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Tantalizing evidence for Cook Inlet oil DGGS team reports on oil-stained Jurassic sandstone on Iniskin Peninsula, implications for oil potential in Mesozoic strata ALAN BAILEY Petroleum News
A recently published report by geologists from Alaska’s Division of Geological and Geophysical Services describes the discovery of some oil stained sandstone in Jurassic rocks exposed in sea cliffs on the south shore of Chinitna Bay, on the north side of the Iniskin Peninsula, on the west side of the Cook Inlet. The discovery came during fieldwork conducted as part of a multi-year investigation that a DGGS-led team is conducting into the petroleum geology of the Cook Inlet basin. The purpose of the investigation is to assemble publicly available data that can aid in the search for new hydrocarbon resources.
All of the producing oil and gas fields in the Cook Inlet region have hydrocarbon reservoirs in rocks of Tertiary age. But, while much of the gas has come from coal seams in the Tertiary rock sequence, most of the oil is thought to have originated from older and deeper oil source rocks in the Tuxedni group, of Jurassic age. There has long been speculation over whether as-yet undiscovered oil may exist in Jurassic or other Mesozoic reservoir rocks, buried under the Tertiary strata.
Reservoir quality There are, however, significant concerns about the possibility of poor reservoir quality in the Mesozoic rocks, which were laid down close to a volcanic arc and tend to have grains cemented together by secondary minerals. Compaction of the rocks as a consequence of deep burial may also have compromised reservoir quality.
Although it has been proposed in the past that oil seeps on the Iniskin Peninsula are associated with oil migration along faults and fractures in the rocks, the absence of fractures in the Chinitna Bay Jurassic rocks where the DGGS scientists observed oil staining suggests that the oil is hosted inside the rock matrix, the DGGS report says. This conclusion raises the possibility that factors inhibiting reservoir quality are not pervasive but may vary, depending on the nature of the sediments involved, the report says.
The sandstone interval with oil staining is in a 100-meter thick rock package, consisting predominantly of sandstone and appearing to have been deposited on a marine shelf adjacent an ancient river delta.
Oil migration Although it is possible that the oil permeated the sandstone prior to subsequent cementation by secondary minerals, the fact that adjacent rocks have been cemented but do not contain oil suggests that cementation took place prior to the migration of oil into the sandstone, the report says. As an alternative explanation for the ability of the oil to penetrate the rock matrix, it is possible that the existence of both coarse grained and fine grained sand particles in the oil bearing rock inhibited the penetration of the rock matrix by the fluids which would have caused the cementation. Cementation of the rock matrix is not visible in every rock stratum at the Chinitna Bay location, the report says.
“Regardless of the ultimate mechanisms controlling reservoir quality in Jurassic strata, this reported discovery of oil-stained sandstone adds to the growing database of hydrocarbon occurrences in Mesozoic rocks of the Cook Inlet, furthering the conclusion that much of the region is underlain by productive source rock,” the report says. “The challenge remains predicting the (hydrocarbon) migration pathways and locating economically significant reservoir facies.”
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