6784.0

33800.0

6784.0

34000.0

6784.0

34200.0

6784.0

34400.0

Line:

Trace:

15000 20000 25000 30000 35000 40000

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

34109.0

6800.0

34109.0

Line:

Trace:

15000 20000 25000 30000 35000 40000

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

3.000

Horizontal

Distance (ft):

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

Figure GC 823-B-1. Inline 6784 and Crossline 34109 Showing Conditions Beneath Proposed Wellsite GC 823-B

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 6784

SALT

SALT

GC

823

GC

867

Southwest Northeast

SEAFLOOR

HORIZON 20

HORIZON 35

HORIZON 40

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 34109

Proposed 

Wellsite GC 823-B

Proposed 

Wellsite GC 823-B

GC

866

GC

824
6800.0

34109.0

Line:

Trace:

25000 30000

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):

Shallow Water Flow and Gas Potential

HIGH

0 1 2 3

0 = NEGLIGIBLE

1 = LOW

2 = MODERATE

3 = HIGH

NEGLIGIBLE

Figure GC 823-B-2. Tophole Prognosis Chart, Proposed Wellsite B, Green Canyon Block 823

Project No. 0411-1924 Filename: 111924_fig_GC832-B-2.cdr

3-D Seismic Section Through

Proposed Wellsite B

X = 2,477,578 FT; Y = 9,854,364 FT

 Crossline 34109 at Inline 6784

SW NE

Crossline 34109

GC

867

PROPOSED WELLSITE B

TWO-WAY TRAVEL TIME (SEC)

GC

823

SEAFLOOR

Water 

Flow Gas

Unit 1

HORIZON 20

HORIZON 40

TOP OF SALT

Unit 2 Unit 4

1.661

1.713

INTERFACE

1.966

2.360

-4,072

-4,207

-4,902

-6,082

135

830

2,010

135

312

1,180

Seafloor Slopes at 2.8° East.

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 2.080 -5,232 1,160

FAULT 1.835 -4,538 466

HORIZON 35 1.854 -4,590 518 383

2.150 -5,439 1,367

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.

Fault at very base of unit.

Mud-rich turbidites.

Turbidites. Alternating layers of clay and silt with thin

sands.

Fault within top half of unit.

Chaotic sequence created by salt movement.

Negligible potential for overpressured sands.

Negligible potential for shallow gas.

Negligible potential for overpressured sands.

Negligible potential for shallow gas.

Faults are possible zones of loss circulation.

Low potential for overpressured sands.

Low potential for shallow gas.

Water flow reported in Unit 4 at GC 823-001, 820 ft to 

East-Southeast.

Moderate potential for overpressured sands.

Low potential for shallow gas.

High potential for overpressured sands.

Low potential for shallow gas.

Faults are possible zones of loss circulation.

Top of salt slopes 41° to East-Northeast

SALT

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 823-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 823-C

Block 823 (OCS-G-33854)

Green Canyon Area

Gulf of Mexico

Apache Deepwater, LLC (Apache) plans to drill an exploration well in Block 823, of the Green Canyon 

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

depth of approximately 893 ft below the mudline (bml) at the Proposed Exploration Wellsite GC 823-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 823-C

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

Proposed Wellsite GC 823-C

Spheroid & Datum: Clarke 1866, NAD27

Projection: UTM Zone 15 North Line Reference Block Calls(SL in GC 867)

X: 2,476,973 ft Latitude: 27.1196541357444o N Inline 6823 2,990 ft FNL

Y: 9,849,490 ft Longitude: 90.4278294471442° W Crossline 34007 5,933 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 823-C-1 through GC 823-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 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 823-C-1: Bathymetry Map

Map GC 823-C-2: Seafloor Rendering

Map GC 823-C-3: Seafloor Amplitude Rendering

Map GC 823-C-4: Geologic Features Map

Figure GC 823-C-1:Inline 6823 and Crossline 34007 Showing Conditions Beneath 

Proposed Wellsite GC 823-C 

Figure GC 823-C-2:Tophole Prognosis Chart, Proposed Wellsite GC 823-C, Green Canyon Block

823
Project No. 0411-1924

 -2- Proposed Well GC 823-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 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 ~0.326 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 823-C

Man-Made Features

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

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

approximately 4,925 ft to the north-northeast (GC 823-Maps C-1 through GC 823-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 823-C
Project No. 0411-1924

 -3- Proposed Well GC 823-C 

Other wells near the surface location of the Proposed Wellsite GC 823-C are the BP Exploration Inc.,

GC 866-001 and GC 823-001 wells. These wells are approximately 9,070 ft west and 10,485 ft eastnortheast, respectively, of the proposed site and lie within the proposed mooring pattern. 

