Alaska's petroleum reserve may be more gas-prone

Oct 26, 2010 12:00 AM

The US Geological Survey has sharply reduced its estimate of conventional, undiscovered oil in the National Petroleum Reserve-Alaska on Alaska’s Central North Slope since the previous assessment in 2002.
The latest estimate, released Oct. 26, is 896 mm bbl of conventional, undiscovered oil and 53 tcf of conventional, undiscovered, non-associated gas in the onshore reserve and in adjacent state waters.

Recent exploratory drilling has revealed gas occurrence rather than oil in much of NPR-A. The survey’s 2002 estimate was for 10.6 bn bbl of oil.
Federal lease sales, 3D seismic surveys, and the drilling of more than 30 exploratory wells provided geologic information that is more indicative of gas than oil, USGS said. Many of the newly drilled wells show an abrupt transition from oil to gas 15-20 miles west of giant Alpine field, which lies just outside NPR-A’s north-eastern boundary.

“These new findings underscore the challenge of predicting whether oil or gas will be found in frontier areas,” explains USGS Director Marcia McNutt.
“It is important to re-evaluate the petroleum potential of an area as new data become available.”

The new assessment also indicates 8 tcf less gas than the 2002 estimate of 61 tcf. New geologic analysis by USGS provides an explanation for the unanticipated predominance of gas in much of NPR-A.
It is likely that oil formed 90 mm years ago in many reservoirs in northern NPR-A and gas in many reservoirs in southern NPR-A. Then 15-60 mm years ago, many parts of NPR-A were uplifted and eroded, thereby changing the pressure conditions in the subsurface.

In areas of modest uplift and erosion, USGS said, gas that was dissolved in oil came out of solution, forming gas caps and displacing oil downward into poorer-quality reservoir rocks. In areas where uplift and erosion were significant, the gas present in the subsurface expanded so much that it completely displaced oil from reservoirs.
The variable nature of the uplift and erosion resulted in some reservoirs retaining their oil, but the majority of the reservoirs instead became gas-bearing. The USGS figures are mean estimates of fully risked, undiscovered, technically recoverable oil and gas resources.

The mid-1990s discovery of Alpine, the largest onshore US oil discovery in the past 25 years, brought new focus to NPR-A. Most recent NPR-A wells are within 50 miles of Alpine and targeted the Alpine sandstone, the main reservoir in that field.
The stratigraphic interval that includes the Alpine sandstone was assessed as the Beaufortian Upper Jurassic play in the USGS 2002 assessment of NPR-A.

Five oil discoveries have been reported in the Alpine sandstone in north-eastern NPR-A. Alpine West, Lookout, and Pioneer are oil accumulations with little or no gas. Mitre appears predominantly a gas accumulation with an oil leg in the south. Spark-Rendezvous is a much larger reservoir system that includes gas-condensate at shallower depths in the north and oil at greater depths in the south.
Spark-Rendezvous represents an abrupt transition of hydrocarbon phase in the Alpine sandstone from oil on the east to gas on the west.

Most known or inferred hydrocarbon accumulations west of Spark-Rendezvous are gas. Data are spotty, but USGS estimated that 120-200 mm bbl of oil and condensate and 1.9-3 tcf of gas may be technically recoverable from these accumulations. Spark-Rendezvous is so large that if it were entirely oil it would rival or exceed Alpine field in recovery.
Several recently drilled wells, most west and northwest from Spark-Rendezvous, evaluated other stratigraphic intervals, including Triassic Ivishak sandstones (Ellesmerian Ivishak play in USGS 2002), Lower Kingak shale (Beaufortian Lower Jurassic Topset play), uppermost Kingak shale (Beaufortian Cretaceous topset play), and the Torok formation (Brookian clinoform play).

Results are proprietary for a recently drilled well in the Wolf Creek gas accumulation discovered in 1952 in the south-eastern NPR-A. Two wells were drilled lately at the 1963 Gubik accumulation just outside the NPR-A south-eastern boundary, and another well was drilled to a deeper target near the 1964 East Umiat accumulation. One recent Guik well indicated producible gas, while the other and the East Umiat test remain proprietary.
NPR-A lies about 115 miles west of the Arctic National Wildlife Refuge.

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