Geochemically Distinct Oil Families in the Onshore and Offshore Santa Maria Basins, California

Abstract

The purpose of this work is to identify genetic affinities and infer petroleum systems among 48 high-sulfur crude oil samples from the Onshore and Offshore Santa Maria basins. A key goal is to resolve the long-standing controversy as to whether geochemical variations among the oils result mainly from secondary processes, such as variable biodegradation or thermal maturation, or organofacies variations inherited during deposition and diagenesis of the source rock. Twenty-one source-related biomarker and stable carbon isotope ratios for each oil sample were assessed to assure that they were unaffected by secondary processes, such as heavy biodegradation or thermal maturation, and were compiled as a training set. The training set allowed creation of a chemometric (multivariate statistical) decision tree that classifies newly collected samples of crude oil or source-rock extracts. Surprisingly, six genetic oil families were identified that reflect organofacies variations within the Miocene Monterey Formation source rock. Three onshore families show systematic map distributions consistent with origins from two synclines, which may comprise one or more pods of thermally mature source rock. Geochemical characteristics of these families indicate generation from organofacies deposited in distinct shale, marl, or carbonate settings under differing conditions of water column oxicity and higher-plant versus planktonic input like those exhibited by outcrops of the Monterey Formation in coastal California. The offshore oil samples consist of one family from the Point Pedernales field and two families from the Point Sal and San Miguel discoveries that are effectively separated from the onshore families by the major northwest trending Hosgri Fault Zone.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018