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Day-Stirrat, R. J., A. McDonnell, and L. J. Wood,
Diagenetic and Seismic Concerns Associated with Interpretation of Deeply Buried Mobile Shales
1Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, U.S.A.
2Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, U.S.A.
3Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, U.S.A.
The authors thank Richie Miller and John Girgis for access to CGGVeritas Gulf of Mexico seismic data and Jennifer Totterdell for access to Bight Basin seismic data. Ken Thies, Paul Murray, Mark Hollanders, Hermann Lebit, and members of the Deep Shelf Gas Industry-Funded Consortium are thanked for valuable discussion and exchange of ideas. We also acknowledge Ursula Hammes and members of the State of Texas Advanced Oil and Gas Resource Recovery (STARR) research team at the Bureau of Economic Geology for discussion and valuable debate. We are grateful to Richard Davies for a thoughtful review of the manuscript. We thank Lana Dieterich for editing support and Nancy Cottington for graphic support. Publication was authorized by the director, Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin.
The study of mudstones is becoming increasingly significant in both conventional and unconventional hydrocarbon exploration. Starting at the microscale, we discuss the chemical and physical properties of mudstones and how they alter with burial and diagenesis. Moving to the macroscale, we address interpretation risks from seismic reflection data in shale tectonic terrains (see Terminology section below). We focus on deeply buried mudstones that experienced temperatures in excess of 50 to 80C (122 to 176F). In these deeply buried settings, large-scale ductile deformation is commonly invoked. Above 80C (176F), chemical burial compaction processes reduce the ability of a mudstone to deform in a ductile manner, even under overpressure conditions. At temperatures above 80C (176F), we envisage shale tectonics as mostly a brittle process; thus, deeply sourced shale diapirs are highly unlikely, as is lateral flowage of shale along deep shale detachments. At the seismic scale, interpretation of mobilized shale at depth is inherently controlled by the geophysical quality or limits of the seismic reflection data. One must consider the survey design and acquisition parameters, migration and velocity issues, and data vintage. Recent advances in seismic technology lead to the conclusion that the actual mobility of deeply buried mudstone features has been overinterpreted in several classic mobile shale basins. Deeply buried mudstones most likely deform by a brittle process, with ductile deformation being highly improbable. When working in a shale tectonic province, workers need to consider (1) mudstone composition and its burial history; (2) chemical compaction and diagenesis; (3) depth, temperature, and timing of the invoked mobilization; and (4) geophysical risks.
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