In 1957 Richfield Oil Co. discovered the Swanson River field in Tertiary sediments on Alaska’s Kenai Peninsula. This event launched the oil and gas industry around the Cook Inlet. The discovery also caused almost all subsequent exploration in the area to focus on the relatively young Tertiary rocks of the region.
But there’s a major sequence of older sediments underneath the Tertiary. Could these older sediments also hold oil and gas?
Deposition of sediments in the Cook Inlet basin started way back in the Triassic period, at the beginning of the Mesozoic era more than 200 million years ago. At that time the Pacific plate of the earth’s crust started sliding under the crust along the line of the present-day Alaska and Aleutian Ranges. The sinking plate caused a trough to form along the general trend of the present-day Cook Inlet. Then from the Triassic through the subsequent Jurassic and Cretaceous periods huge thicknesses of marine sediments accumulated in this trough.
The end of the Cretaceous period about 65 million years ago marked the end of the Mesozoic era and the start of the Tertiary era. During the Tertiary the Cook Inlet area emerged from the sea to become a low landmass that gradually sank under enormous quantities of river borne sands washing down from surrounding highlands. These Tertiary sands provide the reservoirs for all of the known oil fields in the Cook Inlet basin.
Rich Nelson, a veteran petroleum geologist with several decades of experience in Alaska, told Petroleum News that the people who discovered the Swanson River field had really expected to find oil in the marine Mesozoic sediments.
“(At Swanson River) they were looking at 3,000 to 4,000 feet of Tertiary cover and then getting into Mesozoic rocks, which is where they saw the potential,” Nelson said. “What they found was almost 12,000 feet of Tertiary sediments and the oil had migrated into the lower part of those sediments.”
Very few later wells attempted to drill into Mesozoic, Nelson said.
“Historically within the basin if you got to the (Mesozoic rocks) there has been no intention of drilling into them,” he said.
What’s the potential?
So why might you expect to find oil in the older rocks?
Since early in the last century people have known about oil seeps from the Jurassic and Cretaceous rocks, where these rocks outcrop along the edges of the Cook Inlet basin. In addition, there’s general agreement that the characteristics of the oil in the Tertiary of the Cook Inlet indicate a source in the so-called Tuxedni group of the Middle Jurassic.
But although the oil migrated into Tertiary rocks from the Mesozoic, there’s a total absence of Mesozoic gas in the Tertiary — the gas that’s produced in the Cook Inlet and on the Kenai Peninsula originated from the coal seams and other organic-rich sediments in the Tertiary strata.
Nelson believes that the absence of Mesozoic gas suggests that both gas and oil accumulated in Mesozoic reservoirs long before the oil migrated into the Tertiary. The young age of the structure of some of the Tertiary reservoirs also supports that idea — the oil must have migrated into these structures many millions of years after the oil formed.
If you look at Middle Ground Shoal and McArthur River, they have structures that were probably formed in the last 3 million to 5 million years, Nelson said.
“That means that the source and those older reservoirs … have to be buried much deeper … 25,000 to 30,000 feet and at higher temperatures,” Nelson said.
So there’s a high probability that oil still lies in some of these older reservoirs, Nelson said.
Possible reservoirs
And geologists have identified several potential reservoir rocks within the Mesozoic.
Veteran oil and gas geologist Bob Warthen told Petroleum News that the middle Jurassic rock sequence includes sandstones and conglomerates that could act as reservoirs. There are also potential reservoirs in the Upper Jurassic and Cretaceous, Warthen said.
Warthen commented that in the southern part of the Kenai Peninsula pre-Tertiary rocks slope up under the flatter Tertiary strata.
“You could have a large stratigraphic entrapment in that area, stretching from let’s say the Homer area all the way through North Fork up toward Happy Valley and that area,” Warthen said.
However, Warthen thinks that any oil in the southern Kenai Peninsula and the Lower Cook Inlet would source from an area in the southern part of the Inlet, rather than coming from the more northerly source that fed fields like Swanson River.
But any reservoir evaluation needs to take into account the possibility that deep burial of the Mesozoic rocks has degraded their reservoir characteristics. For example, rising temperatures at depth can bake and modify the rocks — a process known as metamorphism.
However, Nelson thinks that with a low thermal gradient in the Cook Inlet basin there’s unlikely to be pervasive metamorphism in the Mesozoic.
