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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 9. (September)

First Page: 1442

Last Page: 1443

Title: Silica and Metal Release During Clay Mineral Diagenesis and Shale Overpressuring: ABSTRACT

Author(s): Kevin C. Beck

Article Type: Meeting abstract

Abstract:

Mud sidewall cores from the northwest Gulf of Mexico, extending down to the upper portion of the overpressured zone (OPZ), were examined mineralogically (XRD) and chemically. The resultant data is limited, but provides some clues on the 17A ^rarr 10A clay transformation and related shale processes.

Squeezed interstitial water concentrations, corrected for temperature effects and evaporative loss during processing, show little deviation from seawater, other than major K and Mg decrease in the upper 3,300 ft (1,000 m). As the upper OPZ dilution is encountered (water release from 17A ^rarr 10A transformation), K, S, and organic C increase markedly relative to C1.

Increased Na2CO3-leachable Si in the upper OPZ suggests that Si released in the 17A ^rarr 10A transformation may reside in part as weakly absorbed material or, as suggested by a broad 5

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to 7.5A XRD band, in a poorly crystalline silicate phase. Concentrations of B in squeezates, water leachates, and different size fractions closely parallel Si, supporting this hypothesis.

Transition metals show decreases with depth in the -0.1 µm (clay mineral-dominated) fractions and equal or lesser increases in the coarse fractions. This trend becomes more marked as the OPZ is encountered. Comparison of total metal with that freed by reducing and oxidizing leach solutions, plus XRD and petrographic observations, suggests removal of some transition metals from the altering 17A phase and their incorporation into coarser sulfidic and possibly carbonate phases.

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