of the matrix between the more easily recognized larger detrital and biogenic grains.

Figure 1.

Map of structural basins in the western United States showing locations of the two study areas. The red stars indicate the locations of the Hentz Family 7-1 Woodford core on the Central Basin Platform of the Permian Basin, and the Java State 16-1 Mowry core in the Powder River Basin. Outlines of the mapped areas in Figure 2 are also shown.

Figure 1.

Map of structural basins in the western United States showing locations of the two study areas. The red stars indicate the locations of the Hentz Family 7-1 Woodford core on the Central Basin Platform of the Permian Basin, and the Java State 16-1 Mowry core in the Powder River Basin. Outlines of the mapped areas in Figure 2 are also shown.

Figure 2.

Generalized depositional thickness maps of the Woodford and Mowry shales. (A) Woodford Shale depositional thicknesses adapted from Wright (1979, p. 62), contour interval is 200 ft (60 m). Star denotes the location of the Hentz Family 7-1 well. (B) Mowry Shale depositional thickness after Nixon (1973), contour interval is 100 ft (30 m). Lettered stars refer to wells mentioned in the text: (A) Educated Guess 11-1H, (B) 1 Bentley Fee 34-1, (C) Java State 16-1, (D) Trans Am 1-16H, (E) Crossbow 2-18H, and (F) Marys Draw 31-23M.

Figure 2.

Generalized depositional thickness maps of the Woodford and Mowry shales. (A) Woodford Shale depositional thicknesses adapted from Wright (1979, p. 62), contour interval is 200 ft (60 m). Star denotes the location of the Hentz Family 7-1 well. (B) Mowry Shale depositional thickness after Nixon (1973), contour interval is 100 ft (30 m). Lettered stars refer to wells mentioned in the text: (A) Educated Guess 11-1H, (B) 1 Bentley Fee 34-1, (C) Java State 16-1, (D) Trans Am 1-16H, (E) Crossbow 2-18H, and (F) Marys Draw 31-23M.

Although silica nanospheres have previously gone largely unrecognized because of their extremely small size, typically just 200–300 nm, they have been noted in other hydrocarbon reservoirs. One particularly significant example is in the very early Cambrian Athel Silicilyte, an anoxic, organic-rich, basin-centered deposit composed of up to 91% authigenic silica in the South Oman Salt Basin (Amthor et al., 2005; Al Rajaibi, 2011). The fact that the silica nanospheres in these silicilyte reservoirs formed millions of years before radiolarians evolved indicates that dissolution and reprecipitation of silica from radiolarians is not necessary to form the nanospheres. Instead some microbial organism, possibly sulfate-reducing bacteria, seems to have played a role in precipitating the silica, either in the water column or directly on the seafloor (cf. Amthor et al., 2005; Al Rajaibi, 2011; Al Rajaibi et al., 2015; Stolper et al., 2017).

These petrographic observations open a new area of research to determine not only the source and origin of authigenic microquartz and silica nanospheres but also their role in deposition and diagenesis, their impact on aspects of reservoir quality such as brittleness and porosity formation/preservation, and perhaps their effect on the preservation and thermal evolution of organic matter and oil.

METHODS

As the search for unconventional hydrocarbon reservoirs in mudrocks has intensified in the past decade, so has the number of cores cut in these extremely fine-grained source and reservoir rocks. This study focuses first on a core cut in the Woodford Shale at depths of 11,468–11,515 ft (3495–3510 m) in the Hentz Family 7-1 (aka Hentz Family A56-0407 N 02WS), a well drilled in Winkler County, Texas, on the Central Basin Platform of the Permian Basin (Figure 2A). This core was analyzed with a variety of techniques including thin-section petrography, crushed rock (GRI) analyses (Luffel and Guidry, 1992), X-ray diffraction (XRD), Rock-Eval analyses, field-emission scanning electron microscopy using both secondary (SE) and backscattered electron (BSE) imagery, and X-ray elemental mapping (EDS). Scanning electron microscope (SEM) imaging was performed on (1) freshly broken surfaces, (2) mechanically polished thin sections (C-coated), and (3) argon-ion-milled surfaces (Ir-coated). Secondary electron images and BSE images were captured at from 400× to 50,000× machine magnification.

We also used cathodoluminescence (CL) with the field-emission scanning electron microscope (FE-SEM) following the technique described by Milliken (2013) on selected Woodford Shale samples to compare with the extensive CL work done on the Mowry by Milliken and Olson (2017). This technique facilitates discrimination of weakly luminescent authigenic quartz from more luminescent detrital quartz and other components. These latter two techniques (FE-SEM and CL imagery) provide many of the images included in this chapter.

To provide a comparison with the Woodford Shale, 14 Mowry core samples from five wells in three counties (Campbell, Converse, and Sheridan) in various parts of the Powder River Basin, and previously studied by Milliken and Olson (2017), provided a wealth of XRD data (Table 1) and FE-SEM images. Locations of their samples ranged from the east side of the basin westward to near the basin axis. Total Mowry thickness in the Powder River Basin ranges from about 130 to 250 ft (40–80 m) and the formation thickens westward (Socianu et al., 2015) toward the major source of the siliciclastic sediments.

TOC and XRD results for 14 selected core samples from the five wells that formed the basis of the study of the Mowry silica diagenesis study by Milliken and Olson (2017).

Table 1.
TOC and XRD results for 14 selected core samples from the five wells that formed the basis of the study of the Mowry silica diagenesis study by Milliken and Olson (2017).
  DepthMaturity Mineral Composition by XRD (wt. %)
Well NameDepthMetersRoTOCQuartzFeldsparCalciteDolomitePyriteTotal Clay
Trans Am 1-16H8340.62542.20.932.16616.40.00.03.829
Trans Am 1-16H8365.42549.80.923.75666.30.00.08.719
Trans Am 1-16H8431.52569.90.922.83638.50.03.28.517
Trans Am 1-16H8446.72574.60.921.55534.30.00.04.636
Bentley Fee 34-192072806.30.793.36697.00.00.04.019
Java State 16-1101563095.50.723.49655.30.00.04.325
Java State 16-11017431010.752.18822.20.21.42.611
Java State 16-1101893105.60.752.47666.90.53.75.917
Crossbow 2-18H110593370.81.133.65407.00.01.09.042
Crossbow 2-18H11069.83374.11.133.31577.00.01.07.027
Crossbow 2-18H11083.63378.31.152.39597.00.03.06.024
Marys Draw 31-23M114213481.11.003.19647.10.01.95.521
Marys Draw 31-23M11433.13484.81.001.62544.30.00.05.836
Marys Draw 31-23M11441.33487.31.002.87525.60.00.77.134
Averages    60.96.10.051.145.9125.5
  DepthMaturity Mineral Composition by XRD (wt. %)
Well NameDepthMetersRoTOCQuartzFeldsparCalciteDolomitePyriteTotal Clay
Trans Am 1-16H8340.62542.20.932.16616.40.00.03.829
Trans Am 1-16H8365.42549.80.923.75666.30.00.08.719
Trans Am 1-16H8431.52569.90.922.83638.50.03.28.517
Trans Am 1-16H8446.72574.60.921.55534.30.00.04.636
Bentley Fee 34-192072806.30.793.36697.00.00.04.019
Java State 16-1101563095.50.723.49655.30.00.04.325
Java State 16-11017431010.752.18822.20.21.42.611
Java State 16-1101893105.60.752.47666.90.53.75.917
Crossbow 2-18H110593370.81.133.65407.00.01.09.042
Crossbow 2-18H11069.83374.11.133.31577.00.01.07.027
Crossbow 2-18H11083.63378.31.152.39597.00.03.06.024
Marys Draw 31-23M114213481.11.003.19647.10.01.95.521
Marys Draw 31-23M11433.13484.81.001.62544.30.00.05.836
Marys Draw 31-23M11441.33487.31.002.87525.60.00.77.134
Averages    60.96.10.051.145.9125.5

In addition to the Mowry data provided by Milliken and Olson (2017), two publicly available (U.S. Geological Survey, Lakewood, Colorado) Mowry cores were examined. These cores are the Java State 16-1 (Section 16, T54N, R80W) and the Educated Guess 11-1 (Section 10, T54N, R83W), both in Sheridan County, Wyoming. The Java State 16-1 (Figures 1, 2B) was also included in the Milliken and Olson (2017) study.

