For biodegraded oil accumulations, field development can be optimized by using geochemical indicators of variations in the extent of bacterial alteration. Biodegradation typically reduces oil producibility by increasing oil viscosity. In the Cymric field (Kern County, California), sidewall core extracts reveal that the extent of oil biodegradation changes substantially over extremely short vertical distances in a shallow, low-permeability reservoir. Zones of more degraded oil can extend laterally for more than a mile. The relationships between oil viscosity and biomarker biodegradation parameters in this field were calibrated from analyses of produced oils, and these relationships were used to convert sidewall core biomarker analyses into quantitative predictions of later l and vertical changes in oil viscosity and gravity. Compositional variations were also used to allocate production to discrete zones. Viscosity prediction and production allocation can be used to optimize (1) the placement of new wells, (2) the placement of completion intervals, (3) the thickness of steam injection intervals, and (4) the spacing between injection intervals in the same well.