6866.0

34000.0

6866.0

34200.0

Line:

Trace:

25000 30000

1.400

1.500

1.600

1.700

1.800

1.900

2.000

2.100

2.200

2.300

2.400

2.500

Horizontal

Distance (ft):

Shallow Water Flow and Gas Potential

HIGH

0 1 2 3

0 = NEGLIGIBLE

1 = LOW

2 = MODERATE

3 = HIGH

NEGLIGIBLE

Figure GC 867-B-2. Tophole Prognosis Chart, Proposed Wellsite B, Green Canyon Block 867

Project No. 0411-1924 Filename: 111924_fig_GC867-B-2.cdr

3-D Seismic Section Through

Proposed Wellsite B

X = 2,473,150 FT; Y = 9,851,000 FT

 Crossline 34079 at Inline 6866

NW SE

Inline 6866

GC

823

PROPOSED WELLSITE B

TWO-WAY TRAVEL TIME (SEC)

GC

867

SEAFLOOR

Water 

Flow Gas

HORIZON 20

TOP OF SALT

Unit 2

1.741

1.775

1.903

-4,267

-4,357

-4,699

90

432

90

221

Seafloor Slopes at 3.9° South.

Lithology/Stratigraphy

WATER

FLOW AND

GAS

POTENTIAL

Horizon

Horizon

TWT

(sec, bsl)

Depth

(ft, bsl)

Depth

(ft, bml)

Units

Unit

(Interval)

Thickness

(ft)

Unit 3

Potential Drilling

Constraints

HORIZON 35 1.821 -4,478 211 121

Very soft, pelagic drape at seafloor. Hemipelagic silty

clays and thin mass-transport deposits.

Alternating layers of hemipelagic silty clays with chaotic

mass-transport deposits.

Chaotic sequence created by salt movement. Mixture of

silts and clays.

Negligible potential for overpressured sands.

Negligible potential for shallow gas.

Low potential for overpressured sands.

Low potential for shallow gas.

SALT

Top of salt Slopes at 17° to Southeast.

GC

822

Unit 1

Negligible potential for overpressured sands.

Negligible potential for shallow gas.

TROUGH PEAK

RELATIVE AMPLITUDE
10615 SHADOW WOOD DRIVE

SUITE 200

HOUSTON, TEXAS 77043

Phone: (713) 468-1410

Fax: (713) 468-1438

E-mail: gems@gemsinc.com

 -1- Proposed Well GC 867-C 

May 20, 2011 Project No. 0411-1924

Apache Deepwater, LLC 

2000 W. Sam Houston Pkwy. South, Suite 1600 

Houston, Texas 77042-3622 

Attention: Mr. John A. Worthen

Site Clearance Letter 

Proposed Wellsite GC 867-C

Block 867 (OCS-G-33858)

Green Canyon Area

Gulf of Mexico

Apache Deepwater, LLC (Apache) plans to drill an exploration well in Block 867, of the Green Canyon 

Area (GC), Gulf of Mexico. This letter addresses specific seafloor and subsurface geologic conditions to a 

depth of approximately 532 ft below the mudline (bml) at the Proposed Wellsite GC 867-C. The following 

discussion is based on findings provided within the main body of the geohazard report for Blocks 823 and 

867 (GEMS Report No. 0411-1924). The surface location for proposed Wellsite GC 867-C is in the 

northwestern quadrant of GC 867. Apache provided the following coordinates: 

Proposed Wellsite GC 867-C

Spheroid & Datum: Clarke 1866, NAD27

Projection: UTM Zone 15 North Line Reference Block Calls

X: 2,474,460 ft Latitude: 27.1210600440824 o N Inline 6855 2,530 ft FNL

Y: 9,849,950 ft Longitude: 90.4355210825508° W Crossline 34043 3,420 ft FWL

Apache will drill this well using a moored vessel. An exact anchor pattern for the proposed well location is 

undetermined at this time. Maps GC 867-C-1 through GC 867-C-4 show an anchor radius representing a 

typical maximum length of chain and anchor on bottom is displayed at 10,000 ft from the proposed 

wellsite. A 1,000 ft buffer zoned beyond the limits of the anchor radius is also included, NTL 2009-G40 

(MMS, 2009a).

Attachments

The maps and figures accompanying this letter have been extracted from the main report’s original maps 

and 3-D data volume and centered on the proposed well location.

