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Surface sediment hydrocarbons as indicators of subsurfacehydrocarbons: Field calibrationof existing and new surface geochemistry methods inthe Marco Polo area,Gulf of MexicoMichael A. Abrams and Nicola F. Dahdah

ABSTRACTMultiple methods are currently used to collect, prepare, ex-tract, and analyze near-surface migrated hydrocarbons frommarine sediments to evaluate subsurface petroleum genera-tion and entrapment. Few have been rigorously tested to eval-uate their effectiveness. A Gulf of Mexico field calibrationsurvey over the Marco Polo field was undertaken as part of anindustry-funded research project to better understand pre-viously published and unpublished seabed geochemical re-sults and determine which gas and liquid hydrocarbon ex-traction methods best characterize migrated hydrocarbons innear-surface sediments.

The Marco Polo calibration data set demonstrates the im-portance of targeted coring and sampling depth. To improvethe detection of seabed migrated thermogenic hydrocarbons,core samples should be collected along major migration path-ways (cross-stratal leakage features) identified by conventionaldeep seismic and high-resolution sea floor imaging. Not alltargeted cores hit the designated feature, and thus, collecting

replicates along key migration features is critical. Collectingsediment samples below the near-surface transition zone knownas the zone of maximum disturbance is also important to

AUTHORSMichael A. Abrams Exploration and Production Technology Apache Corporation Houston, Texas; [email protected]

Michael A. Abrams is currently the managegeochemistry for Apache Corporation. Befo working with Apache, Michael was senior rsearch scientist in the University of Utah s Energyand Geoscience Institute and senior researchgeochemist in Exxon Production Research Cpany. Michael s research interests include surfageochemistry, petroleum systems evaluation,and migration pathway analysis. Michael re-ceived his Ph.D. from theImperialCollegeLona B.S. degree from George Washington Univsity, and an M.S. degree from the UniversitySouthern California.

Nicola F. Dahdah Energy and Geoscien Institute University of Utah Salt Lake City, [email protected]

Nick Dahdah currently manages the organicgeochemistry laboratory at EGI and has beea research scientist with the Institute since 19specializing in oil and source rock charactezation. He previously worked as a well siteologist for the Natural Resources AuthorityJordan. He received a B.S. degree in geolofrom Southeast Missouri State University aan M.A. degree from the University of SoutCarolina.

ACKNOWLEDGEMENTS

We thank the Surface Geochemistry Calibra(SGC) research project industry supporters aEnergy and Geoscience Institute at the Un versity of Utah. Ger van Graas (Statoil), DeMiller (Petrobras), Harry Dembicki (AnadarNeil Frewin (Shell), Andy Bishop (Shell), B

Huizinga (ConocoPhillips), Angelo Riva (Eand Peter Eisenach (Wintershall) have all beextremely helpful during the various phases the multiyear industry-funded research proje We thank Anadarko Petroleum for allowing the SGC research project to collect samples the Marco Polo field and access to key seisand geochemical information. We thank HarrDembicki who arranged permission for theMarco Polo location site, chose and monito the core locations, and participated in the

Copyright 2011. The American Association of Petroleum Geologists. All rights reserved.Manuscript received August 13, 2010; provisional acceptance November 22, 2010; revised manuscript received January 24, 2011; final acceptance March 21, 2011.DOI:10.1306/03211110130

AAPG Bulletin, v. 95, no. 11 (November 2011), pp. 19071935 1907

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avoid possible alteration effects and interference by recent or-ganic matter.

Geochemical analysis should include a full range of hy-drocarbon types: light hydrocarbon gases (C 1 C5 ), gasolinerange (C 5 C10+ ), and high-molecular-weight (HMW) hydro-carbons (C 15+ ). The interstitial sediment gas data should be

plotted on a total hydrocarbon gas ( S C1 C5 ) versus wet gasfraction ( S C2 C5 / S C1 C5 ) chart to identify background,fractionated, and anomalous populations. Compound-specificisotopic analysis on selected anomalous samples is critical tocorrectly identify migrated subsurface gases from near-surfacegenerated microbial gases. Microdesorption bound gases didnot provide gas compositions or compound-specific isotoperatios similar to the Marco Polo reservoir gases, and thus, thebound gas extraction is not recommended. A gasoline rangeanalysis provides a new rangeof hydrocarbons rarely examined

in surface geochemical studies that assist in identifying ther-mogenic hydrocarbons. Extraction gas chromatography andtotal scanning fluorescence (TSF) maximum fluorescence in-tensity provided information on the presence of thermogenicHMW hydrocarbons but did not work as well with the low-level microseepage samples. The TSF fluorogram signaturewas similar for both seep and regional reference (background)samples and did not help to identify migrated thermogenichydrocarbons.

The Marco Polo calibration study provides a framework to better understand how best to collect (targeted deep cores)and extract migrated hydrocarbons from near-surface marinesediments and to evaluate the results.

INTRODUCTION

Multiple methods are currently applied to collect, prepare,extract, and analyze near-surface migrated hydrocarbons frommarine sediments to evaluate subsurface petroleum genera-tion (Horvitz, 1985; Brooks and Carey, 1986; Abrams et al.,

2001; Abrams, 2005; Logan et al., 2009; and Abrams andDahdah, 2010). Many of the sediment hydrocarbon extrac-tion procedures currently used by the industry are based onsampling and laboratory protocols initially designed for wellcuttings and have not always been rigorously tested to evaluatetheir effectiveness with unconsolidated marine sediments. Anextensive series of literature and data reviews, laboratory tests,and field studies have been conducted as part of an industry-funded Surface Geochemistry Calibration (SGC) research proj-ectconducted by theUniversity of Utah s Energy andGeoscience

cruise. We thank W. L. Gore and Associatesand Taxon Biosciences, which provided staff and laboratory analysis at no cost. We thank Fugro and the Seis Surveyor crew as well as the shipboard research staff Harry Dembicki(Anadarko), Matt Ashby (Taxon), Shuanglin Li(Qingdoa Institute of Marine Geology), Rowland

Rincon (Peregrine Ventures), and Chris Reny (Peregrine Ventures). The figures were drafted by Jeffrey Massara with Apache Corporation. Re- views by Harry Dembicki, Barry Katz, and anunnamed reviewer were very helpful.The AAPG Editor thanks the following for their work on this paper: Harry Dembicki Jr., Barry J.Katz, and an anonymous reviewer.

1908 Surface Sediment Hydrocarbons as Indicators of Subsurface Hydrocarbons

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Institute. The multiphase SGC research project was organized to better understand previous sea-bed geochemical results both published and un-

published and determine which sediment hydro-carbon (gas and liquid) extraction methods best characterize migrated near-surface sediment hy-drocarbons (Abrams, 2002).

The SGC Gulf of Mexico Marco Polo field cal-ibration survey was designed to field test specificanalytical procedures examined in the laboratorystudies as well as several emerging and existingtechnologies. The Gulf of Mexico Marco Polo fieldwas chosen because it is an area of known petro-

leum leakage based on previous seismic and geo-chemical surveys (Chaouche et al., 2004; Dembickiand Samuels, 2007), reservoir geochemical dataavailable (Abrams and Dembicki, 2006), and high-quality shallow seismic imaging data acquired(Dembicki and Samuels, 2007).

The near-surface sediment geochemical meth-ods examined in the Marco Polo field calibrationsurvey can be subdivided into three categories:light hydrocarbon gas (C 1 C5 ), gasoline-range hy-

drocarbons (C 5 C10+ ), and high-molecular-weight (HMW) hydrocarbons (C 15+ ). The light hydrocar-bons were examined using two different sediment

gas extraction methods: interstitial (conventionalcan headspace and modified headspace methodcalled disrupter analysis) and adsorbed-bound(microdesorption analysis). The gasoline-range hy-drocarbons were examined by two different ana-lytical procedures, disrupter headspace solid-phasemicroextraction (HSPME) and Gore Module. TheHMW hydrocarbons were examined using sol-vent extraction followed by whole-extract gas chro-matography (GC) and total scanning fluorescence

(TSF).The purpose of this article is to provide anoverview of the Marco Polo field SGC survey re-sults for different seepage types and to better un-derstand how the different methods characterizethe seeping hydrocarbons. This article providesguidance on best practices for seabed geochemicalsurveys based on both previously published lab-oratory experiments and the Marco Polo field cal-ibration survey results.

