When Dave Houseknecht and the U.S. Geological Survey field party went into the field in summer 2019, it was to assess undiscovered oil and gas resources on North Slope state lands between the National Petroleum Reserve-Alaska and the Arctic National Wildlife Refuge.
Those state lands are vast, but not as vast as the Brookian source rocks that cross the Slope from NPR-A to the border of Canada and beyond, to the north and to the northeast, encompassing state, federal and Canadian offshore lands.
Senior research geologist Houseknecht and a multi-disciplinary team from the USGS gathered clues from Brookian source rocks, seeking out available well cores, outcrops, seismic data, geochemical testing and the like for an integrated database designed to more finely model the system, and how it interacts with reservoirs in the region.
The Brookian is the special focus of the USGS study - launched under a secretarial order signed May 2017 by former U.S. Secretary of the Interior, Ryan Zinke. Zinke directed the USGS to do a whole series of updated assessments; this - when released in January, will be the second in that series.
Breakthrough of understanding“We mainly focused on source rocks, reservoir rocks and the occurrence of oil,” Houseknecht said. “One of my colleagues, Kate Whidden, is leading our source rock analysis in particular. She and several other people have focused on the Brookian source rocks all the way across the North Slope, this year focusing on the area from Umiat all the way to the Canning River. That work includes age dating with uranium-lead analysis of zircon grains from volcanic ash beds, so we have a well-established set of ages for the Brookian source rocks; some of that work in the east was done the last two years, in and near ANWR,” he said. “That was the subject of a poster that Kate presented at the AAPG national meeting this year, (soon) to be released in the AAPG search and discovery series.”
The poster will include the ages the team has established, a lot of source rock richness, and a bit of the thermal maturity, Houseknecht said.
“The real breakthrough is that we’ve been able to correlate the different members within the Brookian source rock interval with well logs.” he said. “We’ve got some people who are measuring spectral gamma ray in the outcrop and that enables us to correlate outcrops, to gamma ray logs in exploration wells and to seismic data across the North Slope.
“That’s a real breakthrough, I think, in understanding the Brookian source rock,” Houseknecht said. “Obviously that work is only partially done, but we’re making a lot of progress, and that work will be published over the next few years as all the results are finished up.”
The picture sharpens“We had another crew working on the Brookian sequence stratigraphy in the state lands,” Houseknecht said. “I’ve been doing a lot of the seismic interpretation associated with that, and a couple of our people were working on understanding outcrop belts; we have a good grasp on depositional environments of the various facies that we’ve broken out.
“We can get a good handle on relating the depositional facies to the seismic signature and also to the petrophysical data that are available both from cores that we’ve been able to sample and from the wireline logs,” he said. “That’s advancing well; we expect that prospects on state lands where relatively little exploration has taken place will be focused on Brookian stratigraphic traps; we’ve been expending a lot of effort on understanding those relationships.”
Houseknecht said the team also has collected numerous oil samples from exploration wells and producing fields, to conduct advanced geochemistry on the oil and relate the oil back to the source rock from which it was generated.
“Our geochemist, Palma Botterell, is just about to submit a manuscript for publication that reveals a lot of the geochemical results,” he said. “She’s also given a couple of talks over the last few years and the sample set includes not only a lot of the old established oil fields from the North Slope but also a lot of the new discoveries including the Pikka and Horseshoe. That’s going to be very useful in the state lands because this summer we collected samples from a number of oil saturated sandstones - some of which we were aware of previously and had visited before, but we also discovered a couple of new outcrops of oil saturated sandstone that we were not aware of previously.
“Those samples will be analyzed and fit into the geochemistry framework that Palma has been able to establish, and that will enable us to interpret what source rocks the oils extracted from sandstone outcrops were generated from,” he said, adding, “All of this is part of putting together a three-dimensional framework of stratigraphy and geochemistry sequence stratigraphy and reservoir quality to prepare for conducting this new assessment.”
New outcropsHouseknecht said newly found oil-stained outcrops are west of the Sagavanirktok River.
