Ten shale samples of Frio and Vicksburg (Tertiary) overpressured shales from Brazoria and Hidalgo Counties, Texas, were examined by transmission electron microscopy (TEM) techniques. Initially, illite and smectite (with some mixed-layer component?) are present in subparallel domains, with smectite packets dominating and providing abundant dislocations for ion transport, supplying avenues that contribute to local permeability. As the transition proceeds, illite packets grow within a shrinking matrix of smectite, dislocations decrease, and pathways for ion transport are restricted. We suggest this development of illite packets and diminished ion transport causes loss of local permeability and a rise in the fluid pressure gradient. As illite packets coalesce, local permeabil ty is increasingly lost; hydraulic continuity with the surface is further restricted and a more efficient geopressure seal is formed.

In both Hidalgo and Brazoria Counties, the base of normal hydropressure (0.465 psi/ft or 10.5 kPa/m) coincides with the depth of onset of the smectite-to-illite transition. Geopressure greater than 0.7 psi/ft (15.8 kPa/m) occurs at depths corresponding to 50% illite packets in Brazoria County and 70% in Hidalgo County; in both cases, 0.7 psi/ft (15.8 kPa/m) geopressure occurs prior to the depth at which the transition ceases. These relationships suggest that if the smectite-to-illite transition begins and proceeds to completion, the development of coalescing illite packets during diagenesis will lead to increasing geopressure, with the actual values of illite proportion and corresponding fluid pressure gradient determined by the local geologic setting.