Figure 1. The Marco Polo field is approximately 175 mi (281 km) south of New Orleans, Louisiana, in blocks 563, 607, and 60Canyon Gulf of Mexico in approximately 4000 ft (1219 m) of water.

Abrams and Dahdah 1909

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FIELD OPERATIONS

Study Area

The Marco Polo field is approximately 175 mi(281km) south of New Orleans,Louisiana, inblocks563, 607, and 608 Green Canyon Gulf of Mexico inapproximately 4000 ft (1219m) of water (Figure 1 ).The Marco Polo field is located in a salt-boundedminibasin with petroleum production from supra-

salt Miocene reservoir sands (Chaouche et al., 2004).Geochemical analysis of reservoir fluids by AnadarkoPetroleum indicates that the medium-gravity oiloriginated from a marine type II organic matter typeconsistent with generation from the subsalt upperJurassic source rocks (see Chaouche et al., 2004,for details). Areas of fluid movement from the res-ervoir to near surface had been identified on thewestern side of the field area (Green Canyon Block 607) by the presence of mud mounds (volcanoes),

Table 1. Core Description with Target Classification and Depth Information

Seep Feature Core Depth Subsamples

Core No. (based on seismic feature, previous survey) Target Type Length (m) A (cm) B (cm) C (

1 Outside Marco Polo minibasin Regional reference 3.65 156 239 3222 Outside seep feature Near seep zone 3.90 181 264 347

3 Outside seep feature Near seep zone 4.77 268 351 4344 Edge potential seepage Near seep zone 3.72 163 246 3295 Inside area of seepage Within seep zone 4.96 287 370 4536 Replicate Core 5 Within seep zone 4.19 210 293 3767 Previous survey oil stained Within seep zone 4.17 208 291 3748 Replicate Core 7 Within seep zone 4.23 214 297 3809 Replicate Core 7 Within seep zone 4.04 195 278 361

10 Inside area of seepage Within seep zone 2.46 37 120 20311 Inside area of seepage Within seep zone 4.29 220 303 39612 Outside seep feature Near seep zone 5.33 324 407 49013 Inside area of seepageedge Within seep zone 3.79 170 253 336

14 Previous survey oil stained with hydrate Within seep zone 0.31 NA NA 015 Edge potential seepage Near seep zone 4.02 193 276 35916 Outside seep feature Near seep zone 4.42 233 316 39917 Outside seep feature Near seep zone 3.40 131 214 29718 Near seeping fault along mud volcano Within seep zone 3.67 158 241 32419 On mud volcano Within seep zone 3.85 176 259 34220 Flank mud volcano Near seep zone 3.42 133 216 29921 On mud volcano Within seep zone 0.70 NA 0 3322 Flank mud volcano Within seep zone 1.76 NA 60 14323 Flank mud volcano Within seep zone 0.50 NA NA 1024 Flank mud volcano Within seep zone 3.78 169 252 335

25 Outside north edge mud volcano Near seep zone 5.33 323 406 48926 Outside north edge mud volcano Near seep zone 3.66 157 240 32327 Outside seep feature Near seep zone 3.31 122 205 28828 Outside seep feature Near seep zone 3.73 164 197 28029 Outside seep feature Near seep zone 3.63 154 237 32030 Outside seep feature Near seep zone 3.70 161 244 32831 Outside seep feature Near seep zone 2.89 80 163 24632 Outside seep feature Near seep zone 3.14 105 188 27133 Outside Marco Polo minibasin Regional reference 2.79 70 153 236


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pockmarks (seabed craters), shallow acoustic blank-ing, and bright amplitudes (Dembicki and Samuels,2007, 2008).

Core Selection and Collection

Thirty-threecoreswere collected based on 23 targetsand one location far from known seepage (Table 1 ).The core locations are displayed on a digital ter-rain map generated from high-resolution multi-beam bathymetry (Dembicki and Samuels, 2007)(Figure 2). Two types of features were targeted:within seep zone, sample location within majorseepage zone based on high-resolution acoustic data;and near seep zone, sample location near featurewith major seepage based on high-resolution acoustic

data. A regional reference (local background ref-erence) locationwas selected outside the minibasinfor a baseline sample of recent sediment organicmatter. More details on the core locations and tar-geted seismic features can be found in Dembickiand Samuels (2007, 2008).

Initial positioning of the vessel was done usingshipboard global positioning system to match thelocation of features identified with autonomousunderwater vehicle (AUV) data. Theonboard Chirpsubbottom profiler records were then comparedwith the AUV subbottom profiler records to con-firm the targeted feature.The core samples werecollected aboard the Fugro Seis Survey using amodified Kullenberg piston coring device (troughcorer trap system) with a 6-m core barrel and a

Figure 2. The Marco Polocalibration cruise core locatioon the surface terrain map,Gulf of Mexico, Green CanyoBlock 607.


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Table 2. Marco Polo Surface Geochemistry Calibration Can Headspace and Disrupter Interstitial Sediment Gas Data

Can Headspace Disrupter Headspace

Core No.Core

SectionCore TargetClassification

Methane(ppm)

Sum WetGases (ppm)

Wet GasFraction

Methane(ppm)

Sum WetGases (ppm)

Wet GasFraction

EGI 01 A Regional reference 6 1 0.14 9 2 0.17

EGI 01 B Regional reference 7 1 0.10 16 5 0.23EGI 01 C Regional reference 8 2 0.18 11 4 0.27EGI 02 A Near seep zone 11 2 0.14 15 3 0.18EGI 02 B Near seep zone 19 4 0.16 30 6 0.16EGI 02 C Near seep zone 16 2 0.12 35 4 0.09EGI 03 A Near seep zone 11 1 0.11 11 3 0.22EGI 03 B Near seep zone 22 3 0.10 22 4 0.16EGI 03 C Near seep zone 13 2 0.13 21 4 0.14EGI 04 A Near seep zone 8 1 0.12 12 2 0.12EGI 04 B Near seep zone 59 2 0.03 19 3 0.14EGI 04 C Near seep zone 12 2 0.12 25 3 0.11

EGI 05 A Within seep zone 21 3 0.11 21 4 0.17EGI 05 B Within seep zone 10 2 0.17 17 5 0.23EGI 05 C Within seep zone 22 1 0.05 40 2 0.06EGI 06 A Within seep zone 10 1 0.11 18 3 0.15EGI 06 B Within seep zone 10 1 0.08 21 4 0.15EGI 06 C Within seep zone 16 2 0.10 25 3 0.10EGI 07 A Within seep zone 31 3 0.08 45 2 0.05EGI 07 B Within seep zone 42 3 0.06 111 2 0.02EGI 07 C Within seep zone 112 5 0.04 273 6 0.02EGI 08 A Within seep zone 168 3 0.02 206 3 0.02EGI 08 B Within seep zone 206 9 0.04 831 14 0.02

EGI 08 C Within seep zone 14,262 61 0.00 23,710 66 0.00EGI 09 A Within seep zone 32 3 0.07 92 2 0.02EGI 09 B Within seep zone 53 4 0.06 126 3 0.02EGI 09 C Within seep zone 166 4 0.02 249 4 0.01EGI 10 A Within seep zone 43,083 1177 0.03 71,454 991 0.01EGI 10 B Within seep zone 120,570 7978 0.06 136,500 3843 0.03EGI 10 C Within seep zone 147,738 6384 0.04 181,780 5212 0.03EGI 11 A Within seep zone 191 6 0.03 268 6 0.02EGI 11 B Within seep zone 2736 25 0.01 4364 27 0.01EGI 11 C Within seep zone 79,531 121 0.00 202,040 220 0.00EGI 12 A Near seep zone 407 7 0.02 179 5 0.03EGI 12 B Near seep zone 9206 2421 0.21 21,398 59 0.00EGI 12 C Near seep zone 48,260 149 0.00 105,618 205 0.00EGI 13 A Within seep zone 74,639 224 0.00 110,879 299 0.00EGI 13 B Within seep zone 39,127 553 0.01 490,720 4331 0.01EGI 13 C Within seep zone 41,999 123 0.00 87,892 172 0.00EGI 14 C Within seep zone 5052 89 0.02 4179 58 0.01EGI 15 A Near seep zone 106,017 1309 0.01 161,053 2176 0.01EGI 15 B Near seep zone 187,246 552 0.00 234,324 526 0.00EGI 15 C Near seep zone 64,710 403 0.01 211,839 1199 0.01EGI 16 A Near seep zone 7 1 0.10 3497 22 0.01EGI 16 B Near seep zone 2387 5 0.00 223,696 206 0.00


