3D SEISMIC SURVEY PAYS OFF AT NEVADA DRILLSITES

Independents report glowing success from the first 3D seismic survey in Nevada's Railroad Valley.

Apache Corp., Houston, and Balcron Oil, Billings, Mont., a division of Equitable Resources Energy Co., Pittsburgh, have completed Devonian Guilmette oil wells in recent weeks on locations picked with 3D seismic data.

The wellsites are about 1 mile apart in Nye County, where a group led by the two companies acquired an 11 sq mile 3D seismic survey in December 1992 and January 1993. Processing ended last June.

Balcron's 23 17A well in Bacon Flat oil field flowed 273 bbl of 26 gravity oil in 28 min, or at the rate of 13,000-14,000 b/d. Apache's 79 21 GCU well discovered a new fault block in Grant Canyon field and flowed as much as 200 bbl/hr through drillpipe. Neither well is producing water.

Neither the Bacon Flat area's crestal feature nor the Grant Canyon fault block was visible on 2D seismic lines.

Meanwhile, Apache hopes to drill a 3D wildcat prospect about 1 mile from Grant Canyon field in mid 1994 and is still interpreting data toward the edges of the 2 3/4 by 4 mile survey.

GRANT CANYON FAULT BLOCK

Apache plans a first quarter 1994 spudding of the first offset to its 22 21 Grant Canyon, in 21 7n 57e, which is producing from 30 ft of open hole below 7 in. casing set at 4,035 ft.

It reports pressures at 250 psi shut-in, 202 psi flowing at 2,500 b/d.

The 22 21 well lies on what would appear to be a development location - between the depleted Grant Canyon field discovery well and the 2 Grant Canyon well, a dry hole (see map, OGJ, May 17, 1993, p. 65). Those two wells were about 2,500 ft apart.

Apache's 10 GCU, the first well based on 3D data from the joint survey, came in high to the 99 21 well but hit water because it bottomed in the main reservoir, where the oil water contact (OWC) has risen about 700 ft since discovery. Balcron's 23 17A Bacon Flat was the second well based on the survey.

The 22 21's productive interval is below the main Grant Canyon reservoir but yielded no water. Pressure and production data indicate that the OWC depth in the new fault block is near the original depth of the OWC in the main field, Apache said. The 3D data indicate closure of about 80 acres in the new fault block, compared with 120 160 acres in the main reservoir.

One other well now produces 850 b/d in Grant Canyon field, which has produced 21 million bbl since 1983.

BACON FLAT FLOW

Balcron said its completion indicates ultimate recovery of about 1 million bbl of oil from the Bacon Flat Devonian Guilmette reservoir, which a Balcron geologist believes has an areal extent of no more than 80 acres (OGJ, May 17, 1993, p. 64).

Balcron estimates ultimate recovery from the new well, 23 17A Bacon Flat in 17 7n 57e, at 400,000 bbl or more. That is partly because the well topped Devonian at 4,953 ft, 209 ft high to Balcron's 23 17 well completed in August 1992 and about 350 ft high to the discovery well only 400 ft away.

Recovery was 284,625 bbl from the 23 17 well and 311,428 bbl from the 1981 discovery well, shut in in 1988 when water influx made further production uneconomic. Balcron plans to produce the field through the 23 17A.

NEVADA 3D SEISMIC

Apache said that at 2,500 b/d production, revenue will return its well cost plus the $700,000 to shoot and process the 3D survey in 80 days.

The group used a Vibroseis source and a 60 by 60 ft bin size to obtain high spatial resolution to image steep dips encountered in the area. The survey cost twice as much as a dry hole in this area.

Industry practice has been to shoot 3D costing one third to one half the cost of a dry hole, Apache said.

A 3D survey was shot in the late 1980s over Blackburn oil field in Eureka County. Amerada Hess Corp. has acquired 3D data over about 5 sq nines near the Cenex Sans Spring oil discovery 3 miles west of Bacon Flat field.

Another 3D survey is to be shot later in first quarter 1994 about 6 miles north of Grant Canyon field.

Certain geologic similarities between Sans Spring and Trap Spring fields may help geologists and geophysicists decide whether 3D may be viable over Trap Spring, which produces oil from a truncated trap configuration in Tertiary volcanics.