This paper discusses empirical geochemical correlations used to predict API oil gravities prior to drill-stem testing in Monterey Formation reservoirs, offshore southern California. The primary objective was to eliminate expensive well testing by identifying intervals that contain low-gravity, nonproducible oil (usually <14° API). However, the correlations proved very successful in accurately predicting (within 4° API) oil gravities that range from 5 to 35° API throughout the offshore Santa Barbara and Santa Maria areas.
The primary data are weight-percent sulfur and Rock-Eval pyrolysis of bitumen chemically extracted from reservoir rock samples. In general, reservoirs that contain higher gravity, producible oil have bitumen organic sulfur contents of less than 5 wt. %, Rock-Eval bitumen S1 values greater than 450 mg HC/g bitumen, and Rock-Eval bitumen S1/S2 ratios greater than 1.0. These data are usually supplemented with Rock-Eval pyrolysis of the reservoir rock, where whole-rock S1/S2 ratios greater than 0.30 usually indicate associated oil gravities greater than 14° API. This analytical mix gives a multiple approach for estimating reservoir oil gravities within proposed drill-stem test (DST) intervals.
Using this approach, oil gravities of more than 50 DSTs have been accurately predicted in the offshore southern California area. The technique is also useful for reevaluating API gravities in older wells where Monterey reservoirs were not the primary target. Moreover, the technique should have application elsewhere, provided the range of oil gravities are not the result of obvious biodegradation, and sufficient rock and oil samples are available to establish pertinent correlations.