Useful but not perfect information in regard to the genetic relations and the lack of genetic relationship between crude oils is given by the graph of the deviation-from-normal of the interval between the A.P.I. gravity of the successive cuts of a Hempel analysis. The graph tends to be usably invariant among different samples from a common reservoir and to be usably invariant in the face of drastic oxidation and in the face of drastic weathering. Within certain limitations, crude oils which are of a common stratigraphic age and which occur in the same petroleum province may have recognizably similar

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patterns in their respective graphs, whereas crude oils of different stratigraphic ages commonly have recognizably different patterns. The Permian crude oils of the West Texas basin tend to have a common pattern which, however, differs drastically from the pattern of the Sand Hills Ordovician and from the pattern of the crude from the deeper horizon at Chalk. The Cretaceous crude oils of the Powell district, Texas, show clear correlation in pattern with Cretaceous crude oils of North Louisiana and Arkansas. In the Powell district of Texas, the Corsicana 800-foot crude, the Corsicana 1,250-foot crude and the Powell 1,500-foot crude, the Woodbine crudes (all upper Upper Cretaceous) and the Kosse crude (Lower Cretaceous) have strikingly different patterns and seem not to have been derive one from the other. The serpentine plug crudes tend to have a common pattern. Three different patterns occur among the Woodbine crudes: (1) East Texas, (2) Van and Powell, and (3) Richland, Currie, and Mexia. The pattern of the last three crudes shows similarities to the common "serpentine-plug" pattern; and the Van-Powell pattern has close similarities to Nacatoch patterns of North Louisiana and Arkansas. The gravity interval therefore seems to have considerable possibilities in the study of the genetic relationships of crude oils.

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