Use of Trace Elements for Classification of Crude Oils into Families--Example from Alberta, Canada

Abstract

This study shows that a combination of instrumental neutron-activation analysis and stepwise multiple-discriminant function analysis is equally successful and valuable in correctly classifying crude oils which had previously been grouped into families based on organic geochemical analysis. In the present study, 11 trace elements were determined by instrumental neutron-activation analysis in 75 Alberta crude oils from similar lithostratigraphic situations and the samples grouped into the same families, plus an ungrouped data set. Stepwise multiple-discriminant function analysis shows that only S, Co, Se, V, and Br are significant discriminators of the three grouped data sets. Further, the efficacy of the discriminant analysis is confirmed using multiresponse permutation procedures. Among the grouped data, 89.1% of the crude oils were correctly classified on the basis of their trace-element content, 70% of which were at a probability level of more than 90%. For some misclassified samples, information from the statistical analysis together with knowledge of the local lithostratigraphy and hydrodynamics can be used to indicate mixing or migration phenomena. For example, crude oil in the Lower Cretaceous Basal Colorado Sandstone at Cessford appears to be a mixture from sources within the lower Colorado Group and in the underlying Mannville Group; in contrast, the only source of crude oil in the Upper Devonian Winterburn Group at Youngstown appears to be the overlying Lower Cretaceous Mannville Group. Among the ungrouped data, the trace-element classification for four Triassic crude oils suggests a source in the overlying Lower Cretaceous Mannville Group. The results for crude oils from Carboniferous and Upper Devonian Wabamun Group strata, far below the sub-Cretaceous unconformity, suggest further work is required before a specific source can be identified. In the structurally separated Turner Valley field, the crude oil in Lower Cretaceous Blairmore Group strata seems to have originated from the overlying Colorado Group, whereas that from Carboniferous rocks shows mixed characteristics. Crude oil from the Athabasca oil sand deposit exhibits enigmatic features.--Modified journal abstract.