Shale compaction and the development of abnormal pore pressures can be quantified using exact, rather than empirical, relations. A model of thick, laterally extensive, tectonically undisturbed shales describes their density-depth and pore pressure-depth profiles (1) at the end of shale sedimentation and (2) during burial by a permeable overburden having hydrostatic pore pressures. The model shows that compaction is a transient phenomena controlled not only by the mechanical process of pore-pressure dissipation, but also by such geologic events as the rate of sedimentation, overburden density, shale thickness, and amount of overburden.
Compaction profiles generated by the model show several phenomena. For example, shale density inversions may form after either shallow or deep burial, but can disappear during subsequent burial. Also, thin zones of high-density shale overlying less compacted shale (sometimes called "seals") are shown to be features which can form late in compaction history and are therefore not responsible for abnormal pressure.
The profiles also show that abnormal pressures may originate either at the end of shale deposition or during burial beneath a permeable overburden. Burial rate most affects the shallow part of shale masses; identical shale masses buried under equal amounts of overburden but at different rates have similar pore pressures in their deep parts but dissimilar pressures in their shallow parts. Rapid burial causes higher pore pressures in the upper part of the shale than slow burial.
Consolidation theory may be used to quantify the relation between abnormal pore pressure and effective stress, the compression index to quantify the amount of compaction in response to an increase of effective stress.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC