Experimental Determination of the Rates of Precipitation of Authigenic Illite and Kaolinite in the Presence of Aqueous Oxalate and Comparison to the K/Ar Ages of Authigenic Illite in Reservoir Sandstones

Abstract

Abstract--The importance of precipitation rate as an effective control on illite and kaolanite formation during diagenesis has been examined by measuring precipitation rates, from A1 fluid concentration, in a Dickson fluid-sampling vessel at 160~ ~ and 500 bars (50 MPa). These experiments are considered to be analogues of the precipitation of clays in sandstones from porewaters containing dissolved carboxylic acids, which have a transient stability and may influence aluminosilicate solubility. Precipitated illite had a lath-shaped morphology and its composition was consistent with authigenic iUite in sandstones. Kaolinite formed tabular rather than vermicular shaped crystals. Kaolinite precipitation rate was two orders of magnitude faster than iUite precipitation and was rate-limited by the decomposition of oxalate; kaolinite formation should be equilibrium-controlled at virtually all stages of burial. Extrapolation of illite precipitation rate to burial temperatures indicates that the first appearance of illite in a burial sequence may be kinetically controlled. A model of illite precipitation based on these experimental results has been used to predict the time required to precipitate illite during burial of a sandstone, taking into account temperature changes during burial. For northern North Sea examples, a predicted illitization threshold of -60~ occurring at 60-80 Ma corresponds to the observed initiation of authigenic illite precipitation. Times of around 2-5 Ma would be required to reach a 98% approach to equilibrium at this threshold. The main phase of illite precipitation in the northern North Sea basin is a later, hydrologically controlled event (30-50 Ma). Equilibrium would be approached in around 0.1 Ma during this phase, which is consistent with the narrow illite K/Ar age range (1-5 Ma) recorded for some sequences.

Relation Between Interlayer Composition of Authigenic Smectite, Mineral Assemblages, I/S Reaction Rate and Fluid Composition in Silicic Ash of the Nankai Trough

“…Those relations are consistent with closed-system conditions and minimal water/rock ratios. Organic acids, evolved during early diagenesis, have been shown to be critical in promoting formation of illite from smectite (Small 1993). The lack of reaction in bentonites, which are free of organic matter, and the occurrence of reactions in shales containing organic matter, may therefore be related to such material.…”

“…However, Underwood et al (1993), showed that illite had formed at depths as shallow as 500 m within shales that are interbedded with the bentonites, with transformation to as much as 80% illite at the greatest depths. Many other researchers have found that formation of illite in bentonites lags behind that in shales (Ahn et al 1988;Knox and Fletcher 1978), suggesting that factors such as differences in porosity, activities of alkali ions in pore fluids or the presence of organic acids in shales (Small 1993) may be significant in affecting differences in reaction rates. However, before such factors can be explored, it is necessary to characterize the reactants and products over a full range of sediment depths.…”

Transmission Electron Microscopy Study of Conversion of Smectite to Illite in Mudstones of the Nankai Trough: Contrast with Coeval Bentonites

“…The sequences of phases from smectite to illite involve metastable clays, which react following the Ostwald step rule (e.g., Essene and Peacor, 1995). The rate of transformation may therefore be affected by many variables, including temperature, fluid/rock ratio, and fluid composition (Small, 1993;Velde and Vasseur, 1992;Aja et al, 1991;Huang et al, 1993;Abercrombie et al, 1994), i.e., by any variable affecting reaction rate which promotes reaction of a metastable system toward the stable system.…”

A Novel Approach to Experimental Studies of Mineral Dissolution Kinetics

“…Although the assumption of partial equilibrium has been questioned in the intervening years Nagy and Lasaga, 1993;Small, 1993;Lasaga et al, 1994;Price et al, 2005;Ganor et al, 2007), until now there has never been a rigorous examination of this hypothesis. Here, we systematically analyzed our own feldspar hydrolysis batch experiments and those in the literature.…”