Map GC 867-C-1: Bathymetry Map

Map GC 867-C-2: Seafloor Rendering

Map GC 867-C-3: Seafloor Amplitude Rendering

Map GC 867-C-4: Geologic Features Map

Figure GC 867-C-1:Inline 6855 and Crossline 34043 Showing Conditions Beneath 

Proposed Wellsite GC 867-C 

Figure GC 867-C-2:Tophole Prognosis Chart, Proposed Wellsite GC 867-C, Green Canyon Block

867
Project No. 0411-1924

 -2- Proposed Well GC 867-C 

NTL Requirements

The following report complies with the Bureau of Ocean Energy Management, Regulation, and 

Enforcement (BOEMRE; formerly the MMS) Notice to Lessees (NTLs) 2008-G04, 2008-G05 and 2009-

G40 (MMS, 2008a, 2008b, and 2009a). The proposed wellsite will be required to follow NTL 2008-G06 

with respect to pre- and post-spud ROV visual inspection surveys (MMS, 2008c). Pursuant to the recently 

released Pre-Seabed Disturbance Survey Mitigation issued by BOEMRE, the 2005-G07 NTL (MMS, 2005) 

does not apply to GC 867 in terms of high-probability, (MMS, 2008d), but an archaeological assessment of 

the data must be performed prior to any bottom disturbing activities (BOEMRE, 2011b). 

According to NTL 2008-G05 (MMS, 2008b), 3-D seismic reflection data may be used for shallow hazard 

assessments. A power spectrum analysis was conducted from the 3-D seismic data cube provided by 

Apache to illustrate the data parameters. The power spectrum curve (below) was generated within a 500-ft 

radius of the intersection of the inline and crossline at the proposed wellsite location, over a time interval 

from the seafloor to ~0.198 second below mudline, i.e., the salt/sediment interface. We converted the 

amplitude vs. frequency spectrum, generated by the SMT software, to power vs. frequency by squaring the 

amplitude values as described by J. A. Coffeen, 1978.

Power Spectrum Analysis Curve, Proposed Wellsite GC 867-C

Man-Made Features

There are existing wells in GC 867 and in surrounding blocks. The closest well to Proposed Wellsite 

GC 867-C is the BP Exploration Inc., GC 823-001 well approximately 5,800 ft to the northeast (Maps 

GC 867-C-1 through GC 867-C-4). 

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80 90 100

Power (amp2)

Frequency (hrz)

POWER SPECTRUM

GC 867-C
Project No. 0411-1924

 -3- Proposed Well GC 867-C 

Other wells near the surface location of the Proposed Wellsite GC 867-C are the BP Exploration Inc.,

GC 866-001 and GC 823-002 wells. These wells are approximately 6,570 ft west and 12,520 ft north-east, 

respectively, of the proposed site and lie within the proposed mooring pattern.

Proposed Wellsite GC 867-C and its anchors lie within the “Military Warning Zone W-92”. Military 

warning zones typically do not have any impact on exploration drilling operations or other oilfield 

activities. However, notifications to appropriate military commands need to be addressed by Apache 

(MMS, 2009b). Restrictions would only be enforced during military operations in the area or during an 

emergency.

Water Depth and Seafloor Conditions

The water depth at the proposed location is -4,250 ft (Map GC 867-C-1). The seafloor at the proposed 

location is slightly irregular and dips to the southwest at about 6.8 ̊. The proposed wellsite is located along 

the eastern flank of a southeasterly trending, arcuate valley (Map GC 867-C-2). The valley represents a 

Pleistocene slump scar. The valley occurs along the eastern margin of an extensive topographic high. The 

topographic high is the seafloor expression of a shallow salt diapir. 

Water depths within the potential anchor mooring area range from about -3,660 ft along a ridge line to the 

north to approximately -4,900 ft near the upper part of the Sigsbee Escarpment to the south and southeast

(Map GC 867-C-1).

The seafloor within the potential anchoring area varies from smooth and featureless to very irregular. The 

seafloor irregularities include narrow ridges and gullies, faults, and mounds. Seafloor gradient within the 

mooring pattern will average from 1° to 5°. The highest seafloor gradients in the area are associated with 

fault escarpments. Gradients of over 20 ̊ are associated with fault scarps that occur primarily north and east

of the proposed location. The upper part of the Sigsbee Escarpment to the south will be marked by an 

increase in slope from 10° to around 20°. Near-surface soils along the Sigsbee Escarpment are likely to be 

highly overconsolidated and much stronger than typical slope soils.

Deepwater Benthic Communities

The Seafloor Amplitude Rendering based on the 3-D data volume (Map GC 867-C-3) shows normal or 

ambient amplitude along the seabed, with no indication of any hard-bottom or fluid expulsion events within 

2,000 ft of the proposed GC 867-C location. Based on the renderings and seismic profiles deepwater 

benthic communities are not expected within 2,000 ft of the proposed GC 867-C wellsite. 

