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Outcrop-behind Outcrop (Quarry): Multiscale Characterization of the Woodford Gas Shale, Oklahoma
Roger M. Slatt,1 Nichole Buckner,2 Younane Abousleiman,3 Rafael Sierra,4 Paul R. Philp,5 Andrea Miceli-Romero,6 Romina Portas,7 Neal O'Brien,8 Minh Tran,9 Robert Davis,10 Timothy Wawrzyniec11
1ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, U.S.A.
2ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, U.S.A.
3ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, U.S.A.
4ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, U.S.A.
5ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, U.S.A.
6ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, U.S.A.
7ConocoPhillips, Inc., Houston, Texas, U.S.A.
8Department of Geology, State University of New York, Potsdam, New York, U.S.A.
9Mewbourne School of Petroleum Engineering, University of Oklahoma, Norman, Oklahoma, U.S.A.
10Data Consulting Services, Schlumberger, Oklahoma City, Oklahoma, U.S.A.
11Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, U.S.A.
We thank the Wyche family, who owns the Wyche Shale Pit, for allowing us free and friendly access to the quarry; Erik Kitt and 3D Geophysics for acquiring and processing the seismic data; Schlumberger, Inc., for logging the behind-quarry well; Ashley Merritt for superb field assistance; Ted Champagne of Clarkson University, Potsdam, New York, and Carol McRobbie of State University of New York-Potsdam for technical assistance with SEM analyses; and Andrew Slatt for providing excellent graphics support. Devon Energy Corp. provided financial support for part of this research. Much of the research was accomplished as five Master's theses at the University of Oklahoma by coauthors Buckner, Miceli, Portas, Sierra, and Tran. Analytical work was completed at the Institute of Reservoir Characterization and the PoroMechanics Institute in the ConocoPhillips School of Geology and Geophysics, University of Oklahoma.
An outcrop-behind outcrop study was conducted in and adjacent to a 300 100 16 m (980 330 50 ft) quarry of the gas-producing Woodford Shale to structurally/stratigraphically characterize it from the pore to subregional scales using a variety of techniques.
Strata around quarry walls were described and correlated to a 64 m (210 ft) long continuous core drilled 150 m (500 ft) back from the quarry wall and almost to the Woodford-Hunton unconformity. Borehole logs obtained include neutron and density porosity (NPHI and DPHI) logs, and logs from Elemental Capture Spectroscopy (ECS™), Combinable Magnetic Resonance (CMR-Plus™), Fullbore Formation MicroImager (FMI™), and sonic scanner (Modular Sonic Imaging Platform, or MSIP™)—all manufactured by Schlumberger.
The strata around the quarry are horizontally bedded. Borehole logs were used to identify a basic threefold subdivision into an upper relatively porous quartzose interval; a middle, more clay-rich, and less porous interval; and a lower interval of intermediate quartz-clay content. These intervals correspond to the informally named upper, middle, and lower Woodford. Detailed core and quarry wall description revealed several types of finely laminated lithofacies, with varying amounts of total organic carbon (TOC). The FMI log revealed a much greater degree of variability in laminations than can be readily seen with the naked eye. Organic geochemistry and biomarkers are closely tied to these lithofacies and record cyclic variations in oxic-anoxic depositional environments, which correspond to relative sea level fall-rise cycles. At the scanning electron microscopy scale, microfractures and microchannels are common and provide tortuous pathways for gas (and oil) migration through the shales.
Based on FMI and core analysis, fracture density is much greater in the upper quartzose lithofacies than in the lower, more clay-rich lithofacies. A laser imaging detection and ranging (LIDAR) survey around the quarry walls documented two near-vertical fracture trends in the quartzose lithofacies: one striking N85E with spacings of 1.2 m (4 ft) and the other striking N45E related to the present stress field. The FMI analysis only imaged the latter fracture set.
Both log-derived and laboratory-tested geomechanical property measurements documented a significant relationship between shale fabric (laminations and preferred clay-particle orientation) and rock strength, and a secondary relationship to mineral composition. Porosity and microfractures or microchannels also appear to influence rock strength.
This integrated study has provided insight into the causal relations among Woodford properties at a variety of scales. In particular, a stratigraphic (vertical) segregation of lithofacies can be related to cyclic variations in depositional environments. The resulting stratified zones exhibit variations in their hydrocarbon source and reservoir (fracturable) potential. Such information and predictive capability can be valuable for improved targeted horizontal drilling into enriched source rock and/or readily fracturable reservoir rock in the Woodford and perhaps other gas shales.
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