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Nanushuk reservoir quality
Maximum burial depth, depositional environment of sediments determine storage
Alan Bailey for Petroleum News
Major new oil discoveries, such as Pikka and Willow, have been made in the Nanushuk formation on Alaska’s North Slope in recent years. A new research paper published in Marine and Petroleum Geology documents the results of research conducted by scientists from the Alaska Department of Natural Resources into the factors that determine the oil and gas reservoir quality in the formation - an understanding of these factors can help identify optimum places to drill for new discoveries.
(See diagram in the online issue PDF)
The scientists, Kenneth Helmold of the Division of Oil and Gas and David LePain of the Division of Geological and Geophysical Surveys, comment that it is possible to distinguish two groups of sandstone with distinctly different reservoir qualities within the formation: a low-porosity group with porosities of less than 20% and a higher porosity group with a maximum porosity in excess of 30%. While the environment in which the sediments were deposited is critical in determining variations in reservoir quality within these two groups, the disparity between the two groups is largely determined by the maximum burial depths of the sediments, the paper says.
The rock formation forms part of the Brookian sequence, the youngest and shallowest of the petroleum bearing rock sequences on the North Slope. The sediments that later formed the rocks were deposited in a giant depositional system called a clinothem - a complex system of marine and non-marine sediments associated with ancient river systems - and deposited across the edge and into an ancient marine basin.
River delta complexes The Nanushuk sediments were deposited in two large river delta complexes separated by a geologic high named the Meade Arch that trends south, approximately from the location of Utqiaġvik on the Beaufort Sea coast. The delta complex to the west of the arch is interpreted as formed from rivers flowing from the west of what is now Arctic Alaska. The delta complex to the east of the arch consists of two lobes, with river-dominated sediment deposition systems that also appear to have been impacted by wave action in the marine basin. Sediment in these lobes appears to have originated both from the west, and from the emerging Brooks Range to the south and southwest.
The analysis presented by the DNR scientists used data from 38 exploration wells located across the western and central North Slope. Data obtained included the results of a detailed analysis of the composition of the rocks, including the grains of detritus in the rocks, the material cementing the rocks, and the rock porosities and permeabilities. An interpretation of sedimentary structures provided insights into the environments in which the sediments were deposited.
In general, the depositional environment is a critical determinant of the rock qualities that impact the reservoir quality of a sediment. For example, a high energy environment associated with fast flowing water tends to lead to relatively coarse grained sediments, and hence relatively high porosities. Lower energy environments in which sediment is deposited from slow flowing or still water tend to result in finer grained, lower porosity rocks. The porosity of the rock is the key factor in determining the amount of hydrocarbon a rock can hold.
Burial depth However, the depth of burial of a rock is also critical in determining reservoir quality. The pressure exerted on a deeply buried rock tends to cause compaction, together with chemical processes that can cement the rock grains, thus reducing both the porosity and permeability of the rock.
Because various geologic processes have tended to cause rock formations to rise or sink over time, the current burial depth does not generally equate to the maximum burial depth of the rock. And the maximum burial depth is the determinant of the impact of burial depth on reservoir quality. So, the scientists conducting the research had to obtain estimates of how much sedimentary overburden had been eroded in the past from above the sampled rocks.
Overall, the scientists recognized that the two fundamental porosity groups within the Nanushuk rock samples examined related to maximum burial depths. A low-porosity group had been subjected to maximum burial depths greater than 7,000 feet, and a high-porosity group had been subjected to maximum burial depths less than 7,000 feet. There was also a direct correlation between porosity and permeability - the high porosity rocks tended to have higher permeabilities, and vice versa.
Impact of depositional environment Plots of porosity and permeability against maximum burial depth clearly demonstrate these declining trends with depth. However, at any specific burial depth there are significant spreads of porosity and permeability values. These spreads relate to sediment grain size, a factor of the environment in which the sediments that formed the rocks were deposited. Overall, the data demonstrate that the estimated maximum burial depth of a potential reservoir rock can be used as a key predictor of reservoir quality, prior to drilling, the paper suggests.
However, although the Nanushuk reservoir quality degrades significantly at maximum burial depths approaching 8,000 feet, modern drilling and well completion techniques, including hydraulic fracturing, improve the viability of these tight reservoirs, the paper says.
Carbon dioxide sequestration? The paper also comments that, in addition to their value as oil reservoirs, the Nanushuk reservoirs could probably be used for carbon dioxide sequestration - there is currently much interest in sequestering carbon dioxide underground, as one strategy for reducing atmospheric carbon dioxide. While there is a generally recognized minimum carbon dioxide sequestration depth of 2,625 feet (800 meters), the Nanushuk reservoirs have porosities and permeabilities appropriate for sequestration down to depths of about 8,000 feet, the paper says. Moreover, the carbon dioxide would react with some of the detritus material in the rocks to increase the available porosity. It is also possible that over millions of years the subsequent precipitation of carbonate materials in the rocks could more effectively sequester the carbon dioxide, the paper says.
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