Proposed Wellsite GC 823-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, some permitting issues and/or 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 -4,158 ft (Map GC 823-C-1). The seafloor at the proposed 

location is relatively smooth and dips to the south at about 5.1 ̊. The location lies on a tilted, terrace-like 

area south of a narrow ridge. South of the location is an arcuate valley created by a Pleistocene slump. 

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

north to approximately -5,300 ft on the upper part of the Sigsbee Escarpment to the south-southeast (Map 

GC 823-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, south, 

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 10° to over 25°. 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 823-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 823-C location. Based on the renderings and seismic profiles deepwater 

benthic communities are not expected within 2,000 ft of the proposed GC 823-C wellsite. 

Patches of high-amplitude seafloor returns occur within the 10,000 ft anchor radius. The areas of highamplitude seafloor return occur to the west, southwest, and southeast of the proposed location. They are 

located between 7,100 and 10,000 ft from the location. Some of the seafloor anomalies are associated with 

fault scarps (Maps GC 823-C-1 through GC 823-C-4). The zones of high-amplitude seafloor have a highpotential 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 350 ft to the northwest. Most of the fault escarpments show normal or ambient amplitude 

returns on the rendering created from the 3-D data volume.

Stratigraphy

Inline 6823 and Crossline 34007 are the closest 3-D lines to the proposed GC 823-C location (Figure 

GC 823-C-1). The Tophole Prognosis Chart (Figure GC 823-C-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 893 ft 

(Figures GC 823-C-1 and GC 823-C-2). The section was divided into four units or sequences (Figure 

GC 823-C-1). The sequences represent variations in depositional environments and possible changes in 

soil strength and composition. 
Project No. 0411-1924

 -4- Proposed Well GC 823-C 

The surficial unit, Unit 1 (Seafloor to Horizon 20), to approximately 149 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 and turbidites interlayered 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 500 ft (Figure GC 823-C2).

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

about 500 ft bml. The unit is approximately 275 ft thick beneath GC 823-C.

Unit 4 is a sequence bounded by Horizon 40 and the top of salt (Figures GC 823-C-1 and GC 823-C-2). 

Generally the sequence consists of bedded turbidites and thin mass-transport deposits. At this location the 

unit is thin and has a chaotic character. The sediments will be a mixture of clay and silt layers with thin 

sand or silty sand seams or pockets. Unit 4 occurs between 775 ft and 893 ft bml at the proposed location. 

The sequence is only about 118 ft thick. 

The top of salt is predicted to be 893 ft bml (~5,051 ft bsl). The salt surface dips to the east at 

approximately 7.2°. 

Faults

A fault will be intersected by the well bore beneath proposed Wellsite GC 823-C (Figures GC 823-C-1 and 

GC 823-C-2). The borehole will intersect the fault approximately 673 ft bml, near the base of 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 149 ft bml at proposed Wellsite C (Figure GC 823-C-2). The entire unit is 351 ft 

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

The existing GC 823-001 well is approximately 4,925 ft to the north-northeast of the proposed GC 823-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. 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 was not 

observed until drilling was at the 1,715 ft level.

Unit 4 at proposed GC 823-C wellsite is very thin (118 ft thick) as well as the entire sedimentary section 

above salt (893 ft). The section has been eroded as well as extensively faulted. At this location there is a 

low-potential for overpressured sand layers within Unit 4 or the shallower sequences. 

Shallow Gas 

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

GC 823-C-2). 

Closing

The seafloor and near-surface conditions at Proposed Wellsite GC 823-C, Green Canyon Block 823 (OCSG-33854), 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. One fault will be encountered at depth.
Project No. 0411-1924

 -5- Proposed Well GC 823-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 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.

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.