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Chevron to explore for gas from Anna Following development oil drilling at Granite Point, Chevron looking at drilling untapped shallow gas horizons in aging Cook Inlet field Kay Cashman Petroleum News
In early March, Chevron will spud an oil well to further develop the lower Tyonek formation from its Anna platform in the Granite Point field, which began producing in 1967. The well is the first of a previously announced effort to maintain oil production from the company’s aging offshore fields in Southcentral Alaska’s Cook Inlet basin.
Upon completion of it and a second oil well, Chevron is considering a natural gas well off Anna, targeting shallow gas horizons in the Granite Point field that have never been drilled.
“As far as exploration, the only true exploration well we have in our plan (for 2008 in the Cook Inlet basin) … is off Anna in the Granite Point field,” Chevron’s Alaska General Manager John Zager said in November 2007, referring to the gas well.
He said Chevron’s plans call for “a couple of oil wells” to be drilled first, followed by the gas exploration well.
With gas “a more and more precious commodity” in Cook Inlet, “we’re focused on working to meet our ongoing obligations to our gas markets,” Zager said.
In November 2006 he talked about the gas exploration project in more detail, saying that Granite Point was a big structure, a four-way closure, and is in a good neighborhood for gas with the McArthur River, Beluga River and North Cook Inlet fields in the area. Zager also said Granite Point “has never really been drilled on top as a gas prospect,” and is risky for gas because it’s shallower than some of the other Cook Inlet fields.
Rig 428, a Chevron-owned rig, was moved to the Anna platform “over the last several months,” Chevron spokeswoman Roxanne Sinz told Petroleum News Feb. 7. “The Anna work is commencing after the completion of the McArthur River field Dolly Varden platform well D-47 workover in late January.”
Most of Chevron’s Cook Inlet activity will be development drilling, Zager said, because the object is an “increase in near-term production.”
Untested prospects The U.S. Geological Survey believes only 4 percent (1.3 billion barrels of oil through mid-2006) of the oil that theoretically generated from Cook Inlet source rock has been identified — a theory that has increased interest in, among other things, the deeper rocks where few drill bits have ventured.
The deepest vertical depth of a Cook Inlet basin oil well is approximately 12,000 feet, a state Division of Oil and Gas geologist told Petroleum News. According to Dan Seamount, a commissioner with the Alaska Oil and Gas Conservation Commission, companies will have to drill another 5,000 feet below known reservoirs to test the deeper sands.
In early 2005, Seamount talked to a joint legislative committee about what can be done to continue to extend the life of Cook Inlet’s aging offshore oil platforms, some of which have been shut in.
He described some possible ways to continue use of the platforms to maximize the amount of oil and gas extracted from the Cook Inlet basin, notorious for the complexity of its reservoirs, which often consist of multiple, thin sandstone layers that are difficult to find and trace.
Nonetheless, Seamount pointed out that there has been relatively little exploration drilling in the Cook Inlet compared with oil regions elsewhere in the United States. So the potential for large quantities of undiscovered oil under the inlet should cause people to hesitate before tearing down costly platforms that could play a useful role in future oil development.
“There’s still a good chance that a lot of that 96 percent is still under the drill bit,” Seamount said.
“We’d like to see the platforms used as exploration structures,” he said.
Seamount said that there are many known, untested Tertiary prospects accessible from the platforms. He particularly emphasized the existence of fault blocks, some of which lie right under the platforms — these blocks form fault traps that could contain what is known as “attic oil.”
“There are other fault blocks out there that can be accessed from the platforms,” Seamount said. “We could explore untested fault blocks and now, with new technologies and extended reach drilling, there are a lot of other identified prospects within reach.”
Oil in the Jurassic Seamount also thinks that there is oil in the Jurassic strata under the platforms, below the Tertiary rocks that form the reservoirs for all of the Cook Inlet oil and gas fields. Geologists have established that rocks of the middle Jurassic Tuxedni group sourced all the oil in the Cook Inlet fields. That oil must have migrated through or alongside potential reservoir sandstones that are late Jurassic in age and that are known to exist under Cook Inlet.
“There are very thick sands in the Jurassic,” Seamount said. “The oil would have had to have touched the sandstone within the Jurassic before it got up into the Tertiary reservoirs that have been exploited so far.”
So, there is a high-risk but potentially high-reward play for oil in the Jurassic sandstones, Seamount said. But few wells in the Cook Inlet have penetrated the Mesozoic strata below the Tertiary oil and gas fields. Of these wells, which were drilled by Unocal, only a handful drilled into the Mesozoic for more than a few hundred feet.
“I would recommend we use a non-utilized well bore under every single platform and drill another 5,000 feet below the known reservoir to see what’s down there,” Seamount said, noting that higher oil prices would help alleviate the economic risk.
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