OCS Study 

MMS 2006-044 

 

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 
U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 

OCS Study 

MMS 2006-044 

Effects of Oil and Gas Exploration 

and Development at Selected Continental 

Slope Sites in the Gulf of Mexico 

Volume I: Executive Summary 

Author 

Continental Shelf Associates, Inc. 

Prepared under MMS Contract 

1435-01-00-CT-31034 

by 

Continental Shelf Associates, Inc. 

759 Parkway Street 

Jupiter, Florida 33477-9596 

Published by 

New Orleans

July 2006 
iii 

Disclaimer 

This report was prepared under contract between the Minerals Management Service (MMS) and 

Continental Shelf Associates, Inc. This report has been technically reviewed by the MMS and 

has been approved for publication. Approval does not signify that the contents necessarily reflect 

the views or policies of the MMS, nor does mention of trade names or commercial products 

constitute endorsement or recommendation for use. It is, however, exempt from review and 

compliance with MMS editorial standards. 

Report Availability 

Extra copies of this report may be obtained from the Public Information Office at the following 

address: 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, LA 70123-2394 

Telephone: (504) 736-2519 or 

1-800-200-GULF 

Citation 

Suggested citation: 

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. 45 pp. 
v 

Acknowledgments 

The program was conducted by a large, multidisciplinary team under the direction of Continental 

Shelf Associates, Inc. (CSA). David Gettleson was the program manager and Alan Hart served as 

deputy program manager and data manager. Neal Phillips was the scientific editor for the final 

report. Principal investigators are acknowledged at the head of each report chapter in Volume II 

(Technical Report). 

We appreciate the assistance of MMS staff including Greg Boland (Contracting Officer’s 

Technical Representative), Mary Boatman (Technical Performance Evaluation Committee), 

Debra Bridge (Contracting Officer), and Tom Ahlfeld. Jill Leale with the MMS Mapping & 

Automation Unit provided historical well data and georeferenced maps of well locations based on 

the MMS Technical Information Management System. 

Drilling discharge data were kindly provided by Vince Cottone (ChevronTexaco) and Kent 

Satterlee, Ellen Polanski, and Jane Chady (Shell). Joe Smith (ExxonMobil) sent unpublished 

reports that were helpful in drilling discharge calculations. 

The Scientific Review Board consisted of Andy Glickman (ChevronTexaco), Donald Harper 

(Texas A&M University at Galveston), and Jim Ray (Oceanic Environmental Solutions). 

Additional input in the study design was provided by Peter Arnold (independent consultant), 

Maynard Brandsma (Brandsma Engineering), and Woollcott Smith (Temple University). 

We wish to recognize the following individuals for their roles in the field surveys: 

• Jay Northcutt (C&C Technologies) – Project manager, geophysical cruises 

• Tony George (C&C Technologies) – Manager, geophysical interpretation 

• Scott Melancon – Party chief, geophysical Cruises 2A and 3A 

• Tim MacEwan – Senior operator, geophysical Cruise 2A 

• Scott McBay, David Aucoin, and Paige Melancon – Party chiefs, geophysical Cruise 1A 

• Roger Fay (TDI-Brooks International, Inc.) – Chief scientist, chemical/biological cruises 

• Lynwood Powell (CSA) – Field operations manager, chemical/biological Cruises 1B and 2B; 

navigation/post-plotting and geographic information system development 

• Frank Johnson (CSA) – Field operations manager, chemical/biological Cruise 3B 

Other cruise participants including staff from Continental Shelf Associates, Inc., C&C 

Technologies, TDI-Brooks International, Inc., Florida Institute of Technology, and Louisiana 

State University are too numerous to list individually, but their contributions are sincerely 

appreciated. We also wish to acknowledge Wayne Isphording (University of South Alabama) for 

conducting laboratory sediment grain size analyses. 

The CSA document production team included Melody Powell (document production coordinator 

and technical editor), Heidi Glick (technical editor), Suzanne Short (graphics), and Debbie Raffel, 

Karen Stokesbury, Lynanne Rockhill, and Debbie Cannon (word processing). 
vii 

Contents 

Page 

INTRODUCTION ..........................................................................................................................1 

STUDY SITES ................................................................................................................................3 

 Site Selection .........................................................................................................................3 

 Near-Field and Far-Field Sites...............................................................................................4

DRILLING ACTIVITIES .............................................................................................................7 

 Viosca Knoll Block 916 (Exploration Site) ...........................................................................8 

 Garden Banks Block 516 (Exploration/Development Site) ...................................................8 

 Garden Banks Block 602 (Post-Development Site).............................................................11 

 Mississippi Canyon Block 292 (Post-Development Site)....................................................11 

STUDY METHODS .....................................................................................................................15 

 Sampling Schedule...............................................................................................................15 

 Geophysical Surveys............................................................................................................15 

 Chemical/Biological Sampling ............................................................................................16 

 Box Coring ................................................................................................................19 

 Sediment Profile Imaging..........................................................................................19 

 Seafloor Photographs.................................................................................................20 

 Baited Traps...............................................................................................................20

STUDY FINDINGS......................................................................................................................21 

 Far-Field Conditions ............................................................................................................21 

 Nature and Extent of Impacts...............................................................................................21 

 Anchor Scars .............................................................................................................24 

 Seafloor Equipment and Debris.................................................................................24 

 Mappable Cuttings and Mud Deposits ......................................................................24 

 Sediment Grain Size ..................................................................................................26 

 Sediment Color/Appearance......................................................................................26 

 Thickness of Mud and Cuttings Deposits..................................................................27 

 Drilling Fluid Tracers (Barium and SBF)..................................................................27 

 Polycyclic Aromatic Hydrocarbons (PAH)...............................................................28 

 Sediment Metals Other Than Barium........................................................................28 

 Metals and Hydrocarbons in Biota ............................................................................30 

 Total Organic Carbon ................................................................................................30 

 Sediment Redox Conditions ......................................................................................30 

 Benthic Communities ................................................................................................33 

 Site Comparisons .................................................................................................................38 

 Areal Extent of Impacts.............................................................................................38 

 Severity of Impacts....................................................................................................39 

 Duration of Impacts...................................................................................................39 

CONCLUSIONS...........................................................................................................................43 

REFERENCES CITED ...............................................................................................................45 
ix 

List of Figures 

Figure Page 

 1 Study sites ....................................................................................................................... 1 

 2 Location of near-field (NF) and far-field (FF) sites for (a) Viosca Knoll 

Block 916 and (b) Garden Banks Block 516................................................................... 5 

 3 Location of near-field (NF) and far-field (FF) sites for (a) Garden 

Banks Block 602 and (b) Mississippi Canyon Block 292............................................... 6 

 4 Well locations in and near the Viosca Knoll Block 916 near-field site .......................... 9 

 5 Well locations in and near the Garden Banks Block 516 near-field site....................... 10 

 6 Well locations in and near the Garden Banks Block 602 near-field site ....................... 12 

 7 Well locations in and near the Mississippi Canyon Block 292 near-field 

site ................................................................................................................................. 13 

 8 Schedule including sampling cruises and wells drilled during this study ..................... 16 

 9 Working configuration of the HUGIN 3000 autonomous underwater 

vehicle showing the “free-swimming” HUGIN collecting data from a 

predetermined height above the seafloor....................................................................... 17 

 10 Idealized sampling plans for study sites........................................................................ 18 

 11 Part of the side-scan sonar mosaic for the MC 292 area showing a 

wellsite in the adjacent block (MC 248) where three wells were drilled 

about 1.5 years before Cruise 2A .................................................................................. 25 

 12 Comparison of seafloor photographs from before and after drilling at 

VK 916 .......................................................................................................................... 27 

 13 Sediment concentrations of barium, synthetic-based fluids (SBF), and 

polycyclic aromatic hydrocarbons (PAH) in relation to proximity to 

drilling ........................................................................................................................... 29 

 14 Relationship between total organic carbon (TOC) and synthetic-based 

fluid (SBF) concentrations in near-field sediments at the four sites ............................. 31 

 15 Integrated amounts of oxygen in the sediment column at near-field 

(NF), far-field (FF), and discretionary (DS) stations at all four sites ............................ 32 

 16 Sediment profile image from the near-field site at Garden Banks 516 

during Cruise 1B showing a microbial mat and with the sediment 

column below completely gray to black........................................................................ 34 
List of Figures 

(continued) 

Figure Page 

x 

 17 Meiofaunal and macroinfaunal densities in relation to proximity to 

drilling ........................................................................................................................... 35 

 18 Macroinfaunal community characteristics for the detailed, 24-station 

analysis .......................................................................................................................... 37 

 19 Relationship between post-drilling sediment barium and 

synthetic-based fluid (SBF) concentrations within near-field sites and 

estimated volumes of SBF cuttings discharges ............................................................. 40 

 20 Difference between mean near-field (NF) and far-field (FF) sediment 

oxygen levels in relation to sediment synthetic-based fluid (SBF) 

concentrations and estimated (SBF) cuttings discharges .............................................. 41 
xi 

List of Tables 

Table Page 

 1 Coordinates and water depths of study sites................................................................... 3 

 2 Drilling summary for wells within each near-field site .................................................. 7 

 3 Sampling cruises............................................................................................................ 15 

 4 Numbers of samples collected and transects completed during 

chemical/biological cruises........................................................................................... 19 

 5 Environmental characteristics of far-field sites ............................................................ 21 

 6 Impact summary ........................................................................................................... 22 

 7 Geophysically estimated areal extent of drilling mud and cuttings 

deposits at each near-field site ...................................................................................... 26 
1 

Introduction 

A field study was conducted between November 2000 and August 2002 to assess benthic impacts 

of drilling at four sites on the Gulf of Mexico continental slope (Figure 1). The study was 

conducted for the Minerals Management Service (MMS) by a team of scientists under the 

management of Continental Shelf Associates, Inc. 

