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Origin of Adamantanes and Diamantanes in Marine Source Rock

Li, Yun, Chen, Yuan, Xiong, Yongqiang, Wang, Xiaotao, Fang, Chenchen, Zhang, Li, Li, Jinhua
Energy & Fuels 2015 v.29 no.12 pp. 8188-8194
bitumen, cracking, isomerization, maturity stage, oils, organic matter, shale, thermal stability
Thermal maturation-related variations in the yields of lower diamondoids (adamantanes and diamantanes) in source rock were investigated by thermal simulation experiments based on a marine shale and kerogens obtained from the shale via isolation and artificial maturation, representing different maturity stages of the oil generation window. The simulations show that lower diamondoids are formed and destroyed during thermal maturation of the shale. For example, adamantanes are generated mainly in the maturity range of 0.8%–1.8% EasyRo, then they begin to degrade at 1.8% EasyRo. Diamantanes are produced mainly during the maturity range of 1.0%–2.2% EasyRo and begin to degrade at 2.2% EasyRo. The mineral matrix of shale may have a strong effect on the destruction of diamondoids, leading to a reduction in the peak yield and a reduction in the maturity level corresponding to the peak yield of diamondoids. A comparison of the diamondoid yields from four kerogens at different maturity levels indicates that the lower diamondoids are derived mainly from secondary cracking of extractable organic matter (bitumens) occurring in the source rock. For instance, at the peak stage of adamantane formation (2.1% EasyRo), 75.6% of the total adamantanes is generated from the cracking of bitumens and the remaining 24.4% is from the primary cracking of kerogens. Similarly, the yield of diamantanes generated from the secondary cracking of bitumens accounts for 87.8% of the total diamantanes at the peak stage of diamantane formation (2.5% EasyRo). Almost no diamondoids are detected in the pyrolysates of more mature kerogen (1.3%EasyRo), suggesting that 1.3% EasyRo is the upper limit of maturity for the generation of diamondoids from kerogen. Diamondoid isomerization ratios are maintained at relatively constant levels during the formation stage of diamondoids, whereas a linear correlation with maturity occurs during the destruction stage, suggesting that isomerization ratios of diamondoids are controlled by their thermal stability just in the destruction stage and are unaffected by hydrocarbon generation and expulsion of source rock at early thermal stages. This finding indicates that these diamondoid indices are a potential tool for evaluating the thermal maturity of source rocks at highly mature stages.