--> Abstract: Concepts of Chalk Burial Diagenesis: Porosity Preservation and Pore-filling Cementation; #90063 (2007)

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Concepts of Chalk Burial Diagenesis: Porosity Preservation and Pore-filling Cementation

 

Fabricius, Ida Lykke1 (1) Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark

 

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 mD.

 

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 mD. Introduction of hydrocarbons may halt the cementation process by sorption of polar hydrocarbons to the clay of the stylolites and thus preventing pressure dissolution.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California