There have been few field measurements and observations around deepwater drillsites in the Gulf 

of Mexico (Fechhelm et al. 1999; Continental Shelf Associates, Inc. 2004). Studies on the 

continental shelf have documented impacts of water-based drilling fluids and cuttings on benthic 

organisms near wellsites. However, the applicability of this knowledge to the deepwater 

environment is uncertain (Carney 2001). In addition to environmental differences between 

continental slope and shelf communities, deepwater drilling more frequently uses synthetic-based 

drilling muds (SBMs) (Neff et al. 2000). 

Specific objectives of this study were to document (1) drilling mud and cuttings accumulations; 

(2) physical modification/disturbance of the seabed due to anchors and their mooring systems; 

(3) debris accumulations; (4) physical/chemical modification of sediments; and (5) effects on 

benthic organisms. 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. 

LOUISIANA

MISSISSIPPI

Gulf of Mexico

0 25 50 75 100 NAUTICAL MILES

Houston

Galveston

Mobile

New

Orleans

96o 94o 92o 90o 88o

30o

28o

26o

Corpus 

Christi

N TEXAS

2000 m

1000 m

3000 m

2000 m

1000 m

3000 m

Viosca Knoll 916

Mississippi 

Canyon 

292

Garden Banks 516

Legend

= Study Site

Garden Banks 602

ALABAMA

Figure 1. Study sites.
3 

Study Sites 

SITE SELECTION 

Four study sites were selected within MMS offshore lease blocks on the northern Gulf of Mexico 

continental slope. These were Viosca Knoll Block 916 (VK 916), Garden Banks Block 516 

(GB 516), Garden Banks Block 602 (GB 602), and Mississippi Canyon Block 292 (MC 292). 

Coordinates and water depths are provided in Table 1. 

Site selection was based on MMS requirements and operators’ drilling schedules. The MMS 

initially requested that two exploration sites and three post-development sites be selected (the 

number of post-development sites was ultimately reduced to two). The general requirements 

were as follows: 

• Sites should be located in water depths greater than 1,000 m. 

• Sites should be located at a similar water depth and be biologically and geologically similar. 

• At least one exploration site and one post-development site must be located as far east as 

possible in the Central Planning Area (e.g., near DeSoto Canyon). Ideally, the other sites 

should be selected to represent the central Gulf and western Gulf. 

• Sites drilled with an anchored drilling unit were preferred so that physical effects of 

anchoring could be studied. 

For exploration sites, it was preferred that no previous drilling had occurred in the block. For 

post-development sites, all drilling activities were to have been completed prior to the first cruise. 

To select potential exploration sites, the project team worked with the MMS and operators to 

identify Exploration Plans for wells in water depths of 1,000 to 1,500 m with a projected starting 

date in the fourth quarter of 2000. This range of water depths was selected because anchoring 

impacts were of interest and anchored rigs are infrequently used in deeper water. It also was 

assumed that the geology and biology of sites within this depth range in the Central and Western 

Planning Areas would be generally similar. This process ultimately resulted in the selection of 

VK 916, which fulfills the requirement for a site near the eastern edge of the Central Planning 

Area. No previous wells had been drilled in VK 916. 

Table 1. Coordinates and water depths of study sites. 

X/Y Coordinatesa Latitude/Longitude Site Water Depth 

(m) X (ft) Y (ft) Latitude Longitude 

VK 916 1,125 1,356,696.75 10,564,161.84 29°06'24.31"N 87°53'19.48"W

GB 516 1,033 1,839,531.91 9,975,977.93 27°29'23.81"N 92°23'08.26"W

GB 602 1,125 1,815,625.00 9,934,903.00 27°22'38.02"N 92°27'35.79"W

MC 292 1,034 1,129,042.00 10,419,990.00 28°42'13.08"N 88°35'44.19"W

a

 X/Y coordinates are for Universal Transverse Mercator, North American Datum 1927, Zone 15 (Garden Banks 

Block 516 and Garden Banks Block 602) and Zone 16 (Mississippi Canyon Block 292 and Viosca Knoll Block 916). 
4 

Although a second exploration site with no previous drilling was sought, none was available 

where an operator was planning to drill within the time frame for the pre- and post-drilling 

cruises. Therefore, GB 516 in the Western Planning Area was selected as the best available site, 

even though previous exploration wells had been drilled in the block. This was essentially an 

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

additional development wells were drilled during this study. 

To select potential post-development sites, the project team worked with the MMS and operators 

to identify development/production sites in water depths greater than 1,000 m where drilling 

(with an anchored drilling rig) would be completed by September 2000. Five potential sites were 

identified, and ultimately two were selected. These were GB 602 (in the Western Planning Area) 

and MC 292 (which fulfills the requirement for a site near the eastern edge of the Central 

Planning Area). 

NEAR-FIELD AND FAR-FIELD SITES 

Each of the four study locations consisted of a single near-field site and six far-field (“reference”) 

sites (Figures 2 and 3). The near-field sites were centered on well locations, and the far-field 

sites were 10 to 25 km away in the same water depths. Far-field sites were located using 

bathymetric maps provided by the MMS showing locations of previous wells in the vicinity of 

each near-field site. Although previous wellsites were avoided to the extent practicable, most 

far-field sites had at least one previous well drilled within 10 km. 

For chemical and biological sampling, each near-field site was defined as a circle 500 m in radius 

around the drillsite(s). Each far-field site was a circle 204 m in radius (so that the total area of six 

far-field sites was equal to that of a single near-field site). For geophysical surveys, a broader 

radius of 3,000 m was used for the near-field sites (see Study Methods). 
5 

Near-field (NF) site (500-m radius)

Far-field (FF) site (204-m radius)

LEGEND

516 Lease block

Bathymetry (100-m intervals)

FF1

FF2

FF3

FF4

FF5

FF6

NF

516

916

FF1 FF2

FF3

FF4 FF5

FF6

NF

Viosca Knoll

Destin Dome

Mississippi Canyon Desoto Canyon

916

Figure 2. Location of near-field (NF) and far-field (FF) sites for (a) Viosca Knoll Block 916 

and (b) Garden Banks Block 516. 
6 

Near-field (NF) site (500-m radius)

Far-field (FF) site (204-m radius)

LEGEND 602 Lease block

Bathymetry (100-m intervals)

FF1

FF2

FF3 FF4

FF5

FF6 NF

602

292

FF1

FF2

FF3

FF4

FF5

FF6

NF

292

Figure 3. Location of near-field (NF) and far-field (FF) sites for (a) Garden Banks Block 602 

and (b) Mississippi Canyon Block 292. 
7 

Drilling Activities 

The four sites represent a range of drilling activities: 

• VK 916 was an exploration site sampled before and after drilling of a single exploration well. 

• 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 MC 292 were post-development sites sampled once after several exploration and 

development wells had been drilled. 

Anchored semisubmersible drilling rigs were used for all sites, with an anchor radius of about 

2,400 to 3,000 m. Anchor locations were plotted, where possible, based on information in the 

operators’ exploration plan or development plan submitted to the MMS. At the three 

development sites (GB 516, GB 602, and MC 292), subsea wellheads, manifolds, and flowlines 

had been installed on the seafloor, connecting to remote platforms. 

Table 2 summarizes the number of wells and estimated drilling mud and cuttings discharges for 

each site. Both water-based muds (WBMs) and SBMs were used during drilling at all sites. 

However, at MC 292, most of the drilling was done with WBMs, and the quantity of SBM 

cuttings discharged was relatively small. 

Table 2. Drilling summary for wells within each near-field site. 

Estimated Seafloor 

Discharges (bbl) 

Estimated Drilling Rig 

Discharges (bbl) Site 

Total No. 

Wells Within 

500-m Radius WBM WBM 

Cuttings WBM WBM Cuttings 

SBM 

Cuttings

SBM Type 

Exploration 

VK 916 1 3,074 419 12,807 1,746 2,510 Syn-Teq (IO) 

GB 516 2 6,501 887 14,731 2,009 1,904 Novaplus (IO) or 

Petrofree LE (LAO)

Development 

GB 516 7

a 13,002 1,774 29,462 4,018 7,313 Novaplus (IO) and 

possibly Petrofree LE 

(LAO)

GB 602 7 23,946 3,266 91,301 12,450 28,339 Novadril (IO) or 

Petrofree ester (1 well);

Novaplus (IO) or 

Petrofree LE (LAO) 

(all others) 

MC 292 5 10,331 1,409 93,960 12,813 1,490 Novaplus (IO) 

IO = internal olefin; LAO = linear-α-olefin; SBM = synthetic-based mud; WBM = water-based mud. a Totals for GB 516 post-development include the two exploration wells listed above. 

The general sequence of drilling discharges was similar for most wells. The first part of the well 

was drilled (jetted) using a water-based spud mud. During this time before the marine riser was 

set, there were no returns to the drilling rig, and all mud and cuttings were released at the 
8 

seafloor. Spud muds consisting primarily of water and bentonite clay were used for this jetting 

process. The wellhead was then connected to a marine riser system that returned muds and 

cuttings to the drilling rig during the remainder of the drilling process. Subsequently, one or 

more well intervals were drilled using WBMs, with muds and cuttings discharged from the rig. 

