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Prudhoe discovery debate still rages Finding Alaska’s largest oil field involved two exploration wells, drilling milestones spanning 15 months; actual discovery date contested Rose Ragsdale For Petroleum News
Forty years after a team of explorers first drilled into North America’s largest oil field, the actual date of discovery remains a subject of debate, even among the geologists, engineers and others who participated in the world-changing event.
One problem was that Prudhoe Bay’s discovery, like many great endeavors, did not happen in a single moment. Instead, the process of unleashing the elephant trapped beneath the frozen tundra was gradual, evolving during months of drilling in the harsh Arctic conditions.
Shrouded in secrecy, the project encountered frustrations daily, forcing geologists, geophysicists and engineers to become absorbed in the details of correctly interpreting core samples and puzzling well logs.
“It was about as exciting as watching a tree grow,” recalled John Sweet, then district explorationist for Atlantic-Richfield Co.
Pivotal event in question Sweet, who led the Prudhoe Bay discovery team, said he is convinced the Prudhoe Bay oil field wasn’t actually discovered until the Sag River #1 well was drilled in the summer of 1968.
“The first well provided the leadership, but if that had been all we’d found, it would not have been commercial,” Sweet told Petroleum News April 28.
H.C. “Harry” Jamison, then ARCO’s Alaska exploration manager, disagrees. He said Prudhoe Bay’s date of discovery was the date the well was completed – June 24, 1968, according to Alaska Oil and Gas Conservation Commission records.
“I’ve worked all over the United States, and I’ve always heard that conventional wisdom is that the date of discovery is the date the well was completed,” Jamison said April 29. “Otherwise, you’ll get dates all over the place. One guy will say, ‘It was when we got first oil shows,’ another one will say, ‘It’s when we first hit the Sadlerochit’ (formation), and another will say, ‘No, it’s when we drilled the Mississippian.’ You get the idea.”
People, in fact, can’t even agree on the year in which the oil field was discovered. Some still say it happened in 1967, though most people point to 1968.
Complicated exploration timeline The discovery chronology was complicated by many factors, including the harsh working environment and the constraints on drilling during the Arctic summers.
ARCO and Humble Oil Co. pooled resources to drill the Prudhoe Bay State #1 well in April 1967, after the daunting failure of the Susie No. 1 well to the south and an impassioned plea to try again from then Gov. Walter J. Hickel.
In retrospect, the Prudhoe Bay well has been described as a last ditch effort as the entire oil industry, smarting from the expense and disappointment of 14 dry holes, seemed ready to abandon the North Slope as a frontier for petroleum exploration.
Drillers spud the Prudhoe Bay State #1 well April 22, 1967, but shut down for the summer after a couple of weeks due to breakup conditions.
The explorers re-entered the well in November 1967, and found small oil shows in drill cuttings from thin sandstones in the upper part of the well in late November and early December 1967.
Early oil shows encouraging “This, however, is not uncommon on the North Slope; almost all wells drilled there encounter some shows in the Cretaceous sandstones, thus seeing small oil shows in this part of the section was not particularly significant,” said Gil Mull, who “sat on the well” as a geologist for Humble Oil during the Prudhoe Bay field’s discovery.
On Dec. 8, 1967, a drill stem test run on an interval of thinly inter-bedded Cretaceous age sandstones and shale from 6,876 feet to 6,998 feet deep flowed with some natural gas and recovery of a few barrels of oil from the drill pipe, though no oil flowed to the surface.
“This was an encouraging sign, but again not terribly significant, because other wells previously drilled on the North Slope also had recovered small amounts of oil from rocks of Cretaceous age — those deposited some 65 million to 140 million years ago,” Mull said.
On Dec. 27, 1967, the explorers encountered strong gas shows during drill stem test #2, and the well flowed with a high volume of gas, which marked a significant gas discovery.
Mull said this was the first significant production of hydrocarbons from what has become the main reservoir of the Prudhoe Bay field, but no oil was encountered in this part of the section.
Eyewitnesses recall that the gas rushed out of the well roaring like a jet engine with such force that it shot 50 feet into a 35-knot wind.
After drilling resumed Feb. 1, 1968, oil shows were seen in the drilling mud and in cores from the lower 40 feet of sandstones in the Sadlerochit Formation, but were not evaluated until logs were run on Feb. 8, 1968.
“Evaluation of these logs along with the core data indicated that the formation had sufficient porosity and contained enough oil that it could be considered a discovery,” Mull said.
Field discovery announced ARCO sent out a news release Feb. 16 announcing the discovery of oil, though no flow tests had been conducted.
Two days later, the explorers conducted drill stem test #3 in the top of the Lisburne limestone. It flowed with a large amount of gas and an estimated 100 barrels per day of oil, but this oil and gas flow probably came from the overlying Sadlerochit Formation rather than the Lisburne, according to Mull.
