Sedimentological and geochemical variability of mudrocks in coal-bearing successions of the Carbondale Group (Pennsylvanian) in southwest Indiana: a core-based evaluation

William S. Elliott Jr. Department of Geology and Physics, University of Southern Indiana, 8600 University Blvd., Evansville, Indiana 47712; wselliott@usi.edu

American Association of Petroleum Geologists, Denver, 2015

Sedimentological features observed in cores, along with detailed geochemical data, provide valuable insights into the variability of roof shales in Pennsylvanian rocks of the Midcontinent. Specifically, mudrocks provides valuable insights into their depositional setting, environmental conditions, and diagenetic processes. Furthermore, petrographic examination of thin-sections provide details about sedimentary fabrics and textures preserved in mudrocks. In this study, four cores of mudrocks overlying coal seams were examined to determine the variability of roof shales overlying coals of the Carbondale Group (Pennsylvanian), from southwest Indiana (Fig. 1). Additionally, this study will be used in predicting the ability of roof shales to serve as a tight seal for coalbed methane production, and/or natural gas production from the black shales.

Well USI 1-32 is located near the campus of the University of Southern Indiana (USI) in Vanderburgh County, Indiana. The total depth of the well is 780 feet, with cored intervals from: (1) 395 to 415; (2) 495 to 515; (3) 540 to 560; and (4) 655 to 675 feet (Fig. 2). The studied cores are from the following units of the Carbondale Group: (1) Springfield Coal of the Petersburg Formation; (2) Houchin Creek Coal and overlying Excello Shale of the Petersburg Formation; (3) Survant Coal of the Linton Formation; and (4) Seelyville Coal of the Staunton Formation (Fig. 2). In addition, there is also a core of the West Franklin Limestone (80 to 100 ft), but it was not examined for this study.

MethodsCores with a 3-inch diameter (7.6 cm) from the USI 1-32 well were cut using a 8-inch trim saw using dry techniques. Cut surfaces were then prepared for examination using sandpaper, beginning with 320-grit and progressing to 1,200-grit. A polymer eraser and/or polishing cloth were used to finish preparing the surface of the core. Upon polishing, the cores were examined with a hand lens (10x) and a binocular microscope (20x). From preliminary observations, samples were selected for thin-sections. Billets were cut from slabs of the core, and the surface prepared for mounting. All preparation was conducted using dry grinding techniques. Hillquist epoxy A-B was used to mount the glass slides to the prepared billet surface. An Ingram Thin-Section Machine was used to cut the billet and grind the rock surface to about 40 microns using dry techniques. Final grinding of thin-sections was conducted using 400- to 600-grit sandpaper, and coverslips mounted with Hillquist C-D epoxy. Final thickness of mudrock thin-sections ranged from 30 to 25 microns (25 microns provides better transmission of light for viewing in thin-section). Photomicrographs were captured using an Olympus 10XR37 binocular microscope with digital camera. Image Capture Q Pro software was used to capture photomicrographs and Corel Photo-Paint X7 was used to process images.

Introduction Discussion

Black shales overlying the Springfield and Houchin Creek Coals were probably deposited in a shallow marine environment with aerobic bottom waters. The presence of pyrite is consistent with sulfate reduction of marine waters in an anaerobic environment below the sediment water interface. This is supported by the observation that the pyrite appears to be secondary, disseminated in the sediment. In both cores, the black shale transitions stratigraphically upward to skeletal wackestones and packstones containing abundant marine fauna, including crinoids, brachiopods, foraminifera, and mollusks. Thus, the roof shales above the Springfield and Houchin Creek coals are interpreted to have been deposited in a transgressive marine setting, as evidenced by elevated sulfur content, enriched uranium and heavy metal content, high TOC, phosphatic debris, and the marine fossils.

The mudrock overlying the Survant and Seelyville Coals are light gray to greenish gray with sparse remains of vascular plants including ferns and lycopsids. These mudrocks also contain sparse siderite concretions consistent with brackish water conditions. These mudrocks are interpreted to have been deposited in continental settings as evidenced by low sulfur content and sparse vascular plant debris. This transitional marine setting may be the result of a transgression, or possibly lateral migration of nearshore environments.