Finally, in most cases, the mud system was switched to SBM for drilling of remaining well 

intervals. In some cases, there were bulk discharges of remaining WBMs prior to switching to 

SBM. During SBM intervals, SBM cuttings were discharged, but muds were retained (except for 

those adhering to cuttings). In addition, some quantities of SBMs were lost to the formation, lost 

during casing/cementing operations, or left in the wellbore for future sidetracks. Sidetracks at 

these sites were drilled primarily using SBMs. At MC 292, SBMs were used only during drilling 

of sidetracks. 

VIOSCA KNOLL BLOCK 916 (EXPLORATION SITE) 

Viosca Knoll Block 916 is located in the Central Planning Area, about 170 km SSE of Mobile, 

Alabama. Water depth at this site is approximately 1,125 m. A single exploration well (No. 1) 

was spudded by Shell on 27 November 2001 and reached total depth on 18 December 2001, when 

it was plugged and abandoned. Both WBM and SBM were used during drilling. The SBM used 

was Syn-Teq, an internal olefin isomer. 

There were no previous wells in this block. However, several wells had been drilled within 

10 km. The nearest were about 2.3 km NNW in VK 872 (Figure 4) and were drilled 

approximately 2 years before the start of this study. Any effects on the near-field site are 

assumed to be negligible in comparison with the single well drilled in VK 916 during this study. 

GARDEN BANKS BLOCK 516 (EXPLORATION/DEVELOPMENT SITE) 

Garden Banks Block 516 is located in the Western Planning Area, about 275 km SSE of 

Cameron, Louisiana. The water depth at this site is about 1,033 m. The prospect is named 

Serrano. 

Three exploration wells had been drilled in GB 516 prior to this study, and five development 

wells were drilled during the study (Figure 5). Two of the previous wells (a straight well and one 

sidetrack) were at the center of the near-field site and were drilled in July-August 1999. The 

other was 1,150 m to the southeast and was drilled between September 1995 and July 1996. The 

five new wells included two additional sidetracks of the 1999 well at the center of the GB 516 

near-field site, and one straight well and two sidetracks at a location about 50 m to the northeast 

of the site center. 

Both WBMs and associated cuttings, as well as SBM cuttings, were discharged during drilling. 

Exploration and development plans included the option of using either of two SBMs, Novaplus 

(an internal olefin) or Petrofree LE (a linear-α-olefin). Novaplus was the only SBM used for the 

wells and sidetracks drilled in 2000-2001; it is not known which SBM was used on the 1999 

wells. 
9 

Legend

Previous wellsite

N

S

EW

New well (drilled during this study)

500-m radius

VK 916, Well No. 1

Nov-Dec 2001 500-m radius

VK 916, Well No. 1

Nov-Dec 2001

2.3 km

VK 872 Well No. 1 

and sidetrack

Nov-Dec 1998

Anchor radius

2,440 m

(a) Well within the

VK 916 near-field site

(b) Anchor radius and 

nearby well in VK 872

Approximate anchor location

Approximate anchor locations based 

on radius shown in Exploration Plan 

and scars evident on side-scan sonar 

survey

Figure 4. Well locations in and near the Viosca Knoll Block 916 near-field site. 
10 

Legend

Previous wellsite

N

S

EW

New well (drilled during this study)

(a) Wells within the

GB 516 near-field site

(b) Anchor radius and 

nearby well in GB 516

500-m radius Well No. SE002 and

sidetracks ST01 and ST02

Nov 2000-Feb 2001

Well No. SE001 

and sidetrack ST01

Jul-Aug 1999

Well No. SE001

sidetracks ST02 

and ST03

Nov-Dec 2000

500-m radius

Wells drilled in near-field site

(SE001 and 3 sidetracks,

SE002 and 2 sidetracks)

Well No. 1

Sept 1995-Jul 1996

1.15 km

2,620-m anchor radius

Wellheads and other 

structures placed in this 

area between pre- and

post-drilling surveys

(see Fig. 3.3)

Well SE001 anchor locations (approx.)

Well No. 1 anchor locations (approx.)

Flowline and

umbilical to/from GB 426

Umbilicals to GB

559

Approximate anchor locations 

are based on Exploration Plans

Figure 5. Well locations in and near the Garden Banks Block 516 near-field site. 
11 

GARDEN BANKS BLOCK 602 (POST-DEVELOPMENT SITE) 

Garden Banks Block 602 is located in the Western Planning Area, about 280 km SSE of 

Cameron, Louisiana. Water depth at this site is about 1,125 m. The block is leased by Shell, and 

the prospect has been named Macaroni. 

Seven wells were drilled within the GB 602 near-field site between September 1995 and 

January 2001 (Figure 6). All of these were prior to our study. Three other 

exploration/development wells were drilled in GB 602 approximately 2.3 km NNE of the site 

center. Also, in February 1996, a geotechnical borehole was drilled about 1.3 km northeast of the 

near-field site center. Any effects are assumed to be negligible in comparison with the wells 

drilled within the near-field site. 

Exploration Well No. 1, located 247 m west of the site center, was drilled in September 1995 and 

permanently abandoned (“junked”) after it was lost to shallow water flow. Six other wells (three 

straight wells with corresponding sidetracks) were drilled at three surface locations near the site 

center between September 1995 and January 2001. 

Both WBM and SBM were used during drilling. The Exploration Plan for Well No. 1 included 

the option of two SBMs, Novadril (an internal olefin) or Petrofree ester. The Development Plan 

(for Well Nos. 2, 4, and 5) included Novaplus (an internal olefin) and Petrofree LE (a 

linear-α-olefin). It is not known which SBM was actually used. 

MISSISSIPPI CANYON BLOCK 292 (POST-DEVELOPMENT SITE) 

Mississippi Canyon Block 292 is located in the Central Planning Area, approximately 200 km 

southeast of New Orleans. The water depth is approximately 1,034 m. The block is leased by 

Texaco, and the prospect has been named Gemini. 

Figure 7 shows a diagram of wellsite locations at the MC 292 near-field site. Five wells were 

drilled at three closely-spaced locations near the center of the site between May 1995 and 

July 1999. From south to north, these are Well Nos. 1, 3 (and sidetrack), and 4 (and sidetrack). 

These are producing wells with a subsea manifold and flowlines carrying gas to VK 900, about 

44 km to the north. All three wells and sidetracks were drilled using a semisubmersible rig. The 

Exploration Plan indicates an anchor radius of approximately 3,000 m. 

Well Nos. 1, 3, and 4 were drilled with WBM, which were discharged along with cuttings. These 

included initial discharges at the seafloor during jetting, and subsequent discharges from the 

drilling rig. The two sidetracks were drilled using Novaplus, an internal olefin SBM. Due to the 

short duration of drilling for the two sidetracks (11 days and 8 days), relatively small quantities of 

SBM cuttings were discharged from the drilling rig. 

The nearest other wells are 2.5 km to the north in MC 248 (three wells, January to March 2000). 

Several other wells were drilled in the area, but any effects are assumed to be negligible in 

comparison with the wells drilled within the MC 292 near-field site. 
12 

Figure 6. Well locations in and near the Garden Banks Block 602 near-field site. 
13 

Legend

Previous wellsite

N

S

EW

Anchor radius

(3,000 m)

500-m radius

(a) Wells within the

MC 292 near-field site

Well No. 1

(May-Sep 1995)

Well No. 3 and sidetrack

(Feb-Jun 1999)

Well No. 4 and sidetrack

(Feb-Jul 1999)

Wellheads, manifold,

and other structures 

located in this area 

(see Fig. 3.10)

500-m radius

3 wells in MC 248

Jan-Mar 2000

Well No. 1, No. 3 and 

sidetrack, No. 4 and 

sidetrack (May 1995-Jul 1999)

1 well in MC 291

(Aug-Oct 1997)

Flowlines and 

umbilical to/from

VK 900

(b) Anchor radius and 

nearby wells

Figure 7. Well locations in and near the Mississippi Canyon Block 292 near-field site. 
15 

Study Methods 

SAMPLING SCHEDULE 

Six cruises were conducted during this study – three geophysical surveys and three 

chemical/biological sampling cruises (Table 3). The geophysical surveys mapped the seafloor 

within a 3,000-m radius around each near-field site. The chemical/biological cruises collected 

samples and photographs within a 500-m radius around each near-field site and in smaller circular 

areas (204-m radius) at each far-field site. 

Table 3. Sampling cruises. 

Cruise Site 1A 1B 2A 2B 3A 3B 

Cruise Type: Geophysical Chem/Biol Geophysical Chem/Biol Geophysical Chem/Biol 

Cruise Start: 11/10/2000 10/23/2000 6/24/2001 7/8/2001 8/7/2002 8/4/2002 

Cruise End: 1/1/2001 11/17/2000 7/7/2001 7/25/2001 8/12/2002 8/14/2002 

VK 916 pre-drilling pre-drilling -- -- post-drilling post-drilling 

GB 516 -- post-drillinga post-drilling post-drilling -- -- 

GB 602 -- -- post-drilling post-drilling -- -- 

MC 292 -- -- post-drilling post-drilling -- -- 

a

 Cruise 1B at GB 516 preceded drilling of development wells; however, exploration wells had been drilled previously. 

