With the U.S. Department of the Interior calling for updated assessments of the oil and gas resources on Alaska’s North Slope, most surprisingly (and, perhaps, controversially) the tightly regulated 1002 Area of the Arctic National Wildlife Refuge (ANWR), geologists are preparing for the possibility of exploring a frontier believed to be rich in hydrocarbon resources.
Congress and the president must first agree to open ANWR’s coastal plain, which has remained essentially closed to the industry since 1980, and that hurdle might be difficult to clear even with a Republican-controlled administration. But if the president has his way, operators will get their first glimpse into the largest, unexplored and potentially productive geologic onshore basin in the United States.
That is why the Stinson No. 1 well, drilled in 1990 by ARCO (now ConocoPhillips), is beginning to draw interest. One of the closest wells to the 1002 Area at roughly 3 miles from its northwestern border, the well’s core samples and associated 2-D seismic data suggest it could be a major gateway to understanding and developing part of the 1002 Area.
“The Stinson No. 1 well is undoubtedly our key to understanding what the basement looks like near the northwestern border in the 1002 Area of ANWR,” said Robert B. Blodgett, AAPG Member and geological and paleontological consultant with Blodgett & Associates LLC. “It should be highly significant in light of the Trump administration’s push to develop the area for petroleum exploration.”
The Oldest Play in Alaska
Blodgett and Steve Sutherlin, a petroleum strategist with Strategic Action Associates, both based in Anchorage, began looking at the Stinson No. 1 well almost a year ago.
For nearly two decades, the well’s data was granted extended confidentiality because of its proximity to unleased acreage in the 1002 Area, but some of its core samples were made publicly available in 2008.
After carefully examining the data, Blodgett said it suggests two potential plays from reservoir rocks of the Eocene and Cambrian ages. If the Cambrian play is viable, this basement play would be from the oldest reservoir rock in Alaska.
“It is commonly believed that older rocks of Cambrian and Proterozoic ages have little potential for petroleum. However, this is not true,” Blodgett said. “Cambrian rocks in eastern Siberia, China and North Africa are good producers of hydrocarbons.”
Better understanding the lithostratigraphy of the rocks in the Stinson No. 1 well is key to understanding the basement rocks in the northwestern margin of ANWR and the possible Cambrian play there. Unlike the more weakly lithified sandstone reservoirs of the overlying lower Eocene strata that hold oil in the rocks themselves, the Cambrian basement reservoirs are brittle and fractured and hold oil in their cracks.
The northwestern part of the 1002 Area near the Stinson No. 1 well is an “undeformed” zone where the U.S. Geological Survey believes that anywhere from 3.4 to 10.2 billion barrels of technically recoverable oil may be stratigraphically trapped in rocks younger than the basement, according to its most recent 1998 assessment.
While it has commonly been accepted that the oldest reservoir rock in Alaska is of early Mississippian age, identifying reservoir rocks from the Cambrian age could be a game changer if a discovery is made, Blodgett said.
Blodgett explained that the basement rock of ANWR sits on top of the Barrow Arch and Mikkelsen High – geologically likely places to find oil, hence the drilling of the Stinson No. 1 well nearly 30 years ago. While ARCO made oil and gas discoveries in that well, bringing oil to market at the time was cost-prohibitive, as nearby infrastructure was not in place, Sutherlin explained.
In 2016, ExxonMobil brought the neighboring Point Thompson unit into production and constructed processing facilities and pipeline access to the northern terminus of the Trans-Alaska Pipeline System.
“This infrastructure has transformed the economics of the Stinson well by reducing costs and significantly reducing the time it will take to get oil to market,” Sutherlin said.
Blodgett believes that if a discovery were to be made, the oil would be economically viable at the current price of oil.
A drill stem test from the Stinson No. 1 well determined that a Cambrian play could produce as many as 800 barrels a day, and an Eocene play could produce as many as 700 barrels a day.
What We Know from Stinson
So far, data from the Stinson No. 1 well make Eocene and Cambrian plays in the 1002 Area look hopeful.
