Those pesky oil bacteria at work

Heavy oil, such as that found in the Ugnu formation on Alaska’s North Slope, is formed when bacteria gobble up the lighter, hydrogen-intense components of regular light oil, leaving behind a residue of the heavier oil components and producing large volumes of methane in the process, Eric West, manager of BP’s Alaska renewal team, told a group of state legislators during a presentation on heavy oil on March 10 in Juneau.

The bacteria cannot survive the relatively high temperatures encountered in the deeply buried reservoir rocks of oil fields such as Prudhoe Bay and Kuparuk, so that the oil in these fields has remained relatively light, flowing easily up oil wells and through pipeline systems.

But over time, some oil has spilled from these deep field reservoirs, percolating upwards through the rock strata into relatively shallow rock formations such as the West Sak-Schrader Bluff and the Ugnu, West said. And the shallower the resulting oil pools, the cooler the oil becomes. Conversely, the cooler the oil, the more active and abundant the bacteria become in chomping at the light oil components.

The West Sak-Schrader Bluff formation now hosts what BP refers to as viscous oil, oil with a consistency of maple syrup that can be produced, especially through horizontal wells that access large sections of reservoir. Heavy oil, with the consistency of molasses and unable to flow unaided, is found in the shallower Ugnu formation.

The methane from the bacterial flows upwards to the base of the permafrost, where it combines with water to form methane hydrate, a potential future source of commercial natural gas.

—Alan Bailey