Figure 8 shows the timing of the cruises in relation to drilling at VK 916 and GB 516 during this 

study. (All previous drilling at GB 602 and MC 292, as well as exploration drilling at GB 516, 

occurred before this study began, and the dates are not shown.) Cruises 1A (geophysical) and 1B 

(chemical/biological) were pre-drilling surveys of VK 916 and a pre-development survey of 

GB 516. Cruises 2A and 2B were post-development surveys of GB 516, GB 602, and MC 292. 

Cruises 3A and 3B were post-exploration surveys of VK 916. 

GEOPHYSICAL SURVEYS 

The goal of the geophysical cruises was to determine (1) the areal extent and accumulation of 

muds and cuttings; (2) the physical modification or disturbance of the sea bed due to impacts 

from anchors and their mooring systems; and (3) the accumulation of debris. 

Bathymetric, side-scan sonar, and subbottom profile data were collected at each site and 

interpreted to map possible anchor scars and accumulations of cuttings and drilling muds. An 

area approximately 3,000 m in radius was surveyed at each near-field site, to encompass the 

anchor patterns for typical moored drilling units in this water depth. 

Cruise 1A was the only survey conducted using a deep-tow system, which involved two boats. 

The data collection systems in the deep-tow fish included a dual frequency side-scan sonar, a 

chirp subbottom profiler, and a deepwater precision depth sensor. Cruise 1A was originally 

intended to survey all of the sites. However, due to extended weather delays, as well as 

equipment, software, and vessel problems, VK 916 was the only site surveyed.
16 

Figure 8. Schedule including sampling cruises and wells drilled during this study. Previous 

drilling at GB 516, GB 602, and MC 292 occurred at various times before this study, 

and dates are not shown (see Chapter 3). 

Cruises 2A and 3A were conducted using the C&C Technologies autonomous underwater vehicle 

(AUV), the HUGIN 3000 (Figure 9). Data collection systems included a dual-frequency 

side-scan sonar, a chirp subbottom profiler, and a swath bathymetry system. Cruise 2A visited 

GB 516, GB 602, and MC 292, whereas Cruise 3A revisited VK 916 after exploration drilling. 

CHEMICAL/BIOLOGICAL SAMPLING 

Chemical/biological sampling was intended to document (1) physical and chemical modification 

of sediments; and (2) effects on benthic organisms. Samples and data were collected with four 

types of equipment: 

• Box core sampler (to obtain samples of sediment, microbiota, meiofauna, and macroinfauna) 

• Sediment profile imaging (SPI) camera (to obtain cross-sectional images of the upper 

sediment column) 

• Still camera on a towed/dragged sled (to obtain photographs of the seafloor and megafauna) 

• Baited traps (to obtain animals for tissue analyses) 

Near-field samples were collected within a 500-m radius around the drilling location. This area 

was chosen in order to sample intensively where SBM cuttings are most likely to accumulate at 

concentrations sufficient to produce biological effects. Each far-field site was a circle 204 m in 

radius, such that the total area of six far-field sites combined was equal to that of the near-field 

site, and the number of samples was allocated so that the sampling intensity was the same. 
Figure 9. Working configuration of the HUGIN 3000 autonomous underwater vehicle 

showing the “free-swimming” HUGIN collecting data from a predetermined height 

above the seafloor. The HUGIN remains in constant communication with the 

mother ship through acoustic links. 

Figure 10 shows the idealized sampling pattern. Box cores were collected at 12 randomly 

selected locations within the 500-m radius at each site (eight samples within 300 m and four 

samples between 300 and 500 m radii). Box cores also were collected at 12 far-field locations 

(two locations selected randomly within each of six far-field sites). Sediment profile photographs 

were collected along three drift transects in the near-field and along one transect at each of three 

far-field sites, with approximately 36 images per transect. Still photographs of the seafloor and 

megafauna were collected along three transects at each near-field site and along one transect at 

each of three far-field sites (approximately 375 photographs per transect). Baited traps were 

deployed at the two post-development sites to collect organisms for tissue analyses. For each 

site, eight traps were deployed in the near-field, and four traps were placed at each of two 

corresponding far-field sites. Actual numbers of samples and transects differed from the planned 

number in some cases (Table 4).

17 
18 

>10 km

Far-Field Site GB 602 and MC 292

Post-Development Sites

 Legend:

= Box core (12 per site and 2 per far-field ) near-field site

= Megafauna trap (8 per site and 4 at each of two far-field s) near-field site

 Legend:

= Box core (12 per near-field site and 2 per far-field site)

= Sediment profile imagery transect (3 per site 

 And 1 at each of three far-field s; goal = 36 images

 per transect)

near-field

site

= Megafauna camera sled drift (3 per site and 1 at

 each of three far-field s; goal = 375 photos per transect)

near-field 

site

VK 916 and GB 516

Exploration and 

Exploration/Development Sites

Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site

>10 km

1,000-m diameter

(500-m radius)

1,000-m diameter

(500-m radius)

(sampled before and after drilling)

(sampled only after drilling)

= Sediment profile imagery transect (3 per site and

 1 at each of three far-field s; goal = 36 images

 per transect)

near-field 

site

= Megafauna camera sled drift (3 per site and 1 at

 each of three far-field s; goal = 375 photos

 per transect)

near-field 

site

Figure 10. Idealized sampling plans for study sites.
19 

Table 4. Numbers of samples collected and transects completed during chemical/biological 

cruises. 

Box Cores SPI Transects Sled Transects Trap Setsa

Cruise and Site 

NF FF NF FF NF FF NF FF 

Cruise 1B 

VK 916 12 10 2 3 3 4 -- -- 

GB 516 12 12 3 3 3 5 -- -- 

Cruise 2B 

GB 516 12+3b 12 3 3 3 3 -- -- 

GB 602 12+3b 12 3 3 3 3 2 1 

MC 292 12+3b 12 2 3 3 3 2 2 

Cruise 3B 

VK 916 12+3b 12 3 0c 3 2 -- -- 

NF = near-field; FF = far-field; SPI = sediment profile imaging. a Each trap set included four traps. 

b Three additional “discretionary” box cores samples for subsurface sediment analyses were taken at near-field sites 

during Cruises 2B and 3B. 

c During Cruise 3B, far-field transects were attempted at VK 916 FF2 and FF4; however, due to problems with the SPI 

camera, no usable data were collected. 

Box Coring 

Sediment samples were collected with a 0.25-m2

Gray-O’Hara box core. The box was partitioned 

into subcores for various sample types: 

Deployment of box core 

• Sediment grain size and clay mineralogy 

• Sediment metals and total organic carbon 

• Sediment redox chemistry (direct Eh and pH 

measurements) 

• Sediment hydrocarbons 

• Microbiota 

• Meiofauna 

• Macroinfauna 

• Harpacticoid genetic diversity 

• Sediment toxicity testing (GB 602 and 

MC 292 only) 

In addition, sediment samples for analysis of radionuclides and interstitial water chemistry were 

removed from selected box cores at each site. 

Sediment Profile Imaging 

A sediment profile camera was used to take photographs of the sediment column. The profile 

camera works like an inverted periscope with a deep-sea 35-mm camera mechanism mounted 

horizontally inside a water-tight housing on top of a wedge-shaped prism. The camera provides 

images of up to 20 cm of the upper sediment column in profile. At each site, SPI photographs 

were collected along three drift transects in the near-field and at three of the six far-field sites. 
20 

Seafloor Photographs 

A deep-sea 35-mm camera on a dragged sled was used to collect still photographs of the seafloor 

and benthic organisms. These photographs were used for visual analyses of sediment appearance 

and megafauna. Once deployed on the bottom, the sled was in constant contact with the seafloor. 

This method produced close-up images of the seafloor looking straight down from a height of 

0.6 m. The area of each photograph was about 0.22 m2

. 

Baited Traps 

Baited traps were used during Cruise 2B to collect animals for tissue analyses at the two 

post-development sites (GB 602 and MC 292) and corresponding far-field sites. The traps were 

similar in design to those used in the U.S. Pacific Northwest in the commercial dungeness crab 

fishery. At each trap station, a set of four baited traps was deployed along a common, weighted 

line. After deployment, each trap set was allowed to “fish” for a period of 18 to 24 hours, after 

which the traps were retrieved and any organisms captured were removed. 

Two benthic organisms were obtained from the traps in sufficient numbers for tissue analyses. 

These were the isopod Bathynomus giganteus and the red crab Chaceon quinquedens. 

30 cm

Isopods – Bathynomus giganteus

20 cm 

Red crabs – Chaceon quinquedens
21 

Study Findings 

FAR-FIELD CONDITIONS 

Based on environmental characteristics of the far-field sites (Table 5), the four sites are broadly 

similar. However, water depths at GB 516 and MC 292 were about 120 m shallower than at 

VK 916 and GB 602. Also, proximity to the Mississippi River mouth is an important influence 

on ambient conditions, as exemplified by lower sediment oxygen levels at the two easternmost 

sites (VK 916 and MC 292) and the relatively high sedimentation rate at MC 292. 

Table 5. Environmental characteristics of far-field sites. Multiple values for a given site 

represent data from two different cruises. 

Sediment Characteristics 

Site 

Water 

Depth 

(m) 

Approx. 

Dist. to 

Miss. R. 

