ABSTRACT: The Nature and Distribution of Fluid Inclusions in the Monterey Formation in the Vicinity of the Santa Maria Basin, Califormia: Implications for Petroleum Migration

R. J. Bodnar, C. L. Knight, R. C. Burruss

Fluid inclusions provide a record of fluids that have flowed through rocks at some time in the past. The presence of petroleum fluid inclusions is definitive evidence that oil has migrated through the rocks. By relating the paragenesis of petroleum fluid inclusions to diagenetic and structural characteristics of the rocks, the timing of oil migration relative to these events may be determined. Fluid inclusion characteristics of vein-filling minerals from several coastal outcrops in the Miocene Monterey Formation in the vicinity of the Santa Maria basin, California, are being studied to better understand the nature of hydrocarbon migration in this important oil province. The Monterey Formation constitutes a fracture-controlled petroleum reservoir with intercalated calcareo s and fine-grained siliceous rocks serving as both the source and reservoir for oil accumulations, resulting in relatively short migration distances for Monterey oils. Many fractures in the Monterey are filled with carbonate, quartz, barite, and anhydrite. These same fractures often contain visible tar or oil, and occasionally a thin layer of oil can be seen coating growth surfaces between two generations of vein-filling minerals.

Evidence for migration of fluids through these fractures in the geologic past is provided by aqueous and petroleum fluid inclusions contained within vein-filling minerals. Vein-filling dolomite from Jalama Beach contains three different types of primary petroleum inclusions (based on their fluorescence characteristics), indicating that oils with significantly different API gravities and compositions flowed through the fractures. Petrographic and microthermometric analyses of oil and coexisting aqueous inclusions indicate that the fracture-filling minerals precipitated from aqueous solutions of seawater salinity heated to ^sim100°C and that oil was introduced into the fracture system episodically during mineral growth. A vein from the Lion's Head area consists of early calcite and late quartz, both of which contain aqueous inclusions with seawater salinity. Inclusions in quartz homogenize at slightly higher temperatures than those in calcite, and hydrocarbon inclusions are relatively rare in this sample. Petrographic analysis of samples from several other areas indicates that not all fractures have served as conduits for oil migration in the Monterey.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990