Characteristics of the Woodford Shale

The motivation for this study was an evaluation of a Woodford Shale core from the Hentz Family 7-1 well in Winkler County, Texas. This well was drilled in a structural subbasin located along the north–south trending Central Basin Platform (Figure 2A; Drake et al., 2017) near the center of the Permian Basin. A complete section of the nearly 600-ft-thick (180 m) Woodford Shale is preserved in this subbasin in what at the time of deposition was the deepest, central part of the ancestral Tobosa Basin (Wright, 1979; Comer, 1991), the precursor to today’s Permian Basin.

Only the upper part of the Woodford Shale was cored at a depth of 11,468–11,515 ft (3495–3510 m) in the Hentz Family 7-1 well. The cored interval is a siliceous mudrock containing 55–75 wt. % silica (average 65 wt. %) and 15–30 wt. % total clay (average 24 wt. %) based on bulk XRD analyses. Most of the clays are illite/mica and muscovite, interpreted as detrital components. Other components include pyrite/marcasite (average 5.5 wt. %), dolomite (3 wt. %), albite (2 wt. %), and magnesite (1 wt. %). Calcite is rare (< 1 wt. %). Total organic carbon (TOC) ranges from 4 to 8 wt. % (average 5.4 wt. %). The thermal maturity ranges between 0.91 and 0.94 %Ro vitrinite reflectance equivalent based on Rock-Eval (Tmax) analyses using the equation of Jarvie et al. (2001) and the oil geochemistry, respectively. This places the Woodford in the Hentz Family 7-1 in the peak oil-generation window. With over 4 wt. % average TOC, the Woodford in the well has excellent hydrocarbon source potential.

Study of slabbed four-inch-diameter continuous core from the Woodford Shale with both standard petrographic thin sections and the FE-SEM reveals that the silica occurs in four distinct forms: (1) randomly distributed angular to subrounded detrital silt grains (Figures 3C, D; 4); (2) siliceous microfossils, mainly radiolarians (Figure 5C, D) with local sponge spicules (Figure 4A); (3) euhedral quartz overgrowths on some silt grains (Figure 3C, D), and in and around the quartz silt and radiolarians (Figure 5); and (4) as quartz spherules typically 200–500 nm in size (only observable at > 5000× SEM magnification as shown in Figures 3, 4, 6). These nanometer-size quartz spherules, which we call silica nanospheres, are common in every Woodford sample we examined using the FE-SEM and are two to four times more common than the other three types of silica combined. The silica nanospheres are distinguished in size from pore-filling lepispheres of opal-CT that precipitate in the early diagenesis of bio-siliceous sediments and have sizes about an order of magnitude larger (1–3 μm; Wise and Kelts, 1972; Oehler, 1975). Recrystallized lepispheres were implicated by Milliken and Olson (2017) as a potential source of silica for the authigenic microquartz in the Mowry Shale.

Figure 3.

FE-SEM images of the Mowry from the Bentley Fee 34-1 from 9207 ft (2086 m) and the Woodford from 11,492 ft (3507 m) showing EDS element maps (A and C) and cathodoluminescence (B and D). EDS element maps show sodium (light blue), calcium (dark blue), and silica (red); other elements are shown in gray. Detrital quartz (q) is identified by its colorful luminescence (B and D). Nonluminescent authigenic quartz present as silica nanospheres (circled) in the matrix and as reworked quartz overgrowths (arrows) rimming detrital quartz grains. Dolo = dolomite; m = mica; and py = pyrite framboid.

Figure 3.

FE-SEM images of the Mowry from the Bentley Fee 34-1 from 9207 ft (2086 m) and the Woodford from 11,492 ft (3507 m) showing EDS element maps (A and C) and cathodoluminescence (B and D). EDS element maps show sodium (light blue), calcium (dark blue), and silica (red); other elements are shown in gray. Detrital quartz (q) is identified by its colorful luminescence (B and D). Nonluminescent authigenic quartz present as silica nanospheres (circled) in the matrix and as reworked quartz overgrowths (arrows) rimming detrital quartz grains. Dolo = dolomite; m = mica; and py = pyrite framboid.

Figure 4.

Secondary electron images of broken surfaces from four different Woodford samples (A–D) illustrating microporosity in the siliceous matrix. Scale bars and sample depths are provided at lower right in each photo. (A) Partly pyritized monaxon sponge spicule in a microporous siliceous matrix. (B) Higher magnification view showing silica nanospheres (Si). (C) Euhedral pyrite crystals in a matrix of silica nanospheres averaging < 300 microns in size. (D) Pyrite framboid in a matrix of silica nanospheres and botryoids (Si) and illite.

Figure 4.

Secondary electron images of broken surfaces from four different Woodford samples (A–D) illustrating microporosity in the siliceous matrix. Scale bars and sample depths are provided at lower right in each photo. (A) Partly pyritized monaxon sponge spicule in a microporous siliceous matrix. (B) Higher magnification view showing silica nanospheres (Si). (C) Euhedral pyrite crystals in a matrix of silica nanospheres averaging < 300 microns in size. (D) Pyrite framboid in a matrix of silica nanospheres and botryoids (Si) and illite.

Figure 5.

Comparison of siliceous radiolarians (rad) in the Mowry from the Trans Am 1-16H core from 8447 ft (2575 m) (A and B) and Woodford core from 11,469 (3496 m) (C and D). EDS element maps (A and C) show sodium (light blue), calcium (dark blue), and silica (red); other elements are shown in gray. (C) Orange luminescent matrix reflects presence of K in illite. (D) CL image of the Woodford showing quartz-cemented radiolarians partly replaced with crystalline pyrite (py) and cemented with chalcedony (chal), some of which shows distinct growth bands. Early diagenetic microquartz appears light to dark gray with weak luminescence.

Figure 5.

Comparison of siliceous radiolarians (rad) in the Mowry from the Trans Am 1-16H core from 8447 ft (2575 m) (A and B) and Woodford core from 11,469 (3496 m) (C and D). EDS element maps (A and C) show sodium (light blue), calcium (dark blue), and silica (red); other elements are shown in gray. (C) Orange luminescent matrix reflects presence of K in illite. (D) CL image of the Woodford showing quartz-cemented radiolarians partly replaced with crystalline pyrite (py) and cemented with chalcedony (chal), some of which shows distinct growth bands. Early diagenetic microquartz appears light to dark gray with weak luminescence.

