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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 64 (1980)

Issue: 5. (May)

First Page: 709

Last Page: 709

Title: Shale Mineralogy of General Crude Oil and Department of Energy 1 Pleasant Bayou Geopressured-Geothermal Test Well, Brazoria County, Texas: ABSTRACT

Author(s): Robert L. Freed

Article Type: Meeting abstract


Thirty-three shale samples, ranging in depth Previous HitfromNext Hit 2,185 to 15,592 ft (666 to 4,752 m), were examined by X-ray Previous HitdiffractionNext Hit 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 Previous HitfromNext Hit 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 Previous HitfromNext Hit 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 Previous HitfromNext Hit 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 Previous HitfromNext Hit random interstratification in samples Previous HitfromNext Hit 11,540 ft (3,517 m), and shallower, to an ordered layering Previous HitfromTop 11,750 ft (3,581 m) and deeper.

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