|
New insights into Brooks Range foothills Fieldwork brings new geologic interpretations and determines that gas seep is thermogenic Alan Bailey Petroleum News
A team led by geologists from Alaska’s Division of Geological and Geophysical Surveys is continuing to piece together the complex story of the geology of the Brooks Range foothills. Perched on the southern side of the petroleum-rich Colville Basin that runs west to east across Alaska’s North Slope, the foothills offer surface exposures of rocks that lie deep under the basin — an understanding of the foothills area can provide valuable insights into the subsurface geology to the north.
DGGS geologists Marwan Wartes and Paige Peapples told the Jan. 19 meeting of the Alaska Geological Society about the team’s summer 2005 field season. In addition to the DGGS geologists the team includes geoscientists from Alaska’s Division of Oil and Gas, the University of Alaska Fairbanks. Veteran Alaska geologist Gil Mull has also been involved in the work. The work involves stratigraphic and structural studies that focus on geologic units with economic potential, with the ultimate goal of characterizing the petroleum system of the region, Wartes told the AGS audience.
“The Colville basin remains one of the most promising (petroleum) frontiers left in the whole of North America,” he said. Industry consortium An industry consortium interested in North Slope petroleum geology provides much of the funding for the foothills work. The U.S. Geological Survey also provides some funding through the StateMap program, a national initiative to assemble detailed geologic maps through state agencies.
Peapples explained that over a period of several years the field investigations have been moving westward through the foothills.
“Our strategy has been to march westward and, in doing so, refine models, refine our mapping and get the scale of our mapping down to an inch to a mile,” she said.
The team is now in the process of completing a two-year project, developing a geologic map of the Siksikpuk River area, just west of the Anaktuvuk River that flows north out of the center of the Brooks Range. Petroleum interest The existence of oil and gas leases in the Siksikpuk River area indicates a level of interest in an area long thought to be gas prone. The dolomites of the Lisburne group form one example of an interesting reservoir target, rich in solid hydrocarbons at the surface, Wartes pointed out.
And the team rediscovered a gas seep on the Colville River at Tiglukpuk Creek. The U.S. Navy first discovered the seep in the early 1940s and USGS resampled the seep in the late 1940s and early 1950s, Wartes said. Analysis at that time only determined the basic composition of the gas — it was found to mainly consist of methane. The DGGS-led team has now used modern carbon and deuterium isotope analyses to establish that the gas is thermogenic in origin, thus confirming the existence of a petroleum system in the subsurface.
“The structure’s not real clear in the area but (the seep) does appear to coincide with the broad crest of a very, very large anticline,” Wartes said. Complex structures Although in working the Siksikpuk River area the team has continued its detailed stratigraphic and petrologic studies, this year the geologists particularly investigated the geological structure of the area, Wartes said. There is clear evidence of multiple episodes of faulting and folding of rock strata, he said.
In general, a core of metamorphic rocks within the Brooks Range transitions northward into an area of sedimentary strata that have been sliced and shifted across a series of shallow-sloping thrust faults. These strata used to form a broad area of sedimentary rocks that geologists term the Ellesmerian, laid down in a south-sloping marine basin prior to the uplift of the Brooks Range. The strata telescoped into a relatively narrow zone, as a result of the thrust faulting.
To the north of the thrust belt lies an area of more gently folded strata. These strata are believed to have formed at the same time that the Brooks Range was emerging and they mark the southern edge of the Colville Basin. The Colville Basin itself formed as the Earth’s crust flexed downward under a load of sediment eroded north from the Brooks Range to the south.
However, there are several complexities within this overall structural setting. Peapples emphasized the need to understand the detailed changes in structure across the area, to gain better insights into the overall structure of the region. The team plans to publish its findings on this topic but has already delineated several important structural features such as the Desolation Creek fault toward the northern end of the area mapped and the Tiglukpuk anticline to the south. Gravity model To help with interpreting the overall structure, Rick Saltus and Phil Brown from the USGS conducted a ground-based gravity survey, collecting data at 500-meter intervals along a north-south line through the mapping area. Then the researchers constructed a computer-based gravity model, using an assumed geologic cross-section and rock densities derived from well data in the region.
“From this model they (turned) out a calculated gravity curve … and compared it to the actual measurements,” Peapples said. It was “a pretty darned good fit,” she said.
Peapples said that the most important finding so far from this particular research is that a major slice of rock known as the Endicott Mountain allochthon extends north to an escarpment known as the Tuktu bluff — farther north than had previously been thought. If you model the gravity without that slice of rock the fit is not nearly as good, Peapples said.
“Something has to be there,” she said.
The Endicott Mountain allochthon contains an excellent Triassic source rock and thick Mississippian carbonate rocks with reservoir potential. Detailed stratigraphy The team has spent quite a bit of time investigating the early Cretaceous stratigraphy of the area, Wartes said.
“It’s not well understood,” he said.
As part of its investigations the team has looked for evidence concerning the environments in which the various sedimentary rocks were deposited and for fossil evidence of the ages of the rocks. The fossils are particularly important in clarifying the stratigraphy of the region because rock characteristics tend to change laterally near the edge of the Colville Basin, thus rendering unreliable any stratigraphic correlations done by comparing rocks between different locations.
In doing its stratigraphic investigations the team has found what appear to be some new rock units — a unit containing volcanic debris near the base of the Cretaceous Fortress Mountain formation, for example.
In addition to fieldwork, the team is investigating rock samples by, for example, analyzing the chemistry of the rocks.
The intention of all of this work is to gain insights into the nature of the Colville Basin.
“We hope to characterize the basin configuration and in doing so try to determine the implications for more detailed sedimentation patterns,” Peapples said.
And that may lead to further new ideas.
“In a broad sense we’re trying to apply a lot of new ways to think about the Colville Basin,” Wartes said.
Peapples explained how the team is re-interpreting the Cretaceous mélanges found in the foothills. This type of rock contains blocks of other rocks that geologists have thought had been swept north from the emerging Brooks Range. Field evidence is now leading to some rethinking of the origin of these rocks. Kavik next Rather than continuing west from the Siksikpuk River, the team plans to do one inch to one mile mapping in the Kavik River area, at the eastern end of the North Slope, next summer. Continuing west would have presented issues with working in the Gates of the Arctic National Park.
And the Kavik River area is particularly interesting from a petroleum geology standpoint. The area contains an intersection of all three northern Alaska megasequences: the Ellesmerian, the Beaufortian and the Brookian, Wartes said. The area also contains a known gas field, the Kavik field, with estimated reserves of at least 165 billion cubic feet.
Meantime the team is completing its Siksikpuk River work and expects to publish the results of this at the end of 2006.
| 