(km) 

Sedimentation 

Rate (cm/yr) 

Mean 

Grain Size 

(μm) 

Mean Total 

Organic 

Carbon (%) 

Mean 

Integrated 

Oxygen 

(nmol/cm2) 

Mean 

Barium 

(μg/g) 

VK 916 1,125 120 0.06 2.44 

1.82 

1.10 

1.53 

90 

84 

805 

866 

GB 516 1,033 357 0.07 1.96 

2.40 

0.52 

1.08 

153 

165 

1,281 

1,478 

GB 602 1,125 370 0.04 2.03 1.10 227 821 

MC 292 1,034 69 0.14 1.64 1.44 65 754 

Biological comparisons among sites are complicated by temporal differences. At the VK 916 

site, there was an order of magnitude decrease in microbial biomass as well as meiofaunal and 

macroinfaunal densities between cruises. A similar decrease was evident at GB 516 for microbes 

and meiofauna but not for macroinfauna. There were also some geographic differences in benthic 

community composition. Stations from GB 516 and GB 602, which are very close 

geographically (stations within tens of kilometers), were more similar to each other than to the 

stations from VK 916 and/or MC 292, which are relatively close to each other but about 400 km 

away. 

NATURE AND EXTENT OF IMPACTS 

Table 6 summarizes the physical, chemical, and biological impacts noted during this study. 

These are discussed briefly in individual subsections. 
22 

Table 6. Impact summary. 

Parameter Impact Extent Notes 

PHYSICAL 

Anchor scars Presence of anchor scars Within about 3-km 

radius from wellsites, 

8 radial directions 

Individual scars <100 m to 

>3 km in length 

Seafloor equipment 

and debris 

Presence of structures 

and debris on bottom 

Subsea wellheads etc. 

within few hundred 

meters; flowlines and 

umbilicals extend 

many kilometers to 

nearby blocks; 

occasional debris 

within 3-km radius 

VK 916 had the smallest 

number of side-scan 

“debris” contacts 

Mappable mud and 

cuttings deposits 

Presence of 

geophysically detectable 

mud and cuttings zones 

around wellsites 

Well jetting deposits 

mostly within 100 m 

of wellsites. Rig 

discharge deposits 

extending from 

several hundred 

meters to about 1 km 

Areal extent greatest at 

GB 516 and GB 602, 

smallest at VK 916 

Sediment grain size Increased percentage of 

(mainly coarse) sand at 

some stations 

Primarily within 

300-m radius 

Cuttings similar in grain 

size to surficial sediments 

Sediment color/ 

appearance 

Areas of black bottom 

within near-field 

Extent not 

determined. Most 

consistent at GB 602; 

patchy at other sites 

Black bottom is presumed 

due to the presence of 

cuttings and/or mud 

Thickness of drilling 

deposits 

Isopach maps from 

subbottom profiling at 

VK 916 indicate 

thickness ranging from 

0 to 45 cm 

Extent not 

determined 

Sectioning of a few 

discretionary cores at 

VK 916, GB 516, and 

GB 602 indicates layer of 

mud/cuttings several 

centimeters thick. Full range 

of thicknesses was not 

sampled by these cores 

CHEMICAL 

Sediment Ba Elevated by orders of 

magnitude at nearly all 

near-field stations 

Nearly all stations 

within 500-m radius; 

highest values within 

300-m radius 

One discretionary station 

1,000 m from GB 602 site 

center had elevated Ba; 

far-field data suggest slight 

effects at 2 to 3 km 

Sediment SBF Elevated by orders of 

magnitude at most 

near-field stations 

Most stations within 

500-m radius; nearly 

all stations within 

300-m radius 

One discretionary station 

1,000 m from GB 602 site 

center had elevated SBF; 

far-field data suggest slight 

effects at 2 to 3 km 

Sediment PAH Elevated at two GB 516 

stations 

Within 300-m radius Not attributable to SBM 

since these discharges do 

not contain PAH 
Table 6. (continued). 

23 

Parameter Impact Extent Notes 

Metals other than Ba Elevated concentrations 

of As, Cd, Cr, Cu, Hg, 

Pb, and Zn at some 

stations; lower 

concentrations of Al, Fe, 

Mn, Ni, and V 

Few stations Differences are related to 

concentrations of these 

metals in barite, or reductive 

dissolution in the case of 

Mn 

Sediment TOC Elevated at some 

near-field stations 

Primarily within 

300-m radius 

High TOC is associated 

with high SBF 

concentrations 

Sediment oxygen/Eh Low oxygen and 

negative Eh at some 

near-field sites 

Primarily within 

300-m radius 

Poor redox conditions are 

associated with high SBF 

levels; least severe at 

MC 292 

Redox potential 

discontinuity 

Decreased depth of the 

oxidized layer in the 

near-field 

Patchy within 

near-field 

Poor redox conditions are 

associated with high SBF 

levels; least severe at 

MC 292 

Metals in isopods 

Bathynomus giganteus

Elevated Ba, Cr, and Pb 

in isopods at GB 602 but 

not at MC 292; lower 

Cd and Hg in isopods at 

GB 602, but not at 

MC 292 

Extent not 

determined 

Metals in red crabs 

Chaceon quinquedens

Elevated Ba and reduced 

As, Cd, Cu, Hg, Ni, V, 

and Zn in crabs at 

GB 602; elevated Ba, 

Cd, Cr, and V in crabs at 

MC 292 

Extent not 

determined 

Most consistent finding is 

elevated Ba (in isopods 

from GB 602 and in crabs 

from both GB 602 and 

MC 292). Possibly due to 

sediment particles in gut 

Hydrocarbons in 

isopods and red crabs -- -- No impact on tissue PAH concentrations detected 

BIOLOGICAL 

Benthic communities 

(general conditions) 

Azoic areas; areas of 

pioneering communities; 

poor organism-sediment 

index 

Patchy within 

near-field 

MC 292 appears least 

affected 

Microbiota Elevated microbial 

biomass (ATP) at some 

stations 

Patchy within 

near-field 

Generally, stations with 

high ATP had high SBF 

levels; order of magnitude 

decrease in ATP between 

cruises at VK 916 and 

GB 516 (both near- and 

far-field) 

Microbial mats Microbial mats seen in 

SPI images from several 

near-field stations 

Patchy within 

near-field 

Mats not seen at MC 292 or 

at GB 516 on Cruise 2B 

Meiofauna Increased meiofaunal 

densities in near-field 

including annelids, 

harpacticoids, and 

nematodes 

Primarily within 

300-m radius 

Order of magnitude 

decrease in meiofauna 

between cruises at VK 916 

and GB 516 
Table 6. (continued). 

24 

Parameter Impact Extent Notes 

Meiofauna – 

harpacticoid genetics 

Low genetic diversity in 

near-field populations of 

Bathycletopsyllus sp. 

Extent not 

determined 

Bathycletopsyllus sp. absent 

from far-field sites, but 

abundant at near-field sites; 

low genetic diversity is 

consistent with expansion 

from a small population 

Macroinfauna Increased polychaete, 

gastropod, and bivalve 

densities in near-field; 

reduced amphipod and 

ostracod densities 

Primarily within 

300-m radius 

-- 

Megafauna Higher fish densities and 

lower ophiuroid 

densities within 

near-field 

Extent not 

determined 

Effects evident at VK 916 

and GB 516 but not at 

GB 602 or MC 292 

Al = aluminum; ATP = adenosine triphosphate; Ba = barium; Cd = cadmium; Cr = chromium; Cu = copper; Eh = redox 

potential; Fe = iron; Hg = mercury; Mn = manganese; Ni = nickel; PAH = polycyclic aromatic hydrocarbons; Pb = 

lead; SBF = synthetic-based fluid; SBM = synthetic-based mud; SPI = sediment profile imaging; TOC = total organic 

carbon; V = vanadium. 

Anchor Scars 

Based on the geophysical surveys, anchor scars extended to the limit of the surveyed area (about 

3-km radius) around the wellsites. Individual anchor scars, all in soft bottom areas, ranged from 

less than 100 m to over 3 km in length. VK 916 had only one set of anchor scars associated with 

exploratory drilling. However, other scars remained from previous drilling in adjacent blocks 

(VK 872 and VK 873). The other three sites, surveyed post-development, had several sets of 

anchor scars, reflecting the use of different drilling rigs to drill various wells over time. 

Seafloor Equipment and Debris 

At the three sites where development drilling had occurred and production was ongoing (GB 516, 

GB 602, and MC 292), the geophysical survey identified known structures on the seafloor, 

including pipelines and flowlines, subsea wellheads, umbilical termination modules, etc. The 

wellheads and associated equipment were mostly within 200 m of the site centers. Pipelines and 

flowlines extended many kilometers to platforms or other structures in nearby lease blocks. 

At all four sites, there were side-scan sonar contacts identified as debris. These could include 

pieces of pipe, etc. that presumably fell from the drilling rig or other vessels. VK 916 had the 

smallest number of contacts (four), whereas there were numerous contacts at the other sites. 

Debris locations were not investigated further in this study. 

Mappable Cuttings and Mud Deposits 

Side-scan sonar and subbottom profiler records were used to map deposits of drilling mud and 

cuttings around the wellsites. However, there are no unique geophysical signatures for these 

deposits. The mapped distribution of mud and cuttings was inferred from a combination of 

geophysical data and information about discharges. No “ground truth” sampling or analysis was 
25 

conducted to verify that cuttings or drilling mud particles were present in these zones, or to 

determine the minimum thickness that could be detected. 

