Geochemical rationalisation for the variable oil quality in the Orcutt reservoir, California, USA

Abstract

The relationship between fluid physical properties and hydrocarbon compositions was investigated on a suite of eleven heavy oil samples from the Orcutt Field, onshore Santa Maria Basin in California. The physical properties of oils display a considerable range across the reservoir, with viscosity varying from 46.3 to 611,150 centipoises at 20 °C and API gravity from 16.3 to 25°. Not surprisingly, the molecular composition of the Orcutt Field oils is also highly variable.Present-day fluid physical properties in the field are determined by the mixing of multiple petroleum charges. The bulk of the oil volume in place was generated by source rocks of the Monterey Formation in the early oil window stage of petroleum generation. The mixing of biodegraded and non-biodegraded oils indicates at least two oil charge events, punctuated by episodes of biodegradation. Oil viscosity decreases towards the Orcutt Frontal fault and Solomon Hills fault, which bound the Orcutt anticlinal structure to the northeast. These faults appear to be conduits for a separate diamondoid-rich light oil or condensate charge responsible for localised improvements in oil viscosity towards the NE and in the crest of the structure.Data from this study show the profound impact of charge mixing and in-reservoir alteration on physical properties of oils within a single accumulation.