Patches of high-amplitude seafloor returns occur approximately 4,900 ft to the southwest of the location. 

The patches are associated with seafloor faults. In addition, the base of a mud mound exists approximately 

10,000 ft to the northwest (GC 867-Maps C-1 through GC 867-C-4). These features have a high-potential 

for deepwater benthic communities. Therefore, anchors must avoid these features by at least 250 ft (MMS, 

2009a).

A number of seafloor faults occur within the proposed mooring pattern. The closest seafloor faults occur 

approximately 475 ft to the north. Except for the faults 4,900 ft to the southwest, no high-amplitude returns 

occur along the seabed escarpments; therefore, prolific hydrocarbon seepage or carbonate hardgrounds are 

unlikely.

Stratigraphy

Inline 6855 and Crossline 34043 are the closest 3-D lines to the proposed GC 867-C location (Figure 

GC 867-C-1). The Tophole Prognosis Chart (Figure GC 867-C-2) shows the inline annotated with 

calculated depths to the various horizons and predicted lithology of the sequences, along with their 

potential for shallow water flow and shallow gas. 

The 3-D data volume was used to define the sedimentary section to a depth of approximately 532 ft 

(Figures GC 867-C-1 and GC 867-C-2). The section was divided into three units or sequences (Figure 
Project No. 0411-1924

 -4- Proposed Well GC 867-C 

GC 867-C-1). The sequences represent variations in depositional environments and possible changes in 

soil strength and composition. 

The surficial unit, Unit 1 (Seafloor to Horizon 20), to approximately 92 ft bml, consists of a very soft 

pelagic drape underlain with hemipelagic silty clays and thin, mass-transport deposits. Very soft clays with 

high water content will likely exist at the seabed. However, the underlying section has been uplifted and 

eroded. Beneath the thin (~10 ft) of pelagic drape firm to stiff, overconsolidated soils may be encountered.

Unit 2 (Horizon 20 to Horizon 35) consists of bedded hemipelagic clays interbedded with thin chaotic 

mass-transport deposits. The sediments will be mud-rich; however, some thin sand seams and pockets are 

possible. The combined thickness of Units 1 and 2 is approximately 277 ft (Figure GC 867-C-2).

Unit 3 is a sequence bounded by Horizon 35 and the top of salt (Figures GC 867-C-1 and GC 867-C-2). 

This chaotic interval, affected by shallow salt emplacement, is between 277 ft and 532 ft bml at the 

proposed location. Unit 3 will likely consist of a mixture of silts and clays and is approximately 255 ft 

thick. 

The top of salt is predicted to be 532 ft bml (~4,782 ft bsl). The salt surface dips to the southeast at 

approximately 18°. 

Faults

No faults will be intersected by the well bore beneath proposed Wellsite GC 867-C (Figures GC 867- C-1

and GC 867-C-2). 

Shallow Water Flow

Unit 2 between Horizons 20 and 35 is possibly the regional “Red Unit” slope-fan sequence. However, the 

top of the unit is only 92 ft bml at proposed Wellsite GC 867-C (Figure GC 867-C-2). The entire unit is 

185 ft thick. At this location, Unit 2 is an unlikely zone for overpressured sand layers. 

The existing GC 823-001 well is approximately 5,800 ft to the northeast of the proposed GC 867-C

wellsite. A “low” severity shallow water flow occurred in the GC 823-001 well at 1,715 ft bml (-5,840 ft 

bsl). Based on the reported depth the flow occurred in the lower portion of Unit 4 within a generally lowamplitude interval of discontinuous to chaotic reflections. Unit 4 is not present beneath the GC 867-C

wellsite. The potential for overpressured sands at this location is low to negligible.

Shallow Gas 

The well path will not penetrate any high-amplitude events between the seafloor and the top of salt (Figure 

GC 867-C-2). There is a negligible to low potential for shallow gas in this section. 

Closing

The seafloor and near-surface conditions at Proposed Wellsite GC 867-C, Green Canyon Block 867 (OCSG-33858), appear suitable for exploration drilling operations. The existing wellsites and the potential 

benthic communities will need to be avoided by anchors supporting a drilling vessel at the proposed 

location. Engineers should be aware that the soils above salt may be stronger and more consolidated than 

typical slope sediments. 
Project No. 0411-1924

 -5- Proposed Well GC 867-C 

Sincerely,

GEOSCIENCE EARTH & MARINE

SERVICES, INC.®

Thomas W. Neurauter Ph. D. Daniel Lanier

Senior Marine Geologist Vice-President/Marine Geologist

Attachments (4 Maps, 2 Figures)

Distribution:

Mr. John Worthen, Apache, Houston, TX. (6)

References:

Bureau of Ocean Energy Management and Enforcement, 2011a, ASCII Data Files, published on the BOEMRE Gulf of 

Mexico Region Homepage, <http://www.gomr.mms.gov/homepg/pubinfo/freeasci/freedesc.html>, accessed May, 

2011.