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Table 2. Continued


Core No.Core


Methane(ppm)

Sum WetGases (ppm)

Wet GasFraction

Methane(ppm)

Sum WetGases (ppm)

Wet GasFraction

EGI 16 C Near seep zone 185 2 0.01 234,874 202

EGI 17 A Near seep zone 16 1 0.04 11 2 0EGI 17 B Near seep zone 20 3 0.14 15 4 0EGI 17 C Near seep zone 15 2 0.11 17 4 0EGI 18 A Within seep zone 22 3 0.12 25 5 0EGI 18 B Within seep zone 26 5 0.16 31 5 0EGI 18 C Within seep zone 29 7 0.19 16 6 0EGI 19 A Within seep zone 81 3 0.03 113 3 EGI 19 B Within seep zone 310 8 0.02 427 10 EGI 19 C Within seep zone 11,750 78 0.01 24,238 116 EGI 20 A Near seep zone 180 4 0.02 384 8 0EGI 20 B Near seep zone 4125 31 0.01 13,253 58

EGI 20 C Near seep zone 111,873 110 0.00 206,623 167 EGI 21 B Within seep zone 102,222 10,581 0.09 149,638 7484 EGI 21 C Within seep zone 105,464 16,270 0.13 147,362 11,699 EGI 22 B Near seep zone 85,849 463 0.01 153,412 395 EGI 22 C Near seep zone 68,002 69 0.00 133,143 156 EGI 23 C Near seep zone 49,834 504 0.01 164,486 2163 EGI 24 A Near seep zone 145 7 0.04 222 5 0EGI 24 B Near seep zone 193 3 0.01 325 8 0EGI 24 C Near seep zone 1076 15 0.01 2259 46 0EGI 25 A Near seep zone 294 3 0.01 381 7 0EGI 25 B Near seep zone 9800 19 0.00 45,563 52

EGI 25 C Near seep zone 24,673 21 0.00 51,987 48 EGI 26 A Near seep zone 10 1 0.12 11 4 0EGI 26 B Near seep zone 13 2 0.15 10 4 0EGI 26 C Near seep zone 20 2 0.07 16 6 0EGI 27 A Near seep zone 19 2 0.10 6 3 0EGI 27 B Near seep zone 5 1 0.22 19 4 0EGI 27 C Near seep zone 46 3 0.06 22 4 0EGI 28 A Near seep zone 112 2 0.01 166 4 0EGI 28 B Near seep zone 480 7 0.01 968 17 0EGI 28 C Near seep zone 68,185 75 0.00 87,801 78 EGI 29 A Near seep zone 22 2 0.08 31 2 0

EGI 29 B Near seep zone 56 2 0.03 88 4 0EGI 29 C Near seep zone 91 2 0.02 183 4 0EGI 30 A Near seep zone 7 1 0.14 70 5 0EGI 30 B Near seep zone 9 3 0.25 17 4 0EGI 30 C Near seep zone 13 9 0.40 11 4 0EGI 31 A Near seep zone 14 8 0.35 81 8 0EGI 31 B Near seep zone 17 4 0.18 12 6 0EGI 31 C Near seep zone 21 8 0.28 18 9 0EGI 32 A Near seep zone 25 3 0.10 8 3 0EGI 32 B Near seep zone 22 2 0.07 25 3 0EGI 32 C Near seep zone 37 3 0.07 19 5 0


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6.7-cm internal diameter. The deep-water pistoncorer was launched and retrieved using a rotatingA-frame high-speed hydroelectric dual winch. The33 cores range in length from 0.5 to 5.3 m (1.6 17.4 ft) with an average recovery of 3.5 m (11.5 ft)(Table 1 ). Three core sections were collected from

each sediment core: top ( 0.52.0m[ 1.6 6.6 ft]),middle ( 2.0 3.0 m [ 6.6 9.8 ft]), and bottom( 3.0 5.0 m [ 9.8 16.4 ft]). Each core sectionwas subdivided in the shipboard laboratory andprocessed using the analytical protocols providedby the participating laboratories or established inthe SGC laboratory studies: a 5-cm subsectionplaced in Gore Module sample container with asorber; replicate 10-cm sample splits placed in adisrupter container (500-mLplastic container witha screw on sealing cap, built-in septum, and in-ternal blades; see Abrams and Dahdah, 2010, for

details) and a metal can for interstitial gas analysis;3-cm sample placed in a plastic bag for micro-desorption analysis; and replicate 10-cm samplesplits were wrapped in aluminum foil for solvent extraction.

ANALYTICAL PROCEDURES

Sediment Gas Analysis

The conventional can headspace method is a non-mechanical procedure that uses high-speed shak-ing to release vapor-phase interstitial sediment gasesinto the can headspace (Bernard, 1978). A 500-mLmetal can is filledwith170 mLof sediment, 160 mLof 3.5% NaCl in H2 O solution, and the remainingheadspace is flushed with N 2 gas. The sample can

Table 2. Continued


Core No.Core


Methane(ppm)

Sum WetGases (ppm)

Wet GasFraction

Methane(ppm)

Sum WetGases (ppm)

Wet GasFraction

EGI 33 A Regional reference 14 2 0.14 19 6 0.23

EGI 33 B Regional reference 25 1 0.04 13 2 0.14EGI 33 C Regional reference 34 1 0.03 139 5 0.04

Figure 3. The three major groups forMarco Polo can headspace and disrupterlight hydrocarbon data using the clas-sification scheme from Abrams (2005);background with total gas concentrations less than 10,000 ppm and low wetgas fraction (0.05),anomalous with total gas concentrationsgreater than 10,000 ppm and wet gasfraction less than 0.1 (

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with mud and processed water is stored frozen.The can is thawed for 24 hr before analysis, heatedto 40C for 4 hr, and shaken vigorously using aconventional paint shaker. The headspace gas iscollected by syringe and injected in the gas chro-matograph for compositional analysis. See Bernard

(1978) for more details.The disrupter sediment gas extraction methodwas designed as part of the SGC laboratory studiesto capture interstitial gases not easily released byvigorous shaking (Abrams and Dahdah, 2010). Thedisrupter method uses a 165-mL sediment samplethat is placed in a 500-mL disrupter chamber with165 mL of saturated salt brine solution and remain-ing volume air headspace. The disrupter chamberhas a fixed internal blade that breaks apart the sedi-

ment, releasing interstitial gases without crushing(Abrams and Dahdah, 2010). The disrupter withsample is frozen on the vessel, then shipped andstoredfrozenuntil analysis. Thedisrupter is thawedto room temperature 24 hr before analysis andshaken for 5 min using a high-speed unidirectionalpaint can shaker. A 0.2-mL disrupter headspacesample is collected by a syringe through the dis-rupter cap septum at room temperature and injected into the GC inlet. See Abrams and Dahdah(2010) for more details.

The bound gases are believed to be attached toorganic and/or mineral surfaces, entrapped in struc-tured water, or entrapped in authigenic carbonateinclusions and thus require a more rigorous proce-dure to remove (Horvitz, 1985; Bjoroy and Ferriday,2002; Whiticar, 2002). The microdesorption boundsediment gas extraction method (Whiticar, 2002)uses a 300- to 400-mL bulk sediment sample that has been stored frozen in a plastic bag. A fixedweight of wet sediment sample (1 3 g) is placed ina reaction vessel, sealed, and evacuated. A smallamount of saline water is added, and the sediment-water slurry is mixed using a vortex ultrasonic mixer.The interstitial gases are removed by vacuum. Phos-phoric acid is added under reduced pressure wherethe sorbed gas is released to the vessel headspace.Potassium hydroxide is added before GC to reducecarbonate-generated carbon dioxide. The pressureis increased and sample aliquotsof gas are collectedfor GC analysis. See Whiticar (2002) for details.

