Detrital Mica
Influence on Reservoir-Scale Diagenesis
Boles,
James R.1 (1) University of California, Santa Barbara, Santa
Barbara, CA
Detrital muscovite and biotite are
important to sediment diagenesis at the reservoir scale. Although the two mica
types have similar structures and compositions, surprisingly, they exhibit
different mechanical and chemical behavior. Muscovite tends to remain largely
undeformed throughout the compaction history of the sediment. This is partly
because muscovite resists compaction by dissolving silicate grains upon
contact. In contrast to muscovite, biotite readily deforms during compaction,
and can show both plastic and brittle deformation styles. Early
carbonate-cemented rocks have undeformed biotite whereas the mica is extensively
deformed in deeply buried uncemented sediment. Thus a measure of the relative
timing of carbonate cement can be deduced from the extent of biotite
deformation.
Mica cleavage faces produce a micro
chemical environment which has a different chemistry than the bulk pore fluid.
In certain chemical environments, biotite cleavage is an important nucleation
site for early-formed diagenetic pyrite or carbonate minerals, such as dolomite
and siderite. The carbonate crystals can greatly expand the original mica
causing porosity reduction. The effect is believed to be a result of locally
high pH near the biotite surface due to H+ exchange onto the
surface. Carbonate growth is almost never observed in muscovite, yet in
contrast to biotite, it is an important catalyst for pressure solution of
quartz and feldspar. In all examples of cleavage surface effects, the micas do
not breakdown, but rather provide a substrate which catalyzes reactions.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California