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Suppression of Vitrinite Reflectance by Bitumen Generated From Liptinite During Hydrous Pyrolysis of Artificial Source Rock


Mean random vitrinite reflectance of polished rock or kerogen samples under oil immersion (Ro) is widely used to determine thermal maturity of coal and other sedimentary rocks. However, Ro values of liptinite-rich, oil-prone samples are commonly lower than those measured in samples from adjacent vitrinite-rich coals at the same level of thermal stress. So-called suppressed Ro values have also been observed in hydrous pyrolysis experiments performed on liptinite-rich compared to vitrinite-rich endmembers. Hypotheses to explain Ro suppression, such as sorption of products generated from liptinite during maturation, diagenetic formation of perhydrous vitrinite under anoxic conditions, and retardation by overpressure, remain controversial. To experimentally test for Ro suppression caused by liptinite content, artificial rock was prepared using silica and a calcined blend of limestone and clay with various proportions of thermally immature vitrinite-rich Wyodak-Anderson coal and liptinite-rich kerogen isolated from the oil-prone Parachute Creek Member of the Green River Formation. Hydrous pyrolysis was performed on samples containing various proportions of the coal and liptinite-rich kerogen for 72 hrs. at isothermal temperatures of 300oC, 330oC, and 350°C to simulate burial maturation. The experimental results provide the first evidence that Ro suppression occurs in vitrinite mixed with liptinite-rich kerogen in a rock matrix without variations in redox conditions during diagenesis or overpressure during catagenesis. Compared to artificial rock that contains only coal, samples having different proportions of oil-prone kerogen show suppressed Ro in the 300oC and 330oC experiments. For example, artificial rock containing only vitrinite-rich coal reaches Ro of 0.97±0.07%, while a 90:10 mixture of liptinite-rich kerogen and the same coal shows Ro suppression >0.2% (0.75±0.05% when heated under the same conditions. Solid bitumen generated during heating of the samples containing liptinite-rich kerogen shows distinctly lower reflectance than the associated vitrinite and does not interfere with Ro measurements. Although the precise mechanism for Ro suppression by liptinite remains unclear, free radicals generated during maturation of liptinite may contribute to termination reactions that slow aromatization of polyaromatic sheets in vitrinite, thus suppressing Ro. The experimental results do not preclude that Ro suppression in nature might also result from different redox conditions during diagenesis or overpressure during catagenesis.