The variation in composition, fossil content, sedimentary fabrics, bioturbation, and TOC implies diverse depositional settings and environmental conditions of mudrocks overlying coals in the Middle Pennsylvanian of southwest Indiana. For example, the mudrocks overlying the Springfield and Seelyville coals are interpreted to be the result of deposition associated with a transgressive marine setting. In contrast, the mudrocks overlying the Survant and Seelyville Coals are interpreted to be deposited in a continental setting with minimal marine incursion. The variation in depositional settings of these Pennsylvanian mudrocks ultimately dictates the sulfur content of their respective underlying coal. Finally, the organic content and thickness of Pennsylvanian black shales, especially the Excello, Mecca Creek, and Logan Quarry in the Illinois Basin, may be viable targets for the production of shale gas.

Conclusion

• Mudrocks overlying coals in the Carbondale Group show variation in ichnofabric, fossil content, and mineralogy indicating wide ranging depositional settings following coal deposition.

• Black shales overlying the Springfield and Houchin Creek Coals are interpreted to represent deposition during a transgressive marine setting. These black shales contain disseminated pyrite, phosphate nodules, and abundant organic matter.

• Greenish gray shales overlying the Survant and Seelyville Coals are interpreted to represent deposition in a transitional marine environment that may be induced by a marine transgression or lateral migration of nearshore depositional systems, such as lagoons.

• The sulfur content of coal is directly related to the depositional setting that existed after coal deposition. Low sulfur coals are overlain by continental mudrocks or deposits with limited marine incursion. High sulfur coals are overlain by marine black shales that permitted accumulation of sulfur by reduction of sulfate from marine waters.

Illinois Indiana

Kentucky

Vanderburgh

County

Mt. Vernon

Morganfield

Carmi

NAlbion

Petersburg

Study Site

3

2

1

1

1

2

2

2

33

3

LEGEND

PennsylvanianMissourian Series

PennsylvanianDesmoinesian Series

PennsylvanianAtokan & Morrowan Series

Southern

Limit ofGlaciation

Index Map

0 10 20

20 30100kilometers

miles

88 W

38 N

87 W

EvansvilleUSI 1-32

Ohio River

1

Figure 1: Geologic map of the study area (modified from King and Beikman, 1974) showing the location of USI 1-32. These Pennsylvanian exposures are on the southeast margin of the Illinois Basin.

Figure 2: Stratigraphic column, age designation, and gamma-ray log for USI 1-32 used in this study. Stratigraphic units and nomenclature based primarily from Shaver et al. (1986) and Droste & Horowitz (1998).

Acknowledgements

Internal texture of a “nodule” encased with greenish gray mudrock.

Broken and abraded skeletal fragments of brachiopods, mollusks, and crinoids in a light gray skeletal wackestones to packstone.

I would like to thank Clinton Broach, Ryan Voegrel, and Aaron Feldhaus, former undergraduate geology students at USI, for their initial work on cores. This research also benefited from discussions with Jack Beuthin, James Durbin, William Gilhooly, Norman King, and Kent Scheller. Many thanks to Chris Smith at Weatherford Labs for geochemical data used in this study.

Dry preparation methods were used to maintain the integrity of the mudrock for this study. Proper safety gear was used to minimize exposure to dust. A Shop Vac minimized the amount of particulate dust produced by cutting rock samples dry.

0.5 mm

Sedimentological Features of Roof Shales of Pennsylvanian Coals

USI 1-32 cores used in this study. Samples of core were removed carefully from these boxes for processing, noting their orientation and depth. The slabs were then transferred to new core boxes and the surfaces polished using dry techniques.

Detailed sedimentological study of the roof shales overlying coals in the cores collected from USI 1-32 yielded to types: (1) Black Shales, and (2) Greenish Gray Mudrock with Siderite Concretions. Sedimentary structures, fabrics, body fossils, trace fossils, and mineralogy of these roof shales were compiled using megascopic observations of the core, and petrographic examination of selected thin-sections from the core. Detailed observations for each of these roof shales is provided.

