Compaction mechanisms in mudstones and shales :implications from the laboratory and nature

Abstract

This thesis details the analysis of experimental compaction and measurements upon natural samples to quantify the relative roles of mechanical and chemical processes during porosity loss in mudstones and shales, whilst understanding the concepts behind their behaviour at the pore-scale. The primary controls on the mechanical mechanisms observed are effective stress above the previous maximum, temperature and time. Mechanisms include grain packing/compression and rotation to produce minor fabric development and also thermal hardening. Effects of these mechanisms are quantified in Chapter 3, "Laboratory Simulation of Mudstone Compaction". This work program included extended duration triaxial experiments, testing the results of high effective stresses (max. 50MPa) with varied fluid chemistries and temperatures on compaction behaviour. Primary controls on chemical mechanisms are again stress, temperature and time with the addition of fluid chemistry. The mechanisms include the effects of chemical alteration such as potassium exchange and dehydration, but also the effects of full mineralogical transformation such as the ordering associated with illitization and the proposed link to higher degrees of alignment of phyllosilicates. This is covered in both Chapter 3 and Chapter 4, "Mudstone Compaction in Nature: Malay Basin Case Study", a study to characterise both the chemical/mineralogical and physical properties of fine-grained clastics from a region where chemical processes would be expected to be enhanced. Other processes in addition to lithology dependant degrees of packing and rotation associated with increased effective stress are not generally incorporated in porosity reduction calculations. Their inclusion may be necessary in more challenging regions such as the Malay Basin.