The Plymouth oil field is located in northeastern San Patricio County, Texas, in the Coastal Bend province of the Gulf Coastal Plain. The discovery well was completed in April, 1935, in the Plymouth or 5,500-foot sand. Reflection seismograph led to the discovery. One hundred eighty-eight wells have been completed as of January 1, 1942. The field has a proved area of 3,000 surfaces acres and has produced 21,631,000 barrels of oil to January 1, 1942.

The field is located on strike with the most prolific lower Catahoula (Frio) sand fields of the Coastal Bend province. The sand conditions are excellent along this strike and reach their maximum thickness a short distance downdip from the Plymouth field. The structure is an elongate flat dome of moderately low relief. The maximum proved closure is approximately 100 feet. The dome is bounded on the southwest flank by a graben. The structural uplift is probably caused by a deep, buried salt core. The dome becomes more pronounced and steeper with depth; on the Greta sand there is only a broad nosing, on the 5,600-foot sand there is a broad flat dome, but on the 6,100-foot sand there is a very pronounced doming with steeper dips. There is good evidence of at least one erosional unconformi y.

There are four producing oil sands: (1) Plymouth sand, 5,500 feet; (2) Heep sand, 5,600 feet; (3) Magnolia sand, 5,800 feet; (4) 6,100-foot sand. Of these, the Heep is the most important oil reservoir.

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