The High Island dome was first drilled for oil in 1901; however, the first production was obtained from the porous cap rock in 1922, on the super-dome structure. The Yount-Lee Oil Company in 1931 proved the existence of an overhang at High Island and has since developed the field from the northwest flank to the southeast flank. The development at present is toward the east flank.

The cap rock at High Island is divided into a false cap rock and a true cap rock. The false cap rock is a hard sand and lime rock ranging in thickness from 200 to 1,300 feet. The true cap rock lies underneath the false cap rock and consists of a series of caps. At the base of the series is anhydrite which grades upward into gypsum and a calcite cap above the gypsum.

The overhang has so far been proved to exist on all flanks that have been drilled. There is a cap-rock overhang as well as a salt overhang; the cap-rock overhang extends outward and away from the salt. Production is just as prolific under the cap-rock overhang as under the salt overhang. The dome is somewhat lopsided in that the southeast flank is approximately 1,500 feet higher than the northwest flank. Faulting has been detected on the south and southeast flanks. A theoretical peripheral fault is mapped from the northwest to the southwest flank. Oil has been produced in commercial quantities from the cap rock, Pliocene formation, Miocene formation, M Dv zone, Discorbis zone, and Marginulina zone. The Heterostegina zone at High Island has not yet been penetrated or encountered. The P iocene and Miocene production is very small and practically all of the sands in these two formations contain water.

The M Dv zone is a new paleontological zone mapped by the writer as lying immediately above the true Discorbis zone of the Middle Oligocene, and equivalent to Lower Miocene in age. This new zone is characterized by both Lower Miocene and true Discorbis fauna. Therefore, the name M Dv has been chosen to represent the zone because of the occurrence of Miocene fauna associated with Discorbis cf. D. vilardeboana and its fauna; thus, the name: M Dv.

The M Dv zone has so far been proved to be the most prolific of any productive zone. Six different sand horizons have been found in this zone. The Discorbis zone production has been limited; however, some of the company's latest and best wells are producing from this zone. Only one sand has been found in the Marginulina formation. The oil at High Island is divided into three types, ranging from 24° to 44° A.P.I. gravity.