Bureau of Ocean Energy Management, Regulation, and Enforcement, 2011b, Pre-Seabed Disturbance Survey 

Mitigation <http://www.gomr.boemre.gov/homepg/regulate/environ/archaeological/Conditional-ArchaeologicalMitigation.pdf>.

Coffeen, J. A., 1978, Seismic exploration fundamentals: Tulsa, the Petroleum Publishing Co., p. 125.

Minerals Management Service, 2005, Notice to lessees and operators of federal oil, gas, and sulphur leases and 

pipeline right-of-way holders in the outer continental shelf, Gulf of Mexico OCS region, Archaeological resource 

surveys and reports: U. S. Department of the Interior, Minerals Management Service, Gulf of Mexico, NTL 2005-

G07.

Minerals Management Service, 2008a, Notice to lessees and operators of federal oil, gas, and sulphur leases in the 

outer continental shelf, Gulf of Mexico OCS region, information requirements for exploration plans and development 

operations coordination documents: U. S. Department of the Interior, Minerals Management Service, Gulf of Mexico, 

NTL 2008-G04. Effective Date May 1, 2008. 

Minerals Management Service, 2008b, Notice to lessees and operators of federal oil, gas, and sulphur leases and 

pipeline right-of-way holders in the outer continental shelf, Gulf of Mexico OCS region, Shallow hazards 

requirements: U. S. Department of the Interior, Minerals Management Service, Gulf of Mexico, NTL 2008-G05.

Minerals Management Service, 2008c, Notice to lessees and operators of federal oil, gas, and sulphur leases in the 

outer continental shelf, Gulf of Mexico OCS region; remotely operated vehicle surveys in deepwater, U. S. 

Department of the Interior, Minerals Management Service, Gulf of Mexico, NTL 2008-G06.

Minerals Management Service, 2008d, Notice to lessees and operators (NTL) of federal oil and gas leases and pipeline 

right-of-way (ROW) holders on the outer continental shelf (OCS), Gulf of Mexico OCS region (GOMR), Revisions to 

the list of OCS lease blocks requiring archaeological resource surveys and reports: U. S. Department of the Interior, 

Minerals Management Service, Gulf of Mexico, NTL 2008-G20.
Project No. 0411-1924

 -6- Proposed Well GC 867-C 

Minerals Management Service, 2009a, Notice to lessees and operators of federal oil and gas leases in the outer 

continental shelf, Gulf of Mexico OCS region, Deepwater benthic communities: U. S. Department of the Interior, 

Minerals Management Service, Gulf of Mexico, NTL 2009-G40.

Minerals Management Service, 2009b, Notice to lessees and operators of federal oil and gas leases and pipeline rightof-way holders on the outer continental shelf, Gulf of Mexico OCS region, Military Warning and Water Test Areas: 

U. S. Department of the Interior, Minerals Management Service, Gulf of Mexico, NTL 2009-G06. Effective Date 

April 22, 2009.
6855.0

33800.0

6855.0

34000.0

6855.0

34200.0

Line:

Trace:

20000 25000 30000 35000 40000 45000

1.300

1.400

1.500

1.600

1.700

1.800

1.900

2.000

2.100

2.200

2.300

2.400

2.500

2.600

2.700

Horizontal

Distance (ft):

6800.0

34043.0

7000.0

34043.0

Line:

Trace:

10000 15000 20000 25000 30000 35000

1.500

1.600

1.700

1.800

1.900

2.000

2.100

2.200

2.300

2.400

2.500

2.600

2.700

2.800

2.900

Horizontal

Distance (ft):

Project No. 0411-1924 Filename: 111924_fig_GC867-C-1.cdr

Figure GC 867-C-1. Inline 6855 and Crossline 34043 Showing Conditions Beneath Proposed Wellsite GC 867-C

15034.839

14132.748

13230.658

12328.568

11426.478

10524.387

9622.297

8720.207

7818.116

6916.026

6013.936

5111.845

4209.755

3307.665

2405.574

1503.484

601.394

-300.697

-1202.787

-2104.877

-3006.968

-3909.058

-4811.148

-5713.239

-6615.329

-7517.419

-8419.510

-9321.600

-10223.690

-11125.781

-12027.871

-12929.961

-13832.052

-15034.839

RELATIVE AMPLITUDE

TWO-WAY TIME TRAVEL (SEC) TWO-WAY TIME TRAVEL (SEC)