Gasoline-Range (C 5 C10+ )Hydrocarbon Analysis

Two methods were used in the Marco Polo fieldcalibration survey to evaluate sediment gasoline-range hydrocarbons, the Gore Module and disrupter

HSPME. The Gore method evaluates a full rangeof hydrocarbons from C 2 to C20+ using a speciallyengineered hydrophobic adsorbent encased in amicroporous expanded Gore Module polytetra-fluoroethylene membrane (Anderson, 2006). TheGore Module is placed in a special glass containerwith a designated volume of sediment and analyzedvia thermal desorption coupled with mass spec-trometry (MS). See the W. L. Gore and Associates Web page for additional details. (www.gore.com)

The HSPME headspace sample is collectedafter the disrupter headspace gas analysis. A 1-cmfused silica fiber coated with 100- mm-thick poly-dimethylsiloxane is inserted into the headspace fora 20-min extraction, then manually injected intothe GC inlet for desorption. The SPME fiber is left in the GC inlet for 5 min. See Abrams et al. (2009)for details.

High-Molecular-Weight Hydrocarbon Analysis

A dried sediment sample is ground to a uniform sizeand an aliquot by weight is extracted using hex-ane in an automated extraction apparatus (DionexASE 200 Accelerated Solvent Extractor). Extractsare concentrated to a final volume of 8 mL usingZymark TurboVap II. The final extracts are sub-mitted for hydrocarbon analysis by GC flame ion-ization detection (FID) and TSF. See Brooks et al.(1983) for additional information.

RESULTS AND DISCUSSION

Sediment Gas Analysis

Interstitial Gas EvaluationThe can headspace and disrupter data are summa-rized in Table 2 and plotted on a total hydrocarbongas ( S C1 C5 ) versus wet gas fraction ( S C2 C5 / S C1 C5 ) plot(Figure 3 ). Both interstitial sediment gas


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extraction methods provide similar results. Threemajor groups exist for both interstitial sediment gas data sets using the classification scheme fromAbrams (2005): background with total gas concen-trations less than 10,000 ppm and low wet gasfraction (0.05); and anomalous with total gasconcentrations greater than 10,000 ppm and wet gas fraction less than 0.1 (10%). The cutoffs arebased on an examination of a worldwide surfacegeochemistry database (Abrams, 2005).

Examination of the three sediment gas groupsrelative to the pre-survey core targets (within seepzone, near seep zone, and regional reference) re-veals interesting observations. The regional refer-

ence core samples fall within the background andfractionated samples. The fractionated group wasdefined in Abrams (2005) to represent very lowconcentration samples (

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importance of good seismic data to define the seepfeature and a core collection protocol with theability to hit the targeted feature at 2+ m below thewater-sediment interface.

The disrupter interstitial sediment gas compo-sition (wet gas fraction and C 1 /C2 ratio) and MarcoPolo field reservoir gases are different (Figure 5).Similar observations were made by Abrams andDembicki (2006). The differences between reser-voir and near-surface sediment gas compositionsare most likely related to near-surface affects (phasefractionation and microbial alteration) as well asmixing with shallow microbial gases. Therefore, cau-tion should be used when interpreting near-surface

sediment gas compositions using interpretationcharts designed for reservoir gases (Abrams, 2005).

Compound-specific carbon isotopic analysis(methane, ethane, and propane) of selected high-concentration macroseepage (anomalous) can head-space and disrupter extracted interstitial gases pro-vide similar results except for core number 23C(Table 3 ). The disrupter d 13 C1 for sample 23C is9 lighter, indicating in-situ microbial gas gener-ation. The methane and ethane d 13 Cn values forthe disrupter-extracted interstitial gases are sim-ilar to the 12,200-ft reservoir production gases,except propane (Figure 6 ). The disrupter andcan headspace-extracted hydrocarbon gases havepropane isotopic values heavier ( d 13 C 3 , 15.3 to

Figure 5. The disrupter interstitial sediment gas composition(wet gas fraction and C1 /C2 ratiomethane/ethane) is differentfrom the Marco Polo Field reservoir gases.

Figure 6. The methane and ethane d 13 Cn values for disrupteextracted interstitial gases are similar to the 12,200 ft (371reservoir production gases, except propane (C3 ) (does not include sample 23C).

Table 3. Marco Polo Surface Geochemistry Calibration Interstitial Sediment Gas (Can Headspace and Disrupter) CompouCarbon and Methane Hydrogen Isotopic Data

Core No. Method Laboratory d 13 C1 ( ) d 13 C2 ( ) d 13 C3 ( ) d DC1 ( )

10B Disrupter 56.8 35.9 25.8 203Can headspace 55.6 36.2 29.2 193

10C Disrupter 54.8 36.4 29.3 207Can headspace 52.6 36.8 30.4 203

21B Disrupter 56.2 36.5 15.3 199Can headspace 53.9 36.5 16.3 196




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Table 4. Marco Polo Surface Geochemistry Calibration Microdesorption Sediment Gas Data

Depth Core Target Total Gas Wet Gas Carbon Isotopes

Sample ID Section Classification C1 (nmol/g) (nmol/g) Fraction C1 C2 C3 iC4 NC4

EGI 01 A Regional reference 236 253 0.07 56.0 32.3 32.5 34.0 31.0EGI 01 B Regional Reference 256 278 0.08 52.3 32.0 30.8 33.0 31.0EGI 01 C Regional Reference 277 303 0.09 48.7 33.2 31.5 31.0 31.0EGI 02 A Near seep zone 267 287 0.07 54.6 32.4 31.1 33.0 30.0EGI 02 B Near seep zone 286 309 0.07 51.1 31.9 30.5 33.0 30.0EGI 02 C Near seep zone 200 220 0.09 48.9 32.7 30.7 30.0EGI 03 A Near seep zone 232 248 0.07 52.0 32.7 31.4 32.0 30.0EGI 03 B Near seep zone 182 196 0.07 53.7 32.6 31.0 31.0EGI 03 C Near seep zone 342 388 0.12 45.5 34.3 31.8 31.3 29.9EGI 04 A Near seep zone 187 201 0.07 54.7 32.4 31.6 30.0EGI 04 B Near seep zone 203 218 0.07 50.2 32.4 30.5 30.0EGI 04 C Near seep zone 233 251 0.07 51.2 32.8 31.3 31.0EGI 05 A Within seep zone 221 240 0.08 53.7 32.3 31.0 30.0EGI 05 B Within seep zone 427 475 0.10 45.9 34.3 31.5 31.2 29.8EGI 05 C Within seep zone 247 272 0.09 49.5 33.9 31.8 32.0 30.0EGI 06 A Within seep zone 201 216 0.07 53.3 33.1 30.6 31.0EGI 06 B Within seep zone 223 240 0.07 50.1 32.2 31.1 29.0EGI 06 C Within seep zone 221 244 0.09 49.1 33.9 31.4 32.0 30.0EGI 07 A Within seep zone 243 263 0.07 50.2 32.4 30.5 33.0 28.0EGI 07 B Within seep zone 312 345 0.10 47.8 34.5 32.4 31.0 30.4EGI 07 C Within seep zone 372 418 0.11 49.7 34.5 32.7 32.2 31.0EGI 08 A Within seep zone 227 241 0.06 53.5 32.7 30.4 31.0EGI 08 B Within seep zone 240 257 0.07 62.0 31.9 29.4EGI 08 C Within seep zone 457 513 0.11 49.0 34.8 30.4 31.6 29.1EGI 09 A Within seep zone 258 274 0.06 56.8 32.2 31.0 31.0EGI 09 B Within seep zone 181 194 0.07 52.8 32.6 30.2 30.3EGI 09 C Within seep zone 400 430 0.07 53.0 32.6 30.1 33.0 29.8EGI 10 A Within seep zone 227 717 0.68 52.7 29.9 7.7 31.5EGI 10 B Within seep zone 678 772 0.12 56.2 33.5 23.4 32.4 28.3EGI 10 C Within seep zone 902 992 0.09 55.1 33.8 26.7 32.3 22.1EGI 11 A Within seep zone 362 384 0.06 58.7 32.9 31.8 32.2 29.8EGI 11 B Within seep zone 300 314 0.04 62.7 33.0 26.5 32.0EGI 11 C Within seep zone 725 768 0.06 57.0 34.8 29.1 32.8 30.6EGI 12 A Near seep zone 223 246 0.09 51.3 32.9 28.5 31.5 30.4EGI 12 B Near seep zone 706 807 0.12 48.3 34.6 28.3 32.1 30.2EGI 12 C Near seep zone 898 991 0.09 58.7 35.2 18.1 31.8 29.0EGI 13 A Within seep zone 847 863 0.02 55.2 33.5 24.2 28.4EGI 13 B Within seep zone 602 634 0.05 52.9 33.2 27.8 30.7EGI 13 C Within seep zone 727 771 0.06 48.7 34.6 30.6 30.0 30.1EGI 14 C Within seep zone 109 121 0.10 63.0 31.1EGI 15 A Near seep zone 479 545 0.12 56.4 34.6 15.6 28.3 23.0EGI 15 B Near seep zone 965 995 0.03 56.1 35.3 21.7 30.7 27.7EGI 15 C Near seep zone 609 640 0.05 59.6 34.6 27.4 31.9 29.6EGI 16 A Near seep zone 385 409 0.06 58.3 32.7 29.3 34.1 29.6EGI 16 B Near seep zone 730 757 0.04 62.6 35.3 29.5 31.6 30.1EGI 16 C Near seep zone 872 899 0.03 58.9 36.2 29.5 32.2 29.6EGI 17 A Near seep zone 83 89 0.07 49.2 32.6


