Elemental Geochemistry of Some Canadian Oil Shale

Abstract

Bulk elemental concentrations of oil shale deposits of Cretaceous-Ordovician age from Canada were analyzed using ICP-MS and Prompt Gamma to determine their depositional environment and concentration of hazardous elements. Boron (B) concentration was used as an indicator of depositional environment of the oil shales. Oil shales with B contents of less than 50 ppm indicate fresh water depositional environment and those with >70 ppm a brackish setting. However, some oil shale show high content of B, which is not associated with their depositional environment and is due to secondary enrichment. The organic matter in early Carboniferous oil shales from New Brunswick consists of predominantly filament algae derived from Pediastrium a freshwater alga with a low salinity tolerance (1.7 g/1), indicating their deposition in a freshwater setting, consistent with some of these oil shale samples having low B (38 ppm). However most these oil shales have higher B concentration that is characteristic of slightly brackish to brackish settings (B: 70-332 ppm). The high B content is these oil shale is related to a secondary enrichment of B due to dissolution of evaporite deposits and migration of those saline waters into lakes by river sediments, which resulted in the highest B value (258ppm) occurring in finer sediments deposited in the lower delta fan. There is also possibility of an input by the hydrocarbon brine water as a result of breached reservoir in certain interval in the oil shale deposit. The salinity of water provided an anoxic environment, which restricted bacterial activities and decomposition of organic matter. There are other elements in these oil shales that are indicative of environmental setting such as rare earth elements (both HREEs and LREEs). Concentration of REEs in Canadian oil shales decreases from those deposited under fresh water towards those in brackish water. In general the concentration of hazardous elements such as As, Cd, Cr, Mo, Ni and Pb in these oil shales are low and/or same compared to other shales.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016