GC

823

GC

867

Northwest Southeast

Inline 6855

SALT

SALT

GC

823

GC

867

Southwest Northeast

SEAFLOOR

UNIT 1 HORIZON 20

UNIT 2

UNIT 3

SEAFLOOR

HORIZON 20

HORIZON 35

HORIZON 40

TOP OF SALT

UNIT 1

UNIT 3

UNIT 4

Crossline 34043

Proposed 

Wellsite GC 867-C

Proposed 

Wellsite GC 867-C

GC

866

GC

822

TOP OF SALT

UNIT 2

HORIZON 35

GC

824
6855.0

34000.0

Line:

Trace:

25000 30000 35000

1.400

1.500

1.600

1.700

1.800

1.900

2.000

2.100

2.200

2.300

2.400

2.500

Horizontal

Distance (ft):

Shallow Water Flow and Gas Potential

HIGH

0 1 2 3

0 = NEGLIGIBLE

1 = LOW

2 = MODERATE

3 = HIGH

NEGLIGIBLE

Figure GC 867-C-2. Tophole Prognosis Chart, Proposed Wellsite C, Green Canyon Block 867

Project No. 0411-1924 Filename: 111924_fig_GC867-C-2.cdr

3-D Seismic Section Through

Proposed Wellsite C

X = 2,474,460 FT; Y = 9,849,950 FT

 Crossline 34043 at Inline 6855

NW SE

Inline 6855

GC

823

PROPOSED WELLSITE C

TWO-WAY TRAVEL TIME (SEC)

GC

867

SEAFLOOR

Water 

Flow Gas

HORIZON 20

TOP OF SALT

Unit 2

1.734

1.769

1.932

-4,250

-4,342

-4,782

92

532

92

255

Seafloor Slopes at 6.8° Southwest.

Lithology/Stratigraphy

WATER

FLOW AND

GAS

POTENTIAL

Horizon

Horizon

TWT

(sec, bsl)

Depth

(ft, bsl)

Depth

(ft, bml)

Units

Unit

(Interval)

Thickness

(ft)

Unit 3

Potential Drilling

Constraints

HORIZON 35 1.839 -4,527 277 185

Very soft, pelagic drape at seafloor. Hemipelagic silty

clays and thin mass-transport deposits.

Alternating layers of hemipelagic silty clays with chaotic

mass-transport deposits.

Chaotic sequence created by salt movement. Mixture of

silts and clays.

Negligible potential for overpressured sands.

Negligible potential for shallow gas.

Low potential for overpressured sands.

Low potential for shallow gas.

SALT

Top of salt Slopes at 18° to Southeast.

Unit 1

Negligible potential for overpressured sands.

Negligible potential for shallow gas.

TROUGH PEAK

RELATIVE AMPLITUDE
10615 SHADOW WOOD DRIVE

SUITE 200

HOUSTON, TEXAS 77043

Phone: (713) 468-1410

Fax: (713) 468-1438

E-mail: gems@gemsinc.com

 -1- Proposed Well GC 867-D 

May 20, 2011 Project No. 0411-1924

Apache Deepwater, LLC 

2000 W. Sam Houston Pkwy. South, Suite 1600 

Houston, Texas 77042-3622 

Attention: Mr. John A. Worthen

Site Clearance Letter 

Proposed Wellsite GC 867-D

Block 867 (OCS-G-33858)

Green Canyon Area

Gulf of Mexico

Apache Deepwater, LLC (Apache) plans to drill an exploration well in Block 867, of the Green Canyon 

Area (GC), Gulf of Mexico. This letter addresses specific seafloor and subsurface geologic conditions to a 

depth of approximately 1,021 ft below the mudline (bml) at the proposed Wellsite GC 867-D. The 

following discussion is based on findings provided within the main body of the geohazard report for Blocks 

823 and 867 (GEMS Report No. 0411-1924). The surface location for proposed Wellsite GC 867-D is in 

the southwestern quadrant of GC 823. Apache provided the following coordinates:

Proposed Wellsite GC 867-D

Spheroid & Datum: Clarke 1866, NAD27

Projection: UTM Zone 15 North Line Reference Block Calls(SL in GC 823)

X: 2,473,150 ft Latitude: 27.1384041402277 o N Inline 6833 3,752 ft FSL

Y: 9,856,232 ft Longitude: 90.439152132407° W Crossline 34195 2,110 ft FWL

Apache will drill this well using a moored vessel. An exact anchor pattern for the proposed well location is 

undetermined at this time. Maps GC 867-D-1 through GC 867-D-4 show an anchor radius representing a 

typical maximum length of chain and anchor on bottom is displayed at 10,000 ft from the proposed 

wellsite. A 1,000 ft buffer zoned beyond the limits of the anchor radius is also included, NTL 2009-G40 

(MMS, 2009a). 

