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Lahann, Richard W.1 
(1) ConocoPhillips, Exploration Production Technology, Houston, TX

ABSTRACT: A Broader View of Framework Weakening

Framework weakening/collapse has been suggested as a cause for excess fluid pressure in diagenetically mature shales. Such shales often contain abundant low-expandability smectite/illite, significant plagioclase and chlorite, and relatively little smectite and kaolinite. The replacement/alteration of detrital load-bearing phases creates multiple opportunities for excess pressure development in such shales. 
The conversion of smectite to illite releases bound water to the pore system, while reducing the volume of load-bearing clay. Similarly, generation of fluids from kerogen reduces the kerogen volume while increasing the fluids volume. The additional fluids must escape the shale system or excess fluid pressure results. 
Beginning at about 70o C, illite formation may consume up to 8 % K-feldspar, for a typical shale composition, while creating illite and a smaller amount of quartz cement. If stress collapses the K-feldspar dissolution voids, then fluid must escape the shale or excess pressure develops. The Na+ released from smectite during illitization contributes to albitization of plagioclase, likely weakening the load-bearing plagioclase in the system. 
Increased burial and temperature can cause kaolinite to react with either K-feldspar to form illite or with a Fe/Mg source to form chlorite. In either case, reaction of load-bearing detrital kaolinite may contribute to framework weakening. 
The combined effect of illitization, K-feldspar dissolution, albitization and kaolinite reaction may weaken more than 50 % of the detrital grains in the shale. The increased stress on the remaining quartz grain framework may induce either fracturing/compaction or pressure solution/cementation. Both processes can decrease porosity and contribute to excess pressure.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.