Factors controlling the thermo-mechanical deformation of oil shales: Implications for compaction of mudstones and exploitation

Six Permian to Miocene organic matter-rich mudstones were subjected to unconfined compression tests under three thermo-mechanical regimes. The aims of this study were to assess the role of factors that control deformation and highlight implications for mudstone compaction and oil shale exploitation. Axial strain ranged from 1.9% to 23% at 25 °C (5.3–70 MPa), 12–79% at 25–310 °C (31–42 MPa) and 1.38–40% at 25–350 °C (0–8 MPa). The axial strain data showed that the principal factor controlling mudstone compaction is temperature with a secondary role for effective stress. During high-temperature tests, dehydration of smectite in the 94–150 °C interval resulted in a distinct axial strain effect. Decomposition of organic matter and other minerals caused visible strain only above 280 °C. Tests up to 350 °C resulted in mass loss of 4.54–43.8 wt% with 1.1–8 wt% due to organic matter. A strong positive correlation was found between the petroleum generation indices (4.6–55.8%) and expulsion efficiencies (38.6–96.2%) while compaction (0.68–51.4%) correlated with axial strain (1.38–40%). Solid volume loss (2.55–12.15%) due to petroleum generation from kerogen resulted in an increase in porosity (1.56–6.36%). Low rate heating and low retorting temperature are necessary to optimise yield from oil shale exploitation.