ACCESS NUMBER: 31034 

STUDY TITLE: Effects of Oil and Gas Exploration and Development at Selected 

Continental Slope Sites in the Gulf of Mexico 

REPORT TITLE: Effects of Oil and Gas Exploration and Development at Selected 

Continental Slope Sites in the Gulf of Mexico, Volume I: Executive Summary, Volume 

II: Technical Report, and Volume III: Appendices 

CONTRACT NUMBER: 1435-01-00-CT-31034 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Central Gulf of Mexico, Western Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 2000; 2001; 2002; 2003; 2004; 2005; 2006 

COMPLETION DATE OF REPORT: September 2006 

COSTS: FY 2000: $59,190; FY 2001: $2,180,912; FY 2002: $871,343; FY 2003: 

$370,240; FY 2004: $147,421; FY 2005: $141,438; FY 2006: $81,905; CUMULATIVE 

PROJECT COST: $3,852,449 

PROJECT MANAGER: D. Gettleson 

AFFILIATION: Continental Shelf Associates, Inc. 

ADDRESS: 759 Parkway Street, Jupiter, FL 33477 

PRINCIPAL INVESTIGATORS*: N. Phillips; A. Hart; H. Roberts; I. Williams; J. Blake; D. 

Rhoads; P. Neubert; R. Diaz; J. Trefry; R. Trocine; R. Rember; M. McElvaine; T. 

McDonald; J. Brooks; P. LaRock; Barry A. Vittor & Associates, Inc.; R. Carney; C. 

Gregg; D. Foltz; J. Fleeger; R. Kropp; J. Neff 

KEY WORDS: Central Gulf; Western Gulf; deepwater; benthic ecology; impacts; 

monitoring; drilling fluids; cuttings; synthetic based fluids; geophysical surveys; sidescan sonar; sediment grain size; sediment profile imaging; redox potential; trace metals; 

hydrocarbons; microbes; meiofauna; macroinfauna; megafauna; photography; image 

analysis; copepod genetics; toxicology; risk assessment. 

BACKGROUND: Drilling activities on the northern Gulf of Mexico continental slope 

have increased dramatically in the last decade. However, there have been few 

environmental studies of benthic impacts at deepwater drill sites in the Gulf of Mexico. 

Because of environmental and technological differences, the applicability of previous 

impact studies done on the continental shelf is uncertain. The findings from this study 

will assist the MMS in conducting environmental analyses, as well as in developing 

mitigative measures and regulations specifically tailored to deepwater operations. 
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OBJECTIVES: The objectives of this study were to assess the physical, chemical, and 

biological impacts of oil and gas development at selected exploration and development 

wellsites on the Gulf of Mexico continental slope. Specific objectives were to document 

(1) drilling mud and cuttings accumulations; (2) physical modification/disturbance of the 

seabed due to anchors and mooring systems; (3) debris accumulations; (4) 

physical/chemical modification of sediments; and (5) effects on benthic organisms. 

DESCRIPTION: Four study sites were selected on the northern Gulf of Mexico 

continental slope. Viosca Knoll (VK) 916 was an exploration site sampled before and 

after drilling of a single exploration well. Garden Banks (GB) 516 was an 

exploration/development site that was sampled once after exploration drilling and again 

after several development wells were drilled. GB 602 and Mississippi Canyon (MC) 292 

were post-development sites sampled once after several exploration and development 

wells had been drilled. Each location consisted of a near-field site (500-m radius) and 

six far-field sites located 10 to 25 km away. Three geophysical cruises and three 

chemical/biological cruises were conducted between October 2000 and August 2002. 

The geophysical cruises included site mapping using bathymetry, side-scan sonar, and 

subbottom profiling. The chemical/biological surveys included box coring, sediment 

profile imaging, seafloor photographs, and baited traps (for tissue samples). 

SIGNIFICANT CONCLUSIONS: Physical, chemical, and biological impacts of drilling 

activities were detected at all four sites. Anchor scars were observed within about 3 km 

of wellsites, with individual scars ranging in length from less than 100 m to over 3 km. 

