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Reservoir potential and a new oil source Fieldwork near Kamishak Bay reveals intriguing possibilities for oil exploration in Mesozoic rocks of the lower Cook Inlet region Alan Bailey Petroleum News
In a talk during the American Association of Petroleum Geologists Pacific Section annual conference on May 23, Marwan Wartes from Alaska’s Division of Geological and Geophysical Surveys described the results of some field surveying in the area of the Kamishak Hills, to the south of Kamishak Bay, on the western side of Alaska’s lower Cook Inlet. The research, conducted by a team of geologists from DGGS, Alaska’s Division of Oil and Gas, and the U.S. Geological Survey, has investigated upper Cretaceous rocks with oil and gas reservoir potential, and has brought to light intriguing evidence for the possibility of a new oil source in the region. Petroleum potential While Alaska’s upper Cook Inlet hosts operational oil and gas fields, the lower Cook Inlet to the southwest also has petroleum potential but remains hardly explored. The Tertiary strata that host the reservoirs for the Cook Inlet fields thin considerably in the lower Cook Inlet. But the older Mesozoic rocks of the Cook Inlet basin exist in considerable thickness under the lower inlet and include the Jurassic age Tuxedni group, the rock sequence that sourced the oil found in the upper Cook Inlet fields.
The DGGS team was particularly investigating the Kaguyak formation, a stratigraphic unit of upper Cretaceous age within the Mesozoic succession. The team examined rock outcrops at Swikshak Lagoon, to the south of the Kamishak Hills drainage, where the Kaguyak attains a total thickness of about one kilometer. The team determined that the formation has distinct upper and lower components. The lower component contains abundant fossils and exhibits properties indicating an origin on a shallow marine shelf. The upper component, in contrast, has sandstone layers that would have been deposited on a deeper marine slope or basin floor, Wartes said.
This sudden transition from shallow water to deep water sediment is also observed in rocks of upper Cretaceous age in the Matanuska Valley to the east and on the Alaska Peninsula to the west, suggesting a regional event in which the seafloor sank. The reason for this subsidence is unclear but may be related to the subduction of one of the plates of the Earth’s crust beneath the evolving Cook Inlet basin, Wartes commented.
Oil staining Particularly intriguing were some pieces of rock including some sandstone fragments that the field team found in a creek bed in the Kamishak Hills. These fragments were heavily oil stained. The team was unable to reach any in-situ rocks that might be correlated with the fragments in the creek. However, by mapping the water drainage into the creek, and by examining photographs of rock exposures, the team determined that the fragments probably came from the lower Kaguyak.
An analysis by the USGS of the oil in these rocks showed an origin from a type of plant material that could not be older than the late Cretaceous. Since the youngest known oil source in the region dates from the Jurassic, the Kamishak Hills find seems to indicate the existence of some hitherto unknown oil source. One possibility is that local volcanic heating has generated hydrocarbons in the rocks, Wartes suggested.
The upper part of the Kaguyak in the Kamishak Hills area tends to contain more rock fragments than the lower part and tends to be relatively tight, as well as showing evidence of being baked by a neighboring volcano. The oil stained rocks exhibited reasonable porosity and permeability. And rock samples collected from the COST No. 1 well, offshore in Cook Inlet, indicate that the Kaguyak does contain rock with very good reservoir potential, Wartes said.
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