Abstract

Analyses of a horizontal well drilled in the Shublik Formation within the southern part of the Prudhoe Bay field provide the first glimpse of the unit’s potential as an unconventional resource. Drilled in 1996, the 1450 ft (442 m) long well, PBU X-19B L1, targeted the 17 ft (5 m) thick Shublik Zone B. More than 70% of the lateral well path sampled that target, a feat of precise geosteering years before the unconventional revolution. The well operator’s drilling plan focused on the middle subzone of Zone B. The well history ascribes the focus on the middle subzone to its high silt content, open fractures, and significant hydrocarbon shows. Contrary to the drilling narrative, cuttings from 41 intervals indicate that siltstone monotonically decreases in Shublik Zone B, on average, from 50% to 40% to 30% in the three subzones from top to base. The limestone concentration inversely varies with the siltstone concentration through Zone B, steadily increasing from top to base (40%, 50%, and 60%). Hydrocarbon shows in the upper and middle subzones of Zone B range from good to fair, whereas the shows in the lower Zone B are weaker. Unfortunately, scant geochemical analyses of oil produced from the Shublik flow test combined with engineering difficulties prevent a definitive conclusion as to the source of the oil — the fractured Shublik Formation or the underlying Ivishak Sandstone. However, on the basis of favorable lithology, hydrocarbon shows, and fracture density, the upper and middle subzones of Zone B should be targeted as a potential unconventional accumulation if and when a well is drilled in the pod of active source rock downdip.

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