C₇ Chemistry of Biodegraded Monterey Oils from the Southwestern Margin of the Los Angeles Basin, California

Alan S. Kornacki and Frank D. Mango

Biodegraded Monterey oils in the Los Angeles Basin can be differentiated from unaltered Monterey oils by utilizing new C₇ parameters supplemented with standard geochemical data, such as gas-liquid chromatograms and biological markers. Unaltered oils occur below c. 4000 ft in the Wilmington, Sunset Beach, and Seal Beach fields. These medium-gravity crudes (>25° API) exhibit high C₇ primary test sum values (0.90-1.00), and contain relatively low concentrations of sulfur (0.3-1.0 wt%). Most of these oils were generated at low temperatures (110-115°C). Heavier crudes in relatively shallow pay zones at the Wilmington and Huntington Beach fields are transformed oils that have undergone various degrees of biodegradation and water washing. The r sidual biodegraded oils, which contain 1.2-1.9 wt% sulfur, have lower C₇ primary test sum values that range from 0.51 to 0.88, and exhibit disturbed (elevated) C₇ oil-generation temperatures. As expected, the transformed crudes contain low concentrations of the aromatic gasoline-range compounds benzene and toluene (which are relatively soluble in water), and they also are depleted in the normal alkanes and (in cases of severe biodegradation) the isoprenoid isoalkanes and steranes. Furthermore, the values of several other C₇ parameters -- such as ratios between 'gem' and 'non-gem' structural configurations of C₇ compounds (e.g., 2,3-DMP/2,2-DMP; 1 ,2-DMCP/1,1-DMCP) -- demonstrate the degree to which bacteria preferentially metabolize certain gas line-range compounds in petroleum. These effects must be considered when C₇ source parameters (such as selectivity ratios) are used to perform oil-oil and oil-source rock correlations.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California