Concepts of
Chalk Burial Diagenesis: Porosity Preservation and
Pore-filling Cementation
Fabricius, Ida Lykke1
(1)
Chalk facies
sediments are deposited as fine-grained carbonate ooze, which upon burial recrystallizes and compacts. When recrystallization
dominates over compaction, contact cement forms, compaction is halted, and subsequent contact cementation causes pore
growth at constant porosity. Permeability thus increases with time. If the chalk
is well sorted, the porosity of this aged chalk may be as high as 45% and the
gas permeability as high as 10
If an interval of chalk contains no
clay-rich intervals, stylolites do not form, and
pressure dissolution does not begin until the effective burial stress is high
enough for calcite-calcite pressure dissolution to begin, a process which
requires stress beyond the realm of normal burial diagenesis.
Pure calcitic chalk may thus preserve its reservoir
properties to high burial.
Most chalk intervals, though, are less
well sorted, and most chalk sediments contain clay rich intervals. Upon burial,
the clay may recrystallize to flaser
structures, which are seen as semi-planar concretions. The flaser
structures trigger pressure dissolution and stylolite
formation. The pressure dissolution at these structures releases calcium and
carbonate ions, which causes calcite to precipitate on pore walls away from the
stylolite as pore filling cementation. The calcite
cement is not localized along the stylolite.
The pore filling cementation takes place
over a relatively short burial-stress interval leaving the resulting limestone
with porosity below 25% and permeability below 0.1
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California