Oil south of Walakpa?

Conoco to drill south of gas field; USGS geologists say target could be oil

Kristen Nelson

Petroleum News

ConocoPhillips Alaska has applied for permits to drill exploration wells on Arctic Slope Regional Corp. land in the National Petroleum Reserve-Alaska south of Barrow and just south of the Walakpa gas field.

Neither ConocoPhillips nor ASRC would say what the objective is for the Intrepid wells, but U.S. Geological Survey scientists familiar with the area say the target could be oil — and the sands could be the same as the Brontosaurus well drilled to the south in the 1980s.

ConocoPhillips has applied to drill three wells, Intrepid 1, 2 and 3, during the winter drilling seasons between January 2007 and June 2011 using the Kuukpik No. 5 drilling rig.

Mobilization would begin this year as soon as weather permits and drilling of the first well is planned for Jan. 18-Feb. 6, with a second well possible Feb. 6-May 5.

The winter route to the well sites would include approximately 30 miles of ice road from proposed staging areas. “If constructed, the ice road system will begin from the nearest gravel road system south of Barrow and continue south to the Intrepid exploration well locations,” the company said.

The proposed Intrepid well sites are on oil and gas leases co-owned by ConocoPhillips Alaska, Pioneer Natural Resources Alaska and Anadarko Petroleum; ConocoPhillips will operate the wells.

The company said production tests may be performed after production casing is set; any oil would be held in tanks and re-injected or hauled to existing North Slope facilities; produced gas would be flared.

Gas going to Barrow now

Natural gas production from the South Barrow field began in 1958 and from East Barrow in 1981. The Walakpa field, discovered in the 1980s, was developed in the 1990s.

The most recent production posted by the Alaska Oil and Gas Conservation Commission, for August, shows one producing completion at the South Barrow gas field, with August production of 4.7 million cubic feet, and nine producing completions at the Walakpa field, with August production at 77.8 million cubic feet.

Production from the South Barrow field peaked in 1980 at 1.027 billion cubic feet per year, the East Barrow field peaked at 583 million cubic feet a year in 1991 and Walakpa peaked at 1.352 bcf per year in 2000, according to Alaska Division of Oil and Gas records. Combined production from the fields in 2005 was 1.388 bcf. The division estimates remaining gas reserves of 5 bcf at East Barrow, 4 bcf at South Barrow and 25 bcf at Walakpa, a total of 34 bcf.

Alaska Division of Oil and Gas petroleum geologist Paul Decker provided Walakpa field background in an e-mail. He said the Intrepid prospect well locations are “just downdip” of the Walakpa gas field, which is producing gas from between about 2,100 feet and 2,600 feet subsea from the Lower Cretaceous Walakpa sands, a Kuparuk-C equivalent, with reservoir sand thickness ranging from about 20 feet to almost 35 feet and averaging 34 feet gross and 27 feet net pay in the delineated part of the field.

“The field has no known gas/water contact, and production is limited on the up-structure edge by gas hydrates,” he said.

Decker said “there is a general consensus that the Barrow region is more gas-prone than oil-prone, due to the regional kerogen character of most of the source rocks, high thermal maturity and the structural focusing for late-stage gas migration out of the Colville basin and along the Barrow Arch to the northwest.”

Is the objective gas?

U.S. Geological Survey geologist Dave Houseknecht said in an e-mail that northwestern NPR-A geology presents some puzzles.

“Many of the geologic parameters that influence whether oil or gas may occur — including presence of appropriate source rock and thermal maturity — suggest the area should be oil-prone. Yet, gas is the most common hydrocarbon recovered from exploration wells, although oil shows are also common,” he said.

Houseknecht said conventional wisdom about the area holds that gas migrated north from the “deeper Colville trough, relatively late in basin history, and this gas may have displaced oil that may have accumulated earlier.” The Lower Cretaceous Unconformity, the LCU, is the commonly suggested migration path, he said.

Houseknecht disagrees with the interpretation of the sandstone reservoir at Walakpa as Kemik sandstone or Kuparuk-C.

He said “there are a number of unconformities that converge in this part of the stratigraphic section and some geologists (including me) have suggested that the reservoir at Walakpa is an older sandstone within the upper Kingak Shale (Miluveach and Kuparuk A-B).”

“Bottom line: there are multiple potential reservoir sandstones in the Upper Jurassic to Lower Cretaceous part of the stratigraphic section and these reservoirs are likely lenticular in nature — not present as a blanket over the entire area.”

Houseknecht said there may be additional reservoirs in the overlying Brookian section, including Torok formation turbidite sandstones and Nanushuk formation marine to nonmarine sandstones. There may also be deeper reservoirs, below the Upper Jurassic, such as the Sag River or Ivishak formations. Those, he said, would probably be secondary objectives, if they are even being considered.

Looking for the oil leg?

USGS geologist Ken Bird agreed with Houseknecht’s analysis.

He also said “the Brontosaurus well, drilled about 15 or 20 miles south of Walakpa by ARCO in 1985, encountered a 50-foot thick sandstone at a stratigraphic position similar to that of the gas-bearing sandstone in Walakpa.” Bird said he recalls that the Brontosaurus sandstone was “water-wet.”

“The optimistic view would hold that the Walakpa gas sand and the Brontosaurus sand are the same sand body, thus showing that it extends at least 20 miles north to south.”

The Intrepid wells are proposed for section 6, township 19 north, range 19 west, Umiat Meridian and sections 11 and 22 of T19N-R20W, UM. Brontosaurus was drilled in section 18 of T18N-R20W, UM. It reached a total depth of 6,660 feet.