Feldspar diagenesis and smectite to illite transition in Texas Gulf Coast shales

Conference · Wed Mar 01 04:00:00 EST 1989 · AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States)

OSTI ID:6086026

Freed, R L; Peacor, D R

Twelve shale samples, selected at 1000-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 7000 ft (2134 m). At a depth of 4000 ft (1219 m), feldspars have the appearance of detrital fragments and have a broad range of alkali feldspar compositions; plagioclase compositions range up to An/sub 50/. At 8000 ft (2438 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 calcium content; plagioclase compositions have changed toward the albite end member, with decreasing potassium content. Feldspars from the deepest sample, 12,000 ft (3658 m), are similar to those from 8000 ft (2438 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.

• Research Organization: Trinity Univ., San Antonio, TX (USA)
• OSTI ID: 6086026
• Report Number(s): CONF-890404-
• Conference Information: Journal Name: AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States) Journal Volume: 73:3
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
020200* -- Petroleum-- Reserves
Geology
& Exploration
DIAGENESIS
DISSOLUTION
FEDERAL REGION VI
FELDSPARS
GULF COAST
MINERALS
NORTH AMERICA
ROCKS
SEDIMENTARY ROCKS
SHALES
TEXAS
USA