Following a productive summer field season on the Alaska Peninsula a team of geologists has reported some intriguing findings. The findings point to the existence of Mesozoic petroleum source rocks under the peninsula near Port Moller and provide other tantalizing insights into geology that could form and trap oil or gas.
The team, consisting of geologists from Alaska’s Division of Geological and Geophysical Surveys, Alaska’s Division of Oil and Gas, Purdue University and the University of Alaska, has just published a preliminary report on its 2005 field season. The fieldwork was part of a three-year program that started in 2004, to gain a better understanding of the geology of the Alaska Peninsula and Bristol Bay region through the investigation of rocks that outcrop at the surface.
The State of Alaska is holding an areawide lease sale for state lands and waters along the Alaska Peninsula on Oct. 26. The sale area lies along the southern side of the Bristol Bay basin, a deep, elongated depression filled with Tertiary sedimentary rocks.
This summer’s fieldwork consisted of three phases: geochemical sampling of reported oil seeps in the greater Dillingham area, the sampling of Mesozoic source rocks around Puale Bay on the southern side of the peninsula and extensive geologic studies across the peninsula in the area around Port Moller between Sandy River to the east and Black Hills to the west. The team’s report can be found at http://www.dggs.dnr.state.ak.us/pubs/pubs?reqtype=citation&ID=7190.
Thermogenic gas
In the report Paul Decker from the DOG and Rocky Reifenstuhl of DGGS describe the results of chemical tests that the team carried out on gas from a known seep in the lower Cretaceous Herendeen formation, on the peninsula between Port Moller and Herendeen Bay. Because the gas is known to consist of nearly pure methane people have tended to assume that it is biogenic in origin, formed by bacterial action in coal beds or organic-laden sediments.
However, the new tests show carbon and deuterium isotope values that indicate the gas is thermogenic and was formed by the heating of organic material deep underground. The fact that the gas consists of methane may result from the temperature to which the organic material was heated or from the composition of the organic material that generated the gas — the team has yet to do a more in-depth analysis to determine the thermal maturity of the gas.
Why do the test results matter?
“We shouldn’t (now) think of this as strictly a biogenic play — there are differences in how you’d explore for a thermogenic gas play,” Decker told Petroleum News.
The Herendeen formation that contains the seep is an unlikely petroleum source, so the gas must have bubbled up from some other formation underground. And the fact that the gas is thermogenic points to a Mesozoic source in what’s known as the Chignik subterrane, rather than a source in the thermally immature Tertiary strata of the area, Decker said. There’s long been speculation about how far the source rocks of the Chignik subterrane extend under the northwestern side of the peninsula. Jurassic rocks of the Chignik subterrane source oil seeps near Puale Bay on the southeast side and equivalent strata source the oil fields of the Cook Inlet.
Mesozoic sampling
Reifenstuhl also reported on some new chemical analyses of the Chignik subterrane source rocks that outcrop at Puale Bay. The team analyzed samples from the middle Jurassic Kialagvik formation and the upper Triassic Kamishak formation. The analyses augmented data obtained from some work done in the previous year.
The Kialagvik formation sample showed a total organic content ranging from 0.71 to 3.51 percent, with both type I and type II kerogen — both of the kerogen types would support oil generation and the type II kerogen would support gas generation. The Kamishak formation showed 0.04 to 0.15 percent total organic content, with mixed type II and type III kerogens. However, Kamishak samples from the 2004 field season contained type I and type II kerogens.
The team has done some chemical analysis of samples from Tertiary strata and will report on these at a later date.
Reifenstuhl also reported on an analysis of coal samples from some upper Cretaceous and Tertiary rocks. Preliminary results show that the upper Cretaceous Chignik formation contains high-volatile bituminous C coal while the Tertiary Bear Lake formation contains coal ranging in rank from lignite A to high-volatile bituminous A.
