2014 Eastern Unconventional Oil and Gas Symposium -- November 5-7, 2014 in Lexington, Kentucky, USA
http://www.euogs.org/
Paleoceanographic Controls on Sedimentation in
a World-Class Source Rock and Unconventional
Reservoir: Devonian Shale in the Southern
Midcontinent
2014 Eastern Unconventional Oil & Gas Symposium (EUOGS) – www.euogs.org
Jack C. Pashin1, Sara A. Callner1
1
Boone Pickens School of Geology, 105 Noble Research Center, Oklahoma State
University, Stillwater OK, 74078
KEYWORDS: Devonian, Woodford Shale, Chattanooga Shale, Sedimentation,
Stratigraphy, Upwelling, Oxygen Minimum Zone
ABSTRACT
A range of stratigraphic, sedimentologic, and geochemical variables were analyzed in
core and outcrop to make a critical assessment of the origin of the Woodford Shale and
the Chattanooga Shale in Oklahoma and southwestern Missouri. These strata range in
thickness from 10 to more than 100 meters and are dominated by organic-rich
mudrocks and novaculitic chert ranging in age from Givetian to Tournasian (Middle
Devonian to Early Mississippian). The Woodford-Chattanooga section is interpreted as
a succession of third-order depositional sequences that were deposited in a low
accommodation passive margin setting. These sequences constitute a 2nd-order
retrogradational sequence set that records the Kaskaskia cratonic onlap.
Most features in organic-rich chert and mudrock in the Devonian System of the southern
Midcontinent can be explained by processes with an an ancient oxygen minimum zone.
The upper part of the oxygen minimum zone is interpreted to have intersected a broad
continental shelf along the southern fringe of Laurussia. Radiolarian chert with
phosphate nodules dominates the Famennian section near the Ouachita embayment
and provides evidence for strong oceanic upwelling along the Arkoma shelf and the
Southern Oklahoma Aulacogen. Soft-sediment deformation provides evidence for
unstable slopes in this region, and synsedimentary extension indicates that the
aulacogenic structures, which formed during Iapetan rifting, were reactivated during
Woodford deposition.
North of the aulacogen and Arkoma shelf, facies distribution was influenced by incised
valleys that underlie Woodford-Chattanooga strata. Ripple cross-laminae, inverse
grading, and normal grading indicate that argillaceous mud was deposited by currents
related to upwelling, storms, internal wave propagation, and internal tides. Pyrite forms
include silt-size euhedra and framboids, which probably precipitated in the water
column, and large framboids and nodules, which formed below the sediment-water
interface. Much of the this pyrite has been reworked by bottom currents. Most quartz is
of biogenic origin and originated as opaline silica that cements the mudstone and fills
radiolarian tests, Tasmanites cysts, and acritarchs. Detrital quartz forms include wellrounded silt and sand grains that were probably derived from the Simpson Group and
St. Peter Sandstone, as well as shardlike silt grains of probable eolian origin. A
conodont lag contains bundled laminasets indicative of spring-neap tidal cycles, and the
northward transition from chert-bearing facies to silty argillite indicates that flocculation
and pelletization were important processes affecting sedimentation on the inner shelf.
X-ray fluorescence and spectral gamma ray analyses were used to evaluate redox
conditions and the sources of sediment and organic matter. Organic-bound trace
elements (V, Ni), pyrite forms, and paleobiological parameters indicate that anoxia and
suboxia were widespread and were most intense in the Southern Oklahoma Aulacogen.
In argillaceous strata deposited on the inner shelf, concentrations of trace metals
associated with oxygen deficiency correlate positively with total organic carbon content.
Seafloor oxygenation events are interpreted to have increased in frequency northward,
and the distribution of trace fossils indicates that these events, however brief, had a
lasting effect on sediment fabric.
The distribution of clay-bound elements (Cr, Co) suggests that flocculation occurred in
more oxygenated settings than is indicated by the organic-bound elements. Shoreward
increase of Th levels, moreover, points toward increased sediment input from terrestrial
settings. Hence, estuarine flushing is interpreted as an important mechanism that
helped deliver argillaceous material to areas where the upper part of the oxygen
minimum zone intersected the inner shelf.
Comparison of depositional and geochemical variables with stratigraphic architecture
indicates that the distribution of rock types, and hence fundamental compositional and
geomechanical reservoir properties, was influenced by 3rd-order changes of relative sea
level. Phosphate grains, glauconite grains, and terrigenous quartz grains are most
abundant in lags and associated condensed sections overlying major lowstand surfaces
of erosion and flooding surfaces. Biogenic and eolian silica predominate in late
transgressive and early highstand deposits, which are associated with encroachment of
upwelling currents onto the inner shelf, which nourished blooms of phytoplankton and
holoplankton. These deposits are highly brittle and are commonly selected as targets for
landing and completing wells. Bioturbated mudstone with low total organic carbon
content and elevated Th:U ratios is characteristic of late highstand falling stage
sedimentation, during which estuarine flushing facilitated relatively rapid accumulation
of terrigenous mud and silt. Decreased radioactivity and organic carbon in the
Tournasian section are associated with increased influx of terrigenous sediment.
Argillaceous highstand and falling stage deposits are the most ductile WoodfordChattanooga strata thus impede height growth of hydraulic fractures.
Eastern Unconventional Oil and Gas Symposium, held November 5-7, 2014 in
Lexington, Kentucky, USA.