Geochemical
Data Provide Insights into Oil Migration History of Elk Hills
Field, California
Countryman, Robert L.1, Judy
Russell2, Stephen A. Reid3, John Zumberge4 (1)
Occidental Oil and Gas, Tupman, CA (2) Occidental Oil and Gas, Houston, TX (3)
Occidental Oil and Gas, Muscat, Oman (4) GeoMark Research, Houston, TX
Geochemical analysis of oils from a
mature field when combined with regional and reservoir geology may provide
insights into oil migration routes, relationships between reservoirs, and seal
quality of traps. At the Elk Hills field in the San Joaquin basin, stable carbon
isotopes and biomarkers of oils from Upper Miocene reservoirs define four
families. Most Miocene reservoirs at Elk Hills lack low maturity oil
characteristic of the Monterey Formation, which implies that traps or migration
routes formed after the early pulse of migration. Family A1 oils are present
only in deep-water channel reservoirs and are derived from a source rock of a
consistent composition that became increasingly mature. Oil migrated from the
generative area to the reservoirs updip along the channel system. Family A2
oils, present in deep-water fan lobe reservoirs, are derived from a source rock
of consistent maturity. The consistent maturity indicates the possibility of rapid
filling of the reservoirs along a newly established migration routes from
generative areas. Oil from porcelanite reservoirs represents another family
(SH), and migration must have occurred following an enhancement of permeability
during diagenesis. The presence of low maturity oil (Family C) in one reservoir
presents a paradox: the oil may represent a remnant of the initial pulse of Monterey oil, or may be the last
oil generated from a location immediately adjacent to Elk Hills.
In general, a strong stratigraphic
relationship exists between oil migration routes and reservoir intervals.
Within the field, mixing of oil between stratigraphic intervals occurs but is
rare. The distribution of oil families also confirms that some adjacent
reservoirs have excellent lateral stratigraphic seals.