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ABSTRACT: SiO2 Mass Transport During Early Diagenesis of a Cretaceous Mudstone

J. Bloch, I. Hutcheon

Mass transport of SiO2 may be evaluated by determining the volume and SiO2 solubility of the fluid expelled during burial and compaction of sediments. Petrographic observations indicate that dissolution of detrital quartz and chert is pervasive in the Albian Harmon Member (Fort St. John Group). This suggests a prolonged period of pore-fluid undersaturation with respect to SiO2. The abundance of detrital chert and the early preservation of K-feldspar suggest that the SiO2 may best be represented by chalcedony saturation.

Using existing porosity reduction, burial history, and SiO2 (chalcedony) solubility data, a minimum volume (one pore volume) of expelled fluid is calculated to contain 10.3g of SiO2 per cubic meter of Harmon Member sediment that may be used in adjacent formations as cement. At a shale/sandstone ratio of 10, to produce 1% by volume of quartz cement in a cubic meter of sandstone would require 100 pore volumes of fluid. These calculations indicate that considerable fluid flux is required for significant mass transport of SiO2.

Consideration of burial history indicates that most SiO2 export occurred between 200 and 1000 m of burial in less than 10 m.y. Fluid flow rates of approximately 1mm/yr therefore were required to export significant quantities of SiO2, and presumably other mobile elements, from compacting Harmon Member sediments.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990