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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 998

Last Page: 999

Title: Kaolinite Formation in Clastic Reservoirs: Carbon Dioxide Factor: ABSTRACT

Author(s): Ronald K. Stoessell

Article Type: Meeting abstract


Thermodynamic modeling of diagenesis in clastic reservoirs was used to quantify geochemical and geohydrologic constraints on the formation of kaolinite. The formation of significant amounts of kaolinite depends upon four conditions: (1) a source for aluminum; (2) the presence of acidic pore fluids; (3) the pH buffering capacity of the fluids; and (4) the quantities of such fluids moving through the reservoir. The first condition is usually satisfied by the unstable mineral assemblage. The second and third conditions can depend upon CO2 released during hydrocarbon maturation, and the final condition requires an open system.

The presence of dissolved CO2 buffers the pH so that the pore solution is undersaturated with respect to illites, chlorites, and smectites while kaolinite is being precipitated. Minimum limits of fCO2 in equilibrium with fluids forming appreciable amounts of kaolinite were computed as a function of unstable

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mineral assemblages, temperature, and fluid compositions. Volumes of fluid necessary to form 5 wt. % of kaolinite are presented as a function of fCO2 and the above variables.

Formation of kaolinite can be related to hydrocarbon maturation. Decrease in O/C atomic ratios of different types of kerogen during maturation initiates the eventual release of oxygen as CO2 into the pore fluids. Subsequent precipitation of kaolinite is thus an indicator of hydrocarbon maturation. To quantify this indicator, more research is needed to delineate the controls on the formation of CO2 during maturation.

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