At all four sites, there were side-scan sonar contacts identified as debris. Cuttings and 

drilling fluid accumulations were evident mainly within the 500-m radius near-field zone 

at all four sites, though there was geophysical and chemical evidence for deposits 

extending beyond this area. Impacts noted within the near-field zone included elevated 

barium, synthetic based fluid (SBF), and total organic carbon (TOC) concentrations, low 

sediment oxygen levels, presence of microbial mats, and altered densities of 

meiofauna, macroinfauna, and megafauna. Within the near-field zone, 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. The severity of discharge-related impacts varied depending on 

the volume of SBF cuttings discharged and the time elapsed since drilling was 

completed. 

STUDY RESULTS: Geophysical and chemical measurements indicated that a layer of 

SBF cuttings and muds several centimeters thick was deposited within the near-field 

sites. Geophysically mapped cuttings zones ranged from 13 to 109 ha in area, with 

larger zones observed at post-development sites. 

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 SBF cuttings. 

Areas of SBF cuttings deposition were associated with elevated TOC and anoxic 

conditions, including low dissolved oxygen, negative Eh, and shallow depth of the 
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oxidized layer. Sites with larger volumes of SBF cuttings discharges and higher mean 

sediment SBF concentrations had the greatest reduction in mean sediment oxygen 

levels. 

Sediment profile imaging indicated that the near-field sites had patchy zones of 

disturbed benthic communities, including azoic areas, microbial mats, zones dominated 

by pioneering stage assemblages, and areas where surface-dwelling species were 

selectively lost. Macroinfaunal and meiofaunal densities and microbial biomass 

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). 

Generally, meiofaunal densities in the near-field were not consistently correlated with 

drilling indicators (barium and SBF) or other sediment variables (TOC and grain size 

fractions). A genetic study of the harpacticoid copepod Bathycletopsyllus sp. was 

conducted with samples from GB 602 and MC 292. This species was absent from farfield sites but present in large numbers at the two near-field sites. Within the near-field, 

abundances were positively correlated with sediment SBF concentrations up to about 

2,500 μg/g. Low genetic diversity was observed in near-field Bathycletopsyllus sp. 

populations, a result consistent with expansion from a small population size. 

Polychaete and gastropod densities in the near-field were positively correlated with 

drilling indicators (barium and 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. SBF chemicals in sediments may adversely affect benthic communities by 

direct toxicity or by organic enrichment and associated development of sediment 

hypoxia caused by microbial degradation of the organic chemicals. 

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 
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drilling were dominated by high abundances of one or a few deposit-feeding species, 

including known pollution indicators. 

Impacts attributable to SBF cuttings such as elevated TOC, poor redox conditions, and 

associated biological changes were least severe at MC 292, where the smallest 

quantities of SBF 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. 

STUDY PRODUCTS: Continental Shelf Associates, Inc. 2006. Effects of Oil and Gas 

Exploration and Development at Selected Continental Slope Sites in the Gulf of Mexico. 

Volume I: Executive Summary, U.S. Department of the Interior, Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2006-044. 

51 pp. 

Continental Shelf Associates, Inc. 2006. Effects of Oil and Gas Exploration and 

Development at Selected Continental Slope Sites in the Gulf of Mexico. Volume II: 

Technical Report. U.S. Department of the Interior, Minerals Management Service, Gulf 

of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2006-045. 636 pp. 

Continental Shelf Associates, Inc. 2006. Effects of Oil and Gas Exploration and 

Development at Selected Continental Slope Sites in the Gulf of Mexico. Volume III: 

Appendices. U.S. Department of the Interior, Minerals Management Service, Gulf of 

Mexico OCS Region, New Orleans, LA. OCS Study MMS 2006-046. 938 pp. 

Continental Shelf Associates, Inc. 2002. Deepwater Program: Effects of Oil and Gas 

Exploration and Development at Selected Continental Slope Sites in the Gulf of Mexico. 

Interim Report. U.S. Department of the Interior, Minerals Management Service, Gulf of 

Mexico OCS Region, New Orleans, LA. 74 pp. 

*P.I.'s affiliation may be different than that listed for Project Managers. 
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