Effects of Oil and Gas Exploration and Development at Selected Continental Slope Sites in the Gulf of Mexico. Volume I: Executive Summary

Abstract

From the Conclusions: “The following are the main conclusions of the study:

Geophysical and chemical measurements indicated that a layer of SBM cuttings and muds was deposited within the near-field radius. Geophysically mapped mud and cuttings zones ranged from 13 to 109 ha in area, with larger zones observed at post-development sites. These zones typically extended several hundred meters from wellsites, with the greatest extent (about 1 km) observed at GB 602 and GB 516. Cuttings deposits were estimated to be up to 45 cm thick at one site (VK 916).

Concentrations of drilling fluid tracers (barium and SBF) were elevated by several orders of magnitude within near-field sites. Mean sediment concentrations of barium and SBF were positively correlated with estimated discharge volumes of SBM cuttings.

Areas of SBM cuttings deposition were associated with elevated TOC and anoxic conditions, including low dissolved oxygen, negative Eh, and shallow depth of the oxidized layer. Sites with larger volumes of SBM cuttings discharges and higher mean sediment SBF concentrations had the greatest reduction in mean sediment oxygen levels.

SPI indicated that the near-field sites had patchy zones of disturbed benthic communities, including microbial mats, areas lacking visible benthic macroinfauna, zones dominated by pioneering stage assemblages, and areas where surface-dwelling species were selectively lost.

Macroinfaunal and meiofaunal densities generally were higher near drilling, although some faunal groups were less abundant in the near-field (amphipods, ostracods). Among megafauna, increased fish densities and reduced ophiuroid densities were noted in the near-field of two sites (VK 916 and GB 516).

Microbial biomass (ATP) was elevated in some samples near drilling and positively correlated with SBF concentrations above about 1,000 μg/g at VK 916 and GB 516, but not at GB 602 or MC 292. The ATP data were problematic, however, with major temporal changes and apparent far-field “outliers” complicating the interpretation. Meiofaunal densities in the near-field were not consistently correlated with drilling indicators (barium, SBF) or other sediment variables (TOC, grain size fractions).

Annelid (predominantly polychaete) and gastropod densities in the near-field were positively correlated with drilling indicators (barium, SBF). Some near-field stations with barium concentrations higher than about 10,000 μg/g and/or SBF concentrations greater than about 1,000 μg/g had elevated polychaete densities. A few near-field stations at GB 516 and GB 602 had very high gastropod densities, which were associated with barium concentrations of 55,000 μg/g or higher and SBF concentrations of 4,500 μg/g or higher.

Amphipod densities in the near-field were negatively correlated with drilling indicators (barium and SBF). Generally, near-field stations with barium concentrations higher than about 10,000 μg/g and/or SBF concentrations greater than about 1,000 μg/g had low amphipod densities. Separately, acute toxicity tests with near-field and far-field sediments from MC 292 and GB 602 showed that mean amphipod survival was significantly lower in sediments from near-field stations than in sediments from far-field stations. Amphipod survival in the toxicity tests was negatively correlated with drilling indicators.

Detailed taxonomic analysis of a subset of the macroinfaunal samples showed some stations near drilling had lower diversity, lower evenness, and lower richness indices compared with stations away from drilling. Species composition varied in relation to both geographic location and drilling impacts. Station/cruise groups most likely affected by drilling were dominated by high abundances of one or a few deposit-feeding species, including known pollution indicators.

At all four near-field sites, impacts were patchy, with some stations showing conditions similar to those at the far-field sites. Impacts generally were less extensive and less severe at post-exploration sites than at post-development sites.

mpacts attributable to SBM cuttings such as elevated TOC, poor redox conditions, and associated biological changes were least severe at MC 292, where the smallest quantities of SBM cuttings were discharged. However, the time elapsed since drilling also was longer at this site (about 2 years) than at the other three sites (5 to 14 months), and the less severe impacts may reflect recovery of this site over time.

Observations from the study sites and adjacent lease blocks suggest that geophysically detectable mud/cuttings deposits may persist for 5 years or more and anchor scars may persist for 14 years or more. Because no chemical or biological sampling was done in adjacent blocks, it is not known if the mapped mud/cuttings from older wells are associated with persistent elevations in barium, anoxic conditions, or altered benthic communities.”