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DGGS filling in the Nanushuk details
Outcrop mapping near the Haul Road and data processing should provide new insights into seismic data from the northwestern Slope
Alan Bailey Petroleum News
Geologists from Alaska’s Division of Geological and Geophysical Surveys are using some new state-of-the-art techniques to compile detailed data about the Nanushuk and Torok formations, to help oil explorers understand the significance of what they see in seismic sections. The two formations have become a major focus for exploration efforts in the Colville River Delta region and the northeastern National Petroleum Reserve-Alaska, following major discoveries in the Pikka and Willow prospects, and at Smith Bay farther west.
In the region of exploration interest the configuration of the rocks in the subsurface must be inferred from seismic data, in particular high resolution 3-D surveys. However, interpreting this data involves uncertainty about what exactly lies hidden beneath the ground. And that uncertainty leads to exploration risk.
Slope Mountain model The DGGS geologists are using observations from surface outcrops of the Nanushuk and Torok on the north side of the Brooks Range, in particular a spectacular outcrop at Slope Mountain, next to the Haul Road to Prudhoe Bay. The geologists are linking these observations into what is referred to as a structure-through-motion photogrammetry survey, to construct a three-dimensional model of the Nanushuk and Torok strata at Slope Mountain. That model can in turn be used to generate artificial or synthetic seismic sections, that can be compared with seismic sections obtained from the oil exploration to the northwest. The idea is that this will provide explorers with new insights into what the seismic data are depicting.
DGGS geologist Dave LePain told Petroleum News that the exposure of the Nanushuk and Torok at Slope Mountain, the farthest east location where the formations have been observed, is particularly spectacular. There are about 2,000 feet of stratigraphy on the east face of the mountain, including the lower half of the Nanushuk and the upper 200 feet of the Torok. The rock exposure is about 2 kilometers in length, LePain said.
Marine basin The sediments that form the Nanushuk/Torok system were deposited from the west into what was a massive marine basin, with that basin filling from west to east. In the more northerly region of the North Slope the basin margin migrated west to east, as the basin filled. That migration ultimately ended somewhere to the east of the current Colville River - no Nanushuk and Torok are observed to the east of that ultimate shelf margin.
To the south, however, on the north side of what is now the Brooks Range, where the Nanushuk and Torok are now exposed at the surface, sediment was carried by rivers south to north into the basin from what was at the time the emerging mountain range. Thus, the shelf margin, which has more of a north-south alignment to the north, swings round into more of an east-west direction close to the Brooks Range. Hence the existence of Nanushuk and Torok strata as far east as Slope Mountain.
LePain commented that the fact that the rocks at Slope Mountain were formed in a different basin setting than the rocks in the oil exploration area to the northwest likely results in some differences in the detailed nature of the rocks between the two regions. However, the depositional settings in the two regions were very similar, leading to an expectation that the rocks at Slope Mountain would be an effective analogue to the rocks where oil exploration is being conducted.
And, unlike the rock samples just a few inches across that can be obtained from exploration well cores, the area of rock that can be observed and sampled at Slope Mountain is massive.
Measured sections LePain has now completed three “measured sections” across the strata at Slope Mountain: He completed the first of these in 2001 and the other two last year. Obtaining a measured section involves traversing along a line across the rock strata, making detailed observations about the rocks, measuring the strata and taking rock samples. Last summer, in conjunction with this work, DGGS geologists conducted a structure-through-motion photogrammetry survey of the entire rock outcrop. It is then possible to use this survey, in conjunction with the surface observations, to construct an accurate three-dimensional model of the geology of Slope Mountain.
Synthetic seismic LePain said that DGGS is working with Shuvajit Bhattacharya, a petrophysicist and geophysicist at the University of Alaska Anchorage, who will use the model to develop synthetic seismic sections of the mountain. Bhattacharya will measure the velocity of sound in rock samples gathered in the field. The velocity measurements, coupled with the rock structure developed in the model, can be fed into a computer program that will simulate what a 2-D seismic survey of the mountain would look like. In addition, it will be possible to conduct seismic simulations using a range of sound frequencies, from higher frequencies than would normally be employed in a seismic survey to more typical frequencies.
Then it should be possible to relate features observed in a synthetic seismic section to similar features in an actual seismic section obtained from oil exploration. Similar seismic features would indicate the presence of similar geologic features, with the nature of those features being well understood at Slope Mountain. Hence the potential for a better understanding of what the exploration seismic is indicating.
Further surveys planned LePain said that DGGS plans to extend this line of research by conducting similar surveys at other Nanushuk/Torok outcrops that are observed at a number of locations to the west of Slope Mountain.
“So we hope that this synthetic seismic section that we’re going to generate at Slope Mountain is the first of many that we will generate at other outcrops,” LePain said.
And, intriguingly, in addition to the results of the DGGS research ultimately becoming public, there is now high resolution 3-D seismic data associated with Nanushuk/Torok exploration, available to the public from the Alaska Department of Natural Resources through the state’s exploration tax credit program.
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