Attachments

The maps and figures accompanying this letter have been extracted from the main report’s original maps 

and 3-D data volume and centered on the proposed well location.

Map GC 867-D-1: Bathymetry Map

Map GC 867-D-2: Seafloor Rendering

Map GC 867-D-3: Seafloor Amplitude Rendering

Map GC 867-D-4: Geologic Features Map

Figure GC 867-D-1:Inline 6833 and Crossline 34195 Showing Conditions 

Beneath Proposed Wellsite GC 867-D 

Figure GC 867-D-2:Tophole Prognosis Chart, Proposed Wellsite GC 867-D, Green Canyon Block

867
Project No. 0411-1924

 -2- Proposed Well GC 867-D 

NTL Requirements

The following report complies with the Bureau of Ocean Energy Management, Regulation, and 

Enforcement (BOEMRE; formerly the MMS) Notice to Lessees (NTLs) 2008-G04, 2008-G05 and 2009-

G40 (MMS, 2008a, 2008b, and 2009a). The proposed wellsite will be required to follow NTL 2008-G06 

with respect to pre- and post-spud ROV visual inspection surveys (MMS, 2008c). Pursuant to the recently 

released Pre-Seabed Disturbance Survey Mitigation issued by BOEMRE, the 2005-G07 NTL (MMS, 2005) 

does not apply to GC 823/867 in terms of high-probability (MMS, 2008d), but an archaeological assessment 

of the data must be performed prior to any bottom disturbing activities (BOEMRE, 2011b). 

According to NTL 2008-G05 (MMS, 2008b), 3-D seismic reflection data may be used for shallow hazard 

assessments. A power spectrum analysis was conducted from the 3-D seismic data cube provided by 

Apache to illustrate the data parameters. The power spectrum curve (below) was generated within a 500-ft 

radius of the intersection of the inline and crossline at the proposed wellsite location, over a time interval 

from the seafloor to ~0.371 second below mudline, i.e., the salt/sediment interface. We converted the 

amplitude vs. frequency spectrum, generated by the SMT software, to power vs. frequency by squaring the 

amplitude values as described by J. A. Coffeen, 1978.

Power Spectrum Analysis Curve, Proposed Wellsite GC 867-D

Man-Made Features

There are existing wells in GC 823, GC 867, and in surrounding blocks. The closest well to Proposed 

Wellsite GC 867-D, with a surface location in GC 823, is the BP Exploration Inc., GC 823-001 well 

approximately 5,640 ft to the east-southeast (Maps GC 867-D-1 through GC 867-D-4). 

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80 90 100

Power (amp2)

Frequency (hrz)

POWER SPECTRUM

GC 867-D
Project No. 0411-1924

 -3- Proposed Well GC 867-D 

Other wells near the surface location of the Proposed Wellsite GC 867-D are the BP Exploration Inc.,

GC 866-001 and GC 823-002 wells. These wells are approximately 8,330 ft south-southwest and 12,900 ft 

east, respectively, of the proposed site and lie within the proposed mooring pattern.

Proposed Wellsite GC 867-D and its anchors lie within the “Military Warning Zone W-92”. Military 

warning zones typically do not have any impact on exploration drilling operations or other oilfield 

activities. However, notifications to appropriate military commands need to be addressed by Apache 

(MMS, 2009b). Restrictions would only be enforced during military operations in the area or during an 

emergency.

Water Depth and Seafloor Conditions

The water depth at the proposed location is -3,944 ft (Map GC 867-D-1). The seafloor at the proposed 

location is slightly irregular and dips to the east-southeast at about 5.3 ̊. The proposed wellsite is located 

along the head of a southeasterly trending, arcuate valley (Map GC 867-D-2). The valley represents a 

Pleistocene slump scar. The valley occurs along the eastern margin of an extensive topographic high. The 

topographic high is the seafloor expression of a shallow salt diapir. 

Water depths within the potential anchor mooring area range from about -3,660 ft along a ridge line to the 

north-northeast to approximately -4,300 ft within the floor of a valley to the south (Map GC 867-D-1).

The seafloor within the potential anchoring area varies from smooth and featureless to very irregular. The 

seafloor irregularities include narrow ridges and gullies, faults, and mounds. Seafloor gradient within the 

mooring pattern will average from 1° to 5°. The highest seafloor gradients in the area are associated with 

fault escarpments. Gradients of over 20 ̊ are associated with fault scarps that occur primarily north and 

south of the proposed location. The overburden in this area is relatively thin and has experienced uplift and 

erosion. Near-surface soils beneath a pelagic drape may be overconsolidated and stronger than typical 

slope soils. 

