Environmental Statement - Maersk Oil

Balloch Field Development Environmental Statement
Section 3 Baseline Environment
Pockmarks identified within the Balloch development area
Pockmarks are usually found in soft, fine‐grained seabed sediments, often post‐glacial sediments of
the Witch Ground Formation or Flags Formation. The Balloch development is on the edge of the
Witch Ground Basin, characterised by high densities of pockmarks of up to 40 per km 2 .
Pockmarks are typically greater than 10 m across and several metres deep. They are thought to be
formed by the escape of gas or water from beneath the sediment and as such they are often
associated with MDACs ‐ mineral formations thought to be created by escaping methane. MDACs
usually occur as a result of either microbial decomposition of organic matter (microbial methane) or
the thermocatalytic destruction of kerogens (thermogenic methane) (Judd, 2001).
Pockmarks alone are not considered to conform to any of the Annex I habitats; however, MDAC
structures within pockmarks are often associated with the potentially important ‘submarine
structures’ listed in Annex I.
Pockmarks have been observed as habitats for unusual and prolific fauna which may be related to the
carbon associated with the MDAC and an increase in sulphide compounds being available to enter the
food chain, or the physical presence of the MDAC as a hard substrate (Figure 3‐6). In addition, as a
result of the seabed depression, currents are likely to be reduced within the pockmark and finer
sediments with higher organic content are likely to accumulate.
Figure 3‐6 Photograph of pockmark (with evidence of bioturbation) (Fugro, 2005).
The lower bottom currents can lead to high levels of larval settlement, thus a higher abundance of
deposit feeding organisms is often observed in comparison to the surrounding area. Bivalve species
such as Thyasira sarsi and Lucinoma borealis are dependent on high sulphide concentrations and are
only found within pockmarks, not the rest of the open North Sea. There is also a tendency for higher
levels of suspended solids to be associated with the water within pockmarks, which may lead to
increased abundance of shrimps and euphausids. Fish also may take advantage of the sheltered
conditions within the pockmark, for example cod (Gadus morhua), torsk (Brosme brosme) and ling
(Molva molva) (Dando, 2001).
The Scanner pockmark is a large seabed depression measuring approximately 600 m by 30 m, with a
depth of approximately 20 m below the surrounding sea floor. The area supports species associated
with rock reef structures. Amongst the species to colonise the carbonate structures are anemones,
squat lobsters and Astromonema southwardorum (a specialist in methane‐rich environments and
unique to this site).
The pockmarks in the Balloch area were initially identified during a seabed site survey in 1990 for the
exploration well 15/20a‐m. They were found to be small and shallow, less than 50 m across and
about 1 m deep (BP, 1990). This is in keeping with the trend for pockmark sizes to be smaller towards
the ends of the Witch Ground Basin (Dando, 2001). The density of pockmarks in the 1990 survey was
found to be 14 per km 2 . Photographs taken of the pockmarks, however, showed no unusual features
or evidence of MDAC (BP, 1990).
3 ‐ 9