Seeps near Dillingham
The analysis of some reported oil seeps in the greater Dillingham area, on the north side of Bristol Bay, proved disappointing — the team collected samples from seven sites where people had observed purplish-blue sheens on the water, persistent winter thawing or unusual hydrological patterns in the tidal zone. The team observed sheens at all of the sites.
Stirring of the water at each site caused the sheen to break up in a manner inconsistent with the presence of hydrocarbons. Laboratory analysis supported this finding — the samples were reported to contain “iron bacteria and were devoid of detectable hydrocarbons.” However, the samples will now be tested in a Texas laboratory, using a chlorine test for the presence of petroleum.
Measured sections in the Tertiary
Ken Ridgway of Purdue University, Emily Finzel of DGGS and consulting paleontologist Robert Blodgett reported on some measured sections documented from outcrops of the Tertiary Tolstoi, Stepovak and Bear Lake formations. These sections add considerable detail to the known geology of the formations. That detail enables a more comprehensive understanding of the depositional environments — the situations in which the sediments that formed the rocks were laid down.
The team interpreted a range of depositional environments that often oscillated between terrestrial and shallow water marine settings. The frequently observed marine environment seems to somewhat distinguish the Bristol Bay Tertiary from the more terrestrial Tertiary of the Cook Inlet, Decker said — people often draw parallels between the two areas. But although many of the Bristol Bay rocks are marine in origin, the widespread existence of coal seams provides evidence of an appropriate environment for the production of gas.
“There’s plenty of coal in the system to be behaving as a source of either biogenic or thermogenic gas,” Decker said.
The Bear Lake formation contains particularly promising potential reservoir sandstones. However, geologists have expressed a concern at an apparent paucity of good seal rocks in the Tertiary. But Decker said that working the measured sections had brought to light potential seal units that aren’t as conspicuous as the reservoir sandstones.
“One of the things that’s becoming clear to me is that there is actually a fair amount of mudstone in the system,” Decker said. “… They may not be regional in extent but they’re certainly pretty thick in places and probably extend to the size of a prospect, for example.”
The field geologists collected numerous fossils, ranging from marine shells to a crab shell and a shark’s tooth. The fossils are helping the geologists better understand and refine the stratigraphic relationships between the different rock units in the region. The fossils are also providing the geologists with additional information about the depositional environments.
Intriguing structures
Decker reported on folding and faulting in the Miocene Bear Lake formation. The deformation of this formation is associated with an unconformity between the Miocene strata and the overlying Pliocene Milky River formation — at some locations highly contorted and sliced up Bear Lake rocks lie under almost level Milky River strata.
Decker attributes the distortion of the Bear Lake rocks to the sliding of the relatively unconsolidated sediments under gravitational forces, when land to the southeast along the Alaska Peninsula was uplifting during Miocene-Pliocene times. But from a petroleum geology perspective, the unconformity and the associate folding and fracturing point to some interesting possibilities for structural or stratigraphic traps.
The folding and faulting of Mesozoic rocks to the west and south of the Herendeen Bay-Port Moller area, with structures such as large, open folds, may also have implications for the petroleum geology. Decker thinks that the folding and faulting were a result of compression forces in the Earth’s crust but there is a rapid transition north and east into the more extensional, pulling-apart features associated with the subsidence of the Bristol Bay basin.
Decker also commented that pervasive fracturing seen in the Naknek formation in the region hints that this formation might make a better gas reservoir than the strong compaction of the rocks otherwise suggests. And the Staniukovich formation above the Naknek is fairly muddy and could perhaps provide a regional seal. Indeed the gas seep in the Herendeen formation actually lies on the crest of a major anticline in the Mesozoic — Decker speculates that there may be a Naknek reservoir in the subsurface.
So, overall the results from the summer season have found more pieces of the Alaska Peninsula geologic jigsaw puzzle and seem to confirm the petroleum potential of the area. Many unknowns remain, especially in the subsurface geology along the northwestern side of the peninsula. But finding out what happens in the subsurface will depend on seismic work and someone drilling some holes in the ground.
