A California operator has completed a Cretaceous Austin chalk horizontal oil well near a basaltic cone in the Uvalde volcanic field area of Dimmit County, Tex.
The well is the HDP Inc. 1 Autumn Unit, about 9 miles northeast of Carrizo Springs. HDP, which stands for horizontal development and production, of Palo Alto, Calif., drilled the well on a farmout from American Exploration Co., Houston.
It initially pumped and flowed 1,600 b/d of oil without stimulation from open hole. HDP drilled about 1,500 ft of horizontal and deviated hole in Austin chalk B-1, the producing horizon.
Production in late August was about 500 b/d of oil, pending determination of proration unit size and allowable.
The well, in the greater Pearsall field Austin chalk play along the Dimmit-Zavala county line, is the first horizontal chalk producer in Elaine field. The field has produced mainly from Escondido sand, Olmos sand, and Anacacho limestone, all in the Upper Cretaceous.
Tesoro Petroleum Corp. 82 Little and 84 Crenshaw, two vertical wells in Elaine field, produced a total of 96,648 bbl of oil and 107 MMcf of gas from Austin chalk from the time of discovery in 1977 through 1988.
HDP plans to drill at least three or four more horizontal Austin chalk wells on the 1,500 acre farmout. At least several dozen similar volcanic structures are in the general area, said John Harbaugh, HDP chairman.
DRILLING 1 AUTUMN
Unit HDP drilled the Autumn Unit well through a known basaltic cinder cone.
Correlation of electric logs from two vertical Austin chalk wells in Elaine field showed a spontaneous potential anomaly at the contact between the basaltic cinder cone and the underlying Austin chalk formation, said Paul Martinez, HDP chief geologist.
The assumption was that the SP anomaly might indicate a more permeable, fractured zone caused by the explosive intrusion and deposition of the basaltic volcano. HDP engineered the well to target the contact zone.
It also conducted several pumpoff tests, which determined sufficient basalt integrity to drill a deviated hole.
HDP drilled the vertical hole and set a cement plug in basalt, then started building hole angle at a rate of 16-18/100 ft.
The best hydrocarbon shows were encountered at the contact zone between basalt and Austin chalk. Cuttings showed calcite filled fractures and good fluorescence.
HDP might frac the well after several weeks of production, Harbaugh said.
Phil Kushner, HDP president, said engineering failures dogged the well during drilling. The well experienced seven mud motor failures, including one motor that failed after making 2 ft of hole.
Coincidentally, the well site received 14 in. of rain during 1 day of drilling.
VOLCANIC DEPOSITION
Elaine field is the most prominent of a cluster of oil fields in the greater Pearsall area associated with buried volcanic mounds, Martinez said.
Volcanic plugs are composed of basalt and volcanic tuff that erupted onto the sea floor as the Austin chalk was being deposited. Magma and steam forced vents through the chalk from below and sometimes created craters in the top of the chalk.
Such craters have been mapped and studied at updip locations where the contacts are exposed on the surface.
Explosive excavation of the crater in Elaine field created fractures. It removed or prevented deposition of the upper Austin chalk A and B zones, Martinez said.
The well bore of the 1 Autumn Unit enters the chalk about 500 ft from the south end of the volcanic plug, Kushner said.
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