“They’re actually in the western part of the state lands in the Tuluvak Formation, which typically has very good reservoir quality in the outcrop and the subsurface,” he said. “It’s been sort of an elusive reservoir - it’s only been the target of exploration drilling in a few wells; those wells were relatively older wells,” Houseknecht said. “A lot of companies over the last decade or so have been intrigued by the Tuluvak. Because it tends to be pebbly and relatively coarse-grained sandstone it tends to have somewhat better porosity and permeability then some of the other Cretaceous sands, so it’s very encouraging to find this oil saturation in two significant outcrops.”
Offshore source rocksBecause of the possibility of offshore source rocks charging state waters prospects, and even some onshore prospects, Houseknecht is looking out to sea, as far as offshore Canada, to understand where this offshore oil is originating.
“In terms of the seismic interpretation we are working offshore not only in the state waters but we’re going all the way out to the shelf edge, even though we don’t do the assessments beyond the state/federal boundary,” he said.
Houseknecht said there is evidence of an oil prone source rock offshore that does not have source rock quality on shore.
“We know that this tertiary source rock was the main source for the Kuvlum and the Hammerhead discoveries,” he said.
But there is, he said, also evidence that some of the oil saturated sandstones along the coast in and west of the ANWR 1002 area were charged by that same tertiary source rock.
“The tertiary source rock is almost certainly a distal equivalent of the Mikkleson Tongue of the Canning Formation,” he said. “The Mikkelson Tongue extends all across the northern part of the state lands but it typically is either not of source rock quality because it’s not rich enough, or it is relatively gas prone onshore.
“As you pursue that Mikkelson Tongue offshore we interpret that it becomes richer and more oil prone in terms of source rock quality,” he said.
The same thing happens on the Canadian side, Houseknecht said.
“The Canadian Geological Survey last year released a big report on the geochemistry of a well, one of the two wells closest to the U.S. boundary - the Natsek well,” Houseknecht said. “The Paleocene - Lower Eocene section has elevated organic carbon, so it’s richer than other rocks above and below, and it is relatively gas prone so it is really not an oil prone source rock - but, as you go farther offshore in the Mackenzie Delta that same source rock is thought to have generated most of the oil that has been discovered in the Mackenzie Delta.
“In other words, you can go back to publications 25 years ago and there was good geochemical evidence that this Tertiary source rock interval generated most of the oil that has been discovered,” he said. “The source rock, where we can sample it both in wells and in outcrops in Alaska is similar in character to the section in the Natsek. And, like the Canadian analog, it probably becomes a richer source rock farther offshore, and becomes more oil prone. So, we are interpreting at this point that the Kuvlum and Hammerhead discoveries - as well as some of the oil saturated sections along the coastline of eastern Alaska - were probably generated from the Tertiary source rock.”
“What we’ve been trying to do over the last several months is obtain samples from that well on the Canadian side so that we can do some advanced geochemical analysis to compare that with samples we’ve been collecting on the U.S. side.
“The rocks of this age on the U.S. side are the Mikkelson Tongue of the Canning Formation, and those rocks do not crop out very well in ANWR, but they do crop out farther west in the state lands - in fact one of the best outcrops we had a team working on this summer is in Franklin Bluffs just along the Sagavanirktok River,” Houseknecht said. “We can correlate those rocks outcrops in Franklin Bluffs all the way across the state lands to the east and across the ANWR coastal plain using seismic data. We know those source rocks are present but we have our doubts that they would be oil prone source rocks under the 1002 area, but evidence from Hammerhead and Kuvlum - and the oil saturated sands along the ANWR coast - suggests they do become oil prone source rocks not far offshore.”
The bottom line“The bottom line there is that the Brookian contains two distinct source rock intervals that are separated by rocks that are not source rock,” Houseknecht said. “At the base of the Brookian we have the GRZ, overlain by the Hue shale, and that is a rich source rock all across the North Slope wherever it is present.”
A section above, a younger source rock which the assessment team thinks is offshore, is physically separated from the older Brookian source rocks, at least in the onshore area, he said.
“Now whether or not they merge or coalesce into a single source rock interval offshore we don’t know, but we don’t think they do nearshore, based on seismic evidence and because the geochemistry of the oils that we’ve been able to extract from outcrops and the oils that we’ve analyzed from Kuvlum for example, are distinctively different, Houseknecht said. “In other words, the older Brookian source rock - the Hue, the GRZ - has a geochemistry that is different than the younger source rock up in the Paleocene and Eocene.”