Abstract: How Oil Composition Relates to Kerogen Facies in the World's Most Petroliferous Basin

T. H. McCulloh, D. W. Kirkland, A. J. Koch, W. L. Orr, H. M. Chung

The Los Angeles basin exceeds all others in volume-of-petroleum to volume-of-basin-fill. As an exploration analog, its usefulness hinges on understanding its great richness. Enhanced understanding results when the diverse oil compositions are viewed in a new way, one that reveals distribution patterns rooted in source-kerogen compositions. Oil compositions reflect nearby kerogen compositions and indicate migration vectors less than five miles long, substantially cross stratal.

Accumulation of major oil-prone source-strata began 14-15 my and continued until 6.5 my (locally to 3.5 my). Abundant, lipid-rich, plankton debris rained from nutrient-rich surface-waters into oxygen-deficient bathyal environments to interstratify or intermix with granitoid siliciclastics shed into the basin principally from the north. Vascular-plant detritus was introduced only near paleo-shorelines. Multiple source-strata of divergent organofacies range from Iaminae to sequences hundreds of feet thick and occur through thousands of feet of section.

Oil compositions were determined chiefly by kerogen compositions, which, in turn, were controlled by stratigraphic position and basin location. Basin location dictated the organic detritus mix and the availability of chemically reactive iron for sequestering bacterially generated H₂S as iron sulfides, especially before 6.5 my. At the distal basin-perimeter, away from terrigenous input (the major iron-source), kerogens contain much organic sulfur (Type II-S) and generate sulfur-rich oils at low maturities. Along the landward (northerly) basin flank, kerogens with moderate to-low organic sulfur-contents (Types II and II-III) have generated low-sulfur oils.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994