Is Gas Hydrate Near the Storegga Slide Offshore Norway Connected to a Petroleum System?

Thomas D. Lorenson¹, William Winters², Charles Paull³, and William Ussler^3
1 U.S. Geological Survey, Menlo Park, CA
² U.S. Geological Survey, Woods Hole, MA
³ Monterey Bay Aquarium Research Institute, Moss Landing, CA

On the mid-Norwegian continental shelf, the northern headwall flank of the Storegga Slide exhibits many features associated with fluid expulsion including pockmarks, and acoustic wipe-out zones indicative of gas. Bottom simulating reflections attributed to the presence of gas hydrate are known mainly on the northern and eastern flanks of the Storegga Slide and within the slide. At least two large Tertiary aged structures occur beneath the northeast headwall and adjacent area including the Ormen Lange Dome charged with estimated of 12 TCF gas. Results from a September 2004 piston coring cruise showed that sediment at depths of 10 meters below the seafloor recovered from the northern flank of the slide evolved gas bubbles and thus exceeded surface methane saturation. The carbon isotopic composition of this methane is light, ranging from –65 to –94 ppt. indicating microbial methane sources. Although higher hydrocarbon gases are present, they are at such low concentrations that they are also consistent with a microbial gas source. The gas composition of the Ormen Lange gas field is likely to contain higher molecular weight hydrocarbons and carbon isotopic compositions heavier than –50 ppt. implying that the gas field is not in direct contact with the surface. The concurrence of high methane concentrations in sediment, fluid expulsion features near the headwall of a large slide, and large quantities of subsurface gas lead us to speculate that this area, and the gas hydrate within it, are likely part of a petroleum system modified in the near-surface by microbial activity.