Buffering Capability of Mineral Assemblage (Feldspar + Kaolinite + Quartz) and Its Application to Predicting Illite Occurrence in Arkosic Sandstones
Studies of phase relationships using the activity diagrams involving feldspar, illite, kaolinite, and gibbsite (or boehmite) reveal that the common mineral assemblage of feldspar, kaolinite, and quartz found in arkosic sandstone provides a strong driving force to change the fluid chemistry to conditions thermodynamically favoring illite formation. The driving force involves three major diagenetic reactions: the hydrolysis of feldspar with acidic fluid, the reaction of kaolinite with alkaline fluid, and the saturation of the fluid with respect to quartz. The first and second reactions result in, respectively, the increase and decrease of K+/H+ activity ratio, whereas the third reaction tends to maintain the Si(OH)4 activity in the pore fluid. The resultant fluid chemistry of these reactions coincidentally falls in the stability field of illite. This relationship occurs over a wide range of pressure and temperature conditions with low fluid to rock ratio.
Experimental results show that an increase in the fluid to rock ratio will decrease the rate that the fluid chemistry approaches the illite stability field. The effectiveness of the buffering capability, therefore, depends on the relative rate of fluid movement and fluid/rock reaction. This study demonstrates that the most likely condition for illite formation is a quartzofeldspathic system involving the presence of kaolinite and low fluid movement. In the absence of these conditions, the opportunity to precipitate illite in the pore space is significantly reduced. The commonly observed illitization of kaolinite or feldspar during burial diagenesis could be caused simply by the reduced influx of fluid (with any compositions: alkaline, neutral, or acidic) due to deep burial of the sand tone.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.