Two zones were recognized (Figure 11). In the geophysical cruise reports, these were identified 

as “drilling muds” and “drill cuttings.” However, both zones probably consist of both muds and 

cuttings, and primarily the latter. The first zone, in the area immediately around wellsites, was 

recognized by a combination of a smooth seafloor (little backscatter on side-scan sonar records) 

and a high amplitude response at the seafloor on high resolution subbottom profiles. This zone 

probably represents an area of cuttings and spud mud deposition from initial jetting of wells. 

Cuttings probably account for most of the materials deposited during this stage, because spud 

muds are primarily seawater with low solids concentrations. Typically, these initial deposits, 

along with excess cement from setting of the well casing, create a shallow, compact mound. 

Other activities including installation of subsea wellheads, flowlines, and umbilical modules at 

the three post-development sites (GB 516, GB 602, and MC 292) may also have contributed to 

the observed reflectance pattern around the wellsites. 

Figure 11. Part of the side-scan sonar mosaic for the MC 292 area showing a wellsite in the 

adjacent block (MC 248) where three wells were drilled about 1.5 years before 

Cruise 2A. Though this is not one of the study sites, the image illustrates several of 

the features commonly noted. 

More extensive areas where side-scan sonar showed high reflectivity extending in a radial pattern 

around the wellsites (Figure 11) were interpreted as primarily cuttings (with adhering muds) 

from rig discharges. Although WBMs were also discharged from the rig at all wellsites, they 
26 

probably were widely dispersed before reaching the seafloor and would not likely be detected 

geophysically. 

Generally, areas mapped as well jetting deposits were identified within about 100 m of wellsites. 

Areas mapped as rig discharge deposits typically extended several hundred meters from wellsites, 

with the greatest extent (about 1 km) observed at GB 602 and GB 516. The estimated area of 

mud and cuttings accumulation was greatest at GB 516 (Cruise 2B) and smallest at VK 916 

(Table 7). 

Table 7. Geophysically estimated areal extent of drilling mud and cuttings deposits at each 

near-field site. Both well jetting deposits and rig discharge deposits are assumed to 

consist of both drilling muds and cuttings (primarily the latter). 

Site Cruise Stage Well Jetting 

Deposits (ha) 

Rig Discharge 

Deposits (ha) 

VK 916 3A Post-exploration None mapped 13.37 

GB 516 2A Post-development 2.48 108.53 

GB 602 2A Post-development 1.08 43.18 

MC 292 2A Post-development 0.78 25.61 

Sediment Grain Size 

Sediments at all four sites were predominantly clay and silt. There were no gross changes in 

grain size around the wellsites. A few near-field samples had slightly elevated percentages of 

sand-sized particles. Most stations had <10% sand. Higher percentages were evident at several 

stations at GB 516 on Cruise 2B (up to 18%) and at one MC 292 station (14%). GB 602 had 

higher sand percentages both near-field and far-field, but one near-field station had the highest 

value (21%). Near-field, post-drilling elevations in sand percentages were detected statistically at 

VK 916, GB 516 (Cruise 2B), and MC 292, but not at GB 602 nor at GB 516 on Cruise 1B. 

Grain size variations were patchy within near-field sites, and the spatial pattern did not 

necessarily correspond to the geophysically mapped drilling discharge zones. Also, except for 

VK 916, there were no strong correlations between percentages of sand, silt, or clay and 

concentrations of drilling mud indicators. These findings suggest that grain size distribution 

per se is not a very good indicator of drilling discharge impacts. 

Sediment Color/Appearance 

Color analysis of seafloor photographs showed that areas of “black bottom” (Figure 12) were 

present on post-drilling surveys at all four near-field sites but not at far-field sites (or VK 916 

near-field on the pre-drilling survey). Black surface layers also were seen in some sediment 

profile images. The black bottom was covered in some images by white-to-red mats consistent 

with the sulfide-using bacteria Beggiatoa. These bacterial areas may indicate sites of local 

bottom anoxia supported by bacterial degradation of SBM. Although impacts were patchy at all 

sites, most images at GB 602 showed dark bottom, possibly indicating that this site has cuttings 

more thinly spread over a larger area. The patterns of black bottom coincided with geophysical 

mapping of cuttings reasonably well where direct comparisons could be made.
27 

 

Pre-Drilling Post-Drilling 

Figure 12. Comparison of seafloor photographs from before and after drilling at VK 916. Note 

the darker color of the sediment surface in the post-drilling photograph. 

Thickness of Mud and Cuttings Deposits 

During the post-drilling geophysical survey at VK 916, the thickness of mud and cuttings 

deposits around the wellsite was estimated to range from 0 to 45 cm. Discretionary core samples 

taken in likely mud/cuttings areas during Cruises 2B and 3B also provide information about the 

thickness of mud and cuttings at a few stations. These values ranged from 2 to 10 cm. These 

values do not represent the thickest deposits since the area immediately surrounding the wellsites 

was not sampled. 

Drilling Fluid Tracers (Barium and SBF) 

Barium and synthetic-based fluid (SBF1) concentrations are sensitive tracers of drilling muds. 

Barite (BaSO4) is a major ingredient in both WBMs and SBMs. SBF concentrations are related 

to the amount of SBF adhering to discharged cuttings. 

Far-field barium values typically were between 500 and 1,500 μg/g, except at GB 516, where 

values as high as 2,780 μg/g were observed. Far-field SBF concentrations generally were less 

than 10 μg/g, except for the Cruise 3B values at VK 916, which were in the 10 to 15 μg/g range. 

All of the far-field sites had non-zero SBF concentrations (i.e., above the detection limits). 

However, these values are probably due to small quantities of natural hydrocarbons that eluted in 

the same range as SBFs in the analytical procedure. 

Barium and SBF concentrations were significantly higher at sites near drilling (near-field, 

post-drilling) than at sites far from drilling (i.e., far-field sites or the near-field, pre-drilling site at 

VK 916). Post-drilling, near-field concentrations were as high as 351,000 μg/g for barium and 

117,280 μg/g for SBF. While nearly all near-field stations had elevated barium, SBF increases 

were not as consistently elevated at VK 916 and MC 292 as compared with the other two sites. 

1 The terms synthetic-based mud (SBM) and synthetic-based fluid (SBF) are often used interchangeably. However, in this report, 

SBF refers to chemical measurements of the base fluid in sediment, whereas SBM refers to the drilling muds and cuttings per se.
28 

Figure 13 shows concentrations of barium and SBF for all sites combined, with stations grouped 

into three categories (far-field, near-field 300 to 500 m, and near-field <300 m). Most near-field 

stations had elevated barium and SBF concentrations, with the highest values within 300 m. 

However, some near-field stations had concentrations similar to those at far-field stations. 

There are some data indicating effects beyond the 500-m near-field radius. One discretionary 

station at GB 602 on Cruise 2B was about 1,000 m southwest of the site center and had a barium 

concentration of 14,800 μg/g, compared with far-field values typically less than 2,000 μg/g. The 

SBF concentration was 930 μg/g, compared with far-field values typically less than 15 μg/g. 

Also, one of the VK 916 far-field sites had slightly elevated barium and SBF concentrations on 

both cruises, possibly due to drilling discharges in nearby blocks. 

Polycyclic Aromatic Hydrocarbons (PAH) 

With two exceptions, sediment PAH concentrations ranged from 43 to 748 ng/g dry wt. Two 

samples at GB 516 had considerably higher concentrations (Figure 13). On Cruise 1B, one of the 

random stations had 3,470 ng/g PAH, and on Cruise 2B, one of the discretionary stations had 

23,840 ng/g in the top 2 cm. Both of these stations were within the 300-m radius. The source of 

the PAH is suggested to be from some other contaminant from the drilling or production activity, 

as SBMs do not contain PAH. 

Sediment Metals Other Than Barium 

Near-field concentrations of arsenic, cadmium, chromium, copper, lead, mercury, and zinc were 

elevated in some near-field sediment samples as compared with far-field samples. Generally, 

elevated concentrations of these metals were associated with high barium concentrations (i.e., 

drilling fluid). However, these elevated concentrations are within the expected range of 

background concentrations for uncontaminated marine sediments (Neff 1987). 

Relatively low concentrations of aluminum, iron, nickel, and vanadium were measured in some 

near-field samples and were attributed to dilution of ambient sediments with barite, which 

contains no significant amounts of these metals. Also, concentrations of manganese were lower 

and more variable at near-field sites, a result attributed to reductive dissolution of this metal at 

stations where the presence of drilling discharges created reducing conditions. 

Considerable interest has been generated regarding concentrations of mercury in sediments 

adjacent to drilling sites because concentrations of total mercury near drilling sites are often 2 to 

10 times higher than in nearby background sediments (Neff 2002). This mercury is known to be 

a natural impurity in barite (Trefry and Smith 2003). Calculations indicate that the mercury 

concentrations in barite deposited at these sites are in line with U.S. Environmental Protection 

Agency (USEPA) regulations, which allow a maximum level of 1 mg/kg in barite (USEPA 1993). 
29 

Figure 13. Sediment concentrations of barium, synthetic-based fluids (SBF), and polycyclic

aromatic hydrocarbons (PAH) in relation to proximity to drilling. Points are individual

stations, including non-random "discretionary"stations. "Far-field" includes near-field,

pre-drilling stations at VK916.

