Markers of crude oil migration through low-maturity organic-rich pathways

Abstract

Maturity and source assessment anomalies have been observed particularly in Tertiary/deltaic oils which have migrated through immature, organic-rich sediments. The occurrence of unsaturated hydrocarbons (olefins) and carboxylic acids in crude oils has been previously reported, though there is limited published quantitative data. A two-part study identifies these uncommonly reported compounds in sets of field and laboratory simulated ‘migrated’ crude oils, in order to investigate their potential as migration-contamination markers, useful for helping to resolve many oil assessment anomalies. In the field study, 29 apparently non-biodegraded crude oils from three well-known Tertiary deltas (Beaufort-Mackenzie, Kutei Basin, and Niger Delta) were investigated. In the laboratory study, a simulation of oil migrating through an immature organic-rich rock was carried out by pumping a mature North Sea crude oil through steel HPLC columns packed with organic-rich Kimmeridge Clay mudstone of varying particle sizes: 4 mm-1 mm, 1 mm- 63 μm, and <63 μm. Samples of the resulting ‘migrated’ oils were analyzed for the presence, concentrations and distributions of these typically low-abundance compounds. Olefins (n-alkenes, diasterenes and triterpenes) were detected in nearly all field samples, with summed concentrations ranging from <5 ppm to ~300 ppm. They were considered to have been dissolved from immature, organic-rich sedimentary sequence during migration as suggested by dissimilarities in the n-alkene to n-alkane distributions, and the presence of other olefins (e.g. diasterenes and hopenes) characteristic of immature organic matter. Linear acids (up to 6500 ppm) and C30-C33 hopanoic acids (from <1 ppm to ~120 ppm) were detected, with the hopanoic acids exhibiting variable isomer distributions. An increase in the abundance of olefins and also a relative enrichment in short-chain- (C12- C19) acids in the laboratory ‘migrated’ oils was observed. The acid profiles from the oils that had passed through the finest (<63μm) mudstone size fractions were different from those of the others, indicating that surface area and/or permeability differences were likely causative factors. The results suggest that relatively high abundances of olefins and carboxylic acids in many of the studied non-biodegraded Tertiary deltaic oils are an indication of the occurrence of migration-contamination and that the extent, and actual signatures of this may be affected by variations in rock properties (e.g. permeability) and presence of faults and/or fractures within the migration route, as well as the organic richness of these carrier sequences.