Отправлено: 28.04.10 05:30. Заголовок: Mudstone microstructure evolution during diagenesis: effect on cap rock sealing capacity

Mudstone microstructure evolution during diagenesis: effect on cap rock sealing capacity
Charpentier, D.; Worden, R. H.; Fisher, Q. J.; Aplin, A. C.
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #14167
Mudstone properties control some important characteristics of sedimentary basins, such as how fluids move from or are contained within aquifers and petroleum bearing-rocks. Despite the volumetric dominance of mudstones in sedimentary basins, there are many basic issues that are as yet unresolved such as what controls permeability and how mud become mudstone.

To investigate the relationship between porosity, permeability, fabrics and mineralogy transformations, and to determine the parameters influencing the controls on mudstones properties, a large range of microstructural and petrophysical techniques have been used on core and cuttings mudstone samples from the Gulf of Mexico and North Sea. At the time of deposition, mudstones usually have randomly aligned detrital clay minerals and have high porosity and permeability (respectively 60--90% and 10-11-10-14 m^2).

Mudstones contain a variety of minerals: mainly clay minerals, silt-grade framework silicates, carbonate detritus and sulphides.

By a combination of mechanical and chemical processes mudstone mineralogy and properties change during burial and diagenesis.
Porosity rapidly diminishes although it can still be significant (15--25%) even after several thousand meters of burial. The degree of alignment of clay minerals tends to increase. Mudstone mineral assemblages change with common detrital smectite replaced by illite.

Grain alignment at the microscale (˜400--1,000 μm^2) and the collapse of porosity occur during the replacement of smectite by illite, thousands of meters of overburden alone do not induce the development of aligned clay fabric in mudstones.

Microfabric and porosity content thus seem to be a consequence of the kinetically (thermally) controlled illitization of smectite that is therefore a chemical compaction process.

Mesoscale fabric (˜10,000 μm^2) including bedding, is controlled by the abundance of silt (diminishing the order) and the contrasting effect of the abundance of relatively large detrital mica grains (locally enhancing the order).

Permeability variations seem to be function of lithology and can be correlated with the mesoscale fabric. Our studies suggest that the two major controls on permeability and sealing capacity might be the initial bulk mudstone mineralogy and the thermal history.

http://adsabs.harvard.edu/abs/2003EAEJA....14167C