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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 from 2,185 to 15,592 ft (666 to 4,752 Previous HitmNext Hit), 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 Previous HitmNext Hit), calcite content varies due to fossil fragments. Below 7,800 ft (2,377 Previous HitmNext Hit), 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/Previous HitMNext Hit), 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 Previous HitmNext Hit); and (3) mixed-layer I/Previous HitMNext Hit 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/Previous HitMNext Hit at approximately 8,500 ft (2,591 Previous HitmNext Hit). A major change in illite content from 40% at 11,210 ft (3,417 Previous HitmNext Hit), to 84% at 11.750 ft (3,581 Previous HitmNext Hit), 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/Previous HitMNext Hit layers changes from random interstratification in samples from 11,540 ft (3,517 Previous HitmNext Hit), and shallower, to an ordered layering from 11,750 ft (3,581 Previous HitmTop) and deeper.

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