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Characteristics of ‘Mobile’ Shale in the Deep Stratigraphic Subsurface

Day-Stirrat, Ruarri 1; McDonnell, Angela 1; Wood, Lesli 1
1 Bureau of Economic Geology, University of Texas, Austin, TX.

Shale tectonics, the study of overpressured or fluidized mudstones and shales, is becoming increasingly significant in both conventional and unconventional hydrocarbon exploration. We examine shale tectonics from a micro-to a macro-scale, discussing the chemical and physical properties of mudstones and how these alter with burial and diagenesis; we then address interpretation risks from seismic reflection data in shale tectonic terrains. We focus on mudstones that have been deeply buried (> 2km/60-80C), where large scale ductile deformation is commonly invoked (Fluid expulsion features such as mud volcanoes, pipes or chimneys, and shallow ductile deformation processes are considered separate, distinct processes). When working in a shale tectonic province it is important to consider mudstone composition and its burial history, chemical compaction and diagenesis. Depth, temperature and timing of the invoked mobilization are also important. Critically, even under overpressure conditions, chemical burial compaction processes are highly effective at reducing the ability of a mudstone to deform below 2000 meters (60-80C) in a ductile manner. At depths below 2 km we envisage ‘shale tectonics’ as largely a brittle process, thus deeply sourced ‘shale diapirs’ are highly unlikely, as is lateral flowage of ‘shale’ along shale detachments. Shale movement is not akin to salt movement. At the seismic scale, the interpretation of ‘mobilized sediment’ at depth is inherently controlled by the geophysical quality or limits of the seismic reflection data. When interpreting seismic data in known shale tectonic provinces it is important to consider 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 shale features has been over-interpreted, supporting the observation that ductile deformation of deeply buried shales is improbable.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009