With oil and gas seeps in proximity to obvious major geologic structures, the Alaska Peninsula became a prime initial focus of Alaska oil exploration in the first half of the 20th century. And in the 1940s and 1950s internal oil company reports portrayed the region as having excellent petroleum potential that would likely give rise to numerous large petroleum discoveries, XTO geologist Bryan Sralla told a meeting of the Alaska Geological Society on Sept. 29.
But a series of dry wells dashed those early hopes and created a negative view of the peninsula that has persisted to the present day. Modern sentiment about the Alaska Peninsula is that the region “is largely devoid of significant reservoir targets,” Sralla said.
But what is the true petroleum potential of this huge region?
Known outcropsIt’s well known that Permian, Triassic, Jurassic and Cretaceous sedimentary rocks outcrop along the south side of the peninsula. Those rocks include the mid-Jurassic Kialagvik formation that is equivalent to the Tuxedni group, the source for the oil of the Cook Inlet oil fields.
“You can actually see the rock that gives rise to the (Cook Inlet) petroleum system along the peninsula,” Sralla said.
And the Triassic Kamishak formation of the Alaska Peninsula is equivalent to the prolific Shublik source rock of Alaska’s North Slope.
A gas seep near Herendeen Bay has been shown from isotope data to be geothermal in origin (that is, formed from the heating of rocks at depth). And there is a well known oil seep at Oil Creek near Puale Bay.
Before joining XTO, Sralla carried out an Alaska Peninsula research project for Hewitt Mineral Corp. Hewitt had picked up four tracts near Herendeen Bay at the State of Alaska’s October 2005 Alaska Peninsula areawide lease sale; the company wanted to investigate the potential of its new oil and gas leases.
Working with Alaska geologist Robert Blodgett, Sralla completed some field research on the peninsula. The two geologists also investigated well data and rock samples from Alaska Peninsula exploration wells.
Sapsuk Lake anticlineSralla and Blodgett’s fieldwork particularly focused on a major structure called the Sapsuk Lake anticline in the Hewitt leases. The Sapsuk Lake structure, several miles in length and one of a series of spectacular Alaska Peninsula fold structures, has never been drilled; it forms the main target of Hewitt’s exploration interest.
Mapping of the anticline revealed an elongated dome-shaped structure trending approximately east-west. Upper Jurassic and Cretaceous strata outcrop in the surface exposures of the structure.
An evaluation of mapping results and detailed measurements around the structure indicates that the anticline persists deep underground and was likely formed by compressional forces within the Earth’s crust. It appears that the fold was forming during the Cretaceous era and was reactivated later, during the Tertiary, Sralla said.
Sralla constructed a computer model of the anticline in the subsurface, using stratigraphic data from a nearby well in the Black Hills area to the west of Herendeen Bay. That model enabled Sralla to plot a three-dimensional representation of the likely shape of the upper surface of the Kamishak formation, a possible petroleum source and reservoir.
“On top of the Kamishak in the upper Triassic we’re looking at subsea depths of about 13,000 feet in the axis of this fold,” Sralla said.
So, a company would need to drill a well to at least 14,000 to 15,000 feet to test all of the stratigraphy that has exploration potential, Sralla said.
But what are the chances of encountering hydrocarbons with such a well?
Drill stem testsSralla thinks that drill stem tests done in exploration wells that have been drilled in the past provide valuable insights into the potential for a petroleum find.
“The other thing that I like to look at that I don’t think is appreciated enough today is drill stem tests,” Sralla said. “Lots of times they provided great data. … (But) you’ve got to go back and dig that stuff up.”
Sralla particularly looks for tests that have flowed hydrocarbons to the surface, even if that flow was not sufficient to make a well capable of economic production.
“When I see lots of drill stem tests to the surface, that’s a good sign,” Sralla said. “Often they’re very close to where we have producing oil and gas fields.”
And several wells on the peninsula have shown good gas flows.
The Bear Creek well, for example, drilled to more than 14,000 feet in 1958-59 near Puale Bay, had six different drill stem tests that flowed gas to the surface.
“The best test was what we believe was in the lower portion of the Kamishak. … This flowed gas to the surface and flowed between 400 and 750 mcf a day,” Sralla said.
Another well, the Cathedral River well, drilled to 14,000 feet in the Black Hills area about 50 miles southwest of the Sapsuk Lake anticline, had three drill stem tests that flowed gas to the surface.
“The reason they’re significant is that these tests were recovered without any formation water,” Sralla said. “They had normal reservoir pressures that do show there was gas involved. There are seals; there are source rocks out here that are capable of sourcing hydrocarbons.”
And although the drillers thought that the Cathedral well had penetrated the upper Triassic, Sralla and Blodgett do not think that the well actually reached the prospective Triassic strata — tiny fossils in the rocks from the bottom of the well indicate a younger age.
“We’ve looked at the rocks,” Sralla said. “We think that the rocks look more like lower Jurassic than upper Triassic.”
But the drill stem tests do provide evidence of good reservoir rocks. And Sralla used some of the well log data from the Bear Creek well to calculate porosity for a promising looking interval between 6,000 feet and 6,600 feet in that well — that porosity turned out to be sufficient to support a prolific gas reservoir were it to be saturated with gas.
“So we’ve got evidence of good quality Mesozoic reservoir rock out here in the Alaska Peninsula,” Sralla said. “The prevailing wisdom is that there’s not. But I think if you really look at the data … there’s some reason to believe that there might be.”
But the thermal maturity of the rocks (a measure of the extent to which the rocks have been heated in the past) is critical to the potential for petroleum production. Maturities at the surface range from relatively low maturities near Puale Bay in the northeastern part of the region to higher maturities in the Black Hills area in the southwest of the region.
The thermal maturity of the rocks penetrated by the Cathedral River well in the Black Hills increases with depth and the deeper rocks of the well appear to be in the thermal window for natural gas generation, Sralla said. And, since the shallower rocks of the region do not appear to have particularly effective seals that would create hydrocarbon traps, Sralla and Blodgett think that the region is likely to be more gas prone than oil prone.
Kamishak formationThe upper Triassic Kamishak formation seems a particularly promising exploration target. Rocks within this formation exhibit total organic contents in excess of 3 percent and very high hydrogen indices, Sralla said.
“They appear to be very good source rocks,” Sralla said.
And rocks called calcareous shales in the upper part of the Kamishak would make an effective seal in a petroleum trap.
Carbonate rocks in the lower part of the Kamishak contain large amounts of the mineral dolomite and show porosities suitable for a petroleum reservoir. There is also evidence of a petroleum system that has involved these rocks.
“A lot of the intercrystalline pore space of this carbonate at Puale Bay is saturated with bitumen,” Sralla said.
The limestones and shales of the upper Kamishak might also be prospective as reservoirs if those rocks are fractured, Sralla said. And there’s also the possibility that hydrocarbons have migrated downwards from the Kamishak into fractured Permian rocks underneath.
However, a major challenge in exploring these Triassic and Permian plays would be the discontinuity of the rocks — both the Kamishak and the Permian are absent in places along a major stratigraphic break at the base of the Triassic.
And, with almost no seismic data available for the peninsula, there’s no effective way of determining whether the rocks are present or absent at a particular point in the subsurface.
“What will be important to explore these large structures down the Alaska Peninsula will ultimately be seismic data,” Sralla said. “… It’s still truly a frontier area.”
But petroleum seeps, potential reservoirs, the thermal maturity of the rocks and the presence of good-looking source rocks all point to the existence of a viable petroleum system.
“I think we can say there is some evidence that there are good reservoirs on the Alaska Peninsula,” Sralla said.