The intrusive diapiric shale structures of south Louisiana are related to the undercompacted shales that occur in normal stratigraphic sequence. These undercompacted shales, characterized by low density, low velocity, low resistivity, high porosity, and high formation pressure, are derived from the outer-neritic shale facies deposited on the continental shelf. The critical prerequisite of their occurrence is the absence of porous and permeable interbedded sandstone members that are pressure-connected with the atmosphere. Where present in the section, these undercompacted shales provide the mother or source bed for the diapiric shale structures, which may or may not be associated with diapiric salt derived from the much more deeply buried mother bed, the Louann Salt.

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