Figure 6.

FE-SEM images of two Ar-ion polished samples of the Woodford. (A) SE image overview of a patch of elliptical silica nanospheres (Si) and pore-filling organic matter interpreted as solid bitumen. Shrinkage cracks are indicated by yellow arrows. (B) Magnified view of image A illustrating porous nature of interpreted solid bitumen. Authigenic euhedral quartz crystals (Qtz xls) are also present. (C) Interparticle porosity between silica nanospheres and crystals (Si) is filled with interpreted solid bitumen (O/M) that exhibits varying degrees of secondary porosity development. (D) BSE image of same field of view as SE image in C illustrating compositional contrast by variation in gray scale.

Figure 6.

FE-SEM images of two Ar-ion polished samples of the Woodford. (A) SE image overview of a patch of elliptical silica nanospheres (Si) and pore-filling organic matter interpreted as solid bitumen. Shrinkage cracks are indicated by yellow arrows. (B) Magnified view of image A illustrating porous nature of interpreted solid bitumen. Authigenic euhedral quartz crystals (Qtz xls) are also present. (C) Interparticle porosity between silica nanospheres and crystals (Si) is filled with interpreted solid bitumen (O/M) that exhibits varying degrees of secondary porosity development. (D) BSE image of same field of view as SE image in C illustrating compositional contrast by variation in gray scale.

Detrital silt grains are easily recognized in standard petrographic thin sections as bright white specks in transmitted light. Based on visual estimates, detrital silt comprises less than 10% of every sample studied. The luminescence of detrital quartz contrasts markedly with the nonluminescent biogenic silica and authigenic microquartz (Figures 3, 5). The detrital quartz grains, some of which have quartz overgrowths, show no evidence of sorting. Instead, they appear randomly distributed and we interpret this as evidence that they were once eolian grains transported over the ancestral Tobosa Basin as dust and eventually settling to the basin floor.

Bulk core matrix porosity measurements based on GRI crushed rock analyses range between 5 and 9%. The FE-SEM images reveal that more interparticle nanoporosity is locally present in the siliceous rock matrix than that derived from GRI core measurements (e.g., Figures 3C, D; 6). The Woodford samples also contain nanopores in the amorphous organic matter surrounding the mineral matrix (Figure 6). Some of the organic matter is interpreted as secondary in origin (solid bitumen) where observed as a pore-filling material that surrounds euhedral quartz cement.

The silica nanospheres appear to have formed penecontemporaneously with pyrite framboids, which are widely considered a product of very early seafloor diagenesis (Wilkin et al., 1996; Folk, 2005; Agbi et al., 2015). The occurrence of silica nanospheres between quartz and other silt grains, which generally lack grain-to-grain contacts, suggests that the nanospheres formed as an early authigenic cement that served to bind the silt grains and prevent early sediment compaction. The volume of silica that precipitated is impressive, reducing the estimated 60–70% initial interparticle porosity to just 8–10%, yet preserving abundant microporosity between nanospheres (Figure 6).

Characteristics of the Mowry Shale

Characteristics of the Mowry Shale, and particularly the features that can be observed with FE-SEM, have been well summarized by Milliken and Olson (2017), so they are reviewed only briefly here. A comparison of the Mowry and Woodford FE-SEM images is shown in Figures 3, 5, 8. Bulk mineralogy is compared by the XRD data shown in Table 2 and Figure 7. Figure 7A compares cuttings data for the whole 550-ft (167-m)-thick Woodford section in the Hentz Family 7-1 well with the whole 220-ft (67-m)-thick section of Mowry cored in the Educated Guess 11-1 well. Figure 7B provides XRD data from the cored part of the Woodford Shale in the Hentz Family 7-1 well (thus these data are not susceptible to possible sample/cavings contamination) with a core from the Java State 16-1 cut in the middle more siliceous and relatively homogenous part of the Mowry. There is a much tighter clustering and overlap of the more focused core data sets shown in Figure 7B, but the mineralogic similarity in both plots is clear.

Figure 7.

XRD ternary compositional plots and wireline logs showing location of core and cuttings samples. (A) Ternary XRD diagram showing Woodford cuttings data from the Hentz Family 7-1 well (red triangles) compared with Mowry core data from the Educated Guess 11-1 well (blue circles). (B) Ternary XRD diagram comparing Woodford core data from the Hentz Family 7-1 well (red triangles) with Mowry core data from the Java State 16-1 well. Note similarity of the bulk mineralogical composition of the two formations. (C) Wireline logs showing depths of cutting and core sample intervals. Green intervals shown in A and blue intervals shown in B. GR = gamma ray; ResD = deep resistivity; RhoB = bulk density. Formation tops indicated: WDFD = Woodford; DVNN = Devonian; MWRY = Mowry; SHCK = Shell Creek. Note the displayed depth scale of the Hentz Family 7-1 is half that of the other two well logs.

Figure 7.

XRD ternary compositional plots and wireline logs showing location of core and cuttings samples. (A) Ternary XRD diagram showing Woodford cuttings data from the Hentz Family 7-1 well (red triangles) compared with Mowry core data from the Educated Guess 11-1 well (blue circles). (B) Ternary XRD diagram comparing Woodford core data from the Hentz Family 7-1 well (red triangles) with Mowry core data from the Java State 16-1 well. Note similarity of the bulk mineralogical composition of the two formations. (C) Wireline logs showing depths of cutting and core sample intervals. Green intervals shown in A and blue intervals shown in B. GR = gamma ray; ResD = deep resistivity; RhoB = bulk density. Formation tops indicated: WDFD = Woodford; DVNN = Devonian; MWRY = Mowry; SHCK = Shell Creek. Note the displayed depth scale of the Hentz Family 7-1 is half that of the other two well logs.

Figure 8.

CL images illustrating the weakly luminescent microquartz in the Mowry (A and B) and Woodford (C and D). (A) Weakly luminescent authigenic microquartz between scattered detrital components composed of quartz (q), albite (f), dolomite (d), and organic matter (OM), Bentley Fee 34-1, 9207 ft (2806 m). (B) Higher magnification view of common authigenic microquartz between detrital quartz silt grains (q), dolomite (d), and organic matter (OM), Trans Am 16-1, 8431 ft (2570 m). (C) Luminescent detrital quartz (q) and nonluminescent dolomite (d) silt grains surrounded by nonluminescent authigenic microquartz, Hentz Family 7-1, 11,492 ft (3507 m). (D) Higher magnification view of image C showing luminescent detrital quartz (q) and nonluminescent feldspar (f) silt grains surrounded by nonluminescent authigenic microquartz and pyrite (py).

Figure 8.

CL images illustrating the weakly luminescent microquartz in the Mowry (A and B) and Woodford (C and D). (A) Weakly luminescent authigenic microquartz between scattered detrital components composed of quartz (q), albite (f), dolomite (d), and organic matter (OM), Bentley Fee 34-1, 9207 ft (2806 m). (B) Higher magnification view of common authigenic microquartz between detrital quartz silt grains (q), dolomite (d), and organic matter (OM), Trans Am 16-1, 8431 ft (2570 m). (C) Luminescent detrital quartz (q) and nonluminescent dolomite (d) silt grains surrounded by nonluminescent authigenic microquartz, Hentz Family 7-1, 11,492 ft (3507 m). (D) Higher magnification view of image C showing luminescent detrital quartz (q) and nonluminescent feldspar (f) silt grains surrounded by nonluminescent authigenic microquartz and pyrite (py).

