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Density/depth and pore pressure/depth profiles of thick tectonically undisturbed shales show that, during deposition and during burial by a permeable overburden having hydrostatic pore pressures, compaction is transient, controlled not only by the mechanical process of pore pressure dissipation, but also by such depositional events as the rate of sedimentation, shale thickness, overburden density, and overburden thickness.
These calculated compaction profiles define several aspects of compaction history. For example, they show that shale density inversions may form early if the rate of burial by a hydrostatically pressured overburden is sufficiently rapid; density inversions also may disappear with additional burial. The profiles also show that thin zones of high-density shale overlying less compacted shale (sometimes called "seals") are features that can form late in compaction history and, therefore, are not necessarily responsible for abnormal pore pressure. The profiles also show that abnormal pressures may be present either during shale deposition, or may originate during burial beneath an overburden having normal pore pressures.
These compaction states have been calculated by integrating empiricism with principles of soil mechanics (consolidation theory). The attempt is to use the depositional history (e.g., sedimentation rate, duration of deposition, and unconformities) and the material properties of the shale (e.g., permeability coefficient) to determine the present compaction state and to unravel the compaction history. Although other techniques utilize the depositional history, this technique differs in including the material properties of shale and not using empirical density-depth relations such as the Athy curve.
Others have utilized consolidation theory to model compaction of thick shales. The calculations shown here, however, differ from them primarily by showing the effect of depositing a normally pressured overburden on top of thick shales. Thus, an important result has been to show the effect of using different boundary conditions of pore pressure dissipation.
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