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Exploitation of Oil in a Volcanic Cone by Horizontal Drilling in the Elaine Field, South Texas


The Uvalde volcanic field west and southwest of San Antonio contains many buried volcanic cones that offer attractive places to exploit with horizontal drilling. These buried cones were formed by eruption of basaltic flows and cinders during the Cretaceous when the Austin Chalk and immediately overlying strata were deposited. The cones vary in size, some being as much as 2 mi in diameter and 1000 ft in vertical dimension. During eruptions, flows and ejecta reacted with sea water, producing intermixtures of basaltic material and limestone.

These cones provide attractive targets for oil exploration. They have been recognized for decades in the subsurface, and a number of oil fields in the region produce from domelike structures where strata over the cones are arched upward. By contrast, much less oil has been produced within or beneath the cones.

The Elaine field about 90 mi southwest of San Antonio is associated with a buried volcanic cone that is now being exploited with horizontal drilling. Shallow production at Elaine is from the San Miguel and Anacacho units, which either overlie the cone or are laterally contiguous with it. Production began at Elaine in the 1950s, but only seven of Elaine's numerous vertical wells penetrate the volcanic cone and underlying Austin Chalk. Two of these wells formerly produced oil from the contact where volcanics rest on Austin Chalk, but were plugged back when production declined. These two wells, however, document the presence of an oil zone at the contact and suggested that it could be exploited with horizontal wells.

Horizontal Drilling and Production, Inc. ("HDP") drilled the Autumn Number 1 as the initial horizontal well at Elaine. A vertical hole was drilled through the cone and underlying Austin Chalk, reconfirming the presence of the oil zone at the contact between volcanics and chalk. Moving back up the hole, a cement plug was set within the volcanics, and a horizontal hole was directed on a N 70 degrees W azimuth. The inclined hole's vertical angle had progressively decreased with distance to about 30 degrees from the horizontal when the oil zone was reentered. The hole's inclination continued to decrease within the oil zone, becoming horizontal after about 600 ft. With further distance, the hole passed beneath the oil zone, where its inclination was then reversed so that it climbed gradual y and reentered the oil zone before reaching its terminal distance of 1500 horizontal feet. The well was completed on August 17, 1990, with an indicated initial production of 1609 BOPD.


AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)