Organic and isotope geochemistry analysis of petroleum condensates from the unconventional portion of the Montney Formation, Western Canada

Abstract

The Montney Formation is a rather complex petroleum reservoir because it contains in-situ and migrated hydrocarbons. Previous organic geochemistry studies reported for this play were based on biomarker ratios of petroleum fluids from the conventional part of this reservoir. Furthermore, there is no isotope study on liquid hydrocarbons from this formation reported to this date. Therefore, we determined the organic and isotope geochemistry of condensate petroleum from the unconventional portion of the Montney Formation to fill this knowledge gap. The condensates were found to be normal fluids of increasing thermal maturity with increasing depth and from the northeast to the southwest. Light hydrocarbon parameters such as paraffinicity (n-C7/methylcyclohexane) and aromaticity (toluene/n-C7) showed to be governed by thermal maturity according to the known maturity trends (increasing depth, and northeast to southwest). The molecular fingerprint of the condensates is to some extent bimodal (noticeable at n-C13) suggesting the possibility of two hydrocarbon charges in the reservoir. This is further evidenced by the stable carbon isotope profile of n-alkanes where light hydrocarbons (C8-C13) show a normal distribution (decreasing δ13C with carbon number) whereas heavier compounds (C19+) show a reverse trend (increasing δ13C with carbon number). We suggest that condensate petroleum produced in the study area corresponds to a mixture of hydrocarbons sourced by the Montney Formation plus alteration products from a pre-existing oil charge that migrated into Montney reservoirs before further burial and thermal maturation of this rock. Producing intervals were distinguished using the δ13C values of n-C11, n-C12, n-C22, and n-C23.