Establishing Petroleum Systems: Biomarkers, Isotopes and Chemometrics

Kenneth E. Peters
Schlumberger and Stanford University, California

A petroleum system consists of a pod of active or once-active source rock, all related petroleum, and all geologic elements and processes necessary for accumulations (Magoon and Dow, 1994). Geochemistry is the key to identifying a petroleum system because it is required to establish the genetic link between petroleum and its source rock.

Decision-tree chemometrics is a powerful new approach to assign oil or rock-extract samples to petroleum systems. Chemometrics uses statistics to extract information from multivariate data. A purely statistical approach is unreliable for oil-oil or oil-source rock correlation because crude oil is a quasi-stable mixture that changes composition due to secondary processes, such as biodegradation. However, geochemical expertise can be used to improve the utility of chemometrics. For example, rather than using all compositional data, selected source-related biomarker and isotope ratios for 622 non-biodegraded Circum-Arctic oil samples were used to create a ‘training set’ based on principal component analysis (Peters et al., 2007). Biomarkers are molecular fossils from once-living organisms that occur in crude oil and extracts of petroleum source rock (Peters et al., 2005).

A chemometric decision tree for the Circum-Arctic based on the training set can be used to classify newly collected samples. Each tier in the decision tree consists of a layer of unique K-Nearest Neighbor models (Figure 1). The decision tree assigns confidence limits to each classification. More than 1,000 Circum-Arctic oil samples can be assigned to 31 genetic families identified in the figure. Source-rock extracts submitted to the decision tree allow oil-source rock correlations for many families, thus establishing petroleum systems.

AAPG Search and Discovery Article #90101 © 2010 AAPG Foundation Distinguished Lecturer Series 2009-2010