Geologic questions abound as Maverick basin Glen Rose oil play grows

A private Houston operator said an oil play it is pursuing in perhaps the least explored Gulf Coast basin has the potential to be very large.

By OGJ editors

HOUSTON, Aug. 28 -- A private Houston operator said an oil play it is pursuing in perhaps the least explored Gulf Coast basin has the potential to be very large.

Saxet Energy Ltd., with public partner The Exploration Co., San Antonio, have produced 268,000 bbl of sweet, 42° gravity oil and 116,000 bbl of fresh water since the discovery well went on production in March 2002.

Eight wells are producing, two are shut-in, one is drilling, and three more are to spud before the companies must vacate the leases during deer season, Brian E. O'Brien of Saxet Energy told the AAPG Prospect & Property Expo on Tuesday.

O'Brien did not discuss reserves for the field, in the Maverick or McKnight basin in Maverick County 30 miles east-southeast of Eagle Pass, Tex., but he said 6 wells are producing a combined 3,670 b/d. One well started up in June, two in July, and two in August.

Geologically, the play has raised many questions, the veteran geologist said. The discovery well tried to blow out in February 2002 and produced 5,300 bbl of oil from about 6,600 ft (OGJ, Mar. 11, 2002, p. 44). No cuttings could be recovered.

Named Comanche-Halsell field and initially thought to be a reef, the reservoir now appears to extend over a wide area and not to be a reef, O'Brien said. Another 300 sq miles of 3D data are to be processed by yearend.

To maintain control of the discovery well, the operator pumped in 10,000 bbl of mud, water, and lost circulation materials, none of which has been recovered even though the well has produced more than 150,000 bbl of oil since then.

No gas has been produced. The produced water contains 200-300 ppm salts and is being analyzed for its suitability for irrigation or other uses. The companies are also studying the relationship between the crude and the water in order to more fully understand the geology and the reservoir. Early operations have involved complete suites of logs, conventional coring, and other tests, O'Brien said.

The greater Glen Rose formation contains open and filled vugs and large areas of pyrobitumen. Some have speculated that hot sulfurous waters might have percolated from below and metamorphosed some of the lighter crude, but the companies are not of one mind on many of the key geologic events, O'Brien said.

The 1-139 well had 49 ft of porosity greater than 12% and as high as 36%. Perforated at 6,630-37 ft, it had an initial rate of 978 b/d of oil and 23 b/d of water at 340 psi flowing tubing pressure. It went on production July 10 and today makes 975 b/d oil and 5 b/d water.

The companies hold 100,000 acres of leases in the circular rift basin. It was not clear whether the play might extend across the Rio Grande River into Mexico.

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