Secondary porosity in sandstones and shale diagenisis, Oligocene, south Texas /

Bibliographic Details
Main Author:	McMahon, David Albert, 1951-
Other Authors:	Dixon, Joe B. (degree committee member.), Domenico, Patrick A. (degree committee member.), Spang, John H. (degree committee member.), Tieh, Thomas T. (degree committee member.)
Format:	Thesis Book
Language:	English
Published:	1989.
Subjects:	Sandstone > Texas, South. Diagenesis > Texas, South. Soil porosity > Texas, South. Diagenesis. Sandstone. Soil porosity. Texas, South. Geology Academic theses
Online Access:	Link to OAKTrust copy

Description
Abstract:	Oligocene sandstones in the McAllen Ranch and Hinde fields in south Texas show two zones of secondary porosity. Sandstones in the upper zone, from about 4,500 to 6,500 ft (1,372 to 1,981 m), have porosities higher than 30%, with secondary porosity up to half of the total. Temperatures are estimated to be about 80°C (176°F) to 100°C (212°F), and pore fluid pressure gradients are close to hydrostatic. Thin sections show evidence of the dissolution of unstable grains and possibly analcime cement. Calcite cement is common in normally pressured sandstones below the upper zone of secondary porosity. The lower zone of secondary porosity extends from 8,500 ft (2,591 m) to 11,000 ft (3,353 m) at Hinde field and from 8,500 ft (2,591 m) to 14,000 ft (4,267m) at McAllen Ranch field, and sediments in this zone have abnormally high pressures. The top of this zone is at a temperature estimated to be about 120°C (249°F) and also corresponds to the clay transformation or dehydration depth. At this depth and temperature, randomly interstratified illite-smectite clay becomes ordered and over 20 volume percent water may be expelled from the smectite by potassium fixation. The clays at McAllen Ranch field have undergone more extensive illitization than those at Hinde field, and more secondary porosity is observed in sandstones from the lower zone in the McAllen Ranch field. Formulas for theoretical endmember smectite and illiie were calculated from chemical and mineralogical analyses of clays repesenting the entire section in the Hinde and McAllen Ranch fields..
Item Description:	Typescript (photocopy). Vita. "Major subject: Geology."
Physical Description:	xvii, 177 leaves : illustrations ; 29 cm
Bibliography:	Includes bibliographical references.