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The intricacies of Nanushuk geology
DGGS releases reports describing and characterizing the rock formation that hosts recent major North Slope oil discoveries Alan Bailey for Petroleum News
Alaska’s Division of Geological and Geophysical Surveys has published three new reports, describing in detail the geology and oil reservoir characteristics of the Nanushuk formation, the formation that hosts recent major new oil discoveries on the North Slope. The Willow, West Willow, Pikka, Narwhal and Horseshoe discoveries are all located in this formation. The DGGS scientists have used rock cores from wells drilled through the formation, and from rock samples collected from surface outcrops, to conduct their research.
In general terms, the sediments that now form the Nanushuk were deposited during the Cretaceous period, about 100 million years ago, down the sides and on the edge of an ancient marine basin, termed the Colville Basin, to the north of what is now the Brooks Range.
Riverborn sediment The massive quantity of sediment filling the basin was carried by ancient rivers flowing into the region. The DGGS scientists say that the sediment deposition occurred in two large river delta complexes separated by a north-south trending area, referred to as the Meade Arch. To the west of this arch sediment was carried eastward by river systems originating to the west of current Arctic Alaska. To the east of the arch sediment carrying rivers flowed in a more south to north direction from the emerging Brooks Range. In both cases, major quantities of sediment were deposited in large river deltas on the edge of the basin. Sediment was also carried down the side of the basin and across the basin floor, as the basin filled.
The Nanushuk is particularly associated with these ancient river deltas. Sediments deposited deeper in the basin formed the Torok formation, a formation that lies under the Nanushuk although roughly contemporaneous in age.
Outcrops in the foothills Along the northern foothills of the Brooks Range rocks of the Nanushuk are found in outcrop at the surface along an area 30 to 50 km wide and 650 km in length, the DGGS scientists say. The lower part of the formation tends to consist of a succession of shale, siltstone and sandstone that would have been deposited in marine conditions, on a marine shelf associated with river deltas and the shore faces of the marine basin. However, these marine deposits grade upwards into a succession of mainly nonmarine sediments, including mudstone, coal, sandstone and conglomerate that would have been deposited in a river setting and in a river delta plain.
The oil fields that have been discovered involve oil that has been trapped in sandstone layers by impervious rock strata above.
Given the directions from which the Colville Basin became filled with sediment, the rocks of the Nanushuk become younger to the east. And as the basin filled, sediment that would form the Nanushuk eventually flowed north, over a major geologic high termed the Barrow Arch that runs roughly along the line of the current Beaufort Sea coast. To the east, under the northern North Slope, deposition of the Nanushuk stopped at the ultimate easterly shelf margin near the Colville River - in the northern part of the North Slope rocks of the Nanushuk and Torok are not found east of this area. The major oil discoveries have been made in the area of the Colville River delta and to the west, in the National Petroleum Reserve-Alaska.
Evaluating reservoir quality An evaluation of microscope slides of rocks obtained from well cores and rock outcrops has enabled the DGGS scientists to determine the detailed content and characteristics of the rocks, and hence their likely origins and their oil reservoir qualities. In general the reservoir quality encompasses a wide range from very poor to excellent. The scientists found that the reservoir quality of the sediments improves with increased grain size, with the grain size likely being linked to the energy of the river system that deposited the grains.
However, the reservoir quality is also linked to the depth of burial of the rocks, with deeper burial resulting in greater compression and, hence, lower porosity and permeability. Using rock samples where the rock grains were minimally cemented by carbonate minerals, the DGGS scientists were able to establish approximate numerical correlations linking porosities and permeabilities to maximum rock burial depths.
Thus, the reservoir quality of a Nanushuk sandstone body is largely controlled by the local environment in which the sand was deposited, and by the more regional impact of the maximum depth to which the rock has been buried.
Uncemented or lightly cemented sandstones can have porosities as low as 10% to 15% or as high as 45%, depending on the grain size. However, deep burial in excess of 10,000 feet at some point in the rock’s history would likely reduce the porosity to less than 10%, the DGGS scientists say. Similar parameters determine the rock’s permeability, the scientists have found.
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