This paper presents a case study of how compositional grading was used as a tool to greatly affect the subsurface interpretation of a deepwater field. Originally, the main reservoir at the field was viewed as being compartmentalized. There were also two viscous low API gravity oil reservoirs that were of little economic interest. A new interpretation based on compositional grading suggests that the main reservoir is, in fact, much better connected. There is also a risk of a viscous asphaltene-enriched zone at the oil-water contact that was not previously considered. Fortunately, if it exists, it would likely only delay aquifer drive. The two low API gravity oil reservoirs of little economic interest likely have much better quality Oil updip making them good appraisal candidates. The different interpretation improves the overall view of the field and has implications for optimal development well locations, initial production rates and facility gas capacity. Compositional grading is particularly useful in aiding subsurface characterization in areas where grading is strong, and in deepwater offshore developments where uncertainty is great and development and appraisal costs are high. Fundamental geology and geochemistry are critical to being able to recognize a priori areas where compositional grading is strong.