Experimental Deformation of Oil Shales: Implications for Sediment Burial and Exploitation
Eseme, Emmanuel, R. Littke,
Compressive deformation experiments up to 350°C have been performed on Torbanite, Himmetoglu, Posidonia, Condor and Messel oil shales. Integrated geochemistry, petrology and rock mechanical techniques were used in order to assess the responses of these oil shales to mechanical and thermal stress. Mechanical deformation experiments reveal that oil shales respond to stress variations with elastic and plastic deformation in relation to organic matter content, i.e. the higher the organic matter content the greater is the plastic deformation. This observation was most pronounced in the Torbanite. The unconfined strength shows that most of these oil shales have been subjected to post burial uplift that led to over-consolidation. Thermal deformation demonstrates the reduction in rock strength by thermal stress; i.e. there is a significant chemical compaction due to thermal stress that leads to much higher strain than observed at the same stress at room temperature. Montmorillonite and possibly kaolinite respond to thermal stress by dehydration. Organic matter conversion is still low at the maximum temperature of 350°C. In in situ exploitation, with increasing thermal stress,an even higher proportion of the load-bearing organic matter would be converted into oil, leading to a significant volume reduction of the oil shale and change of landscape morphology if the oil shale used for retorting is thick. The oil shales studied here need auto-fracturing due to overpressure build-up or artificial fracturing to produce the generated petroleum due to the low initial permeabilities of the rock matrix.