Exciting possibilities
DOG scientist sheds light on recent exploration and finds in Nanushuk/Torok
Alan Bailey Petroleum News
Alaska has seen mounting excitement over new major oil discoveries in the Nanushuk and Torok formations in the more westerly region of the North Slope. On April 21 Josef Chmielowski, a geoscientist with Alaska’s Division of Oil and Gas, reviewed what is known publicly about these finds and their significance in terms of continuing exploration. In part, Chmielowski’s insights come from modern seismic data, made public as a result of Alaska’s tax credit system and processed by the division. In particular the Nanuq South 3-D survey, released to the public in December, provides fascinating insights into the Nanushuk/Torok play.
The Nanushuk and Torok, part of the Brookian sequence, the youngest and shallowest of the North Slope petroleum bearing rock sequences, are found across a huge area of the western North Slope. And, with very sparse exploration drilling in the region, there is much scope for similar oil discoveries, Chmielowski commented.
Large finds The new finds are very large. Pikka, discovered through the drilling of the Qugruk No. 3 well by Repsol in 2013, has subsequently been linked with oil discovered to the south by the drilling of the Horseshoe well and sidetracks by Armstrong Energy in 2017 - the entire prospect extends some 40 miles north to south, Chmielowski said. The Willow prospect, which apparently was discovered by Phillips Petroleum, later to become ConocoPhillips, in 2002 through the drilling of the Hunter A well, is about 25 miles in length.
ConocoPhillips drilled the Tinmiaq Nos. 2 and 6 wells in 2016, and the Tinmiaq Nos. 7, 8 and 9 wells this year, to prove out and appraise the Willow discovery. The company has not said why it did not decide to develop the find following the original discovery in 2002. However, it is likely that at that time the company was fully occupied in developing the Colville River unit, Chmielowski suggested, adding that in 2002 there was no infrastructure extending into northeast NPR-A to support a Willow development - nowadays ConocoPhillips is extending the infrastructure west through the Greater Mooses Tooth 1 and planned Greater Mooses Tooth 2 developments.
Caelus Energy made the Smith Bay discovery in 2016 through the drilling of two wells in the bay. The discovery is in the Torok formation.
Net thicknesses of oil bearing sands are some 200 feet for Pikka; 40 to 70 feet for Willow; and perhaps 200 feet for Smith Bay, Chmielowski said.
Stratigraphic traps Oil in these discoveries is caught in what are called stratigraphic traps, traps formed as a consequence of the manner in which the sediments that constitute the rocks were deposited, rather from the way in which the rock strata have been folded and faulted.
Oil gravity is light and well flow tests have produced excellent results. A vertical well flow test at Pikka showed a flow rate of 2,000 barrels per day, with a subsequent horizontal well test flowing at 4,600 bpd. Willow tested last year at 3,200 bpd in a vertical well - ConocoPhillips has yet to release the results of Willow flow testing carried out in this year’s exploration season. Caelus has not yet conducted any flow testing at Smith Bay, Chmielowski said.
Reservoir depths are typically a little over 4,000 feet - Chmielowski commented that there is an interesting question regarding why light oil at such shallow depths has not been degraded by microbes, as might be expected. Pikka/Horseshoe presents another enigma concerning the route whereby oil flowed into the reservoir - the oil was sourced from the Shublik formation, now at a depth of 10,000 feet, some 6,000 feet below the Nanushuk, Chmielowski commented.
Production from Pikka/Horseshoe, once developed, is expected to be some 120,000 bpd; 40,000 to 100,000 bpd for Willow; and perhaps 200,000 bpd for Smith Bay. These projected production rates are very significant in comparison to the current throughput of around 500,000 bpd for the trans-Alaska pipeline. Bringing any of these new discoveries on line would be great for the Alaska economy, for the oil operators, and for the creation of jobs, Chmielowski said.
A moving basin margin The Nanushuk and Torok were formed from the infilling from west to east of an ancient marine basin. Essentially, massive quantities of sediment pouring across the basin from the west formed a basin margin on the basin’s west side. Then, as the basin filled, the margin traversed from west to east, with that movement finally coming to a halt in the region of the present-day Colville River, to form what is referred to as the ultimate or terminal shelf margin: No Nanushuk or Torok rocks are found to the east of that margin, Chmielowski said
As the sediments tumbled across and down the side of the basin, the piles of sediments, in cross section, formed distinctive sigmoidal shapes that geologists call clinoforms. The sediments laid down in the relatively shallow, nearshore section of a clinoform are referred to as topsets, the strata on the downward slope are called foresets, and the deeper water sediments near the base of the slope are called bottom sets. Essentially, in the context of Brookian geology, the Nanushuk is associated with topsets, while the Torok is associated with foresets and bottom sets.
Thus, given the geometry of the sediment deposits, the Nanushuk always overlies the Torok, even although both formations resulted from the same depositional system: Geologists recognize the formations on the basis of their distinctive rock properties, properties that relate to the depositional environments under which the rocks were formed. However, sediments within one formation would have been deposited at approximately the same time as some sediments in the other formation, as the sediments tumbled down the side of the basin.
This phenomenon leads to some potential confusion in the nomenclature, since people tend to refer to the distinctive curved clinoform surfaces observed in seismic cross sections as Nanushuk 1, Nanushuk 2 and so on, even although these time horizons pass through both the Nanushuk and the Torok. Chmielowski showed a horizontal slice through the 3-D seismic data from the Nanuq South survey that depicts subtle lines that mark the shelf edge breaks that the clinoforms are associated with.
Depending on the circumstances, oil can become trapped in sand in a clinoform. A clinoform does not necessarily hold any oil but could, on the other hand, hold one or more oil pools. Hence the challenges and opportunities of exploring the Nanushuk/Torok play.
The link to clinoforms The new oil discoveries are associated with specific clinoforms: Pikka/Horseshoe with Nanushuk 3, and the Horseshoe wells with Nanushuk 2 and Nanushuk 3. The Putu exploration well that ConocoPhillips drilled this winter appears to have targeted Nanushuk 3, with the Stony Hill No. 1 well that ConocoPhillips also drilled this winter appearing to target a seismic amplitude anomaly along the same trend as the Putu and Horseshoe wells. Willow is associated with other clinoforms farther west.
Rock properties, in particular the permeability, change between the topset Nanushuk rocks down through the Torok in the deeper foreset and bottom set regions. Essentially, the permeability worsens with depth, in part because higher burial pressures have compressed the rocks more, and in part because the deeper rocks tend to have become clogged by more mud: In the Nanushuk, deposited in relatively shallow water, wave action may have winnowed dirt out of the sands, Chmielowski suggested.
However, although in the past the poor quality of the Torok reservoir rocks has deterred oil development, the emergence of techniques such as horizontal drilling and hydraulic fracturing from shale oil development has opened up the possibility of future Torok developments, Chmielowski commented.
But the variations in rock property within the Nanushuk/Horseshoe oil discovery, for example, will have important implications for fluid communications through the reservoir, a consideration that will impact strategies for field development and oil production, Chmielowski suggested. Details matter, with those rock variations impacting factors such as well spacing, oil production strategies and estimates of ultimate oil recovery, he said.
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