Black Shale Greenish Gray Mudrock with Siderite Concretions

750

700

650

600

550

500

450

400

350

300

250

200

150

100

50

0

Springfield Coal

Houchin Creek Coal

Excello Shale

Seelyville Coal

Survant Coal

West FranklinLimestone

Inglefied Sandstone

Herrin Coal

Danville Coal

0 150GAPI

300

Patoka

Shelburn

McL

ea

nsbo

roC

arb

ond

ale

Gro

up

Dugger

Petersburg

Linton

Staunton

Racco

on

Cre

ek

FORMATION MEMBER

MIS

SO

UR

IAN

DE

SM

OIN

ES

IAN

GR

OU

P

SE

RIE

S USI 1-32

Gamma Ray Log

LIT

HO

LO

GY

(me

ters

)

DE

PT

H

(fe

et)

Mecca Shale

Colchester Coal

Logan Quarry Shale

450

1

2

3

4

5

Cored Intervals

Well Cuttings

unnamed shale

unnamed shale

unnamed shale

unnamed shale

Ditney Coal

0

50

100

150

200

Spri

ngf

ield

Co

al

Ho

uch

in C

reek

Co

al

Du

gger

Fo

rmat

ion

Exce

llo S

hal

e M

emb

er o

f th

e Pe

ters

bu

rg F

orm

atio

n

Surv

ant

Co

alPe

ters

bu

rg F

orm

atio

n

Seel

yvill

e C

oal

Lin

ton

Fo

rmat

ion

Burrows filled with dark gray mud in greenish gray mudrock. Burrows are probably Phycosiphon.

The top of the Seelyville Coal marks the boundary between the Staunton Formation of the Raccoon Creek Group and the overlying Linton Formation of the Carbondale Group. In this cored interval, the Seelyville Coal is overlain by greenish gray mudrock with sparse siderite nodules. The contact between the coal and mudrock is marked by a 3 to 5 mm thick, wavy layer of disseminated pyrite.

Internal texture of a siderite concretion that is rimmed with disseminated pyrite. Also note the moderate bioturbation preserved in the concretion.

1 mm

1 mm Contact between bioturbated mudrock and overlying skeletal wackestones to packstones. Burrows are filled with calcite.

1 mm

1 mm

Finely laminated, black mudrock with sparse phosphatic debris (reddish brown) and flattened skeletal fragments of forams and/or shell fragments.

Bioturbated finely laminated mudrock with sparse phosphatic debris (reddish brown) and flattened skeletal fragments of forams and/or shell fragments. Note burrows are filled with spar.

The top of the Springfield Coal marks the boundary between the Petersburg Formation and the overlying Dugger Formation. The contact between the coal and overlying black shale is marked by a layer of pyrite.

Laminated skeletal wackestones with broken and abraded skeletal fragments of forams and crinoidal debris. Note that the skeletal debris is concentrated along discrete lamina.

1 mm

1 mm

1 mm

1 mm

Finely laminated, black mudrock with sparse phosphatic debris (reddish brown) and flattened skeletal fragments of forams and/or shell fragments.

1 mm

Calcareous laminated, black mudrock with several vertical fractures filled with coarse spar.

Peloidal, bioturbated, calcareous mudrock with disseminated pyrite.

Very thin bed of calcareous, well sorted, phosphatic, peloidal grainstone. Thin beds and lamina of phosphatic peloids are interbedded with finely laminated, black mudrock (black shale).

A burrow that followed a phosphatic-rich bed in a finely laminate black mudrock. Note the variability of the burrow fill compared to the phosphatic peloids and the black mudrock.

1 mm

1 mm

The top of the Houchin Creek Coal is overlain by the Excello Shale, both of the Petersburg Formation. The contact between the coal and overlying black shale is marked by a layer of pyrite that decreases in abundance upward. The pyrite is finely disseminated.

Note the increase of bioturbation from 409.7 upward to 408.1 feet. The ichnofabric increases from “1” – no bioturbation” to “4” with 60 to 70 percent bioturbation. Burrows include Phycosiphon and Planolites.

The Excello Shale in the USI 1-32 core consists of finely laminated, dark gray, calcareous mudrock and black mudrock. In addition, there are very thin beds and thick lamina of concentrated phosphatic peloidal grainstones. There is minor bioturbation in the Excello Shale, including Helminthopsis, Planolites, and Phycosiphon.

