Rocks that contain bitumen or exude petroliferous odors will grab the attention of most oil explorers. So perhaps it’s surprising that so few people seem to know about petroliferous rocks in Paleozoic strata near the Holitna Lowlands in Interior Alaska. Even fewer may realize that these rocks probably came from Russia.
Geologist Robert Blodgett, who has worked in Alaska since 1975 and has participated in industry and state field investigations around the Lowlands, described occurrences of dead oil in Ordovician rocks in the area.
“There are lumps of bitumen,” Blodgett told Petroleum News. “We had at least four localities of good bitumen development — there’s just tons of it.”
Blodgett described other rocks that smell of oil.
“They crack ‘em open and they smell good, they smell petroliferous — not fetid, but not bad,” Blodgett said.
The Holitna Lowlands
The Holitna Lowlands consists of a flat area of tundra in the Interior of southwestern Alaska, between the Alaska Range to the southeast and the Kuskokwim Mountains to the northwest. Gravity data indicates that a 4,600 meter-deep basin of Cenozoic rocks underlies the Lowlands. This basin, known as the Holitna basin, sits astride the Farewell fault, a major regional fault that connects with the Denali fault to the east.
No bedrocks of the Holitna basin are exposed at the surface. However, Cenozoic rocks in a similar setting to the northeast along the Farewell fault appear in surface outcrop and suggest that the Holitna basin contains Tertiary sandstone, conglomerate, mudstone and coal.
The rolling hills and river valleys around the Holitna Lowlands expose older Paleozoic strata that include the petroliferous rocks that Blodgett described. Geologists think that these Paleozoic strata also lie beneath the Holitna basin. Reconstruction of the stratigraphy from surface exposures indicates that the entire Paleozoic sequence has attained a thickness of many thousands of feet.
The feature that distinguishes the Paleozoic rocks of the Holitna Lowlands area from most similar aged rocks elsewhere in Interior Alaska is a lack of metamorphism. As a consequence the Holitna rocks could have generated and preserved oil or gas.
“The Holitna basin (area)’s the only place in the Paleozoic where the rocks aren’t too mature,” Blodgett said.
Exploration in the 1980s
Little was known about the geology of the area around the Holitna Lowlands until a flurry of exploration activity in the early 1980s. ARCO, Chevron, Sohio, Unocal and the state of Alaska’s Division of Geological and Geophysical Surveys all sent field parties to the area at around that time, Blodgett said. A batch of geological publications about the area appeared in the mid-1980s.
Both Unocal and Sohio found sufficient of interest to plan second field seasons. There were even rumors about plans to drill a well, Blodgett said.
Dolostones with vugs particularly interested the oil companies as potential reservoir rocks, Blodgett said. Dolostone is similar to limestone but is composed primarily of the mineral dolomite, rather than the calcite that’s found in limestone. Vugs are rock pores lined with minerals. The bitumen that Blodgett described occurs in these “vuggy” dolostones.
“The dolostones have lots of vugs in them and so Unocal really got excited — they thought this would be a major play,” Blodgett said.
But as a result of the oil price crash of the mid-1980s and changes in management priorities none of the oil companies returned to the Holitna Lowlands. DGGS did, however, carry out another study of the area in 1998.
A Siberian continental shelf
Early work in the Holitna area identified three distinct sets of Paleozoic rock sequences, known as terranes. However, the fieldwork in the 1980s showed that these three terranes, called the Dixon Fork, Dillinger and Mystic terranes, comprise different depositional settings or subterranes of a single terrain, known as the Farewell terrane.
Rocks such as dolostones in the Dixon Fork and Mystic subterranes provide evidence of a shallow marine platform bounded by algal reefs. The Dillinger subterrane contains rocks such as mudstones and limestones that formed in a deeper marine environment. By plotting out the locations of the various types of rock geologists could map the geography of a continental shelf that existed many millions of years ago, with the reef-bounded shallows rimming a marine basin.
Geologists could also trace the rocks northeast through the area of the Minchumina basin, north of the Alaska Range. However, the Paleozoic rocks in that area have been altered by metamorphism and have little petroleum potential.
Fossil evidence shows that the stable continental shelf represented by the Paleozoic rocks existed over a period of perhaps 200 million years.
“When you take this Paleozoic package and look at it there’s a really good section of late Proterozoic through Devonian rocks, including (algal) reef facies rocks that ring the outer margin of the Dixon Fork (subterrane),” Blodgett said. It’s possible to track the remains of algal reefs all the way from the Cambrian to the lower Devonian, he said.
The rocks contain characteristic Paleozoic fossils. For example, fossils of extinct animals such as trilobites and graptolites occur in the Ordovician strata. However, the fauna bears close similarities to similar aged faunas of Siberia and does not resemble the faunas of North America. This suggests that the Paleozoic rocks of the Holitna and Minchumina areas form a fragment of a Siberian continental shelf that collided with the North American continent at some time in the distant geologic past.
“The rocks and the facies are unlike North America … they’re actually Siberian like,” Blodgett said.
Curiously, the Paleozoic rocks in what’s known as the Alexander terrane in Southeast Alaska show close affinities with those at Holitna, suggesting that the Alexander terrane may be part of the same fragment of the Siberian continental margin. However, volcanic material in the Alexander terrane indicates an association with volcanic arcs, rather than deposition on a stable continental shelf as at Holitna.
Reservoirs and sources
The stable depositional environment that existed over many millions of years in the Holitna area has given rise to an abundance of good, clean reservoir rocks, especially the dolostones. And the occurrences of dead oil and petroliferous rocks provide evidence of a petroleum system.
The area contains many structures that could form traps. And there’s an intriguing possibility that hydrocarbons could have migrated into Tertiary reservoirs in the Holitna basin. Potential source rocks primarily occur in the deep water Dillinger terrane and consist of shales, shaly limestones and limestones, Blodgett said. However, a lack of good source rocks in surface exposures suggests a need for caution in estimating the petroleum potential of the area. We found lots of organics in the rocks but we didn’t get really great total organic content values, Blodgett said.
Some potential source rocks contain hydrocarbons within the oil window, Blodgett said. But thermal maturities suggest that the area is likely to be gas prone.
“The Paleozoics look to be over-mature to weakly over-mature overall,” Blodgett said. However, the Holitna Lowlands area remains substantially under explored — current knowledge of the area has derived from a handful of field studies, each lasting just a few weeks. We don’t have the structural history of the area worked out, Blodgett said.
“We’re still in the very first pioneer phases of getting geologic maps published,” he said.
And with a very thick sequence of relatively unaltered Paleozoic strata, the Holitna Lowlands area may yet yield some surprises.
