Chemometric Recognition of Genetically Distinct Oil Families in the Los Angeles Basin
Peters, K.E.; Ramos, L. Scott; and Zumberge, J.E.
Multivariate statistical analysis (chemometrics) of selected source-related biomarker and stable carbon isotope ratios for 112 nonbiodegraded or mildly biodegraded crude oil samples from the prolific Los Angeles basin identifies five tribes and a total of 12 genetically distinct oil families. These families occur in different parts of the basin and exhibit different bulk properties, such as API gravity, sulfur, and metal content that are principally controlled by source rock organofacies rather than secondary processes, such as thermal maturity or biodegradation. Stable carbon isotope data for saturate and aromatic fractions of the samples suggest Neogene source rock, although not necessarily all from the basal Mohnian nodular shale unit of the Modelo Formation. Biomarker ratios allow indirect assessment of the character to the source rock for each family. For example, tribes 1 and 2 straddle the Central Trough to the east of the Newport-Inglewood Fault Zone and show evidence of elevated clastic and higher-plant input (e.g., elevated C24/C23 tricyclic terpanes and oleanane, respectively). Some families clearly originated from source rock in different depocenters on opposite sides of the Newport-Inglewood Fault Zone and from different stratigraphic intervals. For example, unlike tribes 1 and 2, tribe 5 occurs west of the Newport-Inglewood Fault Zone and shows evidence of a more distal, clay-poor source rock (e.g., lower C24/C23 tricyclic terpanes and oleanane, elevated steranes/hopanes and bisnorhopane). As another example, tribe 5 oils occur in deeper intervals (9007-10,480’) than tribe 3 oils (3531-6842’) in the Wilmington field. The 112 crude oil samples were used as a training set to build a chemometric decision tree, which allowed classification of nearly 50 additional samples that were affected to varying degrees by severe thermal maturation or biodegradation.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013