Geochemical Evaluation of Ocean Surface Slick Methods to Ground Truth Satellite Seepage Anomalies for
Seepage Detection*
Michael A. Abrams1 and Graham Logan2
Search and Discovery Article #40604 (2010)
Posted September 30, 2010
*Adapted from oral presentation at AAPG Convention, New Orleans, Louisiana, April 11-14, 2010
1
Exploration Production Technology, Apache, Houston, TX ([email protected])
Geoscience Australia, Canberra, ACT, Australia
2
Abstract
Synthetic Aperature Radar (SAR) has become a relatively standard tool for petroleum system validation in offshore frontier
exploration areas. SAR sensors send out a radar signal and build an image from radiation reflected back to the satellite. The
dampening