25th International Meeting on Organic Geochemistry IMOG 2011

P-376
Organic geochemical variability of gas shales: Woodford Shale,
Southeastern Oklahoma, USA, a case study
Andrea Miceli Romero, R. Paul Philp
The University of Oklahoma, Norman, United States of America (corresponding
author:andreamiceli@ou.edu)
It has been very common to consider shales
as uniform stratigraphic sequences in terms of
physical and chemical properties. However, a more
detailed analysis of these source/reservoir rocks can
provide greater insight about variations in the
depositional and environmental factors that influenced
source rock deposition. The Woodford Shale (Upper
Devonian-Lower Mississippian), once considered as a
source rock for both oil and natural gas in the United
States Southern Midcontinent, has recently evolved
as an important shale gas play due to its high
hydrocarbon potential [1-2]. The use of geochemistry
to characterize these unconventional reservoirs has
played a significant role in shale gas evaluation in
terms of helping to determine the type of organic
matter, thermal maturity, paleoenvironmental
conditions, and to be effective for establishing
sequence stratigraphic frameworks of these
source/reservoir rocks.
Woodford Shale samples obtained from a
cored outcrop in southeastern Oklahoma were
analyzed geochemically to determine vertical
variations of organic facies, thermal maturity and an
evaluation of the depositional environments. Samples
were characterized on the basis of total organic
carbon (TOC), Rock Eval pyrolysis, and measured
and calculated vitrinite reflectance (%Ro). Additionally,
crushed rock samples were extracted and extracts
were subjected to gas chromatography (GC) and gas
chromatography-mass spectrometry (GCMS) for
biomarker analyses.
The Woodford Shale was subdivided into
lower, middle and upper members based on the
integration of geochemical and geological data.
Moreover, the presence and extent of photic zone
anoxia was determined by the presence of aryl
isoprenoids. Aryl isoprenoids also helped to infer the
position of the chemocline during deposition of the
different members. The relative hydrocarbon potential
(RHP) parameter derived from Rock Eval data was of
use in determining transgressive and regressive
cycles within the Woodford Shale. TOC values ranged
from 5.01% to 14.81% indicating good source rock
potential. In the study area the Woodford Shale is
mature to marginally mature as indicated by vitrinite
reflectance values; therefore, it is unlikely that the
Woodford Shale has generated high quantities of
hydrocarbons in this area. Biomarker ratios based on
pristane, phytane, steranes, and hopanes show a mix
of marine and terrestrial organic matter with the
middle member showing the greatest marine input.
Integration and correlation of Pr/Ph ratios, aryl
isoprenoids ratios and relative hydrocarbon potential
(RHP) indicate that the lower Woodford was
deposited under dysoxic/suboxic conditions and
episodic periods of photic zone anoxia (PZA) during a
major transgressive-regressive-trangressive cycle; the
middle Woodford was deposited under anoxic
conditions and persistent PZA during a major
transgression (sea-level rise); and the upper
Woodford was deposited under dysoxic/suboxic
conditions and episodic PZA during a general
regression (HST with high sedimentation rate). These
interpretations are consistent with a sequence
stratigraphic framework already proposed for the
Woodford Shale in the study area. High salinity
conditions and water density stratification also
prevailed during deposition of this formation as
indicated by the presence of gammacerane. The
chemocline was located at shallower depths during
middle Woodford deposition and fluctuated within
different depths during deposition of lower and upper
members.
This work undoubtedly demonstrates the
significant lithological and chemical variability that
occurs within shales, a factor that needs to be taken
into account when exploring and producing from
these self-sourced reservoirs.
REFERENCES
[1] Cardott B. J. (2005) Ok. Geol. Surv. Circ. 110, 7-
18.
[2] Comer J. B. (2007) GSA <strong>Meeting</strong> – Abstracts with
programs, pp. 356.
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