BOEM doing a new Torok and Nanushuk evaluation offshore the western North Slope
The Bureau of Offshore Energy Management is re-evaluating its assessment of undiscovered oil and gas resources on the outer continental shelf of the Beaufort Sea, in parallel with a similar assessment that the U.S. Geological Survey is conducting for the National Petroleum Reserve-Alaska, Megan Carr, BOEM Alaska regional supervisor of resource evaluation, told Petroleum News Nov. 17. The assessment results will feed into the new five-year oil and gas lease sale program that the Department of the Interior is developing for years 2019 to 2024, helping the planners figure out priorities for lease sales, Carr said.
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Essentially, BOEM conducts a resource assessment by using seismic data to identify potential prospects within various oil and gas plays, and then using statistical techniques to factor the number, sizes and potential of the prospects into an evaluation of the range and associated uncertainty of potential undiscovered resources.
Torok and NanushukBOEM geologist Kirk Sherwood said that the new assessment is entirely focusing on the petroleum potential of the Torok and Nanushuk formations, offshore the western part of the North Slope. Given an absence of any new data or concepts for other oil and gas plays in the region, the results for those plays will remain unaltered from previous assessments, he said.
The Torok and Nanushuk have created great interest among North Slope explorers following major onshore oil finds at Pikka and Willow, and under state offshore waters at Smith Bay. BOEM last published a Beaufort Sea assessment as recently as 2016, but that assessment did not factor in the recent thinking driven by those new oil finds.
The eastern margin of the Torok and Nanushuk plays lies onshore in the area of the Colville River. The reservoirs in the play were formed from sands deposited on ancient marine shelf margins, as massive quantities of sediment poured west to east across the region. That eastern play edge represents the ultimate shelf margin, the farthest east that the ancient margin migrated.
An examination of seismic data suggests that the ultimate shelf margin runs north offshore under the Beaufort Sea but then swings west to run under the OCS roughly parallel to the coastline. Presumably north of the current-day shoreline, the ancient sediments poured out in a more northward direction.
Given the manner that the ancient shelf margin is believed to have migrated, there is the potential for the formation of a huge number of sand reservoirs across the region. But the hydrocarbon traps consist of what are termed stratigraphic traps, traps formed from the geometry of the way in which the rock strata were laid down. This is distinct from structural traps, formed from the manner in which the rocks have been folded and faulted.
Difficult to findStratigraphic traps are more difficult to locate from seismic data than are the more distinct structural traps. BOEM geophysicist Louis Niglio explained that, in general, stratigraphic traps cannot be identified from 2-D seismic data but require 3-D seismic for their discovery. In a survey multiple sets of seismic data are shot for the same subsurface points, with the seismic sources and receivers being offset at different distances apart for different shots. The key to finding subtle stratigraphic traps from 3-D data is the use of seismic amplitude anomalies at different offsets from the same subsurface feature, a technique referred to as AVO analysis, Niglio explained.
Unfortunately, however, there is no 3-D seismic data for the region of the Beaufort Sea OCS where BOEM is evaluating the Nanushuk and Torok plays. Instead, BOEM is using 3-D data shot onshore in NPR-A to evaluate the frequencies and sizes of prospects in the oil and gas plays, and then extrapolating these results into the offshore, Sherwood explained, commenting that a key part of the assessment involves evaluating the extent to which the offshore geology reflects the same trend as what is observed onshore. Apparently BOEM scientists assist the Bureau of Land Management in the conduct of NPR-A lease sales and thus have access to onshore 3-D data.
Huge number of prospectsWork done to date on the Beaufort Sea re-assessment demonstrates the extent to which the amplitude anomaly analysis from 3-D data ups the ante when it comes to finding stratigraphic traps. The number of potential prospects in the OCS Nanushuk and Torok offshore plays has mushroomed from around a dozen to almost 2,000, Sherwood said. However, Sherwood cautioned that many of these prospects are quite small: BOEM is still figuring out some lower size threshold, below which prospects will be discounted as too small for significance.
However, the ability of the seismic amplitude anomaly technique to locate many small prospects is causing the BOEM scientists to think carefully about how to manage the statistical analysis of the assessment data. In particular, while the propensity of small prospects tends to push the focus of the data analysis towards the small prospect end of the size distribution, the prospects that tend to matter when it comes to oil and gas development tend to be the outliers, at the large prospect end of the distribution, Sherwood explained.
Other uncertaintiesCrucial uncertainties that need to be factored into the Beaufort Sea assessment include the nature and location of hydrocarbon sources, and the quality of the reservoir rocks in the various prospects.
Source rock uncertainty particularly revolves around the fact that the Beaufort Sea OCS lies north of a regional geologic structure called the Barrow Arch - less is known about the source rocks north of the arch than south of the arch. There is a series of huge sunken fault blocks known as graben under the Beaufort Sea continental shelf and these blocks may preserve source rock units such as the Kingak and Shublik formations that are well known onshore the North Slope, Sherwood said. For example, oil in the offshore Northstar field comes from one of the grabens, known as the Dinkum graben, he said. The Torok, itself, especially near its base, occasionally shows some oil source potential, Sherwood said.
A key uncertainty for the Torok play is poor reservoir quality. Reservoir quality is not so much of an issue for the Nanushuk, but one important uncertainty for the Nanushuk is the extent to which shale layers in the strata may have impeded the migration of oil up into potential oil reservoirs, Sherwood said.
As quickly as possibleBOEM is eager to move ahead as rapidly as possible with its Beaufort Sea re-assessment but does not yet have an anticipated schedule for completion. The current phase of the assessment is focusing on estimates of volumes of technically recoverable resource. A second phase will likely address economically recoverable volumes, using a range of oil and gas price assumptions, Sherwood said. Carr commented that results from the current phase of the assessment would soon be subject to internal review.
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