--> ABSTRACT: Feldspar Diagenesis and Smectite to Illite Transition in Texas Gulf Coast Shales, by Robert L. Freed and Donald R. Peacor; #91022 (1989)

Datapages, Inc.Print this page

Feldspar Diagenesis and Smectite to Illite Transition in Texas Gulf Coast Shales

Robert L. Freed, Donald R. Peacor

Twelve shale samples, selected at 1,000-ft (305-m) intervals from a single well in DeWitt County, Texas, were examined by x-ray diffraction, SEM, and microprobe techniques to characterize shale mineralogy as a function of depth. The smectite-illite (S-I) transition occurs abruptly over a narrow depth interval near 7,000 ft (2,134 m). At a depth of 4,000 ft (1,219 m), feldspars have the appearance of detrital fragments and have a broad range of alkali feldspar compositions; plagioclase compositions range up to An50. At 8,000 ft (2,438 m), slightly deeper than the S-I transition, microprobe analyses suggest the following general diagenetic changes: alkali feldspar compositions have changed toward albite and orthoclase end members, accompanied by decreasing calciu content; plagioclase compositions have changed toward the albite end member, with decreasing potassium content. Feldspars from the deepest sample, 12,000 ft (3,658 m), are similar to those from 8,000 ft (2,438 m) with compositions clustering slightly nearer to end members. At depths deeper than the S-I transition, feldspars show dissolution and replacement textures but retain anhedral detrital shapes. Most grains have homogeneous compositions.

The data imply replacement (alkali feldspars) and dissolution/crystallization (plagioclases) concomitant with the S-I transition. At depths below the transition, only minor changes occur, suggesting no further feldspar diagenesis. K-feldspar is found at all depths, indicating that the termination of the S-I transition is not equivalent to complete loss of K-feldspar. Textures, metastable compositions, and chemical differences between adjacent grains imply that chemical equilibrium is attained only locally.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.