6823.0

33800.0

6823.0

34000.0

6823.0

34200.0

Line:

Trace:

20000 25000 30000 35000 40000 45000

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):

6600.0

34007.0

6800.0

34007.0

7000.0

34007.0

Line:

Trace:

15000 20000 25000 30000 35000 40000

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

3.000

3.100

Horizontal

Distance (ft):

Project No. 0411-1924 Filename: 111924_fig_GC823-C-1.cdr

Figure GC 823-C-1. Inline 6823 and Crossline 34007 Showing Conditions Beneath Proposed Wellsite GC 823-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 6823

SALT

SALT

GC

823

GC

867

Southwest Northeast

SEAFLOOR

HORIZON 20

HORIZON 35

TOP OF SALT

UNIT 1

UNIT 2

UNIT 3

UNIT 4

SEAFLOOR

HORIZON 20

HORIZON 35

HORIZON 40

TOP OF SALT

UNIT 1

UNIT 2

UNIT 3

UNIT 4

Crossline 34007

Proposed 

Wellsite GC 823-C

Proposed 

Wellsite GC 823-C

GC

866

GC

824

HORIZON 40
6800.0

34007.0

Line:

Trace:

20000 25000 30000

1.500

1.600

1.700

1.800

1.900

2.000

2.100

2.200

2.300

2.400

2.500

2.600

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 823-C-2. Tophole Prognosis Chart, Proposed Wellsite C, Green Canyon Block 823

Project No. 0411-1924 Filename: 111924_fig_GC823-C-2.cdr

3-D Seismic Section Through

Proposed Wellsite C

X = 2,476,973 FT; Y = 9,849,490 FT

 Crossline 34007 at Inline 6823

SW NE

Crossline 34007

GC

867

PROPOSED WELLSITE C

TWO-WAY TRAVEL TIME (SEC)

SEAFLOOR

Water 

Flow Gas

Unit 1

HORIZON 20

HORIZON 40

TOP OF SALT

Unit 2 Unit 4

1.696

1.753

1.981

2.023

-4,158

-4,307

-4,933

-5,051

149

775

893

149

275

118

Seafloor Slopes at 5.1° 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

FAULT 1.945 -4,831

HORIZON 35 1.883 -4,658 500 351

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.

Mud-rich turbidites.

Fault near base of unit.

Chaotic sequence created by salt movement. 

Mixture of silts and clays.

Negligible potential for overpressured sands.

Negligible potential for shallow gas.

Negligible potential for overpressured sands.

Negligible potential for shallow gas.

Low potential for overpressured sands.

Low potential for shallow gas.

Fault possible zone of loss circulation.

Low potential for overpressured sands.

Low potential for shallow gas.

Top of salt slopes 7.2° to East.

SALT

673

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-A 

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-A

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 1,121 ft below the mudline (bml) at the Proposed Exploration Wellsite GC 867-A. 

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-A

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

Proposed Wellsite GC 867-A

Spheroid & Datum: Clarke 1866, NAD27

Projection: UTM Zone 15 North Line Reference Block Calls

X: 2,476,928 ft Latitude: 27.1278025504676o N Inline 6805 27 ft FNL

Y: 9,852,453 ft Longitude: 90.4277812536595° W Crossline 34073 5,888 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-A-1 through GC 867-A-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 zone 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-A-1: Bathymetry Map

Map GC 867-A-2: Seafloor Rendering

Map GC 867-A-3: Seafloor Amplitude Rendering

Map GC 867-A-4: Geologic Features Map

Figure GC 867-A-1:Inline 6805 and Crossline 34073 Showing Conditions Beneath 

Proposed Wellsite GC 867-A

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

867
Project No. 0411-1924

 -2- Proposed Well GC 867-A 

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 ~0.405 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-A

Man-Made Features

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

GC 867-A is the BP Exploration Inc., GC 823-001 well approximately 2,270 ft to the northeast (Maps 

GC 867-A-1 through GC 867-A-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-A
Project No. 0411-1924

 -3- Proposed Well GC 867-A 

Other wells near the Proposed Wellsite GC 867-A are the BP Exploration Inc., GC 823-0022 well and GC 

869-001 wells. These wells are approximately 9,350 ft east-northeast and 9,420 ft west-southwest, 

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

Proposed Wellsite GC 867-A 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 -4,106 ft (Map GC 867-A-1). The seafloor at the proposed 

location is slightly irregular and dips to the southeast at about 1.1 ̊. The proposed wellsite is located within 

a northwest-southeast trending valley. The walls of the valley are formed by seafloor faults. The closest 

fault is approximately 500 ft to the south (Map GC 867-A-2).

