Chemical compaction of Illite shale: an experimental study
Student thesis: Doctoral Thesis
Sediment diagenesis has been classified nearly a century ago as a key process for the formation of rocks. In response to burial by overlying younger deposits, both mechanical and chemical processes contribute to the compaction and consolidation of sediments, the degree of which is controlled by both intrinsic and extrinsic parameters such as time, effective pressure, temperature, mineral composition and grain size distribution. Rock physical properties are strongly affected by clay diagenesis. So far, only the mechanical processes during clay diagenesis that dominate in the top 2-3 km of the sedimentary column have been simulated in the laboratory. However, studies on natural shales and mudstones have also emphasized the importance of chemical processes for diagenesis, although occurring in deeper domains and controlled more by temperature than by effective stress. Foreland basins are typical examples of a tectonic setting where sediments are buried deep enough for the activation of chemical processes. The effect of tectonic forces (deformation) on diagenesis is however enigmatic and to date poorly constrained by experimental simulation. The present study simulates the chemical processes in the laboratory and examines how tectonic forces affect compaction processes that transform porous illite shale powder into compact crystalline metapelite. The experimental compaction procedure consists of three stages. In the first compaction stage, dry illite shale powder (originating from Maplewood Shale, New York, USA) was mechanically compacted in a hydraulic cold-press with a vertical load of 200 MPa. The second stage employed a hot isostatic press (HIP) set at 170 MPa confining pressure and 590 °C, and ensured powder lithification. In the final stage, further compaction was achieved by either repeating HIP treatment or by performing confined deformation tests in a Paterson-type gas-medium apparatus. During the second HIP event temperature and pressure were set at 490 °C and 172 MPa. In the Paterson apparatus three different stress regimes were applied: confined compression, confined torsion or isostatic stress. For the first two regimes, deformation was enforced by applying a constant strain rate ranging from 7×10-6 to 7×10-4 s-1. Experiments were performed at 300 MPa confining pressure and a fixed temperature of 500 °C, 650 °C, 700 °C or 750 °C. These conditions were chosen from a thermodynamic forward simulation of the stability of mineral phases. In some Paterson apparatus tests, the effects of fluid availability and effective pressure were tested by respectively venting the sample or applying a 50 MPa argon pore pressure. Lithified samples were analyzed for their microstructural, chemical and physical development with compaction using scanning electron microscopy, Karl Fisher Titration, X-ray diffraction and gas pycnometry. Sample strength evolution was recorded during Paterson apparatus tests and strain measured afterwards. The magnetic signature of the compacting metapelites was quantified by measuring low and high-field anisotropy of magnetic susceptibility (AMS), and the ferrimagnetic contributions were identified using rock magnetic methods. The three-stage compaction resulted in synthetic metapelites ranging in porosity from 1.0 % to 17.1 %. Linear increase in density and strong correlation between volumetric strain and pore reduction indicate that compaction was accommodated primarily by pore space closing. In deformation experiments axial strain was accommodated first by uniaxial shortening following pore collapse and later by radial extension, which in some cases resulted in total decompaction. Illite transformation to phengite and fluid assisted mass transfer resulted in diffusion creep type strain accommodation. Mechanical pore closing is characterized by pronounced compaction hardening and partial pore recovery upon unloading. Porosity reflects the progress of illite transformation, which is accompanied by enhanced mica alignment and shape preferred authigenic quartz. The AMS signal carried by the phyllosilicates increases linearly with compaction, quantifying the development of texture and reflecting illite preservation. Deformation structures accommodate strain as alternative to exhausted or delayed pore space reduction processes. The smallest visible pores in SEM, associated with illite flakes, diminished as compaction progressed; larger pores were preserved. Chemical compaction can be simulated in the laboratory provided mineral transformation processes are sufficiently enhanced. In this, differential stress (tectonic force) acts merely as accelerant. There is no evidence for otherwise altered fabric and porosity/density development with the application of differential stress, in comparison with isostatically compacted metapelites.