2 mm

1 mm

1 mm

Burrows filled with dark gray mud in greenish gray mudrock. Burrows are probably Planolites.

Burrows filled with dark gray mud in greenish gray mudrock. Burrows are probably Phycosiphon.

The top of the Survant Coal marks the boundary between the Linton Formation and the overlying Petersburg Formation of the Carbondale Group. In this cored interval, the Survant Coal is overlain by greenish gray mudrock containing sparse siderite nodules.

Roof shales of the Survant and Seelyville Coals consist of light gray to greenish gray mudrocks with sparse siderite concretions. These greenish gray mudrocks have TOC values ranging from 0.1 to 0.8 wt. % with total sulfur values ranging from 0.15 to 0.40 wt. %, with little to no gamma ray response (Fig. 2). The mudrock overlying the Seelyville coal is greenish gray to light gray, finely laminated with discrete bioturbated intervals consisting primarily of Chondrites, Phycosiphon, and Planolites with an ichnofabric of 2 to 3. Likewise, greenish gray mudrock overly the Survant Coal. Additionally, disk-shaped, 1 to 4 cm diameter siderite concretions occur sparsely in the mudrock interval above the Survant and Seelyville Coals. In some instances, disseminated pyrite occurs at the edges of a siderite concretion, and may also partially replace it. Preserved fabrics in the siderite concretions suggest biological activity, and siderite appears to be replacing trace fossils in several instances. The mudrocks also contain compacted peloids, consistent with the lack of primary laminations.

Roof shales above the Springfield and Houchin Creek coals consist of laminated to bioturbated, black to gray shale with disseminated pyrite and sparse phosphate nodules. These mudrocks have TOC values ranging from 0.9 to 27.4 wt. % with total sulfur values ranging from 0.2 to 4.6 wt. % with an elevated gamma ray response (Fig. 2). Additionally, there is a gradual increase in bioturbation from an ichnofabric of 1 directly above the Springfield Coal to an ichnofabric of 4 at 1.5 feet above the coal. Discrete laminations within this interval also contain broken skeletal debris, including brachiopod and molluscan fauna. Likewise, the Excello Shale transitions from laminated black mudrock to bioturbated gray mudrock above the Houchin Creek Coal. Trace element analysis revealed the maximum uranium concentration in the black shale above the Springfield Coal to be 47.8 ppm. In addition, elevated concentrations of Mo (>100 ppm), Cr (210 ppm), and V (796 ppm) were also measured for the black shale above the Springfield Coal. Likewise, uranium concentrations in the Excello Shale ranged from 17 to 80 ppm, with corresponding elevated concentrations of Mo (>100 ppm), Cr (90 to 150 ppm) and V (150 to 1,830 ppm).

1 mm

For mudrocks that disintegrate in contact with water, it is essential to use dry preparation techniques. This requires the use of various grit sizes of wet/dry sandpaper, most of which can be purchased from a local hardware store.

Comparison of an unprepared surface of core (left) to a polished surface of core (right). Polishing the surface of the core is essential to observe detailed sedimentological features and sedimentary fabrics.

The overlying mudrock contains abundant Barroisella campbelli, a Lingula brachiopod along laminations from 398.4 down to 400.0 feet depth.

The mudrock of the Petersburg Formation overlying the Survant Coal in the USI 1-32 core consists of finely laminated, light gray to greenish gray, bioturbated to laminated mudrock with sparse siderite concretions and disseminated pyrite. There is minor bioturbation in the mudrock, including Helminthopsis and Chondrites.

The mudrock of the Linton Formation overlying the Seelyville Coal in the USI 1-32 core consists of finely laminated, light gray to greenish gray, bioturbated to laminated mudrock with sparse siderite concretions and disseminated pyrite. There is minor bioturbation in the mudrock, including Helminthopsis and Chondrites.

Pyritization of burrows that have been cemented with siderite in the mudrock above the Seelyville Coal. The burrows are probably Chondrites.

Burrows that have been cemented with siderite in the mudrock above the Seelyville Coal. The burrows are probably Chondrites.

1 mm

1 mm

1 mm

Burrows that have been cemented with siderite in the mudrock above the Survant Coal. The burrows are probably Chondrites.

1 mm

1 cm