Quartz cementation and chemical compaction of mudstones and shales
|Location||International Geological Congress,oslo 2008|
|Author||Jahren, Jens; Peltonen, Christer B.; Marcussen, طyvind; Mondol , Nazmul Haque; Bjّrlykke, Knut|
|Holding Date||23 September 2008|
Quartz cementation and its impact on the rock properties of mudstones and shales is not well understood. The most important mineralogical reactions that will cause precipitation of quartz in mudstones and shales are the smectite to illite and/or mixed layer illite/smectite and kaolinite to illite (around 70oC and 130oC respectively) reactions. Both reactions require the presence of a potassium source (K-feldspar). Additional quartz cementation from dissolution and recrystallization of amorphous silica and redistribution of detrital quartz due to pressure dissolution (PD) or possible clay induced dissolution (CID) of detrital quartz is also expected in mudstones and shales. The petrophysical importance of quartz cement is evident from well logs resulting in increased rock velocities and densities. Deviation from the compaction trends derrived from pure mechanical compaction of synthetic mudstones and single clay minerals indicate that chemical processes dominate compaction of mudstones and clays at temperatures above about 60-80 oC. This could in the case of illite producing mineral reactions partly be due to precipitation of smaller and stiffer illite crystals in pores. Soft mudstones will however be transformed to hard shales without significant amounts of reactive mineral phases producing illite present. This "shalification" process indicates that quartz cementation may be responsible for the observed density and velocity changes below about 60-80 oC. In this study (mudstones from the cretaceous and tertiary Vøring basin, offshore Norway) authigenic quartz cementation sourced from both recrystallization of opal-CT and from dissolution of smectite and precipitation of illite and quartz has been identified. Two distinct authigenic quartz populations (about 20 micrometers and less than 2 micrometer respectively) have been formed. The larger opal-CT sourced population predates the smectite derived smaller quartz crystals. The changes in the velocity and the density gradients are related to the latter quartz population. The velocity and density changes are particularly significant in lithologies with more than 20% bulk smectite due to less mechanical compaction in such lithologies. Correspondence between quartz cementation in mudstones and the stiffening is not well understood but evidence of a quartz cement framework is suggested by scanning electron microscopy elemental mapping of these fine grained sediments. Mapping indicate the precence of a silica (quartz) framework above about 80 oC. The formation of such a framwork require quartz dissolution and precipitation processes in mudstones similar to the processes found in sandstones explaining the "shalification" of mudstones.