Thirty-three shale samples, ranging in depth from 2,185 to 15,592 ft (666 to 4,752 m), were examined by X-ray diffraction methods to determine changes in a mineralogy with depth. Quartz is present in all samples and averages 15 wt. %. Above 7,800 ft (2,377 m), calcite content varies due to fossil fragments. Below 7,800 ft (2,377 m), calcite content varies from 0 to 9 wt. %. Potassium feldspar and plagioclase contents are essentially constant at an average of 3 and 4 wt. %, respectively. Total clay content, combining kaolinite, illite, mixed-layer illite-montmorillonite (I/M), and traces of chlorite, is essentially constant, averaging 65 wt. %. Individual clay minerals have quite variable contents from sample to sample, but distinct trends are noted: (1) kaolinite content s constant at an average of 25% total clay; (2) illite content initially averaged 35% total clay, decreases with depth, and is zero in 10 of the 14 samples below 10,000 ft (3,048 m); and (3) mixed-layer I/M averages 40% total clay in shallow samples and 70% in deeper samples. The top of the geopressured zone, occurring at a pore fluid pressure gradient of 0.465 psi/ft and equilibrium temperature of approximately 190°F (88°C), is marked by a definite increase of illite in mixed-layer I/M at approximately 8,500 ft (2,591 m). A major change in illite content from 40% at 11,210 ft (3,417 m), to 84% at 11.750 ft (3,581 m), corresponds to a pore fluid pressure gradient of 0.7 psi/ft, and equilibrium temperature of approximately 250°F (122°C). In addition, the arrangement of the I/M layers changes from random interstratification in samples from 11,540 ft (3,517 m), and shallower, to an ordered layering from 11,750 ft (3,581 m) and deeper.

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