ABSTRACT: Organic Heterogeneity in a Migrating Depocenter, Ridge Basin, Southern California

CURIALE, JOSEPH A., and SCOTT A. STOUT, Unocal Science & Technology, Brea, CA

The Ridge basin is an elongated, puil-apart half-graben filled with late Miocene-Pliocene clastics, located approximately 60 km north-northwest of Los Angeles. Over 13 km of fluvial and lacustrine sediments were deposited; however, depocenter migration through-out basin development has resulted in no more than 3-4 km of vertical section in any one location. In this paper, we use molecular, isotopic, and petrographic data to help identity variations in nonmarine organic matter type throughout the section, and to relate these variations to oil source rock potential and tectonic development of the basin.

Numerous organic matter parameters were determined for a 45 sample suite. Vitrinite reflectance values (generally less than 0.5% Ro) and biomarker epimer ratios (steranes, hopanes) attest to a migrating depocenter and indicate that almost all samples are thermally immature. TOC (up to 2.6%) and Rock-Eval data (hydrogen indices up to 400 mg/g) show that oil source potential is present in only a few samples. Organic petrographic data indicate that the best potential source rocks contain the lowest relative concentrations of vitrinite and larger amounts of fluoramorphinite, and were probably deposited in a lacustrine setting. The distributions of ethylcholestane, oleanane, isoprenoids, vitrinite, and fluoramorphinite act as monitors for relative input of terrigenous vs. water-column-deri ed (algal) organic matter. Kerogen pyrolysis yields vary directly with percentage of fluoramorphinite, and inversely with 13Cker (-23.5 to -28.0 0/00). Our data suggest that organic matter character and distribution in the Ridge basin are controlled primarily by organic source heterogeneity although isotopic variability may also be due to secular variation.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)