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Sequence Stratigraphy, Distribution and Preservation of Organic Carbon, and Reservoir Properties of the Middle Devonian
Marcellus Shale, of the Central Appalachian Basin; Northern West Virginia and Southwestern Pennsylvania
Roy Sexton Department of Geology and Geography, West Virginia University, Morgantown, WV 26506
Abstract
A transgressive – regressive (T-R) sequence stratigraphic study of the Marcellus Shale interval, using 250
geophysical logs and four cored sections in northern West Virginia and southwestern Pennsylvania, indicates that
the Union Springs and Oatka Creek members of the Marcellus Formation and the Levanna Member of the
Skaneateles Formation define three third-order stratigraphic sequences (i.e., MSS1, MSS2, SKS). The MSS1
stratigraphic sequence encompasses the Union Springs Member of the Marcellus Formation and extends from a
gamma-ray inflection point and/or bulk density base line shift located near the top of the Onondaga Formation to
either an erosional surface at the base of the Cherry Valley Member of the Marcellus Formation, or a gamma-ray
minimum and/or bulk density maximum within the Cherry Valley Member (or stratigraphic equivalent). The MSS2
stratigraphic sequence encompasses the Oatka Creek Member of the Marcellus Formation and extends from the
upper boundary of the MSS1 stratigraphic sequence to either an erosional surface at the base of the Stafford
Member of the Skaneateles Formation, or a gamma-ray minimum and/or bulk density maximum within the Stafford
Member (or stratigraphic equivalent). The SKS stratigraphic sequence encompasses the Levanna Member of the
Skaneateles Formation and extends from the upper boundary of the MSS2 stratigraphic sequence to a gamma-ray
minimum and/or bulk density maximum within a limestone or calcareous grey shale interval overlying the Levanna
Member.
Physical core descriptions and geochemical data (i.e. TOC, x-ray diffraction, and x-ray fluorescence
spectroscopy) provide support for the placement of stratigraphic surfaces and aid in interpretation of depositional
conditions. Specific geochemical proxies for terrigenous sedimentation include the concentrations of calcium,
aluminum, zirconium and silica; the ratio of titanium to aluminum and silica to aluminum; and total clay volume.
Geochemical proxies for paleo-redox states include the concentrations of manganese, molybdenum, and barium; the
ratios of molybdenum to aluminum, barium to aluminum, and thorium to uranium; the degree of pyritization (DOP);
and the depletion of sulfur isotope 34S relative to 32S (δ34S).
Interpretation of depositional conditions of the study interval, utilizing integrated geophysical logs, core
descriptions and geochemical data, suggests a dynamic environment characterized by varying relative sea-level,
oxygen concentration, and sedimentation rates. Organic-rich shale units within the study interval were deposited
under anoxic or possibly euxinic conditions initiated by relative sea-level rise. A significant portion of the silica within
black shale intervals is likely biogenic and/or eolian sourced. Limestone beds and calcareous grey shale intervals
were deposited under dysoxic/oxic conditions initiated by relative sea-level fall. Terrigenous input increases moving
upward stratigraphically from the MSS1 through the SKS stratigraphic sequences and into overlying grey shale
intervals. During periods of relative sea-level fall, carbonate production outpaced terrigenous sedimentation,
resulting in deposition of concentrated skeletal beds. Gravity driven sediment flows and winnowing processes are
interpreted to have commonly introduced oxygenated waters to the basin, resulting in diagenetic fronts and
concentration of heavier mineral grains (titanium).
Variations in the thickness of the MSS1, MSS2, and SKS stratigraphic sequences are controlled by sediment
input originating near the structural front east of the basin and the paleo-topography of underlying strata. The MSS2
and SKS sequences display infilling of paleo-topographic lows in the underlying sequences, suggesting increased
deposition by sediment driven gravity flows. Areas of rapidly increasing thickness of the MSS1, MSS2, and SKS
stratigraphic sequences in the eastern portion of the study area are associated with the distal portions of fluvial
sediment sources located on the basin margins.
Sequence Stratigraphy
Study Area
The study area in northern West Virginia and southwestern Pennsylvania is
highlighted in green. Locations of wells used in the study are indicated by
black circles. Cored wells used in the study are located in southwestern
Pennsylvania, northern West Virginia, and eastern West Virginia. The exact
locations of cored wells are proprietary. The study covers approximately
13,600 square miles (35, 200 square km).