Table 2.

Comparison of compositions of the Mowry, Woodford, and Athel siliceous mudrocks based on XRD and petrographic data.

Stratigraphic TermMowryWoodfordAthel
Study materialCoreCoreCore
Thickness (ft)2205501200
Silica wt. %52.56580
Detrital quartz %64*3
Radiolaria %Unknown6*0
Clay/Illite wt. %27.52410
Pyrite wt. %554
Dolomite wt. %330
TOC (weight) wt. %253
Other wt. %10 (7% albite)33
Porosity %To 10%To 10%To 34%
Oil saturation %8 (Ro = 0.7%)50 (Ro = 0.9)80
Source of XRDJava State 16 #1Hentz Family 7-1Al Rajaibi Data
Stratigraphic TermMowryWoodfordAthel
Study materialCoreCoreCore
Thickness (ft)2205501200
Silica wt. %52.56580
Detrital quartz %64*3
Radiolaria %Unknown6*0
Clay/Illite wt. %27.52410
Pyrite wt. %554
Dolomite wt. %330
TOC (weight) wt. %253
Other wt. %10 (7% albite)33
Porosity %To 10%To 10%To 34%
Oil saturation %8 (Ro = 0.7%)50 (Ro = 0.9)80
Source of XRDJava State 16 #1Hentz Family 7-1Al Rajaibi Data
*

Visual estimate from 18 thin sections.

Episodic influxes of detrital silt and clay from highlands to the west into the Mowry Seaway, particularly during times of relatively low sea level, contributed to the lithologic heterogeneity of the Mowry (Socianu et al., 2015), and also contributed to more dysoxic bottom conditions than were present during deposition of the more anoxic Woodford Shale. Quartz content in the Educated Guess 11-1 Mowry core ranges from 12 to 68 wt. % and the clay content ranges from 13 to 55 wt. %, but it is important to note that this well is located toward the western side of the Powder River Basin, where the influx of detrital silt and clay was high. Clay content varies dramatically within the formation and is reflected in its relatively thin bedding, but in general, there is a reciprocal relationship between the amount of quartz and amount of detrital clay, most of which consists of illite/mica and illite/smectite. The most quartz-rich intervals were deposited during relative highstands of sea level when starved sediment conditions existed in the deep seaway (Socianu et al., 2015), favoring accumulation of biogenic quartz, mainly radiolarians.

Total organic carbon in the Mowry samples (following bitumen extraction) ranges from 1.5 to 3.7 wt. %, although a few samples exceed 4 wt. %. Thermal maturity measured by vitrinite reflectance and vitrinite reflectance equivalent from Tmax (e.g., Jarvie et al., 2001) ranges from 0.72 to 1.1% Ro and indicates the samples have reached the early oil to wet gas generation window.

Scanning electron and CL images of the Mowry samples (e.g., Figures 3A, B; 5A, B) show that the quartz occurs as (1) detrital grains of extrabasinal origin (brightly luminescent), (2) weakly luminescent detrital grains of biogenic intrabasinal origin such as radiolarians (Figure 5), and (3) weak-luminescent pore-filling cements and overgrowths (Figures 8, 9). What we are calling nanospheres can be seen at higher magnification on a broken surface (Figure 10) to be of submicron size with a subspherical shape and no apparent sharp-edged (euhedral) overgrowths. Although identifying true porosity with FE-SEM on broken surfaces is fraught with problems (e.g., grain plucking), at least some of the porosity between the nanospheres in Figure 10 appears to be real, based mainly on the three-dimensional nature of the pores between the silica particles. Overall measured matrix porosity in the Mowry cores, determined from GRI crushed rock analysis, ranges from 2 to 10%. However, the microporosity observed in the high magnification image (50,000×) in Figure 10 appears to exceed 10%.

Figure 9.

CL images showing overviews of selected Mowry samples from the Crossbow 2-18H well. (A–D) Blue and red luminescent detrital quartz silt grains are surrounded by nonluminescent authigenic microquartz with pyrite (py) framboids. Some detrital quartz silt grains exhibit weakly luminescent quartz overgrowths (yellow arrows). A, B, and D are from 11,059 ft (3371 m), and C is from 11,070 ft (3374 m).

Figure 9.

CL images showing overviews of selected Mowry samples from the Crossbow 2-18H well. (A–D) Blue and red luminescent detrital quartz silt grains are surrounded by nonluminescent authigenic microquartz with pyrite (py) framboids. Some detrital quartz silt grains exhibit weakly luminescent quartz overgrowths (yellow arrows). A, B, and D are from 11,059 ft (3371 m), and C is from 11,070 ft (3374 m).

Figure 10.

High-magnification (50,000×) SEM image illustrating clusters of nanoscale quartz crystals on the broken surface of the Mowry Shale. Crossbow 2-18H, 11,059 ft (3371 m). These microquartz crystals are similar in size, but not as round as those observed in the Woodford Shale (cf. Figure 4).

Figure 10.

High-magnification (50,000×) SEM image illustrating clusters of nanoscale quartz crystals on the broken surface of the Mowry Shale. Crossbow 2-18H, 11,059 ft (3371 m). These microquartz crystals are similar in size, but not as round as those observed in the Woodford Shale (cf. Figure 4).

Interpretation of the Authigenic Quartz

The striking similarity of two forms of authigenic microcrystalline quartz in the Woodford and Mowry (Figures 3, 8) argues for a common diagenetic origin. At one scale, the equant shape, the 1–3-micron size, the weakly luminescent character, and the widespread distribution in the mudrock matrix support the interpretation of this material as a pore-filling cement formed from alteration of an amorphous silica precursor (opal A, opal C-T; e.g., Fishman et al., 2013; Milliken and Olson, 2017) at some point during the burial history in which these mudrocks were still highly porous. However, the existence of an even finer submicron structure that comprises the bulk of the authigenic silica in these formations, seen as aggregates of nanospheres with the individual nanospheres mostly 200–400 nm in diameter, raises the possibility of a microbial influence on silica precipitation in these organic-rich mudrocks during very early syndepositional diagenesis. Such nanoscale crystal forms have been attributed to nucleation and growth of minerals in the presence of microbial cells and biofilms (Westall et al., 1995; Cisar et al., 2000; Toporski et al., 2002; Schieber and Arnott, 2003). The fact that the nanospheres occur in relatively sediment-starved, marine deposits that are enriched in pyrite, organic matter, uranium, and molybdenum (based on unpublished XRF data) indicates that dysoxic to anoxic conditions existed during the early burial history. Such an environment would have favored sulfate-reducing bacteria that could influence the precipitation of nanoscale quartz both directly and indirectly through effects on silica concentrations (Birnbaum and Wireman, 1984, 1985).

