From Mud to Shale - Mechanical and Chemical Compaction of Mudstones

Jens Jahren, Knut Bjørlykke, Øyvind Marcussen, Chris Peltonen, and Nazmul Haque Mondol
Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway

The transition from soft clays and mudstone to hard shales is still relatively poorly understood. This change can be observed by testing core samples geomechanically and by examining the change in well log velocity and density. The part of this compaction process which is mechanical can be tested in the laboratory but the chemical compaction is more difficult to reproduce. In sandstones the degree of cementation by quartz and other cementing minerals can readily be examined. In mudstones carbonate cement is easy to detect, but quartz cement is in most cases difficult to observe. Yet we know that significant amounts of silica have been released by diagenetic reactions must have precipitated as quartz. In the absence of carbonate cements it is difficult to explain the increase in stiffness, density and velocity without a cementing mineral. The compaction can not be explained by mechanical processes alone. Sediment composition and diagenesis vary substantially in siliciclastic sedimentary basins but most fine grained sediments contain unstable minerals and amorphous material that will produce quartz cement above 60-80^oC. Quartz precipitation will not take place in most mudstones at lower temperatures for kinetic reasons. Amorphous silica (Opal A) from siliceous organisms is the most obvious silica source. The distribution and amount of amorphous silica from this source will be a function of sedimentary facies. The clay mineral smectite will also if present contribute substantially to silica cementation. Smectite is particularly important in basins with substantial volcanic input. Silica will be released when smectite is illitized (smectite + K+ = Illite + Silica + H₂O). Amorphous silica will recrystallize in steps forming several metastable silica phases before the stable end product micro-crystalline quartz is formed. Micro-crystalline quartz is formed due to low quartz growth rate at low temperatures, thus favouring nucleation of many new micro-crystalline quartz grains rather than overgrowths on detrital sand and silt particles. Neoformed micro-crystalline quartz is readily identified in mudstones by electron optical methods like high resolution scanning electron microscopy. Recrystallization of amorphous silica is probably finished before substantial amounts of smectite are illitized because the illitization reaction requires a silica saturation approaching quartz solubility. Illitization of smectite is for geometrical reasons in tight mudstones unlike in sandstones a relatively slow process. This is probably related to slow diffusion of potassium from dissolving potassium containing minerals like K-feldspars present in the sediment. Mixed layer illite/smectite (I/S) will therefore survive for a while in mudstones before all smectite is transformed to illite. This rather complicated low temperature (60-80^oC) silica diagenesis is clearly seen as a marked velocity increase in well log data indicating sediment stiffening caused by silica cementation. The formation of small stiff illite crystals is also expected to contribute somewhat to the overall stiffening of the sediment. The overall velocity vs. density gradient is also changed by this reaction indicating that mechanical compaction is unimportant compared to chemical compaction below this transition temperature in mudstones. Mechanical smectite compaction tests show no sign of a stress related velocity change as a function of density supporting the above conclusion. Other diagenetic reactions in mudstones like illitization of kaolinite (Kaolinite + K+ = Illite + Silica + H₂O) commencing at higher temperatures (>120-130^oC) continue the chemical compaction processes found in mudstones involving the release of silica and precipitation of quartz.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands