A geochemical survey in the Santa Barbara Channel and its relationship to subsurface heavy-oil deposits

A geochemical survey was conducted in a part of the Santa Barbara Channel that contains three oil fields: Hondo, Pescado, and Sacate. The fields, especially Hondo, contain substantial reserves of low-gravity oil (14-26° API) in the fractured Monterey Formation of middle Miocene age. The light hydrocarbons, methane through pentane, produced relatively weak and incomplete near-surface anomalies over the three oil fields. These results are to be expected considering the low gas-oil ratios in the reservoirs. The most outstanding anomalies were produced by aromatic compounds detected in the sediments about 1.8 m (6 ft) below the water-sediment interface. These compounds, probably consisting of naphthalenes and phenanthrenes and present in appreciable amounts in most crude oils, were measured by fluorescence techniques. Carbon isotope ratios of adsorbed methane extracted from the fine-grained portions of the shallow sediments were also determined. Most of the <513C, values range from 40 to 49 parts per mil (PDB) and are indicative of an oil-prone area. Furthermore, the isotope ratio data produced anomalies over and adjacent to the oil fields. When the geochemical data are compared with the known subsurface geology, close relationships are seen. Particularly striking similarities between the carbon isotope ratios, the heavier aromatic hydrocarbon fraction (fluorescence 365), and the geological data are apparent.