The first actual measured oil flow from the Prudhoe Bay well came in Drill Stem Test #4 in the Lisburne on March 12, 1968. It measured the flow at 1,152 bopd and produced headlines March 13, 1968.
“However, this oil was produced from Lisburne limestone and dolomite that underlies the sandstone and conglomerate of the Sadlerochit Formation, and showed that there are multiple reservoirs in the Prudhoe Bay area,” Mull said. “This had the effect of increasing the likelihood that the Prudhoe Bay field could be economically viable.”
By May 1968, several drill stem tests had measured the flow of oil from several intervals in the Sadlerochit Formation.
About a month later — June 25, 1968 — explorers drilling the Sag River State #1 well encountered oil in the Sadlerochit Formation, seven miles away and 400 feet deeper than crude found by Prudhoe Bay State #1.
On July 18, 1968, ARCO and Humble announced that the Prudhoe Bay field probably contained 5 billion to 10 billion barrels of recoverable oil.
Support for winter discovery So when was the Prudhoe Bay oil field discovered?
“As you can see, there was a succession of events that built to the realization that the Prudhoe Bay State #1 well was a commercial discovery,” Mull said in a letter to author Gene Rutledge in 1998. “But, if I had to pick a date at which one could say that oil was discovered in the Prudhoe Bay field, I would pick Feb. 1, 1968, when the first oil saturated sandstone was encountered in what has become the main reservoir in the field.”
Not so, says Marvin Mangus, ARCO’s well geologist for Prudhoe Bay State #1.
“Oil was discovered at Prudhoe Bay in December 1967,” Mangus said April 28. “We decided not to make it known until we drilled the confirmation well. So the top brass decided to hold off making the announcement until February 1968. But all of us on the well had decided that we had something. It should have been right around Christmas 1967.
“You just don’t have oil shows that big unless it’s really good. We felt that it was a discovery,” Mangus said.
Frigid weather hampered progress Garnett “Gar” Pessel, a geologist who worked with ARCO geophysicists at the time on core samples coming in to Anchorage from Prudhoe Bay State #1, said one possible reason for the conflicting views on the discovery date was the huge amount of uncertainty that loomed over the operation that winter because of frigid temperatures.
“During the early testing, the engineers were unfamiliar with working in the cold. The equipment kept freezing up and giving bogus results, and the guys kept arguing about it,” Pessel said. “But Don Jessup, the district geologist for ARCO kept looking at the cores and getting excited.”
Sweet agrees that the cores looked promising. “I had never seen oil shows like that before in my whole career,” he told members of the Alaska Geological Society at its annual technical conference April 17.
Still, those tantalizing glimpses of something big did not an economic discovery make, according to Sweet.
Part of the problem was that the initial target for the Prudhoe Bay well was the Lisburne Formation, he said.
At 8,700 feet, the drillers encountered the Lisburne and got 150 bopd, but when they reached the Sadlerochit, gas flowed for 23 minutes and oil at 1,152 bpd, Sweet said.
The period was frustrating because the oil shows were in shale with very little sandstone. As the explorers drilled deeper, they reached the Sadlerochit Formation at 9,600 feet.
“The drill had been progressing at a foot an hour,” Sweet recalled. “When it hit the Sadlerochit, the drill rate went … to about a foot a minute. We ran a test and we had gas to surface at 1,250 Mcfpd.
“On Feb. 4, 1968, we reached the lower part of the Sadlerochit and what came out was aggregate, loose sand and oil, some of which ran through the rig floor.
“I received a core analysis on Feb. 7, and it was one of the most dramatic things you’d ever want to see. With that we made the first reserve calculations,” he said.
Incredible signs, cautious calculations Sweet said those first calculations were based on reservoir characteristics such as porosity upwards of 30 percent, permeability in Darcies up to 3 (permeability in oil fields is usually expressed in millidarcies) and 65 percent oil saturation. Sweet’s figures yielded 223 barrels per acre foot with 20 sections of proven, 17 sections of probable and 33 sections of possible crude reserves for a total of 2.3 billion barrels of oil.
“That’s the number we called in to Dallas,” he said.
In March 1968 what we had was an unbelievable reservoir, a little oil value but with good flow of oil and lots of gas. It was exciting but with many questions,” he said. “Almost immediately, everybody’s minds turned to a confirmation well. We had to determine the oil column, if any, the water table and the continuity of the reservoir and an all season location, which required tons and tons of gravel.”
Though the “Dallas people” pored over all the seismic maps, there was no drama and little politics involved in selecting the location of the confirmation well, Sweet said.
“It had to be a long step-out and near a source of gravel. That put the location near the Sag River where geologists determined there was lots of gravel,” he explained.
The explorers spud Sag River State #1 May 3, 1968 and by July, ARCO and Humble geologists had tested the Sadlerochit Formation and confirmed the presence of the Prudhoe Bay oil field.
Says Sweet: “The statistical chance that the Sadlerochit (formation) would occur in conjunction with the Prudhoe Bay structure boggles the mind.”
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