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Geologists reevaluate Bristol Bay area Field investigations,modern analytical techniques point to significant gas potential and possibility of oil in Southwest Alaska Alan Bailey Petroleum News Staff Writer
The existence of several significant oil and gas seeps along the Alaska Peninsula provides tantalizing evidence of a petroleum system in the area. Alaska’s earliest “oil rush” occurred on the peninsula in the 1920s, but a lack of drilling success coupled with oil discoveries elsewhere moved exploration interest first to Cook Inlet and then to the North Slope.
With only 35 wells drilled in the Bristol Bay and Alaska Peninsula region, the last in 1985, this huge area remains substantially underexplored. Could there be a resurgence of exploration in the region? This October the state of Alaska will hold an areawide lease sale on state lands and offshore waters of the Alaska Peninsula. And a team of state and university geologists, headed by Alaska’s Division of Geological and Geophysical Surveys, is currently engaged in a three-year program that is applying some modern techniques to investigate the petroleum geology of the area.
The U.S. Department of Energy is funding the geological program through the Arctic Energy Technology Development Laboratory at the University of Alaska Fairbanks. In addition to the Division of Geological and Geophysical Surveys, Alaska’s Division of Oil and Gas and Bristol Bay Native Corp. are also providing funding for the work.
Rocky Reifenstuhl, geologist and program head from the Division of Geological and Geophysical Services, has talked to the Alaska Geological Society about the study. He also spoke to Petroleum News. Started in 2004 The study kicked off in the summer of 2004 with some fieldwork on the Alaska Peninsula, Reifenstuhl said. This fieldwork focused on doing stratigraphic investigations and collecting field samples. The study team has also been re-interpreting seismic data and well logs, in addition to doing a series of laboratory tests on the samples.
The geology of the Bristol Bay and Alaska Peninsula area bears striking similarities to that of Cook Inlet, with a thick, mainly terrestrial Tertiary sequence overlying a thick sequence of predominantly marine Mesozoic strata.
The first phase of last summer’s fieldwork focused on the Mesozoic section that is well exposed on the east side of the peninsula around Wide Bay and Puale Bay. The team carried out a detailed investigation of some measured stratigraphic sections and is correlating these sections with rock sequences found elsewhere in southern Alaska.
The Mesozoic includes some good petroleum source rocks, especially the Triassic Kamishak formation and the middle Jurassic Kialagvik formation — laboratory analysis of samples from the Kamishak indicated a total organic content of up to 2.38 percent. Evidence for oil Well-known oil seeps at Oil Creek, west of Puale Bay, demonstrate the petroleum generating power of the regional source rocks — Reifenstuhl described how, over time, the flow of oil from the seeps has generated a 150-meter wide delta-shaped fan of tarry material.
“You can take your hand and stick it right in and it’s just like tar,” Reifenstuhl said. The team was also able to torch the gas emitting from the seep, despite the 30 mph wind that was blowing at the time, he said.
Although the seeps occur in the middle Jurassic Shelikof formation, the team’s chemical analysis of oil from the seep has shown that the oil probably originated from the Kamishak formation, thus confirming the Kamishak’s potential as a source rock.
The team also found some undocumented, 50-meter thick, oil-saturated sections in the Naknek which contain some particularly good reservoir sandstones.
“The Naknek formation is a hugely thick formation and locally it’s got some terrific … quartz-rich, oil-charged bands,” Reifenstuhl said.
A sample from the Naknek formation showed a total organic content of 2.19 percent. The Tertiary Phase 2 of the 2004 fieldwork focused on the Tertiary sequence on the Bristol Bay side of the Alaska Peninsula, around Port Moller, Herendeen Bay and Bear Lake. The team took particular interest in the reservoir potential of the Miocene Bear Lake formation and the Pliocene Milky River formation. Thousands of feet of these formations are exposed along the coast.
The Bear Lake formation looks to be a particularly effective reservoir rock with high porosity. In surface outcrop, the permeability of this formation proved a little disappointing because of mineral alteration and pore-clogging clays, but well samples show good permeability at depth, Reifenstuhl told Petroleum News.
“It’s fine for gas, even at the surface,” Reifenstuhl said, “but when you look at the well data the porosity and permeability says that the Bear Lake is a good if not very good quality reservoir.”
The Milky Way formation exhibits particularly high porosity and permeability but doesn’t appear to have good seal rocks, Reifenstuhl said.
In its preliminary report the team has sounded one note of caution regarding reservoir rocks in the area — some sections of the Naknek, Bear Lake and Milky Way formations were fairly unconsolidated, a factor that needs to be considered when evaluating reservoir potential. Oil and gas potential The existence of coal and carbonaceous shale seams in several of the Tertiary rock formations, coupled with striking similarities to the gas-prone Tertiary strata of the Cook Inlet, suggest a strong potential for biogenic gas. The Oligocene Stepovak formation, for example, contains some black coaly material and yields a total organic content of 9.54 percent.
“There’s a lot of carbon in the Tertiary section,” Reifenstuhl said.
And the region contains plenty of fold and fault structures that could form traps.
However, the petroleum potential of the Mesozoic on the Bristol Bay side of the peninsula remains the big unknown. Well and seismic data show that the thick Mesozoic sequence seen on the Kodiak Island side of the peninsula extends under the Tertiary beneath Bristol Bay. And the team sampled gas from a seep in Cretaceous rocks southwest of Port Moller.
But what is the quality of the Mesozoic source rocks under Bristol Bay? And has any Mesozoic oil or gas remained trapped underground? Has Mesozoic oil migrated into Tertiary reservoirs, as in Cook Inlet?
“Our problem is that we have a lot of oil indications on the east side of the peninsula but how does that bear on the west side of the peninsula?” Reifenstuhl said.
The study team has done some new analyses of old well cores from the Bristol Bay side of the peninsula which indicate that the thermal maturity of the Mesozoic on that side of the peninsula would support oil formation. These results might point to the existence of oil in the subsurface, although many unknowns remain.
“We’re optimistic basically that we can get gas out of the basin but the question is can we get oil out of it, at least onshore?” Reifenstuhl said. Continuing investigation The DGGS-led team will continue its investigations in the summer of 2005. Among its activities, the team will sample several oil and gas seeps to try to determine the source rocks for the seeps — this type of analysis may shed further light on the operation of the petroleum system in the region. The team will also carry out some more detailed investigations of the Bear Lake formation, looking at the stratigraphy and the continuity of the rocks, and estimating how much gas the coals might store.
The big issue in the region is the scarcity of information. And, with a tiny number of wells scattered across an area larger than the entire upper and lower Cook Inlet, there are ample opportunities for new exploration.
A preliminary report from the geological study can be found at http://wwwdggs.dnr.state.ak.us/pubs/pubs?reqtype=citation&ID=7001.
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