What do the Lisburne oil field, the Jacob’s Ladder prospect and the Red Dog mine all have in common? Answer: they are all associated with the Lisburne group, a sequence of 280 million to 360 million year old rocks that underlies much of Alaska’s North Slope and the U.S. Chukchi Sea, and which outcrops along the northern side of the Brooks Range from Cape Lisburne into western Canada.
The Lisburne group consists predominantly of carbonate rocks, especially a rock called dolostone. Dolostone contains calcium/magnesium carbonates, rather than just the calcium carbonate of limestone, the more familiar form of carbonate rock. Carbonate rocks typically originate as fragments of marine organisms on subsea shelves.
At the Alaska Geological Society’s October meeting U.S. Geological Survey geologist Julie Dumoulin talked about the Lisburne group and the oil, gas and mineral resources of this intriguing assemblage of rocks. Dumoulin has been researching the Lisburne for more than 20 years.
Ellesmerian succession
The Lisburne group forms part of what geologists term the Ellesmerian succession, the oldest of the three major petroleum-bearing rock sequences of arctic Alaska. And the oldest of the Lisburne rocks occur in the western Brooks Range where they are Mississippian in age, Dumoulin explained. The strata become younger to the north and northeast, with much of the sequence in the northeastern Brooks Range being Pennsylvanian in age; in parts of the National Petroleum Reserve-Alaska the base of the Lisburne is in the Pennsylvanian and the sequence even extends up into the Permian.
The Lisburne strata formed on an ancient seafloor that shelved south from a landmass that lay at that time to the north of the current Beaufort Sea coast — the strata disappear at the northern side of the North Slope, in part because of the location of the ancient shoreline and in part because some of the more northerly rocks were eroded away at some time in the past.
In general a major break in the deposition of sediments marks the top of the Lisburne sequence, with erosion of the carbonate rocks having occurred in an ancient landscape.
“In parts of the northeastern Brooks Range subsurface there’s a huge hiatus of more than 40 million years,” Dumoulin said. “In some outcrop exposures there’s at least 20 meters of erosional relief.”
That erosion is significant to the petroleum geology of the region. A distinctive type of limestone topography called karst formed in places; karst is typified by caves, hollows and holes dissolved out of carbonate rocks by rainwater. The potential petroleum accumulation that Anadarko is exploring for at Jacob’s Ladder, southeast of Prudhoe Bay, probably lies in karst features in the Lisburne carbonates (see “New Lisburne play at Jacob’s Ladder” in the Nov. 13, 2005, edition of Petroleum News).
Changes east to west
Dumoulin said that in the northeastern Brooks Range Lisburne rocks of a somewhat uniform nature extend for hundreds of kilometers, suggesting deposition on a gently sloping submarine shelf. But, to the west things change markedly, with evidence of an irregular subsea topography consisting of a series of basins and platforms. That topography appears to have evolved during the deposition of the Lisburne sediments, as a result of distortion of the Earth’s crust at the time — Dumoulin displayed a seismic section which clearly showed that a thrust fault and an associated fold in the rocks had been actively moving during the deposition of the Lisburne sediments.
The irregular sea floor in the west has given rise to great variations in the thickness of the Lisburne group, from as much as 6,000 feet in one of the basins to less than 300 feet over one of the platforms, Dumoulin said. Major variations in water depth in the ancient sea have also given rise to large variations in Lisburne rock types and a very complex stratigraphy that varies from one part of the region to another.
And, just to add to the difficulties facing geologists who try to unravel the mysteries of the Lisburne group, the uplift of the Brooks Range during the Cretaceous and Tertiary periods caused huge slices of the Lisburne to slide northward many miles across the northern front of the mountains, to form what geologists call allochthons. Each allochthon tends to have its own distinct rock sequence and the allochthons themselves have a quite complicated classification and nomenclature.
“The important thing is that the Lisburne has different stratigraphic successions from one allochthon to another,” Dumoulin said.
Reservoir rocks
The good news from the perspective of petroleum geology is that many of the Lisburne dolostones exhibit porosities and permeabilities appropriate for oil or gas reservoirs. In fact PetroCanada has been researching the potential Lisburne dolostone reservoirs on the north side of the Brooks Range.
In one allochthon, the Endicott Mountains allochthon, an 80 to 100 meter thickness of Lisburne carbonates exhibits porosities as high as 11 to 13 percent, Dumoulin said. And dead oil (oil residue left after volatile hydrocarbons have escaped) in cavities in the northern Brooks Range dolostones provide evidence of an active petroleum system occurring at some time.
“There’s lots of dead oil in these rocks,” Dumoulin said. “… You see dead oil in thin sections (microscope slides), both in fractures and in smaller pores between dolomite crystals and then in smaller vugs.”
