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DNR issues new report on Bristol Bay geology State-led research team confirms the oil and gas potential of the Alaska Peninsula but also recognizes some major uncertainties Alan Bailey Petroleum News Staff writer
If oil and gas exploration is all about taking risk for potential reward, the Alaska Peninsula and Bristol Bay ought to provide enough food for thought to whet any explorer’s appetite.
The geology of the area bears a striking resemblance to that of the productive Cook Inlet to the northeast, with a thick terrestrial to shallow marine Tertiary rock sequence overlying a thick sequence of Mesozoic rocks. The Tertiary sequence lies in an elongated basin along the northwest side of the Alaska Peninsula. This basin is variously known as the Bristol Bay basin or North Aleutian basin.
But oil discoveries in the Cook Inlet area and then on the North Slope, coupled with lack of early drilling success on the Alaska Peninsula, diverted attention from Bristol Bay and the Alaska Peninsula many years ago.
There’s now interest in restarting oil and gas exploration on the peninsula and the state of Alaska is conducting an areawide lease sale in October of this year.
The state has issued a report on the results to date of a multiyear project to investigate the petroleum geology of the lease sale area, on state lands and waters on the south and northeast sides of Bristol Bay. A team of geologists from the state’s Division of Geological and Geophysical Surveys, the state’s Division of Oil and Gas, Purdue University and the University of Alaska Fairbanks is collaborating in this project which includes field investigations, analysis of rock samples and the organic geochemistry of hydrocarbon samples.
Paul Decker, a petroleum geologist with the Division of Oil and Gas, discussed some of the project findings with Petroleum News.
The Bristol Bay area is enormous, Decker said. Overall it’s 325 miles from Moffat Point at the southwest end of the state’s lease sale area to the northeast end of the area — the state acreage consists of more than 2.5 million acres onshore and 1.5 acres of offshore state waters.
Yet only 35 exploration wells have ever been drilled in or around this vast area. No 3-D seismic has been shot in the area and most of the publicly available 2-D seismic was shot offshore, Decker said. But evidence such as oil and gas seeps demonstrates that the area contains hydrocarbons.
“We know it’s an active hydrocarbon habitat — we have both oil and natural gas and it’s probably from more than one source,” Decker said. Source rocks and reservoirs A petroleum-bearing Mesozoic sequence along the southeast side of the Alaska Peninsula is known regionally as the Chignik subterrane. This subterrane contains two excellent source rocks: the Triassic Kamishak formation that’s equivalent to the prolific Shublik shale on the North Slope and the mid-Jurassic Kialagvik Formation that’s equivalent to the oil source rock in the Cook Inlet basin. Coals in the Cretaceous Chignik formation could have sourced thermogenic or biogenic gas.
The Tertiary sequence along the peninsula contains abundant coals and carbonaceous shales that could also have sourced gas.
And the COST No. 1 well, drilled in 1983 offshore Port Moller in one of the deepest parts of the Bristol Bay basin, encountered Tertiary rocks with thermal maturities within the oil window, Decker said. Some of these rocks contain lipid-rich kerogens, the building blocks for oil, he said.
“Below about 12,300 feet in that well you are at oil window maturity,” Decker said. “… There are several hundred feet where there’s a substantial percentage of more lipid rich kerogen that might generate some oil.”
There are several excellent potential reservoir rock units in the area, including the Jurassic Naknek formation, the Tertiary Tolstoi formation and the Tertiary Bear Lake formation. Basin geometry The COST No. 1 well was drilled on the outer continental shelf some distance out from the coast of the peninsula. So a key question is whether deep, thermally mature source rocks also lie under the coastal area or even inland.
Although structural maps show the Bristol Bay basin lying predominantly offshore, there’s great uncertainty regarding the onshore geometry of the basin. There’s no data for the area, Decker said. And although the basin trends northeast-southwest along the coast, structural mapping implies a more east-west trend at depth, he said.
This more east-west trending structural grain can be most clearly seen at the southern end of the basin, where the basin abuts the Black Hills uplift, a structural high south of Port Moller. The east-west trend at depth coupled with the steep basin flank on the southeastern side could have resulted in some of the deeper parts of the basin forming under the northwest side of the peninsula, Decker thinks.
“I have a strong feeling … that the basin is asymmetric in that the deepest portion of the basin where there’s most likely to be gas window maturities or higher oil window maturities is going to be this area right under the lowlands (northeast of Port Moller),” Decker said. “… The lowlands are low because they’re part of the basin.” Traps and seals And there’s an abundance of possibilities for the trapping of oil or gas accumulations.
“From a trap standpoint you’ve got just an incredible range of structural trapping geometries … as a result of a long history of different styles of tectonics from the Mesozoic right on through the present,” Decker said.
As well as structural traps, the area contains many unconformities and other breaks in sediment deposition that could create stratigraphic traps. Also, lateral changes in rock strata from coarse-grained to fine-grained rocks could give rise to sealing barriers within the strata, Decker said. And variations in the diagenesis or chemical alteration of the rocks could create other trap situations.
But a lack of regionally developed seal horizons does present a potential issue, Decker said. He sees analogies with the Tertiary of the Cook Inlet basin, where the strata contain many stacked pay zones with each distinct reservoir unit having its own local seal.