Modern seawater has a relatively low silica content ranging from < 1 parts per million in surface waters up to 10–15 ppm in bottom waters (Siever, 1992). Thus, seawater might seem like a poor fluid for the precipitation of silica by organisms, but this argument against precipitating silica from sea water is clearly specious. Many organisms including sponges, radiolarians, and diatoms, all with well-developed siliceous skeletons or tests, thrive in today’s seas. Devonian seas also had relatively low silica content, determined to be about 6–8 ppm based on study of Devonian brines in the Illinois Basin (Stueber and Walter, 1991), but supported thriving communities of radiolarians and sponges, both of which are common in parts of the Woodford and Mowry. If these siliceous organisms could thrive, it is likely that even smaller silica-precipitating microorganisms such as nanobacteria or other microbes also thrived.

Although rarely completely preserved, radiolarians are common in some, but not all, beds of the Woodford and Mowry. However, the volume of silica represented by the nanospheres would require improbable masses of radiolarians that are now completely dissolved to derive sufficient silica. The action of microbial silicification pathways opens the possibility that direct extraction of silica from sea water may also have been a source of the silica, thus reducing the overall amounts of radiolarians and/or diatoms required. This is exactly the process thought to have caused the precipitation of the silica in the earliest Cambrian Athel Silicilyte (Amthor et al., 2005; Al Rajaibi, 2011; Al Rajaibi et al., 2015), which is discussed in more detail in the following section.

A third source of silica in the Mowry was suggested by Rubey (1927, 1929) and Reeside and Cobban (1960) to be derived from bentonite beds that are common within the formation. Milliken and Olson (2017), however, discounted this possibility, noting the absence of massive silica alteration in the discrete ash beds and a general lack of the authigenic clays and zeolites typically associated with ash alteration in the Mowry.

A POSSIBLE ANCIENT ANALOG: THE ATHEL SILICILYTE

A search for ancient analogs of siliceous reservoir rocks with silica nanospheres similar to those observed in the Woodford and Mowry siliceous mudrocks led us to a series of papers published on the late Precambrian to early Cambrian Athel Silicilyte (aka Al Shomou Silicilyte) in the South Oman Salt Basin (e.g., Amthor et al., 2005; Al Rajaibi, 2011; Ramseyer et al., 2013; Al Rajaibi et al., 2015; Stolper, 2017). Table 2 provides a comparison of the characteristics of the Athel Silicilyte with the Woodford and Mowry.

The Athel was called a “silicilyte” because of its silica-rich nature, but that term is rarely applied to rocks outside of Oman (Alixant et al., 1998). Based on its high silica content and its common organic matter, the Athel can easily be classified as a siliceous mudrock comparable to those in the Woodford and Mowry. Cores of the Athel Silicilyte, which is up to 1300 ft (400 m) thick, reveal wavy discontinuous laminae interpreted as syndepositional microbial layers, deposited in organic-rich, anoxic, bottom waters in the deepest (below storm wave base) part of the South Oman Salt Basin (Al Rajaibi, 2011; Al Rajaibi et al., 2015). Scanning electron microscopy of the silicilyte shows that the rock is dominated by spheres of microcrystalline quartz (Figure 11) similar to, albeit commonly slightly larger at 1–5 microns, the silica nanospheres in the Woodford and Mowry described in this chapter.

Figure 11.

SE images of broken (A) and polished (B–C) early Cambrian Athel Silicilyte core samples from the South Oman Salt Basin showing the matrix to be dominated by submicron to micron-size silica nanospheres. (A) From Amthor et al. (2005, figure 15H, p. 107; reproduced by permission of GeoArabia) from the Al Noor-3, 15,604 ft (4756.1 m). (B and C) From Al Rajaibi (2011, their figure 7.3, p. 184), MK-1(4) and ALNR-3(13) cores, no depths provided.

Figure 11.

SE images of broken (A) and polished (B–C) early Cambrian Athel Silicilyte core samples from the South Oman Salt Basin showing the matrix to be dominated by submicron to micron-size silica nanospheres. (A) From Amthor et al. (2005, figure 15H, p. 107; reproduced by permission of GeoArabia) from the Al Noor-3, 15,604 ft (4756.1 m). (B and C) From Al Rajaibi (2011, their figure 7.3, p. 184), MK-1(4) and ALNR-3(13) cores, no depths provided.

One of the strongest lines of evidence that microbes such as bacteria played a role in formation of the silica nanospheres in the Woodford and Mowry is their similarity to the silica spheres forming most of the Athel Silicilyte in the South Oman Salt Basin. The Athel Silicilyte has been the focus of several key papers because of its importance as a significant source rock and oil reservoir, and its age predating the evolution of significant silica secreting organism such as radiolarians and diatoms (e.g., Amthor et al., 2005; Al Rajaibi, 2011; Al Rajaibi et al., 2015). These authors used many of the same techniques we used to analyze the Woodford and Mowry siliceous mudrocks including study of cores, thin-section petrography, XRD, and SEM with CL. Amthor et al. (1997) suggested that the Athel’s silica matrix was formed by sulfate-reducing bacteria (chemoautotrophs) that had the ability to remove silica from seawater (either as opal or as microquartz) to form the initial silicilyte sediment.

All of these papers conclude that the Athel Silicilyte was deposited on a relatively deep, basinal, anoxic sea floor below storm wave base, estimated at > 300 ft (> 100 m), beneath a stratified water column, and all suggest that microbial activity played a role in forming the silica nanospherules. Further support for the possible importance of microbes in the Athel sediments is the major role they played in precipitation of the platform carbonates that rimmed the Athel basin as described in detail by Grotzinger and Al-Rawahi (2014). Clearly microbial life thrived in sedimentary rocks deposited around the South Oman Salt Basin near the time of the Precambrian/Cambrian boundary and it seems likely that other microbes lived in the deeper, more organic-rich part of the basin.

Stolper et al. (2017) took the analysis of the Athel Silicilyte reservoirs another step forward with a detailed look at the origin of the oil. They concluded on the basis of oil geochemistry and biomarkers that the silica nucleated in the water column on organic matter and then sank to the sea floor where isotopic evidence indicates the original presumably amorphous silica transformed to quartz during kilometers of burial as temperature increased and/or pore water chemistry changed.

DISCUSSION

Because the silica nanospheres so common in both the Woodford and Mowry are generally less than half a micron in diameter, they are invisible to the naked eye and in standard petrographic thin sections. Thus, it is no surprise that they have gone unrecognized during the decades that these hydrocarbon source rocks were studied (e.g., Byers and Larson, 1979; Burtner and Warner, 1984; Davis et al., 1989; Comer, 1991; Hemmesch et al., 2014). Only with high-resolution scanning electron microscopy in recent years has the abundance of very fine-grained (nanometer scale) quartz in the Woodford (Drake et al., 2017) and Mowry (Milliken and Olson, 2017) been recognized and clearly imaged.

The basin-centered deposits of the Upper Devonian Woodford Shale of the Permian Basin, the Lower Cretaceous Mowry Shale of the Powder River Basin, and the earliest Cambrian Athel Silicilyte in the South Oman Salt Basin share several features in common. They were all deposited in relatively deep water below storm wave base; they are all rich in authigenic silica and contain common organic matter and pyrite; they all have limited amounts of detrital clays and quartz silt suggesting starved basin conditions during deposition; and all were deposited under conditions of low oxygenation. The Woodford and Mowry also contain radiolarians and some siliceous sponge spicules, but those organisms are absent in the Athel Silicilyte (Amthor et al., 2005; Al Rajaibi, 2011; Ramseyer et al., 2013).

