DGGS reports on oil stained sandstone

Alan Bailey

Petroleum News

Alaska’s Division of Geological and Geophysical Services has published a report describing a new discovery of oil-stained rock in Jurassic strata near Tuxedni Bay, on the west side of Cook Inlet. The discovery provides more tantalizing evidence for the presence of oil in Mesozoic rocks of the region. The producing oil fields of northern Cook Inlet all have reservoirs in younger rocks that are Tertiary in age, although the oil is understood to have originated in a Jurassic source rock in the Mesozoic.

Early oil exploration in the region targeted the Mesozoic on the west side of the more southerly part of the inlet, but no commercial oil discoveries were made. The Iniskin Peninsula, immediately south of Tuxedni Bay, became a particular focus of that exploration.

The new discovery of oil staining, made during DGGS-led geologic field work in the summer of 2017, was in the Tonnie siltstone member within the middle Jurassic Chinitna formation. The geologists observed petroliferous rocks with a thickness of 35 meters over a lateral extent of at least 250 meters.

“The scale of this exposure is at least two orders of magnitude greater in area than any of the previously discovered oil-stained outcrops in the Iniskin-Tuxedni Bays area,” the DGGS report says.

The oil may have been trapped in sands within the Tonnie by muddy sediments that envelope the sands, the report says.

- ALAN BAILEY