The role of alkenes produced during hydrous pyrolysis of a shale

Hydrous pyrolysis experiments conducted on Messel shale with D2O demonstrat ed that a large amount of deuterium becomes incorporated into the hydrocarb ons generated from the shale kerogen. In order to understand the pathway of deuterium land protium) exchange and the role of water during hydrous pyro lysis, we conducted a series of experiments using aliphatic compounds (1,13 -tetradecadiene, 1-hexadecene, eicosane and dotriacontane) as probe molecul es. These compounds were pyrolyzed in D2O, shale/D2O, and shale/H2O and the products analyzed by GC-MS. In the absence of powdered shale, the incorpor ation of deuterium from D2O occurred only in olefinic compounds via double bond isomerization. The presence of shale accelerated deuterium incorporati on into the olefins and resulted in a minor amount of deuterium incorporati on in the saturated il-alkanes. The pattern of deuterium substitution of th e diene closely matched the deuterium distribution observed in the n-alkane s generated from the shale kerogen in the D2O/shale pyrolyses. The presence of the shale also resulted in reduction (hydrogenation) of olefins to satu rated n-alkanes with concomitant oxidation of olefins to ketones. These res ults show that under hydrous pyrolysis conditions, kerogen breakdown genera tes n-alkanes and terminal n-alkenes by free radical hydrocarbon cracking o f the aliphatic kerogen structure. The terminal ir-alkenes rapidly isomeriz e to internal alkenes via acid-catalyzed isomerization under hydrothermal c onditions, a significant pathway of deuterium land protium) exchange betwee n water and the hydrocarbons. These n-alkenes simultaneously undergo reduct ion to n-alkanes (major) or oxidation to ketones (minor) via alcohols forme d by the hydration of the alkenes. (C) 2000 Elsevier Science Ltd. All right s reserved.