Other published literature further supports the likelihood that bacteria can play a role in silica precipitation. Early experimental work by Birnbaum and Wireman (1984, 1985) showed that bacterial sulfate reduction caused silica to precipitate from solution and suggested that this process played a role in chert formation in the Precambrian banded iron formations. More recently, Fein et al. (2002) noted the role of Bacilis subtilis in precipitating silica on its cell walls. Many other papers dealing with this subject including silica precipitation by bacteria in natural hot springs are also cited by Stolper et al. (2017). Nanobacteria have been implicated in a variety of other depositional environments (Folk, 1993), as well as in the formation of illite clay cement (Folk and Lynch, 1997). Within the past two decades, the importance of microbial precipitation has also increased dramatically and led in 2013 to an AAPG Hedberg Conference on microbial activity in carbonate rocks (Mancini et al., 2013). Furthermore, Grotzinger and Al-Rawahi (2014) emphasized the importance of microbial precipitation in the early Cambrian carbonate platforms surrounding the Athel Basin and show that microbes were active in that area at that time. All of these references indicate that microbial activity can play a major role in precipitation of nanoscale mineral particles comparable to the silica nanospheres we see in the Woodford and Mowry shales.

The source of the common authigenic silica found in the units described here remains open to debate. This silica commonly takes the form of quartz spherules that are relatively consistent in size within a given formation, but range in size from about 200 nm to several microns. Both the Woodford and Mowry also contain radiolarians that have been suggested as the source of the silica in the matrix through a dissolution and reprecipitation process (e.g., Milliken and Olson, 2017), but the Athel Silicilyte predates the evolution of radiolarians by many millions of years. Results presented here suggest that routine inspection of authigenic microquartz in shales (such as the microquartz interpreted as recrystallized lepispheres) at high magnifications using FE-SEM may reveal a more widespread occurrence of this silica nanoscale fabric.

Although numerous papers document the impact that bacteria, particularly sulfate-reducing bacteria, can have on silica precipitation (Westall et al., 1995; Cisar et al., 2000; Toporski et al, 2002; Schieber and Arnott, 2003), few papers other than those focused on the Athel Silicilyte have suggested a bacterial influence on Phanerozoic siliceous reservoirs. This is probably due in part to the common finding of siliceous radiolarians and sponge spicules in post-early Cambrian rocks, which offer an obvious source of silica. However, the abundance and very early formation of the silica nanospheres in the Woodford and Mowry siliceous mudrocks, particularly relative to the paucity or absence of radiolarians and sponge spicules in many of the silica-rich beds, suggests that microbial activity similar to that during Athel Silicilyte deposition occurred in the Devonian, Cretaceous, and other Phanerozoic deep-water siliceous mudrocks.

CONCLUSIONS

  1. Quartz in the Woodford Shale and Mowry Shale includes a mix of biogenic grains (mainly radiolarians) and authigenic silica along with some detrital quartz silt of extrabasinal origin.

  2. Authigenic quartz in these units commonly takes the form of low-luminescent silica nanospheres, a morphological form of microquartz that is less than a half micron in diameter.

  3. The volume of the authigenic quartz, together with “floating” detrital components and the close association of microquartz with pyrite framboids, indicates that the silica nanospheres formed very early, perhaps in association with microbial activity on or in the seafloor sediments.

  4. Where the silicification process ceased prior to complete silica cementation, the early silica nanospheres are associated with up to 15% intercrystalline microporosity. In addition, solid bitumen, where present between the silica nanospheres and where thermally mature, also contains pores within the organic matter. This gives the Woodford and Mowry good potential reservoir quality, at least locally.

ACKNOWLEDGMENTS

The idea to prepare this joint chapter comparing the Woodford and Mowry siliceous mudrocks grew from a presentation seen by the senior author on the Mowry (now published by Milliken and Olson, 2017) at the AAPG Hedberg Conference on Mudrock Diagenesis held in Santa Fe, New Mexico, October 2016. Scanning electron microscopy work on the Woodford Shale completed by QEP Resources in conjunction with Weatherford Laboratories (Jaime Kostelnik and Will Dorsey) revealed silica nanospheres in the siliceous matrix that were similar to those present in images shown by Milliken and Olson. Following the Hedberg Conference, the editors of this AAPG Memoir solicited abstracts for this volume.

The Mowry samples examined by Milliken and Olson (2017) were kindly contributed along with supplementary data (e.g., Rock-Eval, XRD, and GRI core analyses) by three anonymous companies. QEP Resources is thanked for providing access to the Woodford Shale core and its supplemental data. Access to the two Mowry cores discussed in this chapter, the Java State 16-1 and Educated Guess 11-1, and their supplemental data was through the USGS Core Research Center in Lakewood, Colorado. We also thank QEP Resources for permission to publish the results of the company’s work on the Woodford Shale core.

We appreciate the constructive reviews provided by Joan Spaw and an anonymous reviewer and the editorial guidance of Wayne Camp.

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Figures & Tables

Figure 1.

Map of structural basins in the western United States showing locations of the two study areas. The red stars indicate the locations of the Hentz Family 7-1 Woodford core on the Central Basin Platform of the Permian Basin, and the Java State 16-1 Mowry core in the Powder River Basin. Outlines of the mapped areas in Figure 2 are also shown.

Figure 1.

Map of structural basins in the western United States showing locations of the two study areas. The red stars indicate the locations of the Hentz Family 7-1 Woodford core on the Central Basin Platform of the Permian Basin, and the Java State 16-1 Mowry core in the Powder River Basin. Outlines of the mapped areas in Figure 2 are also shown.

Figure 2.

Generalized depositional thickness maps of the Woodford and Mowry shales. (A) Woodford Shale depositional thicknesses adapted from Wright (1979, p. 62), contour interval is 200 ft (60 m). Star denotes the location of the Hentz Family 7-1 well. (B) Mowry Shale depositional thickness after Nixon (1973), contour interval is 100 ft (30 m). Lettered stars refer to wells mentioned in the text: (A) Educated Guess 11-1H, (B) 1 Bentley Fee 34-1, (C) Java State 16-1, (D) Trans Am 1-16H, (E) Crossbow 2-18H, and (F) Marys Draw 31-23M.

Figure 2.

Generalized depositional thickness maps of the Woodford and Mowry shales. (A) Woodford Shale depositional thicknesses adapted from Wright (1979, p. 62), contour interval is 200 ft (60 m). Star denotes the location of the Hentz Family 7-1 well. (B) Mowry Shale depositional thickness after Nixon (1973), contour interval is 100 ft (30 m). Lettered stars refer to wells mentioned in the text: (A) Educated Guess 11-1H, (B) 1 Bentley Fee 34-1, (C) Java State 16-1, (D) Trans Am 1-16H, (E) Crossbow 2-18H, and (F) Marys Draw 31-23M.

Figure 3.

FE-SEM images of the Mowry from the Bentley Fee 34-1 from 9207 ft (2086 m) and the Woodford from 11,492 ft (3507 m) showing EDS element maps (A and C) and cathodoluminescence (B and D). EDS element maps show sodium (light blue), calcium (dark blue), and silica (red); other elements are shown in gray. Detrital quartz (q) is identified by its colorful luminescence (B and D). Nonluminescent authigenic quartz present as silica nanospheres (circled) in the matrix and as reworked quartz overgrowths (arrows) rimming detrital quartz grains. Dolo = dolomite; m = mica; and py = pyrite framboid.

