Lavani SERPENT report

Abstract

In follow-up to work at Zafarani a field study was carried out to describe the deep-sea fauna and the effects of disturbance from seabed discharges associated with exploration drilling at Statoil’s Lavani location, at 2400 m depth in Block 2 offshore Tanzania.

The fieldwork was carried out from the drillship Ocean Rig Poseidon during drilling operations at Lavani. The ship’s remotely operated vehicles were used during standby time to survey the seabed using high definition video cameras. Nine quantitative straight line video transect surveys (up to 260 m in length) were used to determine the abundance and diversity of the fauna, describe the seabed environment and map the distribution of drill cuttings visible on the seabed. Replicated sediment samples were also collected in at five locations up to 180 m from the BOP in order to quantify the disturbance gradient and verify the findings of the video survey.

The background environment was soft sediment consisting of typical deep-sea ooze. It comprised planktonic material such as coccoliths settled to the seabed from the surface water. Many tracks and burrows created by seabed organisms covered the surface of undisturbed sediment. The deep-sea animals observed at the site were similar at Lavani to Zafarani. At Lavani abundance of organisms was lower but more diverse that at Zafarani. The assemblage was dominated by Xenophyophores, Sponges and Echinoderms. Notable observations not encountered at Zafarani include large echinothuriid sea urchins, a large skate and a diverse community associated with sunken wood.

Drilling disturbance resulted in coverage of the seafloor with drill cuttings. This generally extended to approximately 60 m from the well. A pilot well was drilled to the east of the main well and to the east and north east disturbance was visible to a distance of 140 m from the BOP. Within this area the abundance of the megabenthic assemblage was significantly reduced. Distribution of cuttings extended furthest to the east of the well but there was not a single direction in which the cuttings were dominantly distributed as at Zafarani. In contrast to Zafarani, at Lavani background seabed could be observed in all headings. The increased disturbance documented to the east of the well likely results from the drilling of a pilot well in this area rather than the effects of seabed currents. The pilot hole was visible in the video surveys and clearly extended area of disturbed seabed. Sediment sampling showed that there was significantly elevated barium concentration close to the well (indicative of drill mud), which reduced with distance from the BOP.

The abundance and diversity of megafauna was significantly reduced in close proximity to the well following smothering disturbance. Further from the well there was great variability in both parameters owing to varying degrees of disturbance. It is expected that the abundance and diversity of megafauna beyond 140 m from the BOP at Lavani is representative of the background for the area.

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