Paleogeography during the Middle Devonian (385 Ma). Study area is outlined
by a red oval. The Appalachian basin was bound by the (1.) Cincinnati arch to
the west, the (2.) Acadian mountains to the east and the (3.) Rheic ocean to
the south. (Modified from Blakey, Ron:
http://jan.ucc.nau.edu/~rcb7/namD385.jpg)
Simplified stratigraphy of the current study. Includes approximate distribution
of the MSS1, MSS2, and SKS stratigraphic sequences compared with the
stratigraphy of the study interval.
Geochemical Analysis
Sequence stratigraphic framework of Eifelian and Givetian strata within the Appalachian basin. Includes relative sea-
level curve (Modified from Brett et al., 2011) Comparison of base level change with T-R events,
maximum regressive surfaces (MRS) and maximum
flooding surfaces (MFS) (Modified from Embry, 2002).
Type logs in southwestern Pennsylvania (Pa-1) and northern West Virginia (WV-1). Maximum regressive surfaces are identified by high density values and high clay or carbonate
volumes as well as low gamma-ray and resistivity values. The correlation track contains gamma-ray and caliper logs. The gamma-ray log is scaled 0-200 API units and wraps twice,
first when the gamma-ray exceeds 200 API (light green shading) and again when it exceeds 400 API (red shading). The caliper log is scaled 6-16 inches. The porosity track contains
neutron porosity, density porosity, bulk density, and photo electric (PE) logs. The neutron and density porosity logs are scaled 45-(-15) %, the bulk density log is scaled 2-3 g/cm3,
and the PE log is scaled 0-10 barns/electron. The resistivity track contains the deep, medium, and shallow resistivity logs which are scaled .2-2000 ohms (logarithmic). The clay
volume track contains a measurement of clay volume calculated from the corrected gamma-ray log and is scaled from 0-1 in percent decimal form. The carbonate track contains a
measurement of carbonate (Ca) concentration which is scaled from 0-50 percent.
Type logs in southwestern Pennsylvania (Pa-1) and northern West Virginia (WV-1). Maximum flooding surfaces are identified by low density values, and locally low clay or carbonate
volumes (depending on location) as well as high gamma-ray and resistivity values. The correlation track contains gamma-ray and caliper logs. The gamma-ray log is scaled 0-200
API units and wraps twice, first when the gamma-ray exceeds 200 API (light green shading) and again when it exceeds 400 API (red shading). The caliper log is scaled 6-16 inches.
The porosity track contains neutron porosity, density porosity, bulk density, and photo electric (PE) logs. The neutron and density porosity logs are scaled 45-(-15) %, the bulk
density log is scaled 2-3 g/cm3, and the PE log is scaled 0-10 barns/electron. The resistivity track contains the deep, medium, and shallow resistivity logs which are scaled .2-2000
ohms (logarithmic). The clay volume track contains a measurement of clay volume calculated from the corrected gamma-ray log and is scaled from 0-1 in percent decimal form. The
carbonate track contains a measurement of carbonate (Ca) concentration which is scaled from 0-50 percent.
Northern West Virginia WV-1
Southwestern Pennsylvania Pa-1 Northern West Virginia WV-1
Southwestern Pennsylvania Pa-1
Southwestern Pennsylvania Pa-1 Northern West Virginia WV-1
Defined stratigraphic sequences in southwestern Pennsylvania (Pa-1) and northern West Virginia (WV-1). Stratigraphic sequences include: MSS1; MSS2; and SKS. Transgressive
system tracts are highlighted in blue and regressive system tracts are highlighted in gold.
Aluminum (Al) terrigenous sedimentation
Titanium / Aluminum (Ti/Al) heavier mineral grains
Silica (Si) vs. Zirconium (Zr) biogenic and/or eolian silica
Silica / Aluminum (Si/Al) biogenic and/or eolian silica
Manganese (Mn) oxic/dysoxic
Barium/ Aluminum (Ba) oxic/dysoxic
Molybdenum (Mo) and Molybdenum / Aluminum (Mo/Al)
anoxic/euxinic conditions
Degree of Pyritization (DOP)
Depletion of sulfur isotope 34S relative to 32S (δ34S)
Thorium / Uranium (Th/U)
Indicators of bulk sedimentation and paleo-redox conditions (Modified from Sageman et al.,
2003).
