Zafarani SERPENT report

Abstract

The SERPENT Project (Scientific and Environmental ROV Partnership using Existing Industrial Technology) carried out an investigation into deep-sea ecology and the effects of disturbance from hydrocarbon exploration at Statoil’s Zafarani Prospect in Block 2 offshore Tanzania. SEPRENT visited the Ocean Rig Poseidon which drilled the Zafarani well in 2600 m water depth in the western Indian Ocean off Tanzania, an area largely neglected by deep-sea science and exploration, and therefore lacking in the background information required to predict and understand anthropogenic effects on the fauna associated with the seabed.

The fieldwork involved the use of the ship’s remotely operated vehicles during standby time to survey the seabed using high definition video cameras in order to describe the seabed environment and quantify the deep-sea organisms encountered around the drilling operations. Quantitative straight line video transect surveys were carried out to determine the abundance and diversity of the fauna and map the distribution of drill cuttings visible on the seabed. Sediment samples were also collected in four headings in order to verify the findings of the video survey.

The water column temperature reduced from 28?C at the surface to 2?C at the seabed with relatively consistent salinity with depth. The seabed was characterised by soft sediment which, in undisturbed areas, showed abundant signs of the activity of seabed organisms in the form of burrows, tracks and traces. Forty three species of larger organism (megafauna – visible in video footage) were encountered. Xenophyphores, echinoderms and sponges dominated the benthic megafauna. Ten species of deep-sea fish were recorded in the video observations.

Visual observations showed that drilling disturbance had a noticeable effect causing a layer of drill cuttings to overlie the natural seabed. This reduced the variability of the seabed obscuring the evidence of faunal activity. This was predominantly to the north of the well, in which heading it was not possible to reach natural seabed at the full extent of the ROV tether (150 m). In other headings the cuttings disturbance reached approximately 30-50 m from the well. Chemical analysis of the sediment samples verified the observations from ROV video, showing high levels of barium to the north of the well. Examination of the sediment using scanning electron microscopy also showed evidence of high levels of barium sulphate (a key component of drilling mud). In comparison, samples taken in all other headings showed low barium concentration and the sediment samples were dominated by the remains of phytoplankton such as coccolithophores which naturally settle to the seabed.

The abundance of megafaunal organisms was significantly reduced within the area impacted by the drill cuttings, this was of particular importance in the area up to 20 m from the BOP in all headings and for the entire survey length to the north of the well. Other measures of the megafaunal community (species diversity and multivariate analysis of the megafaunal assemblage composition) suggested similar patterns but beyond the visible cuttings the initial effects of drilling disturbance appeared to be local, however it is not clear how far this extends to the north. The presence of lebensspuren (animal tracks and traces) may be a more sensitive measure of drilling disturbance than megafaunal abundance in deep, soft-sediment environments such as Zafarani.

This project provides valuable data about the deep water fauna of the area around the deep water drilling operations at Zafarani that was highlighted as lacking in pre-drilling assessments. It also verifies the predicted impacts of top- hole drilling suggested by modelled discharge calculations, providing local scale resolution to predicted effects.

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