> 10 km from

Drilling

Barium Concentration (μg/g)

300-500 m

from Drilling

<300 m

from Drilling

10

SBF Concentration (μg/g)

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

102

103

104

105

Synthetic-Based Fluids (SBF)

10

102

103

104

PAH Concentration (μg/g)

Polycyclic Aromatic Hydrocarbons (PAH)

Note: PAH could not be analyzed at

Barium

102

103

104

105

106

f

t

*

our <300 m stations at VK 916 due

o high SBF concentration

Far-Field Near-Field

1

Far-Field Near-Field

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

GB 516 Cruise 2B

NF-DS2

1

*

GB 516 Cruise 1B

NF-B02
30 

Metals and Hydrocarbons in Biota 

Concentrations of 11 metals (arsenic, barium, cadmium, chromium, copper, iron, lead, mercury, 

nickel, vanadium, and zinc) were determined in samples of the giant isopod Bathynomus 

giganteus and the red crab Chaceon quinquedens from near-field and far-field stations at sites 

GB 602 and MC 292. 

In isopods from GB 602, levels of barium, chromium, and lead were greater at near-field stations, 

whereas concentrations of cadmium and mercury were greater at far-field stations. No significant 

differences were detected for isopods at MC 292. For red crabs from GB 602, barium 

concentrations were higher in the near-field, whereas concentrations of arsenic, cadmium, copper, 

mercury, nickel, vanadium, and zinc were higher in the far-field. For MC 292, concentrations of 

barium, cadmium, chromium, and vanadium were greater at near-field stations. 

The most consistent finding in the metals data is elevated barium (in isopods from GB 602 and in 

crabs from both GB 602 and MC 292). The elevated barium concentrations detected in isopods 

and crabs may reflect small amounts of sediment particles retained in the gut. 

Both species also were analyzed for tissue PAH concentrations. PAH concentrations ranged from 

38.6 to 416 ng/g dry. No significant difference was found between the total PAH body burden 

concentrations in the two deepwater organisms. No significant difference also was found 

between the total PAH body burden concentrations at near-field and far-field stations. 

Total Organic Carbon 

Sediment TOC concentrations at far-field and pre-drilling near-field stations ranged from 0.28% 

to 1.73%. In contrast, near-field concentrations ranged from 0.26% to 7.16%. Elevated TOC was 

noted at a relatively few near-field stations. With one exception, elevated TOC (greater than 2%) 

was observed only within 300 m of the site center. 

All stations with elevated TOC had high SBF and barium concentrations. TOC was strongly 

correlated with SBF concentration within near-field sites (Figure 14), supporting the conclusion 

that the organic-rich areas were due to deposition of SBM cuttings. Most of the TOC 

concentrations exceeding 2% occurred at either VK 916 or GB 516 (Cruise 2B). There was little 

or no elevated TOC at MC 292 or at GB 516 post-exploration (Cruise 1B). 

Sediment Redox Conditions 

Several study components provided information about sediment redox conditions. These 

included direct measurements of sediment oxygen concentrations and Eh profiles, as well as 

visual observations through SPI. As a useful summary measure, Figure 15 compares integrated 

oxygen levels in the sediment column at each site. With a couple of exceptions, oxygen levels 

were markedly reduced at near-field, post-drilling sites. This effect was most pronounced at 

GB 602 and at GB 516 on Cruise 2B, where all near-field and discretionary stations had much 

lower oxygen than the far-field. 
31 

Figure 14. Relationship between total organic carbon (TOC) and synthetic-based fluid (SBF)

concentrations in near-field sediments at the four sites.

TOC vs. SBF

0

2

4

6

8

0 25000 50000 75000 100000 125000

SBF Concentration (μg/g)

Total Organic Carbon (%)

VK 916 Cr3

GB 516 Cr2

GB 602 Cr2

MC 292 Cr2

Far-Field TOC Range

TOC

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

0

2

4

6

8

Total Organic Carbon (%)
32 

DISC NF FF DISC NF FF

0

50

100

150

200

Integrated O2 (nmol/cm2)

GB 516

Post-Exploration

122

153

25 17

165

Post-Development

NF FF NFDS FF

Figure 15. Integrated amounts of oxygen in the sediment column at near-field (NF), far-field (FF),

and discretionary (DS) stations at all four sites. Markers and numbers show means

and lines show standard deviations. The standard deviation for some DS samples is

smaller than the marker.

Post-Development

Legend

Discretionary stations

Near-field stations

Far-field stations

DISC NF FF DISC NF FF

0

50

100

150

200

250

Integrated O2 (nmol/cm2)

GB 602

22

227

55

38

65

NF FF DS NF FF

MC 292

DS

25

Post-Development

NF MF FF NF MF FF 0

30

60

90

120

150

Integrated O2 (nmol/cm2

)

VK 916

Pre-Exploration

86

44

11

84

NF FF DS NF FF

86 90

Post-Exploration
33 

On Cruise 1B at VK 916, where there was no previous drilling, most near-field stations had 

oxygen levels similar to those seen in the far-field. Similarly, at GB 516, although there was 

previous drilling prior to Cruise 1B (two exploration wells), oxygen levels at many near-field 

stations initially were similar to those in the far-field. However, after subsequent drilling of five 

development wells, the near-field oxygen levels were greatly reduced on Cruise 2B. 

At MC 292, differences between near-field and far-field stations were less striking. Oxygen 

levels were already low in MC 292 area sediments due to the proximity of the Mississippi River 

delta and the associated higher sedimentation rate. MC 292 and VK 916 had lower far-field 

oxygen levels than the two western sites (GB 516 and GB 602). In addition, MC 292 had the 

smallest quantities of SBM discharges and the longest time since drilling. 

Benthic Communities 

This study included several biological components. SPI provided an overview of benthic 

community health and successional status. Box core samples were analyzed for microbial 

adenosine triphosphate (ATP), and meiofauna and macroinfauna. Additional studies of 

harpacticoid copepod genetics were conducted at GB 602 and MC 292. Photographs taken with 

the camera sled were examined for the presence of megafauna. 

Sediment Profile Imaging 

SPI was used to characterize benthic communities at each site. Images were analyzed to 

determine successional stage using a paradigm based on recolonization of disturbed 

shallow-water habitats (Rhoads and Germano 1982). Application of this method to our study 

sites is an extrapolation, as detailed knowledge about infaunal successional stages is not available 

for these sites. 

Post-drilling surveys indicated that the near-field zone at three of the four sites (VK 916, GB 516, 

and GB 602) consisted of a mosaic of poor benthic conditions (patchy areas of low or negative 

organism-sediment index [OSI] and azoic or retrograde benthic communities) alternating with 

areas of moderately high benthic habitat quality. The degraded near-field habitats stand in 

marked contrast to those seen at far-field sites. 

At VK 916, observations suggested that recent surface disturbance (deposition, resuspension, 

and/or erosion) may have selectively compromised near surface-dwelling species relative to 

deeper-living infauna. A similar phenomenon has been noted in studies of the ecological effects 

of bottom trawling (Rosenberg et al. 2003). 

At MC 292, only a small number of stations appeared to have been visibly affected by drilling 

muds and cuttings. Only a few stations along one transect had signs of anoxic conditions. There 

was evidence of infaunal activity at most stations, and no microbial mats were observed at this 

site. The relative lack of impacts is consistent with box core redox measurements and may be 

related to the low volume of SBM cuttings discharged at this site. 
34 

Microbiota 

Microbial data provided mixed results. ATP concentrations measured on Cruise 1B were about 

an order of magnitude higher than those measured on Cruises 2B or 3B. The difference is 

statistically significant for both VK 916 and GB 516. Since the change is noted at both near-field 

and far-field sites, it is not attributable to drilling discharges. However, the temporal difference 

complicates interpretation. 

There is ample evidence that microbial biomass, though patchy, was higher in areas where 

drilling discharges accumulated. For example, sediment profile camera images commonly 

showed microbial mats at near-field sites but not far-field sites (Figure 16). 

Microbial Mat 

Figure 16. Sediment profile image from the near-field 

site at Garden Banks 516 during Cruise 1B 

showing a microbial mat and with the 

sediment column below completely gray to 

black. Image width is 15 cm. 

Within the VK 916 and GB 516 sites, the stations with the highest microbial biomass 

corresponded to those with the highest SBF concentrations. This effect was evident at SBF 

concentrations greater than 1,000 μg/g. A similar effect was not evident at GB 602 or MC 292. 

The increased microbial abundance at some near-field stations is consistent with an organic 

enrichment effect due to the addition of SBM cuttings. The possibility that some inhibition of 

microbial activity also occurs where SBM cuttings accumulate cannot be ruled out. 

Meiofauna 

Meiofaunal densities tended to be higher near drilling (Figure 17). Although there was 

considerable overlap in near-field vs. far-field densities, the highest densities of nematodes, 

harpacticoid copepods, and especially annelids occurred in the near-field. This effect was seen 

mainly within 300 m of the wellsite(s), although a few stations in the 300- to 500-m range also 

had elevated numbers. Meiofaunal densities in the near-field were not consistently correlated 

with drilling indicators (barium, SBF) or other sediment variables (TOC, grain size fractions). 
35 

Figure 17. Meiofaunal and macroinfaunal densities in relation to proximity to drilling. Points

are individual stations including non-random "discretionary" stations. The pre-drilling

survey at Viosca Knoll 916 has been excluded from these plots (higher densities both

near- and far-field would obscure other patterns).