Figure 3.

FE-SEM images of the Mowry from the Bentley Fee 34-1 from 9207 ft (2086 m) and the Woodford from 11,492 ft (3507 m) showing EDS element maps (A and C) and cathodoluminescence (B and D). EDS element maps show sodium (light blue), calcium (dark blue), and silica (red); other elements are shown in gray. Detrital quartz (q) is identified by its colorful luminescence (B and D). Nonluminescent authigenic quartz present as silica nanospheres (circled) in the matrix and as reworked quartz overgrowths (arrows) rimming detrital quartz grains. Dolo = dolomite; m = mica; and py = pyrite framboid.

Figure 4.

Secondary electron images of broken surfaces from four different Woodford samples (A–D) illustrating microporosity in the siliceous matrix. Scale bars and sample depths are provided at lower right in each photo. (A) Partly pyritized monaxon sponge spicule in a microporous siliceous matrix. (B) Higher magnification view showing silica nanospheres (Si). (C) Euhedral pyrite crystals in a matrix of silica nanospheres averaging < 300 microns in size. (D) Pyrite framboid in a matrix of silica nanospheres and botryoids (Si) and illite.

Figure 4.

Secondary electron images of broken surfaces from four different Woodford samples (A–D) illustrating microporosity in the siliceous matrix. Scale bars and sample depths are provided at lower right in each photo. (A) Partly pyritized monaxon sponge spicule in a microporous siliceous matrix. (B) Higher magnification view showing silica nanospheres (Si). (C) Euhedral pyrite crystals in a matrix of silica nanospheres averaging < 300 microns in size. (D) Pyrite framboid in a matrix of silica nanospheres and botryoids (Si) and illite.

Figure 5.

Comparison of siliceous radiolarians (rad) in the Mowry from the Trans Am 1-16H core from 8447 ft (2575 m) (A and B) and Woodford core from 11,469 (3496 m) (C and D). EDS element maps (A and C) show sodium (light blue), calcium (dark blue), and silica (red); other elements are shown in gray. (C) Orange luminescent matrix reflects presence of K in illite. (D) CL image of the Woodford showing quartz-cemented radiolarians partly replaced with crystalline pyrite (py) and cemented with chalcedony (chal), some of which shows distinct growth bands. Early diagenetic microquartz appears light to dark gray with weak luminescence.

Figure 5.

Comparison of siliceous radiolarians (rad) in the Mowry from the Trans Am 1-16H core from 8447 ft (2575 m) (A and B) and Woodford core from 11,469 (3496 m) (C and D). EDS element maps (A and C) show sodium (light blue), calcium (dark blue), and silica (red); other elements are shown in gray. (C) Orange luminescent matrix reflects presence of K in illite. (D) CL image of the Woodford showing quartz-cemented radiolarians partly replaced with crystalline pyrite (py) and cemented with chalcedony (chal), some of which shows distinct growth bands. Early diagenetic microquartz appears light to dark gray with weak luminescence.

Figure 6.

FE-SEM images of two Ar-ion polished samples of the Woodford. (A) SE image overview of a patch of elliptical silica nanospheres (Si) and pore-filling organic matter interpreted as solid bitumen. Shrinkage cracks are indicated by yellow arrows. (B) Magnified view of image A illustrating porous nature of interpreted solid bitumen. Authigenic euhedral quartz crystals (Qtz xls) are also present. (C) Interparticle porosity between silica nanospheres and crystals (Si) is filled with interpreted solid bitumen (O/M) that exhibits varying degrees of secondary porosity development. (D) BSE image of same field of view as SE image in C illustrating compositional contrast by variation in gray scale.

Figure 6.

FE-SEM images of two Ar-ion polished samples of the Woodford. (A) SE image overview of a patch of elliptical silica nanospheres (Si) and pore-filling organic matter interpreted as solid bitumen. Shrinkage cracks are indicated by yellow arrows. (B) Magnified view of image A illustrating porous nature of interpreted solid bitumen. Authigenic euhedral quartz crystals (Qtz xls) are also present. (C) Interparticle porosity between silica nanospheres and crystals (Si) is filled with interpreted solid bitumen (O/M) that exhibits varying degrees of secondary porosity development. (D) BSE image of same field of view as SE image in C illustrating compositional contrast by variation in gray scale.

Figure 7.

XRD ternary compositional plots and wireline logs showing location of core and cuttings samples. (A) Ternary XRD diagram showing Woodford cuttings data from the Hentz Family 7-1 well (red triangles) compared with Mowry core data from the Educated Guess 11-1 well (blue circles). (B) Ternary XRD diagram comparing Woodford core data from the Hentz Family 7-1 well (red triangles) with Mowry core data from the Java State 16-1 well. Note similarity of the bulk mineralogical composition of the two formations. (C) Wireline logs showing depths of cutting and core sample intervals. Green intervals shown in A and blue intervals shown in B. GR = gamma ray; ResD = deep resistivity; RhoB = bulk density. Formation tops indicated: WDFD = Woodford; DVNN = Devonian; MWRY = Mowry; SHCK = Shell Creek. Note the displayed depth scale of the Hentz Family 7-1 is half that of the other two well logs.

Figure 7.

XRD ternary compositional plots and wireline logs showing location of core and cuttings samples. (A) Ternary XRD diagram showing Woodford cuttings data from the Hentz Family 7-1 well (red triangles) compared with Mowry core data from the Educated Guess 11-1 well (blue circles). (B) Ternary XRD diagram comparing Woodford core data from the Hentz Family 7-1 well (red triangles) with Mowry core data from the Java State 16-1 well. Note similarity of the bulk mineralogical composition of the two formations. (C) Wireline logs showing depths of cutting and core sample intervals. Green intervals shown in A and blue intervals shown in B. GR = gamma ray; ResD = deep resistivity; RhoB = bulk density. Formation tops indicated: WDFD = Woodford; DVNN = Devonian; MWRY = Mowry; SHCK = Shell Creek. Note the displayed depth scale of the Hentz Family 7-1 is half that of the other two well logs.

Figure 8.

CL images illustrating the weakly luminescent microquartz in the Mowry (A and B) and Woodford (C and D). (A) Weakly luminescent authigenic microquartz between scattered detrital components composed of quartz (q), albite (f), dolomite (d), and organic matter (OM), Bentley Fee 34-1, 9207 ft (2806 m). (B) Higher magnification view of common authigenic microquartz between detrital quartz silt grains (q), dolomite (d), and organic matter (OM), Trans Am 16-1, 8431 ft (2570 m). (C) Luminescent detrital quartz (q) and nonluminescent dolomite (d) silt grains surrounded by nonluminescent authigenic microquartz, Hentz Family 7-1, 11,492 ft (3507 m). (D) Higher magnification view of image C showing luminescent detrital quartz (q) and nonluminescent feldspar (f) silt grains surrounded by nonluminescent authigenic microquartz and pyrite (py).

Figure 8.

