Predicting Carbonate Cementation Patterns in Shallow Marine Sandstones

Koen Van den Bril and Rudy Swennen
Katholieke Universiteit Leuven, 3001 Heverlee, Belgium

Carbonate cementation in shallow marine sandstones commonly takes place during early diagenesis. This will result in very specific cemented geometries, which are strongly related to the depositional environment. Predicting the distribution and lateral extension of carbonate cementation therefore strongly rely on knowledge of (1) depositional system and (2) diagenetic evolution of the sands.
The possibility to predict compartmentalising cementation patterns is tested on the Luxembourg Sandstone Formation, which is deposited as an offshore sandbar complex in a general transgressive regime during the lower Jurassic. Based on a combination of extensive fieldwork, petrographic and geochemical research the most important parameters which determine cementation distribution were unravelled. Elongated cemented lenses (10-100 m) are parallel to the bedding planes of cross-stratified layers. Coarse grained cemented channels have a lensoid outline. Continuously cemented (>> 100 m) layers are present in fine grained lithologies, which have large lateral extent or they can be found at important lithological boundaries. Small nodules are aligned parallel to the stratification of the sandstones. Petrographic and geochemical observations show that an original marine fringe cement has been replaced by an equant feroan meteoric calcite cement. The nodular forms are composed of sparitic to poikilotopic calcite cements.
The outline and distribution of the lenses is the result of an early cementation process, which affected carbonate-rich strata with a longer residence time close to the sediment-seawater contact. Later redistribution of unstable carbonate from the uncemented strata enhanced the early diagenetic signature. This resulted in zones containing a high amount of up to 100 m elongated lenses, zones with smaller (1–10 m) lenses and zones without any lenses.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece