The subunconformity trap at Oklahoma City long has been known to have leaked a substantial first crop of oil when it was breached in Early Pennsylvanian time, although when discovered the buried Ordovician reservoirs again were nearly full of oil and gas. It can be shown now that the first crop was generated in deeper, already thermally mature parts of the pre-Pennsylvanian section and that the second crop later was generated in the “preservation window,” defined as the shallower part of that section which was both preserved from erosion and still thermally immature when unconformably buried. A simple method is used to estimate the depths and ages at which significant crops of oil started to form.

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