The reservoir for the Lisburne field at Prudhoe Bay occurs in carbonate rocks at the top of the Lisburne succession and has yielded about 3 billion barrels of oil and 3 trillion cubic feet of natural gas, Dumoulin said. At the Lisburne field the upper Cretaceous unconformity, a regional discontinuity in the stratigraphic sequence, cuts across the Lisburne strata — ancient erosion on that unconformity has contributed to the porosity of the reservoir.
But the carbonate rocks that form the Lisburne field reservoir are distinctively finer grained than the equivalent rocks on the northern side of the Brooks Range and formed in a different type of marine environment, Dumoulin said.
Lisburne source rocks
Good potential oil and gas source rocks occur in the Lisburne stratigraphic sequence, close to the potential dolostone reservoirs. Of particular note is the Kuna formation, a dark, shaly rock that attains thicknesses as great as 200 meters in the western Brooks Range and is associated with some of the deeper water of the ancient sea.
“More than two-thirds of our (Kuna) samples have (total organic content) values greater than 2 percent going up as high as 20 percent, so it’s an excellent potential source rock,” Dumoulin said.
The Kuna samples contain mainly type 2 kerogens that would support the generation of oil or gas. And the Lisburne source rocks along the northern Brooks Range have, in general, been heated in the past to temperatures that would cause natural gas to form. But there are considerable variations in the thermal maturities of the rocks, with rocks in some areas showing maturities more conducive to the formation of oil.
“Most of the Lisburne in outcrop and in subsurface is in the gas window, but there are parts of the outcrop that are actually in the oil window,” Dumoulin said.
The widespread occurrence of potential Lisburne source rocks has led some people to investigate the existence of oils that originated from these Lisburne sources, rather than from the younger source rocks of northern Alaska. It now appears that Lisburne sourced oil is somewhat similar to oil from the Triassic Shublik formation, a major oil source for the Prudhoe Bay field.
But Dumoulin cautioned about the uncertainties regarding the petroleum geology of the Lisburne, especially with respect to the timing of trap formation relative to the generation of oil and gas and the upheavals associated with the Brooks Range.
“There’s a lot of excellent source potential,” Dumoulin said. “There’s certainly lots of evidence of generation and migration of oil and gas, but the risk is going to come in the tectonics that may have destroyed any traps that there were.”
The Red Dog mine
The Red Dog mine, a world-class zinc resource with about 40 million tons of contained zinc and lead, sits in the western end of the Brooks Range. The massive sulfide deposits that contain the mine’s mineral resources lie in that same Kuna formation that forms one of the most promising petroleum source rocks of the Lisburne group.
In fact, the mine has been investigating the production of natural gas from the Kuna as a fuel source for powering the mine (see “State approves Sakkan unit at Red Dog” in the Nov. 5 edition of Petroleum News).
The Lisburne strata near Red Dog also contain vast barite deposits.
“Just one of the deposits in the Red Dog district at Anarraaq has more than a billion tons of barite. … It’s probably one of the largest barite deposits in the world,” Dumoulin said.
It seems that these various mineral deposits formed on the sea floor, in water that was likely low in oxygen, or perhaps lacking in oxygen. Radiometric dating of the mineral deposits indicates an age of 338 million years. That age matches fossil evidence for the age of the associated rocks and suggests that mineralization occurred at the time that the sediments that formed the rocks were deposited.
Phosphates
Rocks containing phosphates are widely distributed in the Lisburne group. These phosphatic rocks can be up to 12 meters thick and occur in association with the organic-rich potential petroleum source rocks, Dumoulin said. Intriguingly, fossil evidence indicates that all of the phosphatic rocks formed within the same very precise time interval towards the end of the Mississippian period. And that time period also correlates with the formation of the mineral deposits in Red Dog district.
This coincidence of different types of mineral formation points to some significant event in the geologic history of the region. Dumoulin thinks that upwellings of ocean water occurred in the area at a time when the climate was becoming increasingly arid. Upwellings of this type in contemporary oceans form zones rich in organic material; the over abundance of organic nutrients stresses carbonate forming organisms.
“There were all kinds of things going on at this time — the mineralization in the Red Dog district, the formation of the phosphate and you have the final drowning of the carbonate platform in this part of the Lisburne,” Dumoulin said. “We think all these things are tied into a high-productivity upwelling regime.”
But the complications that result from the upheavals of the Brooks Range make reconstruction of the regional geography at the time when the Lisburne was forming extremely challenging. One geologist has tried to unravel this geography by estimating where the various Lisburne allochthons slid from as the mountain range emerged. The reconstructed distribution of Lisburne rock types then enables the locations of the ancient sea basins to be inferred.
That reconstruction points to mineralization in the Red Dog district near some basin margins, presumably where the upwelling of the ancient ocean occurred.
