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Geologists offer insights into geology of Brooks Range foothills The Alaska Department of Geological and Geophysical Surveys' NPR-A-Foothills program is making some exciting discoveries near the Haul Road Alan Bailey PNA Contributing Writer
A team of geologists working near the Haul Road on the north side of the Brooks Range is piecing together the detailed geology of some of the rocks that underlie much of the North Slope. The investigations are yielding new information about potential oil and gas source rocks and reservoirs.
“What we're trying to do ... is collect baseline geological data in the Foothills belt, north of the Brooks Range, south of the NPR-A (National Petroleum Reserve-Alaska), that are important for understanding the oil and gas resources of the area,” Dave LePain, NPR-A-Foothills program leader in the Division of Geological and Geophysical Surveys, told Petroleum News Alaska.
LePain leads a team consisting of Rocky Reifenstuhl, Ellen Harris and Paige Peapples of the Alaska Division of Geological and Geophysical Surveys and Gil Mull of the Alaska Division of Oil and Gas — Mull was one of the geologists who discovered the Prudhoe Bay field back in the 1960s. This core team is collaborating with several other geologists.
Industry provides much of the funding for the program.
“Our operational support ... and most of our analytical budget ... comes from an industry consortium that we've developed over the past five, six, seven years,” LePain said. Mapping and stratigraphy The program involves detailed geological mapping combined with investigations of the rock stratigraphy.
“We usually map in the area where we're doing the detailed stratigraphic studies — the two kind of go hand in hand,” LePain said.
And the study takes a particular interest in rocks that show potential economic value.
“We focus in on selected stratigraphic units that have economic significance, either as reservoirs or as source rocks,” he said.
Geological mapping by the DGGS team also feeds into a federal program called StateMap, in which the U.S. Geological Survey is assembling detailed maps through state agencies. The StateMap program enables some federal funds to be applied to the mapping component of the DGGS work.
“Every year we submit a project proposal and a budget to the StateMap committee ... for funding,” LePain said. DGGS alternates between energy and mineral related StateMap projects in successive years.
“All of the energy state maps have been up on the North Slope and over the last five years or so they've all been in the area that the NPR-A-Foothills program is working,” he said. “So we use StateMap moneys and our industry consortium moneys to try to leverage each other.”
The Foothills belt The study area sits in the Foothills belt of the Brooks Range, where rocks that lie under the North Slope become exposed at the surface.
Uplift and overthrusting of the Brooks Range more than 100 million years ago loaded and depressed the earth's crust to the north, forming the so-called Colville Basin that extends east to west under the North Slope. Erosion of the newly formed mountains dumped huge quantities of sand, silt and other debris into this basin over an extensive time period.
Later movement in the Brooks Range pushed the rock strata up to the surface along the north front of the mountains. As a result, the Foothills area affords excellent surface access to strata that are buried underground elsewhere. By detailed investigation of the surface exposures, the study team can provide valuable insights into the geology for people doing seismic exploration.
“What we're looking at is basically the outcrop equivalents of the units that occur in the subsurface to the north,” LePain said. “... by looking at them in outcrop you're able to get information that you can't get when you're limited to just the subsurface ... so by providing outcrop details to the industry groups that are up there actively exploring, they can integrate that outcrop data set with their subsurface data and come up with a much better understanding.” Oil-stained sands near the Haul Road The team of geologists has found widespread outcrops of oil-stained Lower Cretaceous sandstones in the area west from the Haul Road to Chandler Lake. In that area, deepwater sands grade upward into shallow water or non-marine sands. Many of these sands exhibit excellent reservoir potential — some U.S. Geological Survey geologists have even suggested that the oil staining provides evidence of a breached, pre-existing oilfield.
“You've got these deepwater sands and shallow marine sands that have good indications of hydrocarbons having been in them in the past, so these are definitely potential reservoir rocks in the subsurface to the north,” LePain said.
And there's an abundance of shales which could provide a rich source of hydrocarbons.
“Gil Mull's ... taken the lead on the source rock part of the stratigraphic study, focussing in on ... a unit that we call the Otuk formation,” he said.