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

north to approximately -4,700 ft on the upper part of the Sigsbee Escarpment to the south (Map GC 867-A1).

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 

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

by an increase in slope from less than 10° to approximately 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-A-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-A location. Based on the renderings and seismic profiles deepwater 

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

Patches of high-amplitude seafloor returns occur approximately 7,700 ft to the southwest of the location. 

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

9,800 ft to the northwest (Maps GC 867-A-1 through GC 867-A-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 500 ft to the south. Except for the faults 7,700 ft to the southwest, no high-amplitude returns 

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

unlikely. 

Stratigraphy

Inline 6805 and Crossline 34073 are the closest 3-D lines to the proposed GC 867-A location 

(Figure GC 867-A-1). The Tophole Prognosis Chart (Figure GC 867-A-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. 

The 3-D data volume was used to define the sedimentary section to a depth of approximately 1,121 ft 

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

 -4- Proposed Well GC 867-A 

(Figure GC 867-A-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 186 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 exist at the seabed. The soils will increase in strength and decrease in water content 

with depth.

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 near the base of the unit as suggested by the slight increase in reflection amplitude. The combined 

thickness of Units 1 and 2 is approximately 294 ft (Figure GC 867-A-2).

Unit 3 (Horizon 35 to Horizon 40) consists of mud-rich turbidites. The top of Unit 3, Horizon 35, is at 

about 294 ft bml. The unit is approximately 561 ft thick beneath GC 867-A.

Unit 4 is a sequence bounded by Horizon 40 and the top of salt (Figures GC 867-A-1 and GC 867-A-2). 

The sequence consists of bedded turbidites that have been affected by shallow salt emplacement. Unit 4

occurs between 855 ft and 1,121 ft bml at the proposed location. Unit 4 will likely consist of a mixture of 

silts and clays and is approximately 266 ft thick.

The top of salt is predicted to be 1,121 ft bml (~5,227 ft bsl). The salt surface dips to the east-northeast at 

approximately 21°. 

Faults

Three faults will be intersected by the well bore beneath proposed Wellsite GC 867-A (Figures GC 867-A1 and GC 867-A-2). The borehole will intersect the faults at approximately 303 ft, 499 ft, and 739 ft bml. 

All faults will be penetrated between Horizon 35 and Horizon 40 (Unit 3). 

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 186 ft bml at proposed Wellsite A (Figure GC 867-A-2). The entire unit is 108 ft 

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

The existing GC 823-001 well is approximately 2,270 ft to the northeast of the proposed GC 823-A 

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 (Figure GC 867-A-2). In addition, the interval is 

bounded by faults that extend from salt to the seafloor. The shallow water flow may have originated from 

an isolated sand pocket or layer within the chaotic interval. An alternative explanation is the flow 

originated in the shallower interval at the top of the unit consisting of bedded, moderate-amplitude 

turbidites. The flow may not have been observed until drilling was at the 1,715 ft level.

Because of the proposed well is within the same valley as the GC 823-001 well, the lower portion of Unit 4 

has been given a high potential for shallow water flow (Figure GC 867-A-2). Sands within the unit are 

likely to be isolated and faulted. Like GC 823-001 any flow will be short-lived and of low severity. 

Shallow Gas 

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

GC 867-A-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-A, Green Canyon Block 867 (OCSG-33858), appear suitable for exploration drilling operations. The existing wellsites and the areas with 
Project No. 0411-1924

 -5- Proposed Well GC 867-A 

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

proposed location. Three faults will be encountered at depth.

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 Deepwater, LLC, 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-A 

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.