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Targets tell a story
DOG exploration drilling targets categorized by play map, database out
Steve Sutherlin for Petroleum News
A recently released map and database of exploration drilling targets categorized by play type from the Alaska Department of Natural Resources’ Division Oil and Gas sent its researchers on a treasure hunt - for data.
Laura Gregersen and her associates postulated that drilling targets of historic exploration wells on the North Slope would shed light on exploration trends and on the thinking of explorers at the time of drilling, she said in a presentation to the Alaska Geological Society in Anchorage May 29.
“What was the drilling objective of the well,” she said. “What were the operators thinking at the time they had drilled the exploration well?”
In compiling the North Slope Map and Database of Exploration Drilling Targets Categorized by Play Type, North Slope and Offshore Arctic Alaska, which Gregersen co-authored with Garrett Brown, the authors and associates had to first find the data.
Oil fields can be a secretive place.
“We looked at public well histories, Petroleum News articles, any book, anything we could find - maps, anything we could find to track down what was the drilling objective of the well,” she said.
The resultant maps and database show the drilling targets categorized by play type for 548 exploration wells on the North Slope, Beaufort Sea and Chukchi Sea. It is designed to be a resource for explorers. The discovery well and production data for each producing pool are integrated into the study to address explorers’ questions.
Exploration target data are solely from public sources. The team did not use protected information from wells that are held confidential by the state, Gregersen said.
“When we pulled all of this together we took those objectives and we categorized them,” She said. “When you look at the history and you combine that with mapping them, you can see regional geologic trends.”
Wells have targeted five main stratigraphic play groups: Ellesmerian clastics and carbonates (Kekiktuk, Lisburne, Ivishak, Shublik, and Sag River), Jurassic shoreface sands (Barrow, Simpson, Kugrua, Nechelik, Nuiqsut, and Alpine), Cretaceous rift sands (Walakpa, Kuparuk, Put River, Kemik, and Thomson), Brookian turbidites (Torok, Seabee, and Canning) and Brookian topsets (Nanushuk, Tuluvak, Schrader Bluff, West Sak, Ugnu, Prince Creek, and Sagavanirktok). A sixth category called ‘Other’ includes the remaining targets (e.g., basement, methane hydrates, and gas hydrates).
The division has released a series of maps based on the project data, which can be downloaded from its website.
In addition to a North Slope overview map of exploration drilling targets of all play types, exploration drilling targets are also mapped in three major categories: Ellesmerian, Beaufortian, and Brookian.
The map of Ellesmerian exploration drilling targets includes the Kekiktuk Formation, Lisburne Group, Sadlerochit Group, Shublik Formation, and Sag River Sandstone.
The Beaufortian Exploration Drilling targets map includes the Jurassic shoreface and the Cretaceous rift.
The map of Brookian exploration drilling targets features the Brookian turbidite and Brookian topset plays.
The maps are accompanied by a database of exploration drilling targets categorized by play type, on Excel software, which also can be downloaded from the division’s web site, as well as a slide sequence from Gregersen’s presentation.
Exploration influences “Exploration was influenced by multiple things - land availability, pipeline access and key oil discoveries that influenced operator behavior,” Gregersen said.
“The story begins in 1943, the state of Alaska wasn’t a state yet, we were still a territory and we were in World War II at that time,” she said. “The federal government decided that north Alaska was no longer available for public entry so only the U.S. Navy was drilling wells at that time.”
The federal government had three periods of active exploration drilling: 1940s and 1950s; the mid-1970s to the early 1980s; and then also from 2007 to 2009, focused on the National Petroleum Reserve-Alaska and the nearby region, she said.
“Luckily for the state oil was discovered in Cook Inlet in 1957, which led to the first federal lease sale, our statehood and the first lease sale in 1964 - that the state held,” she said. “In 1968 Prudhoe was discovered, and in the following year we had more exploration than any other year in north Alaska.
“Prudhoe produced over 12 billion barrels to date, and the flowing year many discoveries were made - one of which is Kuparuk River which is has produced over 2 billion barrels.”
Discoveries rolled in, but a way to market was needed.
“Even though these big discoveries were occurring, drilling really decreased because there hadn’t been approval to construct the oil pipeline,” she said. “Luckily in 1973, it took a world event to make that happen; it took four years and then we had oil going down the pipeline.”
Growing land availability was a factor as well, boosting the pace of exploration drilling.
“We started to have access to our lands offshore, NPR-A became available, and the first OCS sale,” she said. “There have been multiple lease sales between then and now, and in 1999 the state of Alaska started offering our areawide lease sales, which we do every year; we offer our available lands in north Alaska for people to lease.”