Deepwater Benthic Communities

The Seafloor Amplitude Rendering based on the 3-D data volume (Map GC 867-D-3) shows normal or 

ambient amplitude along the seabed, with no indication of any hard-bottom or fluid expulsion events within 

2,000 ft of the proposed GC 867-D location. Based on the renderings and seismic profiles deepwater 

benthic communities are not expected within 2,000 ft of the proposed GC 867-D wellsite. 

Two mud mounds occur to the northwest of the location. The mound approximately 7,600 ft to the 

northwest is a known site of chemosynthetic communities. The second mound, approximately 4,700 ft to 

the northwest, does not have the high-amplitude seafloor returns but is likely caused by upwelling of 

gaseous muds. Both mounds need to be avoided. In addition patches of high-amplitude seafloor returns 

occur approximately 8,600 ft to the south-southwest of the location. The patches are associated with 

seafloor faults (Maps GC 867-D-1 through GC 867-D-4). These features also have a high-potential for 

deepwater benthic communities. Therefore, anchors must avoid these features by at least 250 ft (MMS, 

2009a).

A number of seafloor faults occur within the proposed mooring pattern. The closest seafloor faults occur 

approximately 450 ft to the north. Except for the faults 8,600 ft to the south-southwest, no high-amplitude 

returns occur along the seabed escarpments; therefore, prolific hydrocarbon seepage or carbonate 

hardgrounds are unlikely.

Stratigraphy

Inline 6833 and Crossline 34195 are the closest 3-D lines to the proposed GC 867-D location (Figure 

GC 867-D-1). The Tophole Prognosis Chart (Figure GC 867-D-2) shows the crossline annotated with 

calculated depths to the various horizons and predicted lithology of the sequences, along with their 

potential for shallow water flow and shallow gas. 
Project No. 0411-1924

 -4- Proposed Well GC 867-D 

The 3-D data volume was used to define the sedimentary section to a depth of approximately 1,021 ft 

(Figures GC 867-D-1 and GC 867-D-2). The section was divided into three units or sequences (Figure 

GC 867-D-1). The sequences represent variations in depositional environments and possible changes in 

soil strength and composition.

The surficial unit, Unit 1 (Seafloor to Horizon 20), to approximately 124 ft bml, consists of a very soft 

pelagic drape underlain with hemipelagic silty clays and thin, mass-transport deposits. Very soft clays with 

high water content will likely exist at the seabed. However, the underlying section has been uplifted and 

eroded. Firm, overconsolidated soils may be encountered beneath the thin (~10 ft) pelagic drape.

Unit 2 (Horizon 20 to Horizon 35) consists of bedded hemipelagic clays interbedded with thin chaotic 

mass-transport deposits. The sediments will be mud-rich; however, some thin sand seams and pockets are 

possible. The combined thickness of Units 1 and 2 is approximately 478 ft (Figure GC 867-D-2).

Unit 3 is a sequence bounded by Horizon 35 and the top of salt (Figures GC 867-D-1 and GC 867-D-2). 

The upper portion of the sequence (478 ft to 876 ft bml) is primarily composed of mud-rich turbidites. The 

lower portion of Unit 3 (876 ft to 1,021 ft bml) is a chaotic interval affected by shallow salt emplacement 

and will likely consist of a mixture of silts and clays. Unit 3 is approximately 543 ft thick. 

The top of salt is predicted to be 1,021 ft bml (~4,965 ft bsl). The salt surface dips to the northeast at 

approximately 43°.

Faults

Two faults will be intersected by the well bore beneath proposed Wellsite GC 867-D (Figures GC 867-D-1

and GC 867-D-2). The borehole will intersect the shallowest fault near the base of Unit 2, at 

approximately 336 ft bml. A deeper fault will be intersected in the upper portion of the Horizon 35 to Top 

of Salt sequence (Unit 3), at a depth of about 737 ft bml. These faults may be possible loss circulation 

zones.

Shallow Water Flow

Unit 2 between Horizons 20 and 35 is possibly the regional “Red Unit” slope-fan sequence. However, the 

top of the unit is only 124 ft bml at proposed Wellsite D (Figure GC 867-D-2). The entire unit is 354 ft 

thick. At this location, Unit 2 is an unlikely zone for overpressured sand layers. 