“There is beautiful oil staining present in the fractures in the older Cambrian play, which consists of fractured, pure orthoquartzite,” Blodgett said. “No analog to the Cambrian orthoquartzite is known elsewhere in northeastern Alaska, suggesting that the source area is to the north, farther offshore.”
Conventional wisdom suggests that the oil should source from the Paleocene and Cretaceous age strata. However, the presence of common bitumen at great depths in the basement rocks, well below the oil migration pathway that may have been provided by an existing lower Eocene unconformity, suggest that another older petroleum system is present, Blodgett said. Oil in analogous strata in eastern Siberia is sourced to the Cambrian and the upper Proterozoic age strata.
A 2-D seismic analysis by the late geophysicist Robert Klipping suggests that the Cambrian basement at the Stinson site is quite large, roughly 18 miles from east to west, located on the Barrow Arch and plunging to the southeast.
“A bounding high angle reverse fault near the south edge of the acreage alone would provide closure for the entire anticlinal trend,” said Sutherlin, who obtained the some of the seismic data from Donkel Oil and Gas, which currently holds the lease position on the Stinson well site. In other words, the play could extend into the northwestern part of the 1000 Area.
According to Klipping’s analysis, the Stinson No. 1 well also had very good oil shows in the base Eocene sand at the site of the unconformity, which is present in the vast majority of the acreage, Sutherlin added. Klipping suggested that an Eocene play extends into the 1002 Area.
A Second Opinion
While he agrees there is evidence of multiple oil charges into the Barrow Arch trap, David Houseknecht said Stinson is not the only well to offer clues into the basement of the 1002 Area, including its northwest corner.
Houseknecht is an AAPG Member and USGS senior research geologist who is overseeing the current North Slope assesments.
Roughly 20 wells have been drilled in the nearby Point Thompson field reservoir, offering more data to those looking to understand the basement of the 1002 Area, he said. From these wells, multiple phases of hydrocarbons have been encountered and thoroughly documented.
In fact, the oil leg found in the basement rocks at Point Thomson has API gravity as light as 18 degrees – which is among the lighter “heavy oil” accumulations on the North Slope, meaning it might be producible under the right conditions, Houseknecht said.
However, information provided by wells at both Point Thompson and Stinson might not paint the most accurate picture of any of the 1002 Area, simply because of the unique nature of its basement.
“The basement rocks have been badly beaten tectonically, Houseknecht said. “They are severely deformed.”
He explained that during the Devonian Period, a tectonic event cracked the rocks to the point where, if they held oil, the oil likely seeped out. Furthermore, the rocks were heated to temperatures that exceeded 125 degrees Celsius, likely breaking the hydrocarbon chains and reducing the oil to pyrobitumen and gas.
Hypothesizing aside, however, additional oil and gas samples are needed for geochemical analysis in order to have a better understanding of a possible oil charge in the basement rocks of the 1002 Area.
What’s Next?
Blodgett and Sutherlin are continuing to analyze core samples from the Stinson No. 1 well and the data from five neighboring wells to the west: Alaska State A No. 1, Alaska State D No. 1, Alaska State F No. 1, Alaska Island No. 1 and Challenge Island No. 1 – all of which have Cambrian basement rocks that might have fractured reservoir potential. All are located along the coastline of the Beaufort Sea.
They are currently examining the ages and environments of the basement rocks of these wells to get an idea of how far they might extend along the northern fringe of the 1002 Area. Confirmation wells need to be drilled in the offshore state waters with guidance from 3-D seismic data. 3-D seismic data also is needed inside the 1002 Area to help confirm either the Cambrian or Eocene play, or both.
According to Blodgett, operators are beginning to express interest in the area. The chance to explore ANWR’s coastal plain would be a once-in-a-lifetime opportunity for many geologists.
“The most enticing, unleased acreage near Stinson is the 1.5 million-acre 1002 Area,” Sutherlin said. “We feel we have taken the first step to understanding its northwest corner, and we are looking forward to further exploration.”