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Table 4. Continued

Depth Core Target Total Gas Wet Gas Carbon Isotopes

Sample ID Section Classification C1 (nmol/g) (nmol/g) Fraction C1 C2 C3 iC4 NC4

EGI 17 B Near seep zone 293 312 0.06 55.6 33.3 30.9 30.8EGI 17 C Near seep zone 259 279 0.07 51.4 33.0 31.2 29.9EGI 18 A Within seep zone 293 311 0.06 57.5 32.5 31.3 33.2 30.7EGI 18 B within seep zone 362 391 0.07 48.0 32.4 30.6 31.4 28.9EGI 18 C within seep zone 302 325 0.07 53.7 33.4 32.0 32.5 32.1EGI 19 A within seep zone 293 311 0.06 55.4 32.5 32.2 30.3EGI 19 B within seep zone 294 316 0.07 53.1 33.0 30.9 31.5 29.3EGI 19 C within seep zone 381 410 0.07 53.0 33.8 31.3 28.7EGI 20 A near seep zone 270 287 0.06 59.8 32.5 31.9 30.5EGI 20 B near seep zone 346 373 0.07 50.6 31.8 28.7 32.1 28.8EGI 20 C Near seep zone 822 844 0.03 59.8 33.0 28.5 33.6 29.7EGI 21 B Within seep zone 436 835 0.48 46.5 33.9 28.2 31.7 26.7EGI 21 C Within seep zone 591 1736 0.66 54.2 34.3 14.8 31.8EGI 22 B Within seep zone 862 937 0.08 52.0 37.0 26.5 30.3 27.3EGI 22 C Within seep zone 993 1020 0.03 58.0 35.6 27.5 30.8 27.7EGI 23 C Within seep zone 215 234 0.08 55.5 33.7 28.9 30.6 26.5EGI 24 A Within seep zone 138 150 0.08 54.8 34.0 30.6 29.1EGI 24 B Within seep zone 205 227 0.10 51.2 33.3 25.5 32.4 29.3EGI 24 C Within seep zone 619 701 0.12 44.2 34.6 32.1 32.1 30.2EGI 25 A Near seep zone 215 233 0.08 54.3 33.1 30.3 33.4 27.9EGI 25 B Near seep zone 262 285 0.08 57.7 33.9 31.8 30.7EGI 25 C Near seep zone 684 759 0.10 52.9 35.0 32.0 32.2 30.2EGI 26 A Near seep zone 280 296 0.05 58.3 32.6 32.1 31.9EGI 26 B Near seep zone 284 305 0.07 49.3 32.3 31.8 29.6EGI 26 C Near seep zone 185 204 0.09 49.6 32.6 31.4 32.4EGI 27 A Near seep zone 83 91 0.08 51.4 32.9 30.6EGI 27 B Near seep zone 245 262 0.07 54.6 32.7 29.3 32.9 31.2EGI 27 C Near seep zone 262 283 0.07 51.5 32.6 31.0 33.9 30.9EGI 28 A Near seep zone 174 179 0.03 62.7 31.9EGI 28 B Near seep zone 237 252 0.06 56.2 33.6 31.0EGI 28 C Near seep zone 398 414 0.04 64.4 34.1 28.8 28.5EGI 29 A Near seep zone 243 256 0.05 59.2 32.3 31.5EGI 29 B Near seep zone 210 223 0.05 58.2 33.5 31.0EGI 29 C Near seep zone 199 213 0.06 57.3 32.4 30.8 30.1EGI 30 A Near seep zone 287 301 0.05 59.2 32.3 31.8 29.7EGI 30 B Near seep zone 227 244 0.07 46.6 32.5 30.3 32.1 30.0EGI 30 C Near seep zone 220 237 0.07 53.8 32.3 30.5 33.0EGI 31 A Near seep zone 70 77 0.09 42.5 31.9EGI 31 B Near seep zone 256 270 0.05 58.4 33.0 31.6 30.5EGI 31 C Near seep zone 194 206 0.06 55.5 32.5 29.7 30.9EGI 32 A Near seep zone 61 67 0.09 44.6 31.7EGI 32 B Near seep zone 318 338 0.06 58.5 33.4 32.9 30.5EGI 32 C Near seep zone 228 244 0.07 53.3 32.9 29.9 30.5EGI 33 A Regional reference 66 74 0.11 42.0 31.0EGI 33 B Regional reference 246 263 0.06 57.9 32.5 30.9 33.0 29.1EGI 33 C Regional reference 233 249 0.07 52.8 31.6 30.2 29.4


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29.3 ) relative to the reservoir gases (d 13 C 3 , 27.8 to 31.1 ) (Table 3 ; Figure 6). This is most likely related to near-surface microbial alterationof propane. The preferential attack of propanewas noted by James and Burns (1984) in reservoirgases, and microbial fractionation of near-surfacegases is not uncommon for seabed sediment gases(Abrams, 1989).

Microdesorption (Bound) GasesThe microdesorption bound gas data (Table 4 ) areplotted on the same total hydrocarbon gas ( S C1 C4 ) versus wet gas fraction ( S C2 C4 / S C1 C4 )evaluation plot using the same core designations(Figure 7). Note that the microdesorption sedi-ment gases are reported in nanomoles per gram

by weight, whereas the disrupter headspace gas isreported in parts per million by volume.

The bound gases display a very different trendthan the interstitial gases. No bimodal distributionis present and total microdesorption hydrocarbongas displays significant variability from low- andhigh-concentration samples (Figure 7 ). Most of the samples have wet gas fractions less than 0.12(

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important target for most surface geochemicalsurveys (Abrams et al., 2009).

To date, few surface geochemical surveys at-tempt to evaluate the gasoline-range hydrocarbonsin near-surface marine sediments. Conventionalheadspace light hydrocarbon analysis is not an ef-

fective method to evaluate the C 6 to C12 hydro-carbons because of higher boiling points and lowvapor pressures relative to the hydrocarbons gases(C 1 C5 ) (Abrams and Dahdah, 2010).

Headspace Solid-Phase MicroextractionThe HSPME data are reported as the area sum of a single carbon number (SCN) within the mainboiling point range detected using the SPME fiber.

The unresolved complex mixture (UCM) is in-cluded in the area of each SCN (Abrams et al.,2009) (Table 5 ). A plot of the disrupter total hy-drocarbon interstitial gas ( S C1 C5 ) versus HSPMESCN (Figure 9) displays moderate correlation be-tween sediment interstitial gas and the concentra-tion of gasoline-range hydrocarbons.