The existing GC 823-001 well is approximately 5,640 ft to the east-southeast of the proposed GC 867-D

wellsite. A “low” severity shallow water flow occurred in the GC 823-001 well at 1,715 ft bml (-5,840 ft 

bsl). Based on the reported depth the flow occurred in the lower portion of Unit 4 within a generally lowamplitude interval of discontinuous to chaotic reflections. Unit 4 is not present beneath the GC 867-D

wellsite. However, an interval within Unit 3 (669 ft to 876 ft) shows bedded reflectors with moderate 

amplitudes. This interval consists of turbidites and may have thin sand layers. The potential for 

overpressured sands within this interval is moderate. The potential for the rest of the section is low to 

negligible.

Shallow Gas 

The well path will not penetrate any high-amplitude events between the seafloor and the top of salt (Figure 

GC 867-D-2). There is a negligible to low potential for shallow gas in this section. 

Closing

The seafloor and near-surface conditions at Proposed Wellsite GC 867-D, Green Canyon Block 867 (OCSG-33858), appear suitable for exploration drilling operations. The existing wellsites and the potential 

benthic communities will need to be avoided by anchors supporting a drilling vessel at the proposed 

location. Engineers should be aware that the soils above salt may be stronger and more consolidated than 

typical slope sediments.
Project No. 0411-1924

 -5- Proposed Well GC 867-D 

Sincerely,

GEOSCIENCE EARTH & MARINE

SERVICES, INC.®

Thomas W. Neurauter Ph. D. Daniel Lanier

Senior Marine Geologist Vice-President/Marine Geologist

Attachments (4 Maps, 2 Figures)

Distribution:

Mr. John Worthen, Apache, Houston, TX. (6)

References:

Bureau of Ocean Energy Management and Enforcement, 2011a, ASCII Data Files, published on the BOEMRE Gulf of 

Mexico Region Homepage, <http://www.gomr.mms.gov/homepg/pubinfo/freeasci/freedesc.html>, accessed May, 

2011.

Bureau of Ocean Energy Management, Regulation, and Enforcement, 2011b, Pre-Seabed Disturbance Survey 

Mitigation <http://www.gomr.boemre.gov/homepg/regulate/environ/archaeological/Conditional-ArchaeologicalMitigation.pdf>.

Coffeen, J. A., 1978, Seismic exploration fundamentals: Tulsa, the Petroleum Publishing Co., p. 125.

Minerals Management Service, 2005, Notice to lessees and operators of federal oil, gas, and sulphur leases and 

pipeline right-of-way holders in the outer continental shelf, Gulf of Mexico OCS region, Archaeological resource 

surveys and reports: U. S. Department of the Interior, Minerals Management Service, Gulf of Mexico, NTL 2005-

G07.

Minerals Management Service, 2008a, Notice to lessees and operators of federal oil, gas, and sulphur leases in the 

outer continental shelf, Gulf of Mexico OCS region, information requirements for exploration plans and development 

operations coordination documents: U. S. Department of the Interior, Minerals Management Service, Gulf of Mexico, 

NTL 2008-G04. Effective Date May 1, 2008. 

Minerals Management Service, 2008b, Notice to lessees and operators of federal oil, gas, and sulphur leases and 

pipeline right-of-way holders in the outer continental shelf, Gulf of Mexico OCS region, Shallow hazards 

requirements: U. S. Department of the Interior, Minerals Management Service, Gulf of Mexico, NTL 2008-G05.

Minerals Management Service, 2008c, Notice to lessees and operators of federal oil, gas, and sulphur leases in the 

outer continental shelf, Gulf of Mexico OCS region; remotely operated vehicle surveys in deepwater, U. S. 

Department of the Interior, Minerals Management Service, Gulf of Mexico, NTL 2008-G06.

Minerals Management Service, 2008d, Notice to lessees and operators (NTL) of federal oil and gas leases and pipeline 

right-of-way (ROW) holders on the outer continental shelf (OCS), Gulf of Mexico OCS region (GOMR), Revisions to 

the list of OCS lease blocks requiring archaeological resource surveys and reports: U. S. Department of the Interior, 

Minerals Management Service, Gulf of Mexico, NTL 2008-G20.

Minerals Management Service, 2009a, Notice to lessees and operators of federal oil and gas leases in the outer 

continental shelf, Gulf of Mexico OCS region, Deepwater benthic communities: U. S. Department of the Interior, 

Minerals Management Service, Gulf of Mexico, NTL 2009-G40.
Project No. 0411-1924

 -6- Proposed Well GC 867-D 

Minerals Management Service, 2009b, Notice to lessees and operators of federal oil and gas leases and pipeline rightof-way holders on the outer continental shelf, Gulf of Mexico OCS region, Military Warning and Water Test Areas: 

U. S. Department of the Interior, Minerals Management Service, Gulf of Mexico, NTL 2009-G06. Effective Date 

April 22, 2009.