“Our temperature gradient here is about one degree Fahrenheit for every 100 feet of depth,” Nelson said. “That’s how you can go pretty deep into the section without really worrying about cooking it.”
Compaction of the rock at depth will reduce both the porosity, the ability of the rock to hold oil, and the permeability, the ability of the rock to allow oil to flow. But Nelson thinks that the porosity of the Jurassic sands, for example, should be quite reasonable and that fracture systems in the rock should provide permeability. Nelson cited evidence from one well that penetrated some Jurassic rock with low permeability — the drillers measured a flow rate of 850 barrels a day of salt water, presumably out of rock fractures.
Growths of secondary minerals in the rocks may prove a problem: an exploration well in the Lower Cook Inlet found mineral growths known as zeolites in Mesozoic sandstone. By blocking the gaps between pores in the rock these zeolites reduce the permeability.
Zeolites crystallize from mineral laden water. U.S. Geological Survey geologist Les Magoon told Petroleum News that the abundance of a mineral called feldspar in the Cook Inlet sediments results in a chemical mix that’s likely to create a particular type of zeolite.
“It depends on certain pressure and temperature regimes and also on feldspar-rich sandstones,” Magoon said. However, “you can’t condemn the entire section based on just one well,” he said.
And Nelson doesn’t think that there’s any reason to assume that this type of mineral formation is especially widespread. The need for specific temperature and pressure conditions and the potential for oil to displace mineral-laden water would both limit the formation of secondary minerals, he said.
Few wells
With few wells drilled into the Cook Inlet Mesozoic, much of the debate about potential oil reservoirs remains speculation. Outside of the fields we’ve probably got less than 20 penetrations where we see Mesozoic within the basin and only a couple of those go very far, Nelson said.
In the early 1900s a well on the Iniskin Peninsula, on the west side of the Cook Inlet, opposite Homer, produced about 50 barrels a day of oil from the Cretaceous, Nelson said. He also said that a well that Chevron and Richfield drilled south of Swanson River found a little oil in the top of the Cretaceous.
However, Nelson thinks that oil found in the Jurassic at Trading Bay probably flowed back into the Jurassic rocks from the Tertiary rocks.
“There’s been at least two wells at Trading Bay that have drilled into the top of the Jurassic and produced some oil,” Nelson said. “Now whether that is oil that has come directly there from the Jurassic ... I’m a little skeptical on that because those are pretty ugly rocks.”
Magoon thinks that the Starichkof well near Homer found some oil in the Mesozoic.
“There were some turbidite sands down there in the Cretaceous that had some oil in them,” Magoon said.
And according to a Minerals Management Service report, two exploration wells in the Lower Cook Inlet encountered oil pools in Late Cretaceous strata.
However, with a wide scattering of wells offshore in the Lower Cook Inlet, Magoon is skeptical about making a major oil find south of Kalgin Island, in the middle of Cook Inlet.
“My feeling is that from south of Kalgin if there were some big oil accumulations either in the Tertiary or the Mesozoic they most likely would have been found by now, whereas in the Upper Cook Inlet the Tertiary oil has blinded everybody to the Mesozoic,” Magoon said.
Exploration challenges
So what are the challenges for anyone interested in exploring in the older rocks of the Cook Inlet?
Lack of data probably presents the biggest difficulty.
Surface Mesozoic rock exposures only occur at the edges of the basin; folding and faulting of the strata coupled with the potential for lateral changes in the nature of the rocks make estimations of the geology in the center of the basin extremely unreliable. And the shortage of wells limits well control of the subsurface geology.
A lack of detailed, deep seismic subsurface information compounds this problem: Magoon said that the relatively steep dips of the rocks in the basin and weak stratification make it difficult to shoot good seismic.
“So you don’t always get the energy back that you want,” Magoon said. “The seismic data is less than clear cut.”
Warthen thinks that a shift of attention from the Cook Inlet to the North Slope following the discovery of Prudhoe Bay has limited the use of modern seismic techniques in the Inlet. And Nelson believes that modern 3-D seismic could help clarify the deep Mesozoic structures.
However, to really find out what’s going on down in the Mesozoic someone’s going to have to take some risk and drill a deep hole.
And Nelson feels confident that the oil’s there — it’s all a question of finding an economic accumulation.
“For me I can’t see any alternative to it, it has to be down there,” Nelson said.
