Field trip gets close to the rocks

Cook Inlet Outcrops Exceptional

Exploration interest in Alaska’s Cook Inlet Basin is on the rebound – or so it seemed from the mood of the recent AAPG Pacific Section annual meeting held in Anchorage.

The oral session on “Oil and Gas Fields of the Cook Inlet, Alaska” was standing-room-only for several of the talks – and the post-meeting field trip “Sedimentology, Reservoir Quality and Tectonic Setting of Late Miocene-Early Pliocene Gas-Bearing Formations, Upper Cook Inlet, Alaska” was a sellout.

The trip was led by David LePain, sedimentologist at the Alaska Department of Natural Resources, Division of Geological and Geophysical Surveys in Fairbanks. He showcased the effects of compressional tectonics in a forearc basin with a fascinating tour of the Seward and Sterling highways and Kenai Peninsula geology.

There was excellent weather (unusual for spring in south-central Alaska) and the rocks didn’t disappoint.


With clear blue skies and temps in the 50s, the group of 25 started in Anchorage en route to Homer, 220 miles away near the southern end of the Kenai Peninsula. Mesozoic and Tertiary outcrops on the eastern side of the Cook Inlet were the focus of three days in the field.

Although the trip’s main objective was outcrop examination of the onshore equivalent of Miocene-Pliocene reservoirs productive in the basin, the clear skies enabled long distance visibility (50-plus miles) and appreciation of surrounding terrain.

Clearly visible were the mountains of the Aleutian-Alaska Range, home to Mount McKinley (Denali), which mark the northern extent of the Aleutian Island Arc. All four snow-covered active volcanoes along the western basin margin and within the Alaska Range were visible. From north to south, they are Mount Spur, Mount Redoubt, Mount Illiamna and Mount Augustine.

Located at the compressional boundary between the down-going Pacific Plate and the North American Plate, the Cook Inlet is a tectonically active forearc basin. It trends NE-SW parallel to the trend of the inlet and is bounded by regional faults on both sides that extend for hundreds of miles.

Since 1957, 23 gas fields and eight oil fields have been discovered in the basin. These have provided energy to fuel the local economy and growth of south-central Alaska, the most populated part of the state.

In 1968, Prudhoe Bay field was discovered on Alaska’s North Slope, and as a result the exploration program slowed considerably – exploration dollars were diverted from the Cook Inlet to exploration and production on the North Slope.

Since then large areas of the Cook Inlet basin have remained unexplored. Approximately 95 percent of the existing fields in the basin were found pre-1970.

The 2004 South Central Alaska Natural Gas Study concluded that the resources found to date are less than half of what may actually be there. An estimated ultimate recovery (EUR) of approximately 8.5 TCF has been produced with, as of 2004, 1.8 TCF proven and remaining.

They conclude that a total Cook Inlet resource endowment of 25-30 TCF OGIP might be more reasonable. The EUR for oil is reported to be 1.4 BBO.

Many in the industry think these numbers are conservative.


Exceptional exposures of the Mesozoic-age Chucagh terrain accretionary complex were visited on the first day. Bright sunshine and snow-covered peaks highlighted the dramatic relief along the Seward Highway, which follows the north shore of Turnagain Arm, just outside Anchorage.

(Turnagain is named after the action of Captain James Cook in 1778. He was not successful at finding a Northwest Passage back to Europe through the inlet waters and had to “turn again.”)

Seward Highway exposures included highly sheared conglomerate and greywacke of the McHugh Complex and near vertical, highly compressed and sheared turbidite beds of the Valdez Group. The turbidites are interpreted as proximal channel deposits that occur near the base of slope.

Spectacular deformed beds were visible in outcrops of the trench-filling Valdez Group, including slaty cleavage in shales, smeared shale beds and intensely fractured tabular sandstone beds. These deposits, believed to be Maastrictian to Campanian in age, were scraped off the seafloor as the Pacific Plate subducted, and uplifted as if on a regional conveyer-belt.

Both the McHugh Complex and the Valdez Group comprise the most impressive Chugach terrain.


The stratigraphic record of the Cook Inlet Basin includes a thick Mesozoic succession overlain by nearly 26,000 feet of Tertiary section. All significant petroleum production to date has come from Tertiary age reservoirs in upper Cook Inlet.

The stratigraphy of the Sterling Formation reservoirs are exposed in shoreline bluffs several hundred feet thick near Clam Gulch and the town of Ninilchik. They contain tabular sand bodies with conglomeratic lags deposited in braided channels, lignite beds and abundant sedimentary features.

Broadly lenticular sand bodies of the Beluga Formation are exposed in the bluff northwest of the town of Homer.

The sands in both formations are largely well-sorted and unconsolidated with abundant sedimentary structures. Dewatering features such as disturbed bedding and flame structures are particularly abundant, raising the question of their origin – they may have been generated by natural fluvial depositional processes or perhaps generated by regional seismicity common to this tectonic setting.


Overall the Cook Inlet Basin oral session and field trip left many with a greater appreciation of this tectonically active basin, its oil and gas potential and the quality of future potential reservoirs.

Thanks to the Alaska Geological Society and Apache Corporation for their sponsorship of these efforts.

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