Targeted Plays Most North Slope drilling activity has been along the Barrow Arch, “which has been the focus area which the hydrocarbons have migrated to,” she said.
In the first of the play categories, the Ellesmerian, “you have a series of non-marine to marine clastics, then you have carbonates or deposits in the Mississippian and Permian time frames,” she said. Sediments coming from the north were deposited on the south facing slopes, and at that time the shelf was relatively stable.
The second category, Jurassic shoreface is “represented by the beginning of the Barrow Arch starting to rise because of rifting and opening of the Arctic Ocean,” she said. The third category, “Cretaceous rift sediment are also deposited during this time, with the continued rifting and opening of the Arctic Ocean.”
“The fourth and fifth targets are the Brookian peroxites and Brookian turbidites,” she said. “You can see the turbidites are the toe slope prodelta deposits, and your topset including non-marine to shallow marine to deltaic deposits.”
She said 74% of 18 billion North Slope barrels that have been produced to date have come from the Ellesmerian, 28% has come from Cretaceous rift deposits or reservoirs, 2% came from the Brookian topset, 1% from Brookian turbidites, and 3% from the Jurassic shoreface.
The production makeup is evolving, she said. In 2018, of 187 million barrels produced, 48% came from the Ellesmerian, 20% came from the Cretaceous rift, Brookian topsets represented 12% of production, Jurassic 10%, and Brookian turbidites were 2% of North Slope production.
Discoveries beget exploration Significant discoveries in each play type have led to a flurry of drilling activity, Gregersen said.
Most of the early fields that were discovered were nearby Prudhoe Bay and there were two gas fields located in the foothills. In the late 70s to mid-80s you had discoveries in the Beaufort Sea.
Exploration activity targeting the Ellesmerian was greatest after the discovery of Prudhoe Bay in 1968 and continued at an elevated rate through 1986.
The Ellesmerian targeting map reaches “far and wide; the Ellesmerian has been targeted by folks trying to find other Prudhoe Bays,” she said. The Sadlerochit was the most targeted play in the Ellesmerian, the Lisburne was targeted along the Barrow Arch, and in NPR-A multiple wells in the foothills targeted Lisburne outcrops.
Many plays have been known, but not developed for years.
The Jurassic shoreface was the target in 1940s and 1950s by the U.S. Navy, with first production in the 1980s and 1990s. The Jurassic formation has produced up to 120,000 bpd.
After the giant Alpine field was discovered in 1994, the delineation going to the west occurred. Jurassic exploration continued from 1992 to 2015, spurred by the Alpine discovery.
Gas was discovered in the 1940s and the 1970s within the Colville Delta. The Nuiqsut sand was actually first discovered in 1985, Gregersen said. “It was known about but there wasn’t room for it (in the pipeline) so the exploration continued on.
“Greater Mooses Tooth Alpine was discovered in 2002, that is why I came up with Anadarko in the winter of 2001/2002,” Gregersen said. “It has taken a long time for development to happen so I’m happy about that - and happy to see that the Pikka has also discovered Jurassic (oil) in 2014.”
The late 1960s to mid-1980s found discoveries of gas and oil at Point Thomson, and around the Prudhoe discovery, Gregersen said. In the late 1980s to early 1990s oil was found in many existing units. The Burger accumulation was discovered in 1990 - another OCS discovery that has not been developed.
Cretaceous rift sands have regularly been a drilling target since the Kuparuk River field was discovered 1969.
“Most targets are called Kuparuk, but if you want to think back in time about what was going on, this is the subcrop under the LCU, and you can see that the Barrow Arch was rising and the sediments were coming off of the Barrow Arch and being deposited,” she said. “The recent discovery - still undeveloped - of Placer and Mustang are within the cretaceous group target.”
Brookian topset and turbidites were the earliest targets starting in the 1940s with spikes in exploration activity in every decade since 1964.
“Many discoveries by the U.S. Navy in the ’40s and ’50s, and more wells targeted the Brookian at that time,” Gregersen said. “There were peaks in the ’60s and ’70s the ’90s, and after the discovery of Nikaitchuq by Kerr McGee there was increased drilling effort by multiple companies in that region looking for a topset trend.”
Large recent discoveries at Willow and the Nanushuk topset play have garnered much attention for Alaska.
“We hope that this continues, that the trend keeps on going,” she said. “Most of the oil from the topset has been heavier oil but the nice thing about these recent discoveries is that they are lighter oil.”
After the 2018 cutoff for the study, there were more exploration wells targeting the Brookian topset than any other target, Gregersen said.
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