When Ardmore, Okla.-based Hewitt Mineral Corp. picked up four tracts near Herendeen Bay at the State of Alaska’s October 2005 Alaska Peninsula areawide lease sale, the company said that it was going to embark on a geologic investigation of the area of its new leases. And, as a consequence of that investigation, the company now thinks it has found a new natural gas play that exists under the Peninsula and may extend under the lower Cook Inlet.
“We now have identified some evidence that there could be a significant carbonate reservoir under the Peninsula that has not been recognized,” Hewitt petroleum geologist Bryan Sralla told Petroleum News on Feb. 27.
The Hewitt leases lie over what geologists call the Sapsuk Lake anticline, a major fold in Mesozoic strata that outcrop at the surface in that area. The Mesozoic rocks are known to have petroleum potential, but presence of pore-clogging minerals formed from volcanic fragments in the rocks has long caused concerns about reservoir quality.
But following some advice from Alaska geologist Robert Blodgett, Sralla conducted a field investigation of Triassic strata of the Kamishak formation, where these rocks are exposed on the coast at Puale Bay on the southeastern side of the Alaska Peninsula.
Carbonate rocksAt Puale Bay a thick sequence of carbonate rocks particularly intrigued Sralla. Carbonate rocks consist predominantly of calcium or magnesium carbonate — rock containing calcium carbonate is generally known as limestone, while rock containing calcium/magnesium carbonate (the mineral dolomite) is known as dolostone.
“At Puale Bay there is a thick carbonate sequence that is about 2,000 to 2,500 feet thick,” Sralla said.
And in the lower sections of the Triassic Kamishak formation, Sralla found carbonate rocks, crammed with fossil corals and shellfish. Fractures, cavities and pores in the rocks indicated significant petroleum reservoir potential — a feature of particular importance since carbonate rocks would not become clogged with altered volcanic material in the same way as the potential reservoir sandstones of the region.
Later examination of microscope slides of the rocks confirmed a prevalence of dolomite in the lower Kamishak sequence. But what particularly caught Sralla’s attention was microscope evidence that the rocks contain what geologists term “hydrothermal dolomite,” formed when magnesium from warm underground fluids reacts with calcium carbonate in limestone.
Because the fluids that pervaded this type of rock tend to cause extensive fracturing, hydrothermal dolomites are associated with particularly good petroleum reservoirs.
“Very prolific reservoirs are often associated with these hydrothermal dolomites,” Sralla said. “…Hydrothermal dolomites have been very much in the literature in the last several years. There is pervasive theory that a lot of prolific dolomite reservoirs are in fact hydrothermal dolomites.”
And the known existence in the Alaska Peninsula area of very deep faulting that could have supported the movement of chemically laden fluids provides a plausible explanation of how hydrothermal dolomite could have formed.
Source and seal potentialAbove the carbonate rocks lie upper Kamishak shales. These shales are equivalent to the prolific Shublik oil source rock of Alaska’s North Slope and contain a large amount of organic material. The shales would make an excellent source rock in the Alaska Peninsula Area and may be the source for a well known oil seep at Oil Creek, west of Puale Bay.
“We think that this looks every bit as good or better than the Shublik,” Sralla said.
And just to confirm the source rock potential of the region, Sralla’s microscope slides revealed pervasive bitumen staining and dead oil in pore spaces of the Kamishak carbonates. In addition, the upper Kamishak shales could effectively seal a Kamishak carbonate reservoir.
“So potentially the Triassic has three elements: a source, a seal and now a potential reservoir rock in a hydrothermal dolomite,” Sralla said.
Evidence from the pastTheir interest piqued by an apparent lack of previous reports of dolomite in the Triassic of the Alaska Peninsula, Sralla and Blodgett embarked on a detective adventure through the archives of Alaska Peninsula petroleum geology, pooling information from several companies that had been active in the region in the past.
“We’ve got a lot of data that’s been buried for 50 years now,” Blodgett said.
