Unfortunately models are just that, and mother nature is not so easily pinned down. In a series of shallowing anticlines with a continuous formation of reservoir quality, this is the case since the deepest anticline will first be charged with oil. Subsequent gas influx will flush the oil which will then migrate upwards to the next trap, and so on. Hence oil in the upper beds, gas in the lower. Elementary stuff. However in fault compartments with stacked individual reservoirs, each can be face charged by source rocks of differing quality. Deeper sands may be charged by oil sources, shallower beds by more gassy sources. In general they will be independent accumulations with their own water contacts. As an example (and if I remember correctly) the Tidepole fault ledge to the Rankin Platform has upper sands gas bearing, lower sands oil bearing with a few water sands thrown in for good measure. I am sure I could rigorously document a few more if I were interested to do so. Chaos rules.
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