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Diagenesis of organic matter and fine clay minerals: a comparative study

Snowdon, L.R, P. R. Gunther, T. G. Powell, Foskolos Antonis

Πλήρης Εγγραφή

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Περίληψη

A geochemical study has been made of fine clay minerals and organic matter in subsurface shale samples from the Canadian Northwest Territories. The mixed layer clays comprise smectite-vermiculite-illite but are transformed during thermal diagenesis to a pseudo-quaternary system (smectitevermiculite-illite-chlorite) by incorporation of amorphous inorganic material. The first clay dehydration occurs prior to hydrocarbon generation and is accompanied by adsorption of K+ and substitution of Al3+ for Si4+ in the clay lattice. Vermiculite is an intermediary in the transformation of smectite to illite and in the presence of Ca2+ ions delays the second dehydration step to the zone where cracking of liquid hydrocarbons to gas occurs.Hydrocarbon generation commences at a vitrinite reflectance level of ≅ 0.5% R0 maximum in both amorphous and woody-herbaceous organic matter but does not reach a significant level in the latter case until reflectance levels of 0.7% R0 maximum. The pristane to phytane ratio and proportion of n-alkanes and acyclic isoprenoids increase during hydrocarbon generation from woody-herbaceous organic matter. Anomalous hydrocarbon yields from certain samples are attributed to high concentrations of plant resins. Destructions of liquid hydrocarbons by cracking commences at a reflectance level of 1.0% R0 maximum and is essentially complete by a reflectance level of 1.4% R0 maximum. The proportion of n-alkanes in the saturates decreases during cracking of the liquid hydrocarbons.

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