USGS North Slope assessment gearing up
Team is finalizing assessment units; anticipates publishing results of its estimation of undiscovered oil and gas in January
Having conducted a meeting in Anchorage, gathering input to the assessment process, the U.S. Geological Survey is completing its plans for its new evaluation of undiscovered oil and gas resources in the central North Slope. The assessment team plans to finalize its identification of assessment units to use for the evaluation around Dec. 2, with the intent to be able to release the results of the assessment in January, USGS geologist Dave Houseknecht told Petroleum News in a Nov. 25 interview. The assessment will encompass North Slope lands between the eastern border of the National Petroleum Reserve-Alaska and the western border of the Arctic National Wildlife Refuge, as well as nearshore state waters of the Beaufort Sea.
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An assessment unit consists of an oil and gas play associated with specific geologic features. And, as in previous assessments, the USGS will evaluate potential volumes of undiscovered, technically recoverable resources in each unit, using available geologic information from sources including well and seismic data. A statistical analysis will result in an estimated range of possible resource volumes, with the breadth of that range providing insights into the uncertainty of the resource estimates. Economic factors will not be considered.
Planned assessmentsThe USGS last conducted an assessment of the central North Slope in 2005. The new assessment results from an administrative order from then-Interior Secretary Ryan Zinke in May 2017, requiring new assessments across the entire North Slope region. USGS has already published a new assessment for the NPR-A and had planned to next conduct an assessment of the ANWR coastal plain. However, because of the opening of the ANWR coastal plain for oil and gas lease sales, the plan now is to delay the ANWR assessment in expectation of new 3D seismic data becoming available - currently the only seismic available for ANWR is old 2D data, Houseknecht explained.
The plan had also been to complete the central North Slope assessment by the end of this year, but a government shutdown earlier in the year had delayed the process, Houseknecht said.
A key new play and associated assessment unit is that of the Torok and Nanushuk formations, in the Brookian sequence, the youngest and shallowest of the North Slope petroleum bearing rock sequences. But the USGS fully evaluated this play, the focus of recent major oil discoveries, in its recent NPR-A assessment and will not, therefore, include the play as an assessment unit in its central North Slope evaluation. The Torok and Nanushuk extend eastward, to the east side of the Colville River. But east of that area the Brookian rocks, while extensive, are generally younger.
Producing area well understoodHouseknecht pointed out that the central North Slope contains most of the producing oil fields of the region. Hence, the petroleum potential along the northern part of the region is already well understood, other than a few offshore opportunities that warrant further evaluation. In particular the Ellesmerian sequence, the host rock sequence for fields such as Prudhoe Bay and Endicott, has been thoroughly explored in areas where the oil sources are in the pressure and temperature window appropriate for oil generation. However, there is gas potential in the Ellesmerian to the south, and in the fold belt to the north of the Brooks Range, Houseknecht commented.
Focus on the BrookianMuch industry interest is currently focused on the Brookian, which is found across much of the region and represents the largest block of underexplored geology with oil potential, Houseknecht said. The sequence includes the Seabee, Tuluvak, Canning and Sagavanirktok formations. This rock sequence was deposited in the same evolving marine basin system as the older Torok and Nanushuk, but in a significantly different geologic environment. In particular, the region became less tectonically active, leading to very thick potential oil source rocks but relatively sparse sand relative to the Nanushuk, Houseknecht said. On the other hand, there are rock formations such as the Schrader Bluff and Prince Creek with good potential oil and gas reservoirs. And, in a third, young and shallow “slug” of the Brookian, rejuvenation of the emerging Brooks Range resulted in much sand of reservoir quality in the Sagavanirktok, Houseknecht said.
Regional variationsThere is a contrast in the nature of the petroleum systems between the region to the west of the trans-Alaska pipeline and the region to the east. To the west the Brookian rocks typically dip to the south, causing oil to tend to migrate northward into traps associated with the Barrow Arch, a major structure that runs along the Beaufort Sea coastal region. To the east, the Brooks Range steps out farther north, causing the rock strata to dip more northward. That, in turn, raises uncertainty over trapping mechanism and the possible loss of oil resources to the surface, Houseknecht said.
Another key factor relates to the fact that in the northeastern sector of the central North Slope a major geologic discontinuity, the Lower Cretaceous Unconformity, has eroded out the older oil source rocks, especially the Shublik and the Kingak, that are key to the petroleum system to the west. There are thick and rich Brookian source rocks in the northeast, but the lack of the older sources introduces a risk factor into the petroleum potential.
Under explored blockParticularly intriguing is a 2,000-square-mile block east of the trans-Alaska pipeline where there has been significant recent industry interest and land leasing, but where relatively little is known about the subsurface. There have only been four wells drilled in the region, with the youngest of these wells drilled several decades ago.
“So there’s a big block there about which we know very little, based on subsurface penetrations,” Houseknecht said.
And such seismic that does exist for this region is 2D seismic, rather than the 3D seismic that is typically used to located the subtle stratigraphic oil traps characteristic of the Brookian. The result will presumably be high levels of uncertainty in the petroleum assessment for a region that has good oil potential - companies are in the process of collecting 3D seismic data in the region, but this proprietary data is not available to the USGS scientists, Houseknecht commented.
“It should be exciting to watch over the next few years,” Houseknecht said, reflecting on the high level of new industry activity south of the traditional fairway of oilfield development. “I’m sure there will be prospects identified with the 3D that we really can’t see in the 2D, and that should lead to some interesting wells.”
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