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Horseshoe unit approved
Oil Search-operated unit is most westerly of Slope’s Nanushuk-focused plays
Kristen Nelson Petroleum News
The Alaska Department of Natural Resources’ Division of Oil and Gas has approved formation of the Oil Search-operated Horseshoe unit on the North Slope. This will be the third unit with the Nanushuk formation as the primary target. In a March 17 unit approval, signed by division Director Derek Nottingham, the division said Horseshoe covers some 156,000 acres on both sides of the Colville River.
The division received the application Oct. 21 (see story in Oct. 31 issue of Petroleum News) and deemed it complete Dec. 7 after submission of “all pertinent geological, geophysical, engineering, and well data, and interpretations of those data, directly supporting the application” as required by state regulations.
Horseshoe straddles the Colville River, with tracts to the west of the river in the National Petroleum Reserve-Alaska, managed by the U.S. Department of the Interior’s Bureau of Land Management.
The Oil Search-operated Pikka unit, the most northerly of the three Nanushuk-focused units, was formed in 2015 and expanded to the south and west in 2016.
Most recently Oil Search applied to form the Quokka unit, south of Pikka (see story in Jan. 9 issue of Petroleum News).
Horseshoe is the most westerly of the three, with state tracts south and east of the Colville River where the river bends and NPR-A tracts north of the river and then west along the river.
The division said the state and BLM require a new multi-royalty unit owner agreement including the state and BLM. Oil Search created a multi-royalty owner unit agreement, with the division and BLM providing feedback on a draft agreement, but the agreement was not finalized prior to submittal of the unit application, the division said.
Oil Search, a Santos subsidiary, is operator and holds a 51% working interest in the Horseshoe leases; Repsol E&P USA holds 49%.
Early exploration The division said the first wells drilled in the Horseshoe area were in the early 2000s on federal leases, targeting Jurassic reservoirs.
The Tofkat wells, on state acreage, targeted younger formations.
Phillips drilled the Rendezvous 2 in 2001 targeting the Jurassic Alpine northwest of Horseshoe acreage. ConocoPhillips re-entered and flow tested Rendezvous 2 in 2008, and after frack stimulation the well averaged 1,250 barrels per day during cleanup and 630 bpd during main flow. The division said that well is now within the Greater Mooses Tooth unit, with Alpine sands expected to contribute to GMT production.
In 2001 Anadarko drilled Altamura 1, targeting Jurassic reservoirs west of Horseshoe, with oil shows noted in the Alpine, Torok and Nanushuk.
ConocoPhillips drilled Pioneer 1 in 2009 northwest of Horseshoe targeting the Alpine and achieved a stabilized rate of 440 bpd after cleanup. The well was plugged and abandoned.
In 2008 Brooks Range drilled the Tofkat 1, 1A and 1B wells to evaluate Kuparuk and Brookian deposits north of Horseshoe. The wells were not flow tested and were plugged and abandoned. The division said there was evidence of hydrocarbons in logs and cuttings “over the Nanushuk interval, fluid samples in the Nanushuk suffered from oil0based mud contamination.”
Exploration 2017 to present Recent exploration began in 2017 with the Horseshoe 1 and 1A wells drilled by Armstrong - the first wells within the proposed Horseshoe unit area - targeting Brookian Nanushuk topsets.
Horseshoe 1 and 1A both “encountered potential Nanushuk reservoir in the top set facies with oil shows but neither well was flow tested,” the division said. Both wells reached total depth in Torok facies beneath the Nanushuk.
“The Horseshoe 1 and 1A wells extended the Nanushuk play more than 20 miles south of previous discoveries made in the Pikka Unit,” the division said.
To the north of the Horseshoe unit, ConocoPhillips drilled Putu 2 and 2A in 2018 “to evaluate Nanushuk topsets to the north” of the Horseshoe unit in the area of the Tofkat wells. Putu 2A was cored in the Nanushuk and tested at more than 2,000 bpd. The well was plugged and abandoned.
In 2018, east of Horseshoe, ConocoPhillips drilled Stony Hill 1. “The data from this well is currently held confidential, and the well was plugged and abandoned,” the division said.
Captivate Energy, an operating subsidiary of 88 Energy, drilled Winx 1 east of Horseshoe in 2019, targeting Nanushuk topsets, with a secondary target of Torok basin floor deposits. Captivate plugged and abandoned the well without flow testing but did encounter oil shows in the Nanushuk and Torok sections, the division said.
Oil Search drilled Stirrup 1 in 2020 “to test the Nanushuk in the center of the HSU acreage,” the division said, at a location some 7.5 miles west of Horseshoe 1 and 1A, some 28 miles southwest of the Pikka unit. “The well was tested and flowed at a stabilized rate of 3,520 bopd after a single-stage hydraulic fracture stimulation,” and plugged and abandoned after sidewall and conventional cores were taken.
Unit potential “Based on non-confidential well control and other data, there is one known reservoir and several potential hydrocarbon accumulations within the proposed Horseshoe Unit,” the division said.
Brookian potential includes Nanushuk and Torok.
The division noted that within the Colville River unit, “one zone of the Nanushuk Group, the informally designated Qannik sandstone, is currently being developed” and the CD4-595 at the Narwhal participating area “also currently producing from the Nanushuk.”
“Based on interpretation of available seismic data, regional subsurface mapping, and multiple flow tests, OSA believes that several Nanushuk sandstones are prospective within the Pikka Unit. The Colville River Unit, Nanuq-Nanuq PA and the modest development of the Oooguruk Unit Torok PA represent the only sustained Torok Formation production to date,” the division said.
The Stirrup 1 flow test proves the Nanushuk top reservoir is present in Horseshoe, and the division said Oil Search “has also identified several potential hydrocarbon accumulations in both the Nanushuk topsets and the basinal Torok deposits across the proposed unit.”
There is also Alpine reservoir potential at Horseshoe, although it has not been penetrated at wells inside Horseshoe but has been in surrounding wells. The division said Oil Search “has identified a potential hydrocarbon accumulation in the Alpine inside the HSU.”
Nanushuk initial target The Nanushuk is Oil Search’s initial target at Horseshoe, the division said.
The company’s plan of exploration lists non-drilling activities, “including additional seismic interpretation, inversion, creating static and dynamic models of the main reservoir targets, reservoir quality studies, and field studies to support future drilling,” the division said.
One exploration well is also planned, no earlier than the first quarter of 2023, but the division said no further details on the location or target of that well were provided.
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