Diagenetic and Seismic Concerns Associated with Interpretation of Deeply Buried ‘Mobile Shales’
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 80°C (122 to 176°F). In these deeply buried settings, large-scale ductile deformation is commonly invoked. Above 80°C(176°F), chemical burial compaction processes reduce the ability of a mudstone to deform in a ductile manner, even under overpressure conditions. At temperatures above 80°C(176°F), 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.