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Delineation of a Large Asphaltum Resource, NW Casmalia field, Santa Barbara County, California

Ramon Elias and Mark L. Wilson

Santa Maria Pacific, LLC had discovered and delineated a large oil-saturated diatomite reservoir near the Casmalia field in northern Santa Barbara County, California in late 2005 and 2006. Production testing using high pressure steam stimulation in 2007 surfaced a significant viscosity problem. Although it was originally hoped that the problem would be isolated, chromatography of oil samples from the wells with cuttings samples, sidewall cores and cores suggest that the conversion of this oil to asphaltum has been extensive.

As it turns out the high resistivity (saturation) trends that were pursued by development drilling were caused by the conditions that probably led to the degradation of the reservoir fluids to asphaltum. The oil accumulation is characterized by very high resistivities, resistivities that are much higher than usually observed in commercial oil-saturated diatomites such as those at Belridge and Cymric fields in the San Joaquin Valley of California. In speculating about how the reservoir oil became degraded, it was suggested that fractures may have allowed entry of a biologic contamination that led to degradation. The theory was tested by mapping the highest resistivity in each well where asphaltum was suspected or confirmed. The highest resistivity “marker” is a planar surface that probably describes a fracture plane.

Tests of the commerciality of producing the asphaltum are underway. In addition, the chromatography studies identified portions of the reservoir that have not been degraded and additional delineation drilling is planned. Our protocols for evaluating these stepout wells have been modified to include retaining cuttings samples and shooting a large number of sidewall cores. These samples will be examined using chromatographic analysis and correlations that can be made to viscosity.


AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California