CL images illustrating the weakly luminescent microquartz in the Mowry (A and B) and Woodford (C and D). (A) Weakly luminescent authigenic microquartz between scattered detrital components composed of quartz (q), albite (f), dolomite (d), and organic matter (OM), Bentley Fee 34-1, 9207 ft (2806 m). (B) Higher magnification view of common authigenic microquartz between detrital quartz silt grains (q), dolomite (d), and organic matter (OM), Trans Am 16-1, 8431 ft (2570 m). (C) Luminescent detrital quartz (q) and nonluminescent dolomite (d) silt grains surrounded by nonluminescent authigenic microquartz, Hentz Family 7-1, 11,492 ft (3507 m). (D) Higher magnification view of image C showing luminescent detrital quartz (q) and nonluminescent feldspar (f) silt grains surrounded by nonluminescent authigenic microquartz and pyrite (py).

Figure 9.

CL images showing overviews of selected Mowry samples from the Crossbow 2-18H well. (A–D) Blue and red luminescent detrital quartz silt grains are surrounded by nonluminescent authigenic microquartz with pyrite (py) framboids. Some detrital quartz silt grains exhibit weakly luminescent quartz overgrowths (yellow arrows). A, B, and D are from 11,059 ft (3371 m), and C is from 11,070 ft (3374 m).

Figure 9.

CL images showing overviews of selected Mowry samples from the Crossbow 2-18H well. (A–D) Blue and red luminescent detrital quartz silt grains are surrounded by nonluminescent authigenic microquartz with pyrite (py) framboids. Some detrital quartz silt grains exhibit weakly luminescent quartz overgrowths (yellow arrows). A, B, and D are from 11,059 ft (3371 m), and C is from 11,070 ft (3374 m).

Figure 10.

High-magnification (50,000×) SEM image illustrating clusters of nanoscale quartz crystals on the broken surface of the Mowry Shale. Crossbow 2-18H, 11,059 ft (3371 m). These microquartz crystals are similar in size, but not as round as those observed in the Woodford Shale (cf. Figure 4).

Figure 10.

High-magnification (50,000×) SEM image illustrating clusters of nanoscale quartz crystals on the broken surface of the Mowry Shale. Crossbow 2-18H, 11,059 ft (3371 m). These microquartz crystals are similar in size, but not as round as those observed in the Woodford Shale (cf. Figure 4).

Figure 11.

SE images of broken (A) and polished (B–C) early Cambrian Athel Silicilyte core samples from the South Oman Salt Basin showing the matrix to be dominated by submicron to micron-size silica nanospheres. (A) From Amthor et al. (2005, figure 15H, p. 107; reproduced by permission of GeoArabia) from the Al Noor-3, 15,604 ft (4756.1 m). (B and C) From Al Rajaibi (2011, their figure 7.3, p. 184), MK-1(4) and ALNR-3(13) cores, no depths provided.

Figure 11.

SE images of broken (A) and polished (B–C) early Cambrian Athel Silicilyte core samples from the South Oman Salt Basin showing the matrix to be dominated by submicron to micron-size silica nanospheres. (A) From Amthor et al. (2005, figure 15H, p. 107; reproduced by permission of GeoArabia) from the Al Noor-3, 15,604 ft (4756.1 m). (B and C) From Al Rajaibi (2011, their figure 7.3, p. 184), MK-1(4) and ALNR-3(13) cores, no depths provided.

TOC and XRD results for 14 selected core samples from the five wells that formed the basis of the study of the Mowry silica diagenesis study by Milliken and Olson (2017).

Table 1.
TOC and XRD results for 14 selected core samples from the five wells that formed the basis of the study of the Mowry silica diagenesis study by Milliken and Olson (2017).
  DepthMaturity Mineral Composition by XRD (wt. %)
Well NameDepthMetersRoTOCQuartzFeldsparCalciteDolomitePyriteTotal Clay
Trans Am 1-16H8340.62542.20.932.16616.40.00.03.829
Trans Am 1-16H8365.42549.80.923.75666.30.00.08.719
Trans Am 1-16H8431.52569.90.922.83638.50.03.28.517
Trans Am 1-16H8446.72574.60.921.55534.30.00.04.636
Bentley Fee 34-192072806.30.793.36697.00.00.04.019
Java State 16-1101563095.50.723.49655.30.00.04.325
Java State 16-11017431010.752.18822.20.21.42.611
Java State 16-1101893105.60.752.47666.90.53.75.917
Crossbow 2-18H110593370.81.133.65407.00.01.09.042
Crossbow 2-18H11069.83374.11.133.31577.00.01.07.027
Crossbow 2-18H11083.63378.31.152.39597.00.03.06.024
Marys Draw 31-23M114213481.11.003.19647.10.01.95.521
Marys Draw 31-23M11433.13484.81.001.62544.30.00.05.836
Marys Draw 31-23M11441.33487.31.002.87525.60.00.77.134
Averages    60.96.10.051.145.9125.5
  DepthMaturity Mineral Composition by XRD (wt. %)
Well NameDepthMetersRoTOCQuartzFeldsparCalciteDolomitePyriteTotal Clay
Trans Am 1-16H8340.62542.20.932.16616.40.00.03.829
Trans Am 1-16H8365.42549.80.923.75666.30.00.08.719
Trans Am 1-16H8431.52569.90.922.83638.50.03.28.517
Trans Am 1-16H8446.72574.60.921.55534.30.00.04.636
Bentley Fee 34-192072806.30.793.36697.00.00.04.019
Java State 16-1101563095.50.723.49655.30.00.04.325
Java State 16-11017431010.752.18822.20.21.42.611
Java State 16-1101893105.60.752.47666.90.53.75.917
Crossbow 2-18H110593370.81.133.65407.00.01.09.042
Crossbow 2-18H11069.83374.11.133.31577.00.01.07.027
Crossbow 2-18H11083.63378.31.152.39597.00.03.06.024
Marys Draw 31-23M114213481.11.003.19647.10.01.95.521
Marys Draw 31-23M11433.13484.81.001.62544.30.00.05.836
Marys Draw 31-23M11441.33487.31.002.87525.60.00.77.134
Averages    60.96.10.051.145.9125.5
Table 2.

Comparison of compositions of the Mowry, Woodford, and Athel siliceous mudrocks based on XRD and petrographic data.

Stratigraphic TermMowryWoodfordAthel
Study materialCoreCoreCore
Thickness (ft)2205501200
Silica wt. %52.56580
Detrital quartz %64*3
Radiolaria %Unknown6*0
Clay/Illite wt. %27.52410
Pyrite wt. %554
Dolomite wt. %330
TOC (weight) wt. %253
Other wt. %10 (7% albite)33
Porosity %To 10%To 10%To 34%
Oil saturation %8 (Ro = 0.7%)50 (Ro = 0.9)80
Source of XRDJava State 16 #1Hentz Family 7-1Al Rajaibi Data
Stratigraphic TermMowryWoodfordAthel
Study materialCoreCoreCore
Thickness (ft)2205501200
Silica wt. %52.56580
Detrital quartz %64*3
Radiolaria %Unknown6*0
Clay/Illite wt. %27.52410
Pyrite wt. %554
Dolomite wt. %330
TOC (weight) wt. %253
Other wt. %10 (7% albite)33
Porosity %To 10%To 10%To 34%
Oil saturation %8 (Ro = 0.7%)50 (Ro = 0.9)80
Source of XRDJava State 16 #1Hentz Family 7-1Al Rajaibi Data
*

Visual estimate from 18 thin sections.

Contents

GeoRef

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