Abstract: Oil Gravity Segregation in the Monterey Formation, California
J. Scott Hornafius
The Monterey Formation is a fractured siliceous shale that is the principal reservoir and source rock for oil fields in the Santa Maria basin and the western Santa Barbara Channel. Monterey crudes in producing offshore fields are high-sulfur oils that range from 10° to 35° API. The oils in Monterey fractured reservoirs display a systematic increase in API gravity with increasing height above the oil-water contact. The rate of change in API gravity with depth in Monterey oil fields generally ranges from 0.5° to 1.2° API/100 ft. The oil-water contact usually occurs at an oil gravity of 10° API (the gravity at which the density of the oil and the water is equal).
The maximum API gravity in a Monterey oil field is related to the level of thermal exposure experienced by the formation in the adjacent depocenter. Monterey oils are sourced by high-sulfur kerogens that generate heavy oils at low levels of thermal exposure, but generate progressively higher gravity oils at higher levels of thermal maturity. Comparison of the maximum API gravity found in 33 Monterey-sourced oil fields with the maximum temperature experienced by the Monterey Formation within three
miles of the field (the most likely migration distance) suggests that a temperature of 260°F (127°C) is required to generate 20° API oil, and a temperature of 330°F (166°C) is required to generate 30° API oil.
AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994