Proposal for a mechanical model of mobile shales

This paper has benefited greatly by the thorough and helpful comments and reviews by Chris Morley, Peter Connolly, and an anonymous reviewer. Andrea Billi is also thanked for his editorial support. We thank Nancy Cottington for initial figure drafting and Cari Breton for creating the map for Fig. 1. The project was funded by the Applied Geodynamics Laboratory (AGL) Industrial Associates program, comprising the following companies: Anadarko, Aramco Services, BHP Billiton, BP, CGG, Chevron, Condor, Ecopetrol, EMGS, ENI, ExxonMobil, Hess, Ion-GXT, Midland Valley, Murphy, Nexen USA, Noble, Petrobras, Petronas, PGS, Repsol, Rockfield, Shell, Spectrum, Equinor, Stone Energy, TGS, Total, WesternGeco, and Woodside (http://www.beg.utexas.edu/agl/sponsors).The ideas and content of this contribution have been strongly benefited from various fruitful discussions with colleagues such as Peter Flemings, Doug Jerolmack, Maria Nikolinakou, and Demian Saffer. Publication authorized by the Director, Bureau of Economic Geology, The University of Texas at Austin.
Structural systems involving mobile shale represent one of the most difficult challenges for geoscientists dedicated to exploring the subsurface structure of continental margins. Mobile-shale structures range from surficial mud volcanoes to deeply buried shale diapirs and shale-cored folds. Where mobile shales occur, seismic imaging is typically poor, drilling is hazardous, and established principles to guide interpretation are few. The central problem leading to these issues is the poor understanding of the mechanical behaviour of mobile shales. Here we propose that mobile shales are at critical state, thus we define mobile shales as “bodies of clay-rich sediment or sedimentary rock undergoing penetrative, (visco-) plastic deformation at the critical state”. We discuss how this proposition can explain key observations associated with mobile shales. The critical-state model can explain the occurrence of both fluidized (no grain contact) shales (e.g., in mud volcanoes) and more viscous shales flowing with grain-to-grain contact (e.g., in shale diapirs), mobilization of cemented and compacted shales, and the role of overpressure in shale mobility. Our model offers new avenues for understanding complex and fascinating mobile-shale structures.
Applied Geodynamics Laboratory (AGL) Industrial Associates program