Insights into North Slope oil sources

Alan Bailey

Petroleum News

The North Slope of Alaska has a reputation for being particularly rich in oil source rocks, the rocks that have generated the oil in the region’s prolific oil fields. And among the valuable resources in the state’s Geologic Materials Center in Anchorage are samples of various North Slope oil types and of the rocks that

sourced the oil. It is possible to use the samples to, for example, characterize different oil types, to figure out where the oil associated with oil discoveries has likely come from. That, in turn, can provide insights into the nature of a particular discovery, and into the potential for making further, similar oil finds.

Geologist Art Banet showed Petroleum News some of the oil samples in the center’s collection and explained some of the nuances of oil categorization and identification. There are perhaps 10 different types of oil across the North Slope, Banet said.

The oil found in the Prudhoe Bay oil field, for example, is relatively dark and heavy, a factor that led explorers to seek similar oil elsewhere on the North Slope.

Thus, while people were looking for reservoir rocks stained with that particular oil type, they may have overlooked lighter and relatively colorless oil that is also prolific in the region, Banet suggested. This possible “bypass oil” is now being discovered in the northeastern National Petroleum Reserve-Alaska, he said.

The center’s collection includes oil samples from various natural oil seeps in Arctic Alaska. The biggest seeps in the region are at Cape Simpson. However, the second and third largest seeps are two locations on the coast of the Arctic National Wildlife Refuge. Then come seeps at Umiat and a location known as Fish Creek.

At one time the state was able to use oil samples at the center to demonstrate to federal officials that the oil from Simpson came from a natural seep, and not from oil drilling, Banet said. Rock specimens in the center may also provide insights to the origin of oil in the Triassic Shublik, one of the most prolific oil source rocks under the North Slope. The center’s collection includes tasmanites, a kind of algae-rich oil shale, from strata in the Brooks Range equivalent in age to the Shublik. Could this algal material have generated the Shublik oil?

Ken Bird, a geologist who is an expert on Alaska and North Slope geology, has told Petroleum News that there was initial skepticism over the tasmanite origin hypothesis, given a lack of reports of tasmanite being found in well penetrations of the Shublik. However, it is likely that when the Shublik was heated to oil generating temperatures or above, the tasmanite algae were destroyed, Bird said. And, in general, tasmanite algae have been found in surface and subsurface rocks across the North Slope region, he said.

The conclusion is that tasmanite algae have likely contributed to some undetermine extent to North Slope petroleum generation, Bird said.