40 Years at Prudhoe Bay: Picture of field emerges

Long summer of drilling in 1969 reveals impressive, complex subsurface geology

Petroleum News

As drilling gathered pace through the summer of 1969, most of it within a 10-mile radius of Prudhoe Bay, the oil companies began to build up a clearer geological picture of what they had found.

The whole Prudhoe Bay field lies within an area roughly 45 miles long, from west to east, along the coast and 18 miles wide. It is one of a series of broad anticlines underlying the North Slope and known collectively as the Barrow Arch. There are four main oil and gas bearing reservoirs in formations of lower Cretaceous to Mississippian ages, between 120 million to 220 million to about 350 million years old that partly underlie each other and are sealed by a stratigraphic unconformity that is overlain by a cap-rock plunging from west to east.

The shallowest is the Kuparuk River Formation, some six miles to the northwest of the western end of the Prudhoe Bay field. It is up to 800 feet thick, below a depth of 6,765 feet, and extends over an area of about 128,000 acres. The Kuparuk field produces about 150,000 barrels a day and remains the second-largest oil field in North America.

Next is the most important and biggest formation, the multi-pool Prudhoe Bay group, formed in undulating sandstone up to 600 feet thick below a depth of 8,110 feet, and covering some 368,640 acres. Today, this field has produced more than 13 billion barrels of oil and ultimate recovery is estimated to exceed 15 billion barrels.

Slightly to the east is a deeper reservoir, formed in what has been classified as Lisburne limestone, up to 1,700 feet thick below a depth of 8,758 feet and covering 181,750 acres.

And still farther to the northeast is the deepest reservoir in the area — the Kekiktuk Conglomerate, which forms the reservoir in the Endicott oil field.

Geologists subsequently identified other crude reservoirs in roughly the same area, including the vast, much shallower heavy oil pools of West Sak and Ugnu.

Of the 23 wells that had been completed by the end of the summer of 1969, nine had been drilled by BP, seven by ARCO-Humble, four by Mobil-Phillips, two by Standard Oil of California and one by Hamilton Brothers. Only two failed to find any oil or gas — one each drilled by BP and ARCO — though not all would necessarily prove to be commercial. Seven wells encountered two of the reservoirs and one, Socal’s Kavearak Point wildcat, found oil in all three reservoirs.