The South Thornwell field is an excellent example of the increasing complexity with depth of geologic structures in South Louisiana and also of the occurrence of an unusual and unexpected type of structure in the deeper beds which bears very little relation to the configuration of the younger sediments.

The field was discovered in 1942 and produced gas from a sand in the Planulina zone of the lower Miocene, the structure being that of a relatively flat dome. Deeper drilling in 1955 led to the discovery of prolific high-pressure gas sands in the Marginulina and Miogypsinoides "A" zones of the Oligocene; however, the main gas sands, which are productive over the eastern part of the field are missing in the western wells where they have been replaced by a body of disturbed shale, which, from paleontological evidence, was originally deposited below the Miogypsinoides "A" zone. This shale closely resembles a salt dome in size and shape and in effect on the adjacent beds. It is intrusive or diapiric and is probably associated with a salt plug which has not as yet been penetrated by drillin .