Calcite Cement in Shallow Marine Sandstones: Growth Mechanisms and Geometry
- Sadoon Morad
Published Online: 17 APR 2009
Copyright © 1998 The International Association of Sedimentologists
Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution
How to Cite
Walderhaug, O. and Bjørkum, P. A. (1998) Calcite Cement in Shallow Marine Sandstones: Growth Mechanisms and Geometry, in Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution (ed S. Morad), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304893.ch8
- Published Online: 17 APR 2009
- Published Print: 29 MAY 1998
Print ISBN: 9780632047772
Online ISBN: 9781444304893
- calcite cement in shallow marine sandstones;
- occurrence of calcite cement;
- calcite cement sources;
- nucleation and calcite cement growth;
- spacing between concretions
Calcite cement in shallow marine sandstones normally cannot be derived from sources outside the sandstones owing to a lack of viable transport mechanisms for significant amounts of dissolved calcium carbonate. Within the sandstones the only significant source of calcite cement is usually biogenic carbonate, which is consequently considered to be the dominant source of calcite cement within shallow marine sandstones. Influx of carbon dioxide into a sandstone will not lead to precipitation of additional calcite cement unless a source of calcium other than biogenic carbonate is present, but the carbon isotopic composition of the calcite cement may be strongly affected.
Geometrically, calcite cementation in shallow marine sandstones typically occurs as continuously cemented layers, as layers of stratabound concretions, and as scattered concretions. All these forms can be explained by local diffusional redistribution of biogenic carbonate originally present within the sandstones. Biogenic carbonate is less stable than calcite cement, and once a calcite cement nucleus has formed it will lower the concentration of dissolved calcite within its range of influence. Biogenic carbonate will then dissolve around the growing nucleus, diffuse down the concentration gradient and precipitate on the surface of the growing nucleus or concretion. This process will continue until the available biogenic carbonate is consumed or all porosity is filled with calcite cement.
If biogenic carbonate is concentrated in layers, stratabound concretions or continuously cemented layers form, as calcite cement nuclei are then concentrated within the biogenic carbonate-rich layers. Nucleation within these layers may take place either because the biogenic carbonate provides favourable nucleation substrates or because calcite supersaturations are highest within these layers. Stratabound concretions form when the supply of biogenic carbonate is exhausted prior to merging of concretions. If more biogenic carbonate is present, concretions merge and form a continuous calcite cemented layer. Scattered concretions form when biogenic carbonate occurs scattered throughout a sandstone, as preferred levels of nucleation will then be absent. Concretions occur with a certain spacing because, once a calcite cement nucleus has formed, the level of dissolved calcite in the pore water will be reduced around the nucleus, thereby inhibiting the formation of new nuclei within the range of influence of the first nucleus. Flattening of concretions parallel to bedding, which traditionally has been ascribed to permeability anisotropy and fluid flow, may rather be a result of more extensive growth of concretions in the direction of greatest supply of biogenic carbonate.
The presented nucleation and growth model implies that the geometry of calcite-cemented zones is controlled by the original distribution of biogenic carbonate, and prediction of the geometry of calcite cementation in subsurface reservoirs therefore largely depends upon an understanding of the depositional environment.