Secondary porosity in sandstones and shale diagenesis, Oligocene, south Texas

Thesis/Dissertation · Sun Jan 01 04:00:00 EST 1989

OSTI ID:5425205

McMahon, Jr, D A

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 80C (176F) to 100C (212F), 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 (4,267 m) at McAllen Ranch field, and sediments in this zone have abnormally high pressures. 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 illite were calculated from chemical and mineralogical analyses of clays representing the entire section in the Hinde and McAllen Ranch fields. A proposed composite reaction, beginning with end-member smectite and including the reduction of ferric iron, was shown to release 0.12 moles of H{sup +} and 2.15 moles of H{sub 2}O per mole of smectite. Mass-balance calculations indicate that the amount of acid from smectite diagenesis in a typical Gulf Coast shale is approximately 11 times greater than the acid from CO{sub 2} formed by the decarboxylation of organic matter. This is enough acid to produce 13.5% secondary porosity due to dissolution, which is about the same amount of secondary porosity found in the McAllen Ranch field.

• Research Organization: Texas A and M Univ., College Station, TX (United States)
• OSTI ID: 5425205
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
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03 NATURAL GAS
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58 GEOSCIENCES
580000 -- Geosciences
CENOZOIC ERA
CHEMICAL REACTION KINETICS
CLAYS
DEVELOPED COUNTRIES
DIAGENESIS
DISSOLUTION
FEDERAL REGION VI
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GROUND WATER
HYDROGEN COMPOUNDS
ILLITE
INTERSTITIAL WATER
KINETICS
MINERAL RESOURCES
MINERALS
NATURAL GAS DEPOSITS
NORTH AMERICA
OXYGEN COMPOUNDS
PETROLEUM DEPOSITS
PH VALUE
REACTION KINETICS
RESERVOIR ROCK
RESOURCES
ROCK-FLUID INTERACTIONS
ROCKS
SANDSTONES
SEDIMENTARY ROCKS
SHALES
SILICATE MINERALS
SMECTITE
TERTIARY PERIOD
TEXAS
USA
WATER