Type logs in southwestern Pennsylvania (Pa-1) and northern West Virginia (WV-1). Track 1 contains gamma-ray and caliper logs. The gamma-ray log is scaled 0-200 API units and wraps twice, first when
the gamma-ray exceeds 200 API (light green shading) and again when it exceeds 400 API (red shading). The caliper log is scaled 6-16 inches. Track 2 contains calcium (Ca) concentration which is scaled 0-
50%. Track 3 contains aluminum (Al) concentration and total organic carbon (TOC). The Al concentration is scaled 0-15% and TOC is scaled 0-25 weight percent (w%). Track 4 contains Zirconium (Zr)
concentration and silica (Si) concentration. The Zr concentration is scaled 0-150 ppm and the Si concentration is scaled 0-35%. Track 5 contains a ratio of titanium (Ti) to Al (Ti/Al) which is scaled 0-(0.20).
Track 6 contains a ratio of Si to Al (Si/Al) which is scaled 0-10. Track 7 contains a measurement of total clay which is scaled 0-65 w%.
Southwestern Pennsylvania Pa-1 Northern West Virginia WV-1
Southwestern Pennsylvania Pa-1 Northern West Virginia WV-1
Type logs in southwestern Pennsylvania (Pa-1) and northern West Virginia (WV-1). Track 1 contains gamma-ray and caliper logs. The gamma-ray log is scaled 0-200 API units and wraps twice, first when
the gamma-ray exceeds 200 API (light green shading) and again when it exceeds 400 API (red shading). The caliper log is scaled 6-16 inches. Track 2 contains manganese (Mn) concentration which is
scaled 0-1000 ppm. Track 3 contains molybdenum (Mo) concentration and the ratio of Mo to aluminum (Al) (Mo/Al). The Mo concentration is scaled 0-250 ppm and Mo/Al is scaled 0-(0.015) (Pa-1); 0-(.02)
(WV-1). Track 4 contains barium (Ba) concentration and the ratio of Ba to Al (Ba/Al). The Ba concentration is scaled 0-2000 ppm and Ba/Al is scaled 0-(0.4). Track 5 contains total organic carbon (TOC) and
degree of pyritization (DOP). TOC is scaled 0-15 weight percent (w%) (Pa-1); 0-20 w% (WV-1) and DOP is scaled 0-3 (Pa-1); 0-2 (WV-1). Track 6 in Pa-1 contains the depletion of sulfur isotope 34S
compared to 32S (δ34S) which is scaled (-35)-0 (‰) S(V-CDT) (sulfur isotopic differences relative to a reference sample expressed in the Vienna Canyon Diablo Troilite scale). Track 7 contains the spectral
gamma-ray log and includes concentrations of uranium, thorium, and potassium. The uranium concentration is scaled 0-100 parts per million (ppm), thorium is scaled 0-25 ppm, and potassium is scaled 0-
10 %. Track 7 contains the ratio of thorium to uranium and is scaled 0.01-100 (logarithmic).
Type logs in southwestern Pennsylvania (Pa-1) and northern West Virginia (WV-1). Track 1 contains gamma-ray and caliper logs. The gamma-ray log is scaled 0-200 API units and wraps twice, first when
the gamma-ray exceeds 200 API (light green shading) and again when it exceeds 400 API (red shading). The caliper log is scaled 6-16 inches. Track 2 contains manganese (Mn) concentration which is
scaled 0-1000 ppm. Track 3 contains the ratio of Ba to Al (Ba/Al) which is scaled 0-(0.4) (Pa-1); 0-(.2) (WV-1). Track 4 contains a ratio of titanium (Ti) to Al (Ti/Al) which is scaled 0-(0.2). Track 5 contains a
ratio of Si to Al (Si/Al) which is scaled 0-10.
Southwestern Pennsylvania Pa-1 Northern West Virginia WV-1
Stratigraphy
Stratigraphy of study interval adapted from New
York (Modified from Brett et al., 2011)
Isopach map of the MSS1 stratigraphic sequence. The MSS1
stratigraphic sequence decreases in thickness from a maximum of 165
feet (50.3m) in the east and 86 feet (26.2m) in the north to a minimum
of 12 feet (3.7m) in the southwest. A trend of decreased MSS1
thickness, oriented southwest to northeast, is evident in southwestern
Pennsylvania (indicated by dashed red line).
Isopach map of the SKS stratigraphic sequence. The SKS stratigraphic
sequence decreases in thickness from a maximum of 99 feet (30.2m) in
the east and 151 feet (46m) in the northeast to a minimum of 13 feet
(4m) in the southwest. A trend of decreased SKS thickness, oriented
southwest to northeast, is evident in southwestern Pennsylvania
(indicated by dashed red line).
Isopach map of the gross MSS1, MSS2, and SKS stratigraphic
sequence. The gross thickness of the MSS1, MSS2, and SKS
stratigraphic sequences decreases from a maximum of 470 feet (143m)
in the east to a minimum of 52 feet (15.8m) in the west. A trend of
decreased gross thickness, oriented southwest to northeast, is evident
in southwestern Pennsylvania (indicated by dashed red line).