The Otuk formation, a deepwater deposit exposed just west of the trans-Alaska pipeline, a short distance north of the Brooks Range front, contains organic-rich shales and shaly limestones.
“Some of these have upwards of 10 to 15 percent total organic carbon, so they're rich marine source rocks and they're correlative with the Shublik,” LePain said.
The Shublik is a stratigraphic unit that people recognize in the subsurface geology throughout the North Slope and that forms the source rock for much of the oil in the Prudhoe Bay area, he said.
Although people generally think that the thermal history of the rocks in the Foothills belt favors gas generation, Mull's analyses of the Otuk rocks reveals intriguing evidence of the formation of oil.
“For a long time the Foothills belt was largely regarded as a gas province ... Gil's work on these Otuk exposures suggests that that's probably the case,” LePain said. “But some of his geochemical results are showing that there might be more oil potential than was originally recognized — that's very economically significant.” Oil indications east of the Haul Road The area east of the Haul Road between the Sagavanirktok and Ivishak Rivers is yielding some equally interesting finds. The rocks in this area post-date the rocks to the west and generally exhibit the depositional features of deeper water.
“The Colville Basin, north of the Brooks Range, was filled from west to east, so the older rocks are in the central part of the basin and the western part,” LePain said. “As you go east, the rocks get younger and you get a predominance of the deep water facies.”
The NPR-A-Foothills team has found a well-exposed 3,900-meter rock sequence on an unnamed drainage east of the Haul Road — the geologists are trying to correlate this sequence back to the detailed stratigraphy that they have established to the west. The fossils in the eastern sequence indicate an Upper Cretaceous age.
And there's ample indication of oil formation in these younger rocks.
“There's oil-stained sands in the base of that section ... and then there's also some brown paper shales that are probably correlative with the Hue shale gamma ray zone,” LePain said. The Hue shale gamma ray zone is a very prominent North Slope subsurface log marker that people think to be an important hydrocarbon source rock.
Indeed, the paper shales that the team has discovered contain substantial organic material and may have generated oil that can be detected in an adjacent layer of a type volcanic rock called a tuff.
“(There are) volcanic tuffs in the same part of the section and they just reek of oil,” LePain said, “... you break them open and they smell like the floor of a gas station.” Assessing the reservoir potential Following some initial funding from the Division of Oil and Gas for laboratory work, the team has supplemented its fieldwork with tests on rock samples, to determine the reservoir potential of the various sandstones in the study area. The lab tests measure the porosity and permeability of the samples — the porosity determines how much fluid the rock can hold, while the permeability provides a measure of how easily fluids can flow through the rocks.
“So now what we do routinely when we're out in the field is we collect the samples from selected sand units and then we send off a limited suite for laboratory analysis,” LePain said.
Team member Rocky Reifenstuhl has been assembling the results of the tests and also looking at microscope slides of the rocks, to determine the internal features that give rise to the porosity and permeability.
LePain pointed out that weathering can make porosity and permeability measurements of surface exposed rocks misleading. However, he believes that the results of the lab tests yield generally useful information for assessing the reservoir potential at depth.
Future direction So how will the NPR-A-Foothills program progress in the future?
When the program started about four years ago the team planned to investigate the surface geology all the way from the Haul Road to the DeLong Mountains at the far western end of the Brooks Range. However, the discoveries in the rock outcrops in the area near the Haul Road have caused the team to focus on that area rather than moving further west.
“We've kind of become bogged down in the eastern part of that whole trend in the Chandler Lake, Sag River area, because there's so much geology there of economic significance,” LePain said. “... each year as we think about the upcoming field season we can't really justify moving further out to the west, because there's so much left to be done.”
With the program now entering the fifth year of what was initially a five-year plan, the project team will need to assess the future direction. The program could move west, as originally envisaged. However, economic interest in the geology east of the Haul Road and the possibility of the coastal plain of the Arctic National Wildlife Refuge opening for exploration could favor an eastward move.
So, future direction will depend in part on where industry exploration goes — the program team will seek input from its sponsoring industry consortium and the Division of Oil and Gas.
Meanwhile the NPR-A-Foothills team continues to extend the knowledge of rocks that may turn out to be critical in future oil and gas discoveries.
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