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Gil still contributing his expertise: An old oil target: Nanushuk formation prime drilling objective prior discovery of Prudhoe Bay
Alan Bailey - Reprint from Jan. 13, 2019, PN for Petroleum News
The Nanushuk formation, the rock unit that has become the focus of new major oil discoveries on the North Slope, has acquired a reputation as something of a new kid on the block, a new oil play, overlooked in the past but now the known reservoir for finds such as Pikka and Willow. Apparently, however, the Nanushuk was a prime target of early North Slope exploration, prior to the discovery of the massive Prudhoe Bay field in 1968.
Veteran North Slope geologist Gil Mull explained to Petroleum News that early interest in the Nanushuk emanated from U.S. Navy exploration in the 1940s and 1950s in what is now the National Petroleum Reserve-Alaska, a region underlain by a vast quantity of lower Cretaceous Nanushuk strata. The Nanushuk had been a focus of that Navy exploration, which discovered high quality oil in Nanushuk sands in the Umiat oil field, in what is now the southeastern corner of NPR-A.
Surface outcrop mapping from Umiat found the Nanushuk to become thinner and more fine-grained towards the north and east, an observation that appeared consistent with a theory that the sediments, formed from ancient river deltas, were sourced from the emerging Brook Range to the south, pouring northward into a marine basin, Mull said.
The Susie well This particular theory in part underlay the drilling in 1966 and early 1967 by ARCO and Humble Oil & Refining of the Susie No. 1 well, in the more southerly part of the coastal plain, immediately west of the Sagavanirktok River and some distance east of Umiat. The Susie well sought two main targets: the Nanushuk and several hundred feet of shallower and younger medium to coarse-grained sandstone, upper Cretaceous and Tertiary in age, Mull said. A north-south seismic line, shot in the winter of 1963-64 by Richfield (subsequently to merge with Atlantic Refining Co. to form ARCO), had revealed an un-breached anticline in the subsurface near the Susie location, he said. As a geologist working for Humble, Mull had sat the Susie well.
Those shallower strata turned out to include sands with good oil shows but no commercial oil pools. However, underneath, instead of Nanushuk sands, came black silty mudstone for thousands of feet. At a depth of 12,900 feet the well encountered the lower Cretaceous Pebble Shale, with the Kemik sandstone appearing at 13,000 feet: These formations are older, and hence should be deeper, than the Nanushuk.
Changing geologic model Subsequent exploration and geologic research have revealed the reason for the absence of the Nanushuk in the Susie well. It turns out that the sediments that formed the Nanushuk in the NPR-A actually flowed in from the west, and not from the south — river delta sands deposited high on the ancient basin margin formed the Nanushuk, while sand and mud layers deposited at the base of the margin formed what is now called the Torok formation. As the basin filled, the basin margin migrated from west to east, eventually coming to a halt to the east of what is now the Colville River. It is now obvious that the Susie well location is some distance east of that ultimate Nanushuk/Torok basin margin — the silty mudstones found where the Nanushuk had been expected were probably deposited far out in the deeper part of the basin.
On the other hand, to the south of Umiat the eastward migrating margin does merge with a region in which Nanushuk equivalent sediment did flow north from the emerging Brooks Range. Mark Myers, formerly commissioner of the Alaska Department of Natural Resources and director of the U.S. Geological Survey, commented to Petroleum News that the presence of the age-equivalent Fortress Mountain sandstone, deposited northward from the Brooks Range, does indicate Nanushuk potential to the south of the Susie well.
Prudhoe Bay discovery The discovery of the Prudhoe Bay oil field shortly after the drilling of the Susie well caused attention to shift to the coastal region of the central North Slope, and to the reservoir potential of rocks older and deeper than the Nanushuk.
The main reservoir at Prudhoe Bay is in the Triassic Ivishak formation, in what geologists refer to as the Ellesmerian sequence. Subsequently, the Kuparuk River field was discovered, with oil in lower Cretaceous sands in what is referred to as the Beaufortian sequence, above the Ellesmerian. The Nanushuk and Torok are in the Brookian sequence, the youngest and shallowest of the petroleum bearing rock sequences.
Some smaller fields, such as Meltwater, were found in Brookian reservoirs.
Re-emergence of the Nanushuk The Nanushuk seems to have been largely forgotten until 2015, when Armstrong Oil & Gas Inc. and Repsol E&P USA Inc., taking a contrarian view of conventional North Slope exploration strategies, made the Pikka discovery to the east of the Colville River delta. Myers, who a number of years earlier had worked as a geologist for ARCO, predecessor company to ConocoPhillips, commented at the time of the discovery that the company had been aware of evidence for oil in the Nanushuk but did not realize the size of the accumulation.
The Pikka discovery, and subsequent finds in the Nanushuk at Horseshoe and Willow, have upended the North Slope oil exploration scene. After many decades, attention has returned to the Nanushuk, the rock formation that was a focus of early exploration on the North Slope.
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