“That could end up being terrific if we have similar sized structures and gas charge or oil charge,” Decker said. “… (but) it’s a departure from the kind of exploration that a lot of companies are doing in some other basins.” Uncertain timing Lack of detailed information about the timing of trap formation and hydrocarbon migration adds significant uncertainty to exploration in the Bristol Bay area. The rocks have been deformed, faulted and heated at several different times over a period of many millions of years.
“That’s a good thing in the sense that it gives you a lot of different opportunities to create traps but it can also be a dangerous thing if you’re concerned about breaches in earlier formed traps by later structures,” Decker said.
In fact, when the project team assembled an events chart for the petroleum geology history of the area, the team elected not to try to identify a critical moment at which hydrocarbons generation and migration synchronized with trap formation.
“We don’t understand the history of the generation and migration completely enough … we have had more than one phase of generation and migration,” Decker said “… It’s going to take a fairly area-specific basin modeling approach.”
There’s also major uncertainty about the thermal maturity of the potential source rocks in different parts of the region. For example, there’s a body of opinion that thinks that all the oil and gas from the Mesozoic rocks was generated back in the late Cretaceous, prior to the deposition of the Tertiary rocks. But Decker thinks that this view is too much of a sweeping generalization for such a large region.
“It may be a dangerous speculation to say that all the Mesozoic source rocks throughout the Alaska Peninsula cooked off everything they had to give back in the late Cretaceous,” Decker said. “It strikes me as likely to be the case that that’s not true, that there are still areas where you could have active pods of the source rock.” What’s under the Tertiary? Indeed there’s only very limited knowledge of the true nature of the Mesozoic rocks where they’re obscured under the Tertiary strata in the Bristol Bay basin — exploration wells drilled into the Tertiary rocks on the northwest side of the Alaska Peninsula and in the Bristol Bay have not penetrated the Mesozoic to any extent.
Substantial thicknesses of Chignik subterrane strata with good petroleum potential occur along the southeast side of the Alaska Peninsula, right next to the side of the basin. And the Cathedral River No. 1 well in the Black Hills area southwest of Port Moller proved that there’s a major Chignik subterrane sequence on the south side of the basin.
“In that well you basically have Jurassic rock at the surface and within a few hundred feet of the surface all the way down to 14,301 feet you have a lot of oil and gas shows,” Decker said. “Even though we haven’t typed that (oil and gas) chemically it’s almost certain that that’s been generated by these … Chignik subterrane Triassic and Jurassic sources.”
All this evidence hints to the possibility of the Chignik subterrane underlying the Tertiary in the basin. And according to the project report, “seismic data in various parts of the basin show reflectivity below the inferred top Mesozoic horizon that is consistent with, ‘though not necessarily diagnostic, of Chignik subterrane units.”
But the Becharof well, located on the peninsula 200 miles to the northeast of the Port Moller area, appeared to bottom in metamorphic Mesozoic rocks known as the Iliamna subterrane and the Chignik subterrane is absent north of Becharof Lake on the west side of the peninsula. The Iliamna subterrane shows little or no petroleum potential.
It’s possible that the Iliamna subterrane rather than the Chignik subterrane extends southwest of the Becharof Lake area, offshore and under the peninsula lowlands — in the absence of evidence it would be dangerous to make assumptions about what’s there, Decker said. However, geochemical analysis of oil shows in wells is planned for a later phase of the project and may shed some light on whether this oil originated from the Chignik subterrane.
But some confusion regarding the stratigraphic interpretations from well samples compounds the uncertainties. Different companies have disagreed regarding even the biostratigraphy on the same well, and quite a few of the stratigraphic interpretations have depended on comparing rock types, Decker said.
In conjunction with the state-led project, a team from Micropaleo Consultants Inc. has prepared comprehensive micropaleontology interpretations of the stratigraphy from some of the old well samples. This work has pointed to significant errors in some of the earlier interpretations and according to the project report should “represent a much more reliable and consistent stratigraphic zonation of the basin.” Intriguing potential Because of all the geological uncertainties the state-led team of geologists has not done a quantitative resource assessment for the lease sale area.
“There’s very little information to go on for making reliable assessments,” Decker said.
But the petroleum potential of the Bristol Bay basin does present intriguing possibilities for explorers, especially using modern data acquisition techniques. I would expect that industry could do marvelous things now, Decker said.
And access to tidewater would make it possible to export oil or gas by sea.
“Here you’re in a place where you’d basically be at tidewater for natural gas production, as opposed to having to transport it down some hundreds-of-mile pipeline system,” Decker said.
Decker also views the support of the local population in the Bristol Bay area as key factor in encouraging development in the area.
“They really want to see economic development now,” Decker said.
And, with so little previous exploration in the region, Decker sees exciting opportunities for exploration companies.
“It’s one of the few places left, not just in Alaska, but anywhere in North America where industry can … start applying some of these new technologies and play concepts for the first time,” Decker said. “… There aren’t that many basins left where a company can go in and lock up a large trend of underexplored acreage.”
The state’s interim report is available at www.dggs.dnr.state.ak.us/pubs/pubs?reqtype=citation&ID=7184 and the micropaleontology stratigraphic report is available at www.dog.dnr.state.ak.us/oil/products/publications/akpeninsula/biostrat.htm.
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