Igneous rocks associated with the Balcones fault zone, extending from Kinney County in southwest Texas through Travis County in central Texas, have produced oil from more than 40 different fields dating from 1915. These rocks, referred to as serpentine by petroleum geologists, have been the subject of continued debate regarding their origin. Abundant evidence exists to support the conclusion that these igneous bodies are of volcanic origin related to crustal tension. Production is shallow and in many places economic. Most discoveries were made by surface geology or random drilling. Geophysical data have been used in exploration and have proved to be accurate.

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