The regional reference HSPME gas chromato-grams contain very low overall signal responses(

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Table 5. Marco Polo Surface Geochemistry Calibration Headspace Solid-Phase Microextraction and Gore Module Gasoline Plu with Statistical Grouping Classifications

Core No.Core

SectionCore TargetClassification Core Depth Gore Grouping

Total GoreHydrocarbons

Total DisrupterC1 C5

Total SPMESCNUCM

EGI 01 A Regional reference 156 Background 142 15 158EGI 01 B Regional reference 239 Background 303 21 181EGI 01 C Regional reference 322 Background 243 11 76EGI 02 A Near seep zone 181 Background 131 18 493EGI 02 B Near seep zone 264 Background 174 36 129EGI 02 C Near seep zone 347 Background 148 39 189EGI 03 A Near seep zone 268 Background 131 14 70EGI 03 B Near seep zone 351 Background 205 26 299EGI 03 C Near seep zone 434 Background 216 25 175EGI 04 A Near seep zone 163 Background 146 14 119EGI 04 B Near seep zone 246 Background 159 22 73EGI 04 C Near seep zone 329 Background 370 28 154EGI 05 A Within seep zone 287 Background 214 26 82EGI 05 B Within seep zone 370 Background 278 22 271EGI 05 C Within seep zone 453 Background 134 42 153EGI 06 A Within seep zone 210 Background 173 21 86EGI 06 B Within seep zone 293 Background 193 25 74EGI 06 C Within seep zone 376 Background 160 28 163EGI 07 A Within seep zone 208 Background 129 48 252EGI 07 B Within seep zone 291 Background 134 113 146EGI 07 C Within seep zone 374 Background 185 279 119EGI 08 A Within seep zone 214 Background 124 209 83EGI 08 B Within seep zone 287 Background 179 845 1467EGI 08 C Within seep zone 380 Medium Aliphatic 1558 23,775 2334

EGI 09 A Within seep zone 194 Background 150 94 178EGI 09 B Within seep zone 278 Background 154 129 188EGI 09 C Within seep zone 361 Background 155 253 171EGI 10 A Within seep zone 37 High aliphatic 6212 72,445 4588EGI 10 B Within seep zone 120 High aliphatic 132,986 140,343 4859EGI 10 C Within seep zone 203 High aliphatic 153,739 186,992 34,294EGI 11 A Within seep zone 220 Medium Aliphatic 443 274 116EGI 11 B Within seep zone 303 Background 331 4391 547EGI 11 C Within seep zone 396 Medium aliphatic 1106 202,259 2426EGI 12 A Near seep zone 324 Background 159 184 92EGI 12 B Near seep zone 407 Background 202 21,456 116

EGI 12 C Near seep zone 490 Medium aliphatic 856 105,824 151EGI 13 A Within seep zone 170 High aliphatic 24,625 111,178 9948EGI 13 B Within seep zone 253 High aliphatic 59,660 495,051 9155EGI 13 C Within seep zone 336 High aliphatic 47,206 88,064 6387EGI 14 C Within seep zone 10 Medium aliphatic 516 4237 1807EGI 15 A Near seep zone 193 High aliphatic 34,715 163,229 9502EGI 15 B Near seep zone 276 High aliphatic 42,592 234,851 12,214EGI 15 C Near seep zone 359 High aliphatic 23,301 213,038 16,784EGI 16 A Near seep zone 233 Background 150 3519 510EGI 16 B Near seep zone 316 Medium aliphatic 1390 223,903 921EGI 16 C Near seep zone 399 Medium aliphatic 517 235,076 508


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Table 5. Continued

Core No.Core

SectionCore TargetClassification Core Depth Gore Grouping

Total GoreHydrocarbons

Total DisrupterC1 C5

Total SPMSCNUCM

EGI 17 A Near seep zone 131 Background 133 13 EGI 17 B Near seep zone 214 Background 318 18 EGI 17 C Near seep zone 297 Background 215 22

EGI 18 A Within seep zone 158 Background 244 29 EGI 18 B Within seep zone 241 Background 201 36 EGI 18 C Within seep zone 324 Background 256 22 EGI 19 A Within seep zone 176 Background 145 116 EGI 19 B Within seep zone 259 Background 111 437 EGI 19 C Within seep zone 342 Medium aliphatic 1757 24,355 1EGI 20 A Near seep zone 133 Medium aliphatic 2332 392 EGI 20 B Near seep zone 216 Background 115 13,312 EGI 20 C Near seep zone 299 High aliphatic 882 206,790 EGI 21 B Within seep zone 10 High aliphatic 19,134 157,122 1EGI 21 C Within seep zone 33 High aliphatic 226,976 159,061 2

EGI 22 B Within seep zone 60 High aliphatic 26,428 153,807 1EGI 22 C Within seep zone 143 High aliphatic 6983 133,299 EGI 23 C Within seep zone 10 High aliphatic 79,866 166,649 1EGI 24 A Within seep zone 169 Background 147 227 EGI 24 B Within seep zone 252 Background 247 333 EGI 24 C Within seep zone 335 Background 277 2305 EGI 25 A Near seep zone 323 Background 124 389 EGI 25 B Near seep zone 406 Background 201 45,616 EGI 25 C Near seep zone 489 Medium aliphatic 482 52,034 EGI 26 A Near seep zone 157 Background 269 15 EGI 26 B Near seep zone 240 Background 291 15

EGI 26 C Near seep zone 323 Background 200 22 EGI 27 A Near seep zone 122 Background 206 9 EGI 27 B Near seep zone 205 Background 116 9 EGI 27 C Near seep zone 288 Background 175 26 EGI 28 A Near seep zone 164 Background 102 170 EGI 28 B Near seep zone 197 Background 210 985 EGI 28 C Near seep zone 280 Background 281 87,879 EGI 29 A Near seep zone 154 Background 130 34 EGI 29 B Near seep zone 237 Background 120 92 EGI 29 C Near seep zone 320 Background 112 187 EGI 30 A Near seep zone 161 Background 184 75

EGI 30 B Near seep zone 244 Background 193 21 EGI 30 C Near seep zone 328 Background 144 15 EGI 31 A Near seep zone 80 Background 255 88 EGI 31 B Near seep zone 163 Background 351 18 EGI 31 C Near seep zone 246 Background 313 27 EGI 32 A Near seep zone 105 Background 151 12 EGI 32 B Near seep zone 188 Background 162 28 EGI 32 C Near seep zone 271 Background 235 24 EGI 33 A Regional reference 70 Background 195 15 EGI 33 B Regional reference 153 Background 180 25 EGI 33 C Regional reference 236 Background 246 144


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MS analysis can detect much lower concentra-tions than other seabed geochemical analysis meth-ods. However, it could not discriminate the low-concentration within-seep-zone and near-seep-zonesamples any better than the disrupter headspacegas or HSPME gasoline-range analysis.

The 8, 11, 16, 19, and 25 within-seep-zone andnear-seep-zone core shallow subsamples were clas-sified by Gore in the background group, whereasthedeeper subsamples fall in themedium aliphaticgroup. This observation reinforces the importanceof sampling depth and placing the corer directly onthe targeted feature.

High-Molecular-Weight Hydrocarbon Analysis

Extract GC: The sediment extract GC evaluationincludes the chromatogram signature, total UCM,and total n-alkanes. Table 6 contains extract GCtotal UCM and total n-alkanes for the Marco Polocalibration samples. Samples with UCM valuesless than 25 mg/g are considered to be background,whereas samples with UCM greater than 100 mg/g

are associated with migrated hydrocarbon seepage(Coleetal., 2001; Abrams, 2005).Six of the 15corescollected at the within-seep-zone targets did not have elevated UCM (>100 mg/g), indicating that the samples may not have hit the targeted feature(Table 6 ). One near-seep-zone sample contains

elevated UCM ( Table 6 ).Sediments containing migrated thermogenicHMW hydrocarbons typically have GC-FID chro-matograms characterized by a large unresolved com-plex mixture (UCM) with some discernible C 15 C32 n-alkanes and isoprenoids peaks, depending onthe severity of microbial alteration (Figure 13A )(Brooks and Carey, 1986). All but one of the within-seep-zone samples have extremely high UCM val-ues(>1000 mg/g) and a sum of total alkanes less than

1. This is a common observation with sediment ex-tract chromatograms and is related to the severenear-surface bacterial alteration that has destroyedmost of the resolvable normal alkane compoundsvery quickly. Sediments containing mainly the re-cent organic matter (ROM) signature with somemigrated thermogenic signal will contain lowerUCM and overprint of odd n-alkanes greater thanC 23 (Figure 13B) (Brooks and Carey, 1986).

Extract TSF: The sediment extract TSF evalu-ation includes examination of the fluorogram sig-nature and maximum fluorescence intensity (MFI).Samples with significant seepage require dilutionbefore TSF analysis. The MFI values are adjustedby multiplying the measured MFI by the dilutionfactor to obtain a corrected MFI (Brooks et al.,1983).

The extract TSF MFI ranges from 30,140 to73,260,000MFI units (Table 6 ) for the SGC phase IIIGOM calibration samples. Most of the samples col-lected in the within-seep-zone targets contain high-extract TSF MFI values (100,000 MFI units) (Coleet al., 2001; Abrams, 2005).Several near-seep-zonesamples as well as one shallow regional-referencesample also have extract TSF MFI greater than100,000 units.

