Clastic sediments, which are fine-grained or clayey, are capable of retaining fluids at pressures considerably greater than hydrostatic. The excess pressures can be induced by any of several mechanisms. A numerical model is developed which considers simultaneously the effects of compaction disequilibrium and aquathermal pressuring. Energy transport by conduction only is used to provide temperature profiles. The pressure and temperature dependency of isobaric thermal expansivity and isothermal compressibility are integrated in the solution. Simulations were conducted for a variety of heat flux, permeability, stratigraphic, and sedimentation conditions.

It is shown that, while compaction disequilibrium itself explains the general pressures in Gulf Coast sections, aquathermal pressuring can lead to fluid pressures greater than lithostatic. Fluid release by hydraulic fracturing must then occur. This combination of processes provides an explanation for the observed variations in shale bulk density, excess pressure, and thermal gradient. A Mohr failure diagram, using a two-part failure envelope combined with horizontal versus vertical stress data provides a means of determining when fracturing is initiated and the orientation of the fractures. A variety of stress conditions that result in both horizontal and vertical fractures are considered. The depth of fracture initiation is highly dependent on the sedimentation rate, the sand versus hale ratio of the sediments, and on the tensile strength and hydraulic conductivity of the shale.

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