Chinitna reservoir potential evaluated

A team of geologists from Alaska’s Division of Geological and Geophysical Surveys, Alaska’s Division of Oil and Gas and the U.S. Geological Survey has reported the results of an investigation into the oil reservoir potential of the Chinitna formation, a rock unit that outcrops in the area of the Iniskin Peninsula and Tuxedni Bay, on the west side of Cook Inlet. The Chinitna, middle Jurassic in age, lies directly above the Tuxedni formation, the formation containing the source rocks for Cook Inlet oil.

Although the operational oil fields of the upper Cook Inlet have reservoirs in younger Tertiary sands, there is potential for finding oil in older Mesozoic strata, including rocks within the Jurassic succession.

From surface exposures the geologists found the Chinitna to be more than 2,000 feet thick and composed predominantly of fine grained sandstone deposited in a marine environment. Within the formation cycles of deposition have resulted variations in grain size. At least two episodes involving the deposition of coarse grained detritus can be recognized. The geologists observed structures derived from marine channels. In general, porosities and permeabilities in the fine sands are quite low, although oil staining has been observed in the Chinitna at Chinitna Bay.

The geologists concluded that the oil staining coupled with the nature of the stratigraphy within the Chinitna demonstrate the presence of viable scenarios for the existence of oil reservoirs in the formation.

- ALAN BAILEY