Total drilling decreased slightly in South Texas from 2,835 wells in 1965 to 2,640 wells in 1966, or 6.9%. Exploratory drilling increased from 887 in 1965 to 1,050 in 1966. Development drilling decreased from 1,952 in 1965 to 1,590 in 1966. Different reporting and counting of exploratory tests and development wells are partly responsible for the difference in drilling categories from 1965 to 1966.

Oil production increased 10.8% in 1966. Production of gas and natural gas liquids increased 9.7% and 4.7%, respectively.

Seismic activity decreased 17.4%. Gravity exploration increased 150%, but still represents a minor part of the geophysical exploration.

No exploratory wells were drilled to the Paleozoic, and only 1 well was drilled to test the Jurassic. No major fields were indicated by the Cretaceous or Eocene discoveries. As in 1965, most exploratory drilling and major discoveries were in the prolific Oligocene-Miocene trend, and activity was concentrated in the offshore and bay areas. These areas are expected to be most active through 1967.