Fluid-rock constraints on porosity models for diagenesis of siliciclastic rocks

Conference · Fri Mar 01 04:00:00 EST 1991 · AAPG Bulletin (American Association of Petroleum Geologists); (United States)

OSTI ID:5677024

Hutcheon, I; Shevalier, M ^[1]; Abercrombie, H J ^[2]

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1. Univ. of Calgary, Alberta (Canada)
2. Geological Survey of Canada, Calgary, Alberta (Canada)

Fluid phase buffering was described by Greenwood (1975) who showed that undetectable amounts of reacting minerals can buffer the composition of a coexisting fluid phase. During diagenesis, buffering of pH must be understood to predict porosity by mineral dissolution and precipitation reactions. Potential buffers of pH in pore waters are carbonates, silicates, and aqueous organic species. Water-rock interactions in sediments suggest that silicate mineral assemblages may buffer fluid compositions. Particular mineral reactions can be identified by comparing theoretical phase stabilities to measured aqueous activities. This process indicates that although the entire rock-water system may not come to equilibrium, metastable silicate reactions equilibrate in geologically short times. The buffer index ({beta} = {minus}dC{sub a}/dpH), where C{sub a} is the concentration of strong acid, is a measure of the effectiveness of pH buffers. Changes in pH affect the distribution of aqueous species and, to accurately estimate {beta}, the aqueous distribution must be recalculated at each step. Hypothetical solutions containing varying amounts of aqueous carbonate and organic species were allowed to equilibrate with the identified silicate mineral assemblages as HCl was added. Silicates, such as albite-smectite and kaolinite-illite, have the highest {beta}'s over the normal pH range. Local, rock-dominated processes thus are interpreted to be more important to dissolution than the import of acids from external sources. A forward model for the dissolution and precipitation of minerals in a progressive burial sequence can be used to estimate changes in volume, and therefore porosity, due to chemical reactions. Physical compaction was not considered in the calculation.

• OSTI ID: 5677024
• Report Number(s): CONF-910403--
• Conference Information: Journal Name: AAPG Bulletin (American Association of Petroleum Geologists); (United States) Journal Volume: 75:3
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
CHEMICAL REACTION KINETICS
CHEMISTRY
DIAGENESIS
DISSOLUTION
GEOCHEMISTRY
GEOLOGIC MODELS
GROUND WATER
HYDROGEN COMPOUNDS
INTERSTITIAL WATER
KINETICS
OXYGEN COMPOUNDS
PH VALUE
POROSITY
PRECIPITATION
REACTION KINETICS
ROCK-FLUID INTERACTIONS
ROCKS
SEDIMENTARY ROCKS
SEDIMENTS
SEPARATION PROCESSES
WATER