The Darst Creek oil field, the fourth Edwards limestone field in southwest Texas, was discovered July 18, 1929, in eastern Guadalupe County. It is located along one of a series of en echelon faults parallel with, and southeast of, the Balcones fault zone of Texas.

After migrating up-dip from the southeast, the oil in this field was trapped in the upper 50 feet of the porous Edwards limestone, which was faulted in juxtaposition with the impervious beds on the downthrown side of the fault. In addition to wells producing from the Edwards, there are several producing from fault plane cavities, and two supposedly producing from reworked serpentine deposits. The maximum vertical displacement along the fault is calculated on top of the Austin chalk to be approximately 550 feet.

The surface beds in the area are Middle and Upper Indio sandy clays of Eocene-Tertiary age. Aside from fault planes, the structure is evidenced on the surface by steep dips, a decided down-dip swing in the Indio-Carrizo contact, and a repetition of Upper Indio.

The productive area of the field, including the Appling area to the northeast, consists of approximately 1,670 acres. It is 6 miles in length and in places exceeds 4,000 feet in width. Out of the 291 wells drilled in the area, only 19 are dry holes. The total production through December, 1931, was 19,700,340 barrels. The oil has a paraffine base, a deep green color, and a gravity of 36° Be.

As proration has been in effect since the field was discovered, the development has been very slow. This, however, has proved beneficial by delaying the encroachment of sulphur water, and tending to increase the ultimate recovery of oil.