They discovered that legendary prospector and geologist Earl Grammer had championed a Triassic oil play on the Alaska Peninsula back in the 1950s; Grammer had, in fact, persuaded the Standard Oil Company of California to pursue that play.
But Sralla and Blodgett’s investigations led them to the Bear Creek No. 1 well, drilled near Puale Bay in 1959 by Exxon; at a depth of 14,000 feet, this was the deepest well in Alaska at the time.
“They were drilling for the Triassic and drilled deep and didn’t find any reservoir rock there,” Sralla said. According to “Crude Dreams,” Jack Roderick’s account of the history of the Alaska oil industry, Shell partnered with Exxon in the drilling of Bear Creek No. 1 and the lease was held in the name of Earl Grammer’s sister.
The rig used to drill the Bear Creek well later drilled the discovery well at Prudhoe Bay, Blodgett said (the Prudhoe Bay discovery in 1968 diverted the attention of oil explorers away from the Alaska Peninsula and Cook Inlet for decades to come).
But Sralla and Blodgett dug through boxes of core chips from the Bear Creek well, archived at the Alaska Geologic Materials Center, and concluded from fossil evidence that the well did in fact penetrate a Triassic reservoir. The prevalence of dolomite in the core chips confirmed that conclusion.
Sralla said that well logs from Bear Creek show good permeability over a 500-foot interval at the inferred level of the Kamishak. A drill stem test in the top of that interval flowed some gas and also flowed saltwater. Sralla’s calculations from the flow rate of water indicate a good permeability of about 21.4 millidarcies.
“What that tells us is that this Kamishak is a very good reservoir in this Bear Creek well where it was tested,” Sralla said. “… We feel really good that we’ve uncovered some significant evidence that had been overlooked out there. We say with a pretty good degree of certainty that this test was in the lower Kamishak.”
So what does all of this mean when it comes to oil and gas prospects under Hewitt’s acreage near Herendeen Bay, or elsewhere?
Sralla said that the Sapsuk Lake anticline in the Hewitt acreage has a closure area of about 60 square miles, much larger that the structures associated with the oil and gas fields of the Cook Inlet. But the Triassic Kamishak formation appears to be buried to a depth of about 14,000 feet, likely placing it within the temperature window where the organic source material would have been cooked into natural gas rather than oil.
Assuming a reservoir thickness of about 120 feet, a porosity of about 7 percent and the high pressure resulting from deep burial, the reservoir could contain huge amounts of gas.
“With those sorts of pressures you could have multiple trillions of cubic feet of gas in a structure of this size,” Sralla said.
Perhaps even more intriguing is the regional implication of a widespread Triassic reservoir rock. Carbonate rocks of the type found in the lower Kamishak form on stable marine platforms that can extend over large areas. And geologists have found Triassic carbonate rocks to the west of the Iniskin Peninsula (on the west side of the Cook Inlet, opposite Kachemak Bay), and at Port Graham on the eastern side of the lower Cook Inlet.
“What gives us some reason to believe that there might be an extensive areal extent of this carbonate deposition is that it appears that there was a fairly pervasive carbonate platform during the Triassic,” Sralla said.
And it appears that none of the wells in the lower Cook Inlet ever drilled deep enough to penetrate the Triassic.
“If it is shown that this lower Triassic carbonate is extensive it might have pretty large ramifications for exploration in the lower Cook Inlet,” Sralla said. “… The interval seems to extend across the lower Cook Inlet. If that could be a seal, a source and a reservoir, it could make some of those anticlines down there attractive targets.”
Meantime, Sralla and Blodgett are publishing their findings for peer review by geologists and industry.
“I can say … that of all of the rocks that we’ve looked at on the Peninsula so far, this is by far and away the most interesting thing we’ve seen in the field,” Sralla said. “… We think it could be significant. It argues that these structures need to be tested down to the Triassic to fully evaluate their potential.”