An extract GC total UCM versus TSF MFI plot demonstrates a relatively strong correlationbetweenthe two HMW screening tools (Figure 14 ), but ex-amination of the TSF fluorogram signatures indi-cate a potential problem with the extract TSF

Figure 9. The headspace solid-phase microextraction (HSPME)data reported as sum of single carbon number (SCN) within the

detection range of SPME fiber. The unresolved complex mixtureis included in the area of each SCN. A plot of disrupter totalhydrocarbon interstitial gas ( S C1 C5 ) versus HSPME SCN dis-plays a moderate correlation between sediment interstitial gasand concentration of gasoline-range hydrocarbons with notableexceptions.


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method ( Figure 15). The fluorogram signatures fora regional-reference sample with low-interstitialsediment gas, UCM, and TSFMFI; and within-seep-zone sample with highly elevated interstitial sedi-ment gas, UCM, and TSF MFI should have verydifferent shapes and locations of the maximum ex-

citation wavelength (MFI Max Ex) and maximumemission wavelength (MFI Max EM), yet they aresimilar (Figure 15A, B). This indicates that thefluorogram shape, MFI Max Ex, and MFI Max EMmay not assist in the identification of thermogenicseepage in marine sediments.

KEY OBSERVATIONS

Sediment GasesThe can headspace and disrupter extraction meth-ods provide similar interstitial sediment gas data,indicating that the disrupter plastic container withscrew cap and sealing gasket, built-in septum, andblades to break up the sediment did not providesignificantly better results than the conventionalcan headspace method. Both methods providehighly variable gas compositions compared withthe Marco Polo reservoir gases. Very few of thehigh-concentration within-seep-zone samples havesediment gas compositions similar to the MarcoPolo reservoir gases. In contrast, the can headspaceand disrupter interstitial sediment gas compound-specific isotopes are similar to the Marco Polo res-ervoir gases, except for the propane carbon iso-tope value. The propane isotopic values are muchheavier most likely because of preferential micro-bial alteration (James and Burns, 1984; Abrams,1989, 2005). The bound gas extraction methoddid not provide gas compositions or compound-specific isotopes similar to the Marco Polo reser-voir gases. This could be related to the bound gasremoval process, which may fractionate the sedi-ment gas sample (Abrams and Dahdah, 2010).

Proper identification of anomalous versus back-ground interstitial (free) sediment gas is requiredtoseparate background samples that may seem to bethermogenic because of phase fractionation. Phasefractionation (preferential loss of methane relative

to ethane plus gases) results in background low-concentration sediment gases having elevated wet gasfractions. The fractionated samples are commonlyconfused with migrated thermogenic hydrocarbons(Abrams, 2005)because of the higher relativewet gascomponent. Onlythehigh-concentrationsamples are

likely to be derived from migrated thermogenic gas.Compound-specific isotopic measurements are crit-ical to confirm thethermogenic origin, assuming that isotopically distinctive changes are present.

Many samplescollected at within-seep-zone ornear-seep-zone targets identified by conventionaland high-resolution surface seismic data have onlylow concentrations (background) of interstitial gas. We believe that these samples did not hit the in-tended target or were collected within the ZMD.

Gasoline-Range Sediment Hydrocarbons

Both the HSPME and Gore Module extractionmethods provide strong gasoline-range seepage sig-nals for the high-concentration within-seep-zoneand near-seep-zone samples and minimal to no sig-nal in the regional reference and low-concentrationwithin-seep-zone and near-seep-zone samples. TheGore Module thermal extraction combined withMS provides much greater compositional detailthan the HSPME GC-FID method.

The HSPME chromatograms show evidencefor significant near-surfacemicrobial alteration. Thelack of an unaltered oil, even in zones of highfluxmacroseepage, leads ustobelieve that the rateof alteration is rapid. It is our belief that despite mi-crobial alteration, gasoline-range hydrocarbons pro-vide key information for a boiling point range not examined in most offshoresurveys, andthis type of

data is very important to help identify subsurfacehydrocarbon generation (Abrams et al., 2009).

High-Molecular-WeightSediment Hydrocarbons

The within-seep-zone samples with elevated HMW hydrocarbons (macroseepage) have extremely highUCM values and a distinctive GC signature. Theregional reference samples have much lower UCM


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with a different and distinctive GC signature. Whenconcentrations of migrated HMW hydrocarbonare low relative to the in-situ ROM material, theidentification of migrated thermogenic hydrocar-bons is difficult. Reworked or transported hydro-

carbons can be confused with locally migratedhydrocarbons (Abrams, 2005). Reworking andtransported hydrocarbons have been identifiedwithin the Marco Polo Green Canyon area (Coleet al., 2001; Dembicki, 2010).

Figure 10. (A) The head-space solid-phase micro-extraction (HSPME) datafor regional reference core.(B) The HSPME data for the within-seep-zone core.


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The Marco Polo extract TSF data indicate that MFI measurements provide information on thepresence of anomalous hydrocarbons, but TSFfluorogram shapes do not change with target typeor other geochemical measurements (interstitial

gas, gasoline-range hydrocarbons, or extract UCM).Edwards and Crawford (1999) demonstrated that a linear relationship between oil concentration andtotal fluorescence intensity can only be obtainedbetween 0.10 and 10 ppm oil. Within this range,

Figure 10. Continued.


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fluorescence intensity is proportional to concen-tration, but outside this range, TSF fluorescenceintensity (MFI) and shape can vary because of thedilution factor (hydrocarbon concentration). Otherfactors that affect TSF fluorescence intensity (MFI)and shape include absorbance (compounds in ex-

tract that can absorb either excitation or emittedlight), quenching (energy can be transferred non-radiatively to coexisting molecules instead of beingemitted as fluorescence), and secondary alteration(water washing, biodegradation, evaporative frac-tionation, and weathering).

Figure 11. Three end-membergroupings defined by the GoreModule data for the Marco Polocalibration data set: (A C) highaliphatic, medium aliphatic, andbackground.


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Table 6. Marco Polo Surface Geochemistry Calibration Extract Gas Chromatography and Total Scanning Fluorescence Data

Extract GCExtract TSF

Core ID Depth (cm) Core Target n Alkanes UCM ( mg/g) UCM > n-C23 ( mg/g) MFI

EGI 01 156 Regional reference 875 11 7 60,920EGI 01 239 Regional reference 486 6 5 51,430EGI 01 322 Regional reference 1841 12 8 56,480EGI 02 181 Near seep zone 1414 19 16 89,960EGI 02 264 Near seep zone 1209 9 8 43,670EGI 02 347 Near seep zone 1618 11 9 34,140EGI 03 268 Near seep zone 1037 10 9 94,720EGI 03 351 Near seep zone 1562 18 16 30,215EGI 03 434 Near seep zone 3044 19 13 68,370EGI 04 163 Near seep zone 1110 17 13 57,310EGI 04 246 Near seep zone 1233 32 21 57,770EGI 04 329 Near seep zone 1255 9 7 49,170EGI 05 287 Within seep zone 881 8 6 26,205EGI 05 370 Within seep zone 1953 12 8 127,420EGI 05 453 Within seep zone 1071 10 8 54,600EGI 06 210 Within seep zone 1120 9 7 54,440EGI 06 293 Within seep zone 833 8 6 46,760EGI 06 376 Within seep zone 2259 12 9 107,660EGI 07 208 Within seep zone 1216 10 8 47,810EGI 07 291 Within seep zone 996 8 7 51,410EGI 07 374 Within seep zone 2034 14 11 119,600EGI 08 214 Within seep zone 1260 16 12 53,760EGI 08 287 Within seep zone 0 6359 4876 42,712,000EGI 08 380 Within seep zone 0 3971 2815 26,892,000EGI 09 194 Within seep zone 1026 16 11 55,900EGI 09 278 Within seep zone 1269 13 9 50,920EGI 09 361 Within seep zone 1236 81 55 716,400EGI 10 37 Within seep zone 0 664 399 4,288,000EGI 10 120 Within seep zone 0 1070 598 6,926,000EGI 10 203 Within seep zone 1508 20 15 4,558,000EGI 11 220 Within seep zone 0 322 264 90,100EGI 11 303 Within seep zone 0 941 469 2,171,500EGI 11 396 Within seep zone 2302 63 28 563,000EGI 12 324 Near seep zone 989 17 14 49,420EGI 12 407 Near seep zone 3869 26 22 230,600EGI 12 490 Near seep zone 2880 17 14 201,650EGI 13 170 Within seep zone 0 2497 1648 20,516,000EGI 13 253 Within seep zone 0 1130 712 7,032,000EGI 13 336 Within seep zone 0 224 126 2,382,000EGI 14 10 Within seep zone 0 3700 2456 36,080,000EGI 15 193 Near seep zone 0 3660 2344 27,768,000EGI 15 276 Near seep zone 0 3393 2265 27,560,000EGI 15 359 Near seep zone 0 5254 3400 46,752,000EGI 16 233 Near seep zone 1098 20 14 57,700EGI 16 316 Near seep zone 935 10 8 71,900EGI 16 399 Near seep zone 1793 19 12 121,320EGI 17 131 Near seep zone 696 16 12 195,600


