ABSTRACT: Mineralogical Examination of Shale in Core from the Geopressured Frio Formation, Brazoria County, Texas

ALLEY-MCREYNOLDS, P. D., and R. M. CAPUANO, University of Houston, Houston, TX

Core samples from the geopressured portion of the Frio Formation were collected from 5415-5430 m and studied in approximately 1-m intervals by XRD, FTIR, thin sections, and SEM with EDX. The subsurface pressure of these samples is 12,500 psi and temperature is 161 degrees C. Additional core samples were examined by XRD from 3585-3601 m (7500 psi, 92 degrees C) and 3709-3726 m (8800 psi, 124 degrees C) from a nearby well. Although the Frio Formation sandstone mineralogy is well described, data are sparse on the mineralogy of the shales, which comprise about 80% of the Frio Formation. The shale is composed of 35% quartz, 10% orthoclase/albite, 5% illite, 17% Na-montmorillonite, 30% Ca-montmorillonite, with traces of kaolinite, pyrite, chlorite, and biotite. XRD examination of the <2- icrometer-size fraction showed distinct differences between the clay mineralogy of the shales and sandstones. The sandstones <2-micrometer fraction is composed of predominantly kaolinite with little or no montmorillonite and illite. In contrast, the shales' <2-micrometer fraction is composed of predominantly montmorillonite with less than 10% each of kaolinite and illite. Shales layers of less than 2 ft thick showed higher proportions of kaolinite relative to montmorillonite and illite. SEM photos show authigenic dolomite, pyrite, anhydrite, and illite. Microfractures are present in all samples. Several of these fractures are filled with authigenic anhydrite.

AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)