ABSTRACT

Organic-rich rocks have long been recognized as source rocks for clastic reservoirs, but more recently they have gained importance as reservoirs. However, the processes of kerogen maturation and hydrocarbon transport and storage are still poorly understood. Some empirical relations have been developed to relate the increase in elastic modulus with increasing maturity. A systematic study of the cause for this increase in elastic modulus is still lacking, and information about seismic and mechanical properties of kerogen and its alteration products is scarce. Consequently, any rock models must rely on anecdotal or extrapolated data about various types of kerogen. Our experiments address this paucity of data by grain-scale modulus measurements coupled with careful field emission scanning electron microscopy (FESEM) microstructural assessments on organic rich Bakken formation shale samples with a range of maturities. Carefully acquired and detailed FESEM images help to understand the microstructural controls on the reduced (nanoindentation) Young’s modulus of minerals, clay particles, and kerogen matter with maturity in naturally matured shales. Using hydrous pyrolysis, we further investigate the cause for change in modulus with maturity and the mobility of the pyrolized organic matter. In naturally matured shale samples, we find a direct relationship between the reduced Young’s modulus and the total organic content or hydrogen index. Significant lowering of Young’s modulus is observed after hydrous pyrolysis due to bitumen generation. We measured modulus of the extruded bitumen to be less than 2 GPa. The presence of bitumen comingled with the organic matter also reduces its modulus, in excess of 30%. These results are critical to help understand how organic-rich sediments evolve with burial and maturation. The modulus measurements can be used for modeling modulus variations during maturation.

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