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Table 6. Continued

Extract GCExtract TS

Core ID Depth (cm) Core Target n Alkanes UCM ( mg/g) UCM > n-C23 ( mg/g) MFI

EGI 17 214 Near seep zone 1194 15 10 99EGI 17 297 Near seep zone 988 12 9 76

EGI 18 158 Within seep zone 1314 18 13 7EGI 18 241 Within seep zone 1293 16 14 24EGI 18 324 Within seep zone 979 7 7 22EGI 19 176 Within seep zone 1419 22 17 9EGI 19 259 Within seep zone 1661 115 100 1,14EGI 19 342 Within seep zone 3375 207 139 2,33EGI 20 133 Near seep zone 1364 19 14 63EGI 20 216 Near seep zone 2213 258 228 2,75EGI 20 299 Near seep zone 1240 13 10 24EGI 21 10 Within seep zone 0 3537 3016 35,51EGI 21 33 Within seep zone 0 7423 4789 73,26

EGI 22 60 Within seep zone 54,864 6754 4452 67,8EGI 22 143 Within seep zone 0 2678 1745 22,89EGI 23 10 Within seep zone 0 1983 1523 18,77EGI 24 169 Within seep zone 1425 11 9 7EGI 24 252 Within seep zone 2048 10 8 9EGI 24 335 Within seep zone 3008 32 18 12EGI 25 323 Near seep zone 1382 15 14 76EGI 25 406 Near seep zone 2621 12 10 15EGI 25 489 Near seep zone 3720 27 23 368EGI 26 157 Near seep zone 1370 24 18 72EGI 26 240 Near seep zone 1397 13 11 39

EGI 26 323 Near seep zone 827 6 4 53EGI 27 122 Near seep zone 958 19 14 118EGI 27 205 Near seep zone 1553 16 13 85EGI 27 288 Near seep zone 1133 11 9 77EGI 28 164 Near seep zone 1096 14 9 66EGI 28 197 Near seep zone 1140 16 14 150EGI 28 280 Near seep zone 1016 9 7 74EGI 29 154 Near seep zone 1243 19 13 117EGI 29 237 Near seep zone 1245 15 13 57EGI 29 320 Near seep zone 1157 12 9 82EGI 30 161 Near seep zone 1385 24 16 84

EGI 30 244 Near seep zone 1446 15 11 60EGI 30 328 Near seep zone 1016 10 8 34EGI 31 80 Near seep zone 460 8 7 58EGI 31 163 Near seep zone 1211 17 11 58EGI 31 246 Near seep zone 1408 11 9 26EGI 32 105 Near seep zone 436 9 8 30EGI 32 188 Near seep zone 1367 20 14 32EGI 32 271 Near seep zone 1607 13 10 54EGI 33 70 Regional reference 424 18 15 26EGI 33 153 Regional reference 1467 22 16 8EGI 33 236 Regional reference 1101 14 11 6


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Geochemical analysis should include a full rangeof hydrocarbon types; light hydrocarbon gases (C 1 C5 ), gasoline range (C5 C10+ ), and HMW hydro-carbons (C 15+ ).

The two interstitial sediment gas extractionmethods, can headspace and disrupter, providesimilar results in both laboratory (Abrams andDahdah, 2010) and field calibration studies. The

Figure 13. The MarcoPolo Surface Geochem-istry Calibration (SGC) extract gas chromatograms.(A) Upper: within seepzone. (B) Lower: regionalreference.


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interstitial sediment gas data should be plotted ona total hydrocarbon gas ( S C1 C5 ) versus wet gasfraction ( S C2 C5 / S C1 C5 ) chart to identify back-ground, fractionated, and anomalous populations(Abrams, 2005). The Marco Polo survey anom-alous interstitial sediment gases have variable gas

compositions compared with the reservoir gases,thus, caution should be used when plotting seabedgases on conventional gas interpretation charts.The methane and ethane stable carbon isotopesfrom selected anomalous samples are similar tothe Marco Polo reservoir gases. However, propaneisotope values are much heavier, indicating that propane is more easily modified by in-situ micro-bial alteration.

The microdesorption bound gases have gas

compositions and compound-specific isotopes un-like the Marco Polo reservoir gases. The microde-sorption sediment gases tend to have more wet gasfraction and highly variable methane carbon iso-topes with heavier ethane carbon isotopes. TheSGC laboratory studies indicate that prewashing toremove interstitial gases can have a major impact on bound gas results (Abrams and Dahdah, 2010). We do not recommend using bound gas extrac-tion methods to evaluate subsurface hydrocarbonsbased on the Marco Polo and previous laboratorycalibration studies reported in Abrams and Dahdah(2010).

The gasoline-range analysis provides a newrange of hydrocarbons rarely examined in surfacegeochemical studies. Both the HSPME and GoreModulemethods used in theMarco Polo calibrationstudies provide strong gasoline-range seep signalshaving useful information in the macroseepage(high-flux) andmicroseepage (low-flux) seep sites.The Gore Module thermal extraction combined

with MS provides more compositional detail thanthe HSPME GC-FID method, which may be help-ful to evaluate the seep hydrocarbon source andmaturity.

Extraction GC and TSF analyses provide in-formation on the presence of HMW hydrocarbonsin the Marco Polo calibration survey. The GCchromatogram signature and total UCM trackedthe migrated thermogenic hydrocarbon macroseep-age but did not work as well with the low-level

Figure 14. The extract gas chromatography (GC) total unre-solved complex mixture (UCM) versus total scanning fluorescence(TSF) maximum fluorescence intensity (MFI) plot demonstratesrelatively strong correlation between two high-molecular-weightscreening tools for Marco Polo extract GC and TSF data.

Figure 15. Fluorogram signatures for regional reference sple with low interstitial sedimentgas, unresolved complex m(UCM), and total scanning fluorescence (TSF) maximum fcence intensity (MFI); and within-seep-zone sample with elevated interstitial sediment gas, UCM, and TSF MFI shohave similar shapes and locations of maximum excitation length (MFI Max Ex) and maximum emission wavelengthEM). (A) Regional reference target EGI-1 (322 cm [127 in.]56,480 and DIL 1:10. (B) Within-seep-zone target EGI-22 [24 in.]): MFI = 67,845,000 and DIL 1:4000.


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microseepage samples. The TSF MFI data also di-rectionally tracked migrated thermogenic hydro-carbon macroseepage and microseepage samples,but the fluorogram shape could not distinguishwithin-seep-zone and regional reference samples. We do not recommend extraction TSF to evalu-

ate migrated HMW hydrocarbons in near-surfacemarine sediment seep surveys based on the aboveresults.

Follow-up studies by Dembicki (2010) withAnadarko examined extract saturate and aromaticfraction GC-MS to evaluate biomarker signaturevariability. Results of Dembicki (2010) demon-strate the importance of biomarker data to assist indetecting and interpreting low-concentration pe-troleum seepage (microseepage). Biomarker data

provide a means to characterize the ROM con-tribution and in turn allow for the identificationof thermogenic hydrocarbons when the seep oilconcentration is low relative to ROM.

The above conclusions are based on the MarcoPolo calibration study as well as previous labora-tory calibration studies (Abrams et al., 2009; Loganet al., 2009; Abrams and Dahdah, 2010). Theyprovide a framework to better understand howbest to collect and extract migrated hydrocarbonsfrom shallow-marine sediments and evaluate theresults. However, it is also very important to in-tegrate sediment hydrocarbon results with basingeology to fully understand how surface geochem-istry observations relate to subsurface generationand potential entrapment (Abrams, 2005). A fullyintegrated evaluation provides the best petroleumsystems interpretation model.

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