Isopach map of the MSS2 stratigraphic sequence. The MSS2
stratigraphic sequence decreases in thickness from a maximum of 180
feet (54.9m) in the east to a minimum of 16 feet (4.9m) in the west and
8 feet (2.4m) in the southwest. A trend of increased MSS2 thickness,
oriented southwest to northeast, is evident in southwestern
Pennsylvania (indicated by dashed red line).
Isopach Maps
Northeast to southwest cross section datumed on the upper boundary of the SKS stratigraphic sequence. Observe the
thinning of the MSS1, MSS2, and SKS stratigraphic sequences to the southwest.
Pronounced thinning to the southwest
Condensation in distal portion of basin
Removal of strata related to Taghanic
unconformity
Northeast to southwest cross section datumed on the upper boundary of the SKS stratigraphic sequence.
Observe the thinning of the MSS1, MSS2, and SKS stratigraphic sequences to the southwest.
Thinning to the southwest
Increased sediment input
Dilution of organic material
West to east cross section datumed on the upper boundary of the SKS stratigraphic sequence. Observe the
variations in thickness of the MSS1, MSS2, and SKS stratigraphic sequences.
Variations in thickness
Infilling of paleo-topographic
lows
Basement faulting
West to east cross section datumed on the upper boundary of the SKS stratigraphic sequence. Observe the
rapid, westward thinning of the MSS1, MSS2, and SKS stratigraphic sequences in the eastern portion of the
cross section.
Pronounced thinning to west
Condensation in distal portion of basin
Removal of strata related to Taghanic
unconformity
Stratigraphic Cross Sections
Conclusions Characterization of the study interval using a transgressive – regressive sequence stratigraphic model combined with analysis of geochemical, lithologic, sedimentologic, and paleontologic properties lead
to the following conclusions:
Significant stratigraphic surfaces can be identified on common wireline logs, allowing for construction of a basin-wide sequence stratigraphic framework.
Reservoir properties, including mineralogic composition and total organic carbon (TOC) concentration, vary in a predictable manner based on their spatial and temporal relation to specific sequence
stratigraphic system tracts and surfaces.
Relative sea-level change was the dominant factor in controlling sedimentologic properties and paleo-redox states.
Organic-rich black shale intervals were deposited under periods of anoxic/euxinic conditions during relative sea-level rise.
A significant portion of the silica within black shale intervals is interpreted to be biogenic and/or eolian sourced.
Skeletal concentrations and calcareous grey shale intervals were deposited during dysoxic/oxic conditions initiated by relative sea-level fall.
Gravity driven sediment flows and winnowing processes are interpreted to have commonly introduced oxygenated waters to the basin resulting in diagenetic fronts and concentration of heavier
mineral grains (titanium).
Variations in the thickness of the MSS1, MSS2, and SKS stratigraphic sequences are generally controlled by sediment input originating near the structural front located east of the basin and the
paleo-topography of underlying strata.
Isolated areas of rapidly increasing thickness of the MSS1, MSS2, and SKS stratigraphic sequences are associated with the distal portions of fluvial sediment sources located on the basin margins.
250 wireline logs
Two strike lines (I, II)
Two dip lines (III, IV)
Acknowledgments
Dr. Tim Carr; Dr. Richard Smosna; Randy Blood
West Virginia University & EQT Production
References Brett, C. E., G. C. Baird, A. J. Bartholomew, M. K. DeSantis, and C. A. V. Straeten, 2011, Sequence stratigraphy and a revised sea-level curve for the Middle Devonian of eastern North America:
Palaeogeography, Palaeoclimatology, Palaeoecology, v. 304, p. 21-53.
Embry, A. F., 2002, Transgressive-regressive (T-R) sequence stratigraphy: Gulf Coast SEPM Conference Proceedings, p. 151-172.
Lash, G. G., and T. Engelder, 2011, Thickness trends and sequence stratigraphy of the Middle Devonian Marcellus Formation, Appalachian Basin: Implications for Acadian foreland basin development:
AAPG Bulletin v. 95, p. 61-103.
Sageman, B. B., A. E. Murphy, J. P. Werne, C. A. V. Straeten, D. J. Hollander, and T. W. Lyons, 2003, A tale of shales: the relative role of production, decomposition, and dilution in the accumulation of
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Palaeogeography, Palaeoclimatology, Palaeoecology, v. 304, p. 54-73.
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