State team assesses oil reservoir potential on west side of Cook Inlet

Having indications of a petroleum system and being the site of some exploration drilling in the early 1900s, the Iniskin Peninsula on the west side of Alaska's Cook Inlet has long intrigued oil explorers. And, as part of a multi-year program of research into the geology of Cook Inlet, led by Alaska's Division of Geological and Geophysical Services, a team of geologists has been conducting field studies in and to the north of the peninsula. DGGS has now published the results of fieldwork that the team conducted in 2014.

The results shed some new light on the petroleum potential of rocks of Mesozoic age in the Cook Inlet basin. While all of the producing oil and gas fields of Cook Inlet have reservoirs in rocks from a younger Cenozoic sequence, geologists have speculated on the potential for finding hydrocarbons in the older Mesozoic rocks, which are generally buried deep below the prolific and shallower Cenozoic strata.

Mesozoic rocks, including the rock units that sourced the oil found in Cook Inlet oil fields, are exposed at the surface around and near the Iniskin Peninsula.

The DGGS-led research has been characterizing the petroleum potential of both the Cenozoic and Mesozoic rocks of the Cook Inlet basin. By conducting independent research and making its findings public, the agency hopes to encourage oil and gas exploration in Alaska, while also providing useful data to companies interested in the potential of regions such as Cook Inlet. The work is being funded by the state of Alaska and the U.S. Geological Survey. Apache Corp. has also provided funding for the research program.

Tuxedni group

During the 2014 field season, geologists examined the Red Glacier formation, a rock unit that occupies the base of the middle Jurassic Tuxedni group and that outcrops on the east side of Iliamna Volcano, to the north of the Iniskin Peninsula. Previous geochemical analysis has confirmed a view that the Red Glacier formation has been the primary source of Cook Inlet oil, the DGGS report says. In outcrop, the DGGS-led team measured a total thickness of 681 meters for the formation, although the lower 150 meters to 200 meters did not prove accessible because of steep terrain, the DGGS report says.

The lower part of the section that DGGS measured includes massive sandstone beds, with sand grains exhibiting evidence of a volcanic source. The middle part of the section includes 573 meters of laminated mudstone, with features that indicate deposition in a deep marine environment with an abundance of organic material. The rocks contain ammonite, belemnite and bivalve fossils. Sandstone dikes and sheet-like accumulations of a secondary mineral cut across the mudstone layers.

The upper part of the Red Glacier formation in the measured section consists of 41 meters of a light-brown weathered rock containing varying amounts of sand and silt.

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The Gaikema sandstone, another rock formation within the Tuxedni group, directly above the Red Glacier formation, is well exposed in the northern part of the Iniskin Peninsula where it ranges in thickness from 150 to 260 meters. The rock exhibits evidence of being deposited from an ancient river delta and contains plentiful fossil fragments, the DGGS report says.

The Gaikema sandstone is of interest as a potential petroleum reservoir rock, especially given the fact that it lies directly over the Red Glacier formation, a presumed source of Cook Inlet oil. An analysis of samples of the sandstone collected from the field indicates that the rocks may have been buried in the past to depths of 4.5 to 7.7 kilometers. These findings and further investigations should provide insights into the potential of the Gaikema as an exploration target, the report says. However, initial impressions are that cementation of the sand in the rock has resulted in only minimal potential for the rock acting as a conventional hydrocarbon reservoir. But clay content in the rock could provide potential for the discovery of "tight gas," gas that is trapped within the rock lattice but that cannot readily flow, the report says.

Naknek formation

During the 2014 field season, the DGGS-led team also examined surface exposures of the upper Jurassic Naknek formation, another potential Mesozoic hydrocarbon reservoir. The Naknek is well exposed along the north side of Chitina Bay, to the north of the Iniskin Peninsula. At Chitina Bay a lower sandstone unit, 317 meters thick and composed of very fine to fine sand grains, forms light-gray-weathering cliffs. The sands appear to have been deposited in a near-shore setting, the DGGS report says.

At Hickerson Lake on the flanks of Iliamna Volcano the team found a coarse conglomerate in the Naknek formation. The channel-shaped geometry of this conglomerate body suggests that it resulted from the infilling of an ancient submarine canyon, a canyon that may have acted as a conduit for coarse sediment to flow into a marine basin floor. This interpretation of the field observations provides a model for the prediction of where effective reservoir rocks may exist within the Naknek formation, the report says.

Because of the relatively poor permeability and porosity generally observed in the Mesozoic rocks of the Cook Inlet basin, some geologists have suggested that fracturing within the rocks may be important in identifying potential hydrocarbon exploration plays. With that in mind, the DGGS-led team has been measuring and assessing geologic faults and rock fracture patterns observed in the Iniskin Peninsula area.

The patterns discovered are providing insights into both the deformation history of the Cook Inlet basin and into possible fluid migration paths in the subsurface of the basin, the DGGS report says.

The scientists involved in the Cook Inlet research are still conducting laboratory analysis work on rock samples gathered during the field work and will publish the results of this work in due course.

This story originally appeared in Petroleum News. It has been republished with permission.

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