Total Macroinfauna

0

10000

20000

30000

40000

50000

Macroinfaunal Density

(no. per m2)

Total Meiofauna

0

100

200

300

400

Meiofaunal Density

(no. per 10 cm2)

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field
36 

A separate study of harpacticoid genetic diversity was conducted at GB 602 and MC 292. The 

study focused on Bathycletopsyllus sp., which was absent from far-field sites, but abundant at 

near-field sites. Results showed low genetic diversity in near-field populations of 

Bathycletopsyllus sp., which is consistent with expansion from a small, pre-drilling population. 

Macroinfauna 

Macroinfaunal densities tended to be higher near drilling (Figure 17). Although there was 

considerable overlap in near-field vs. far-field densities, the highest densities occurred in the 

near-field. This effect was seen almost entirely within 300 m of the wellsite(s). 

The highest densities of annelids (predominantly polychaetes), gastropods, and bivalves tended to 

occur in the near-field. The opposite was true for amphipods and ostracods, with the highest 

densities generally occurring at one or more far-field stations. 

Annelid 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 annelid and/or 

gastropod densities. Conversely, amphipod densities were negatively correlated with drilling 

indicators, with reduced densities associated with barium concentrations greater than about 

10,000 μg/g and/or SBF concentrations greater than about 1,000 μg/g. 

A detailed taxonomic analysis of 24 stations showed that species composition reflects both 

geographic location and drilling impacts. Some stations “near drilling” had lower diversity, 

lower evenness, and lower richness indices compared with stations “away from drilling” 

(Figure 18). Station/cruise groups most likely affected by drilling (as indicated by high barium 

and SBF concentrations) were dominated by high abundances of one or a few deposit-feeding 

species. For example, at VK 916, Station NF-B07 was numerically dominated by the polychaete 

Capitella capitata, a classic indicator of organic enrichment, and at GB 516, Station NF-B02 was 

numerically dominated by the gastropod Solariella sp. A. 

Megafauna 

Overall, small megafauna identified in photographs showed little or no impact. However, 

localized significant shifts were found for two groups. Fishes were more abundant in the 

near-field, consistent with attraction to disturbance and structure (and possibly, increased food 

supplies in the form of elevated macroinfaunal densities). Ophiuroids were less abundant in the 

near-field, perhaps because these flat organisms were buried by cuttings, or perhaps there was a 

behavior shift, which resulted in fewer animals being seen in the near-field. Near-field vs. 

far-field differences for fishes and ophiuroids were seen mainly at VK 916 and GB 516. There 

was little or no difference between near- and far-field at GB 602 or MC 292. 
37 

Diversity (H')

(a) Density

0

10,000

20,000

30,000

40,000

Near

Drilling

Density (no./m2)

Away from

Drilling

(b) Diversity

Away from

Drilling

Near

Drilling

0

1

2

3

4

Figure 18. Macroinfaunal community characteristics for the detailed, 24-station analysis.

(d) Richness

Richness (D)

0

5

10

15

20

Away from

Drilling

Near

Drilling

"Away from Drilling" = all far-field

stations plus near-field pre-drilling

stations at VK 916

"Near Drilling" = all near-field stations

at GB 516, GB 602 and MC 292, plus

near-field post-drilling stations at

VK 916

Evenness (J')

0

0.2

0.4

0.6

0.8

1

(c) Evenness

Away from

Drilling

Near

Drilling
38 

Discussion of Biological Impacts 

Biological observations consistent with burial and/or stressed conditions include 

• azoic areas (lacking visible benthic macroinfauna) observed in near-field sediment profile 

images from VK 916, GB 516, and GB 602; 

• absence or rarity of surface-dwelling macroinfauna in near-field sediment profile images 

from VK 916; 

• reduced densities of macroinfaunal amphipods and ostracods in the near-field box cores; and 

• reduced densities of ophiuroids in near-field photographs. 

Because SBM cuttings were discharged, toxicity, organic enrichment, and anoxia (including 

potentially toxic concentrations of sulfide and ammonia) are possible contributing factors (Neff et 

al. 2000). 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 was negatively correlated 

with drilling mud and cuttings indicators (barium and SBF) for both sites. However, the 

bioassays were conducted with estuarine amphipods (rather than indigenous deep-sea fauna) and 

do not simulate actual field conditions. 

On the other hand, there is evidence of elevated biological activity associated with a disturbed, 

organically enriched habitat. This includes 

• microbial mats observed in sediment profile images; 

• elevated microbial biomass associated with high SBF concentrations; 

• elevated near-field densities of meiofauna, including annelids, harpacticoid copepods, and 

nematodes; 

• elevated near-field densities of macroinfauna, including annelids, bivalves, and gastropods; 

• predominance of pioneering stage macroinfaunal assemblages at some near-field stations; and 

• elevated near-field densities of fishes at VK 916 and GB 516. 

SITE COMPARISONS 

Areal Extent of Impacts 

Overall, the areal extent of impacts was greatest at two post-development sites (GB 516 and 

GB 602). These two sites had relatively large mapped mud and cuttings zones, and most stations 

had high barium and SBF concentrations. Impact extent was smallest at the VK 916 exploration 

site, where only a single well was drilled. Among the post-development sites, MC 292 had the 

smallest extent of impacts based on both the mapped extent of mud and cuttings deposits and the 

relatively few stations having very high barium and SBF concentrations. The latter result may 

reflect the limited use of SBM at this site and the longer elapsed time since drilling ended there. 
39 

Severity of Impacts 

Generally, the two Garden Banks post-development sites were the most severely affected, having 

high mean barium and SBF concentrations and the greatest reduction in sediment integrated 

oxygen levels. By all measures, MC 292 was least severely affected, having low maximum and 

mean barium and SBF concentrations and the smallest decrease in sediment integrated oxygen 

levels. Mean barium and SBF concentrations in near-field sediments were positively correlated 

with estimated SBM cuttings volume (Figure 19). Decreases in mean sediment oxygen levels 

also were strongly negatively correlated with both mean sediment SBF concentrations and the 

estimated SBM cuttings discharges (Figure 20). 

Duration of Impacts 

The timing of drilling activities vs. sampling cruises differed among sites. The interval between 

cessation of drilling and the date when the surveys began ranged from 5 months to nearly 2 years 

depending on the site. Over time, SBF residues are decomposed by microbes, resulting in 

significant decreases in SBF concentration (Candler et al. 1995). Our observations indicate that 

sediment SBF concentrations tended to increase with increasing SBM discharge volume and 

decrease with longer elapsed time since drilling. 

Observations from the study sites and adjacent blocks suggest that geophysically detectable 

mud/cuttings deposits 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 deposits from older wells are associated with persistent elevations in 

barium and SBF, anoxic conditions, or altered benthic communities. 
40 

0

500

1000

1500

2000

2500

0 5000 10000 15000 20000 25000 30000

Estimated SBM Cuttings Discharge (bbl)

Sediment SBF Concentration (μg/g)

(geometric mean)

GB 602

LEGEND

VK 916

GB 516 Cruise 1B

GB 516 Cruise 2B

MC 292

Sediment Barium vs. SBM Cuttings Discharges

0

10000

20000

30000

40000

50000

0 5000 10000 15000 20000 25000 30000

SBM Cuttings Discharge (bbl)

Sediment Barium Concentration (μg/g)

(geometric mean)

Sediment SBF vs. SBM Cuttings Discharges

Figure 19. Relationship between post-drilling sediment barium and synthetic-based fluid (SBF) 

concentrations within near-field sites and estimated volumes of synthetic-based mud 

(SBM) cuttings discharges. 
41 

-250

-200

-150

-100

-50

0

0 500 1000 1500 2000 2500

Sediment SBF Concentration (μg/g)

(geometric mean)

-250

-200

-150

-100

-50

0

0 5000 10000 15000 20000 25000 30000

Estimated SBM Cuttings Discharge (bbl)

Difference between Mean 

Near-Field and Far-Field 

Integrated Oxygen (nmoles/cm2

)

Difference between Mean 

Near-Field and Far-Field 

Integrated Oxygen (nmoles/cm2

)

GB 602

GB 516 Cr. 2B

GB 516 Cr. 1B

MC 292

VK 916

GB 602

GB 516 Cr. 2B

GB 516 Cr. 1B

MC 292

VK 916

Figure 20. Difference between mean near-field and far-field sediment oxygen levels in relation 

to sediment synthetic-based fluid (SBF) concentrations and estimated synthetic-based 

mud (SBM) cuttings discharges.
43 

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 
44 

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. 

• Impacts 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. 
45 

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synthetic-based mud discharged in the western Gulf of Mexico, pp. 51-69. In: SPE/EPA 

Exploration & Production Environment Conference. Houston, TX, 27-29 March 1995. 

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program. Report for the SBM Research Group, Houston, TX. 3 vol. 

Fechhelm, R.G., B.J. Gallaway, and J.M. Farmer. 1999. Deepwater sampling at a synthetic drilling mud 

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28 February-3 March 1999. SPE 52744. Society of Petroleum Engineers, Inc., Richardson, TX. 

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Rhoads, D.C. and J.D. Germano. 1982. Characterization of organism-sediment relations using sediment 

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System). Mar. Ecol. Prog. Ser. 8:115-128. 

Rosenberg, R., H.C. Nilsson, A. Grémare, and J-M. Amouroux. 2003. Effects of demersal trawling on 

marine sedimentary habitats analysed by sediment profile imagery. J. Exp. Mar. Biol. Ecol. 

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Trefry, J.H. and J.P. Smith. 2003. “Forms of mercury in drilling fluid barite and their fate in the marine 

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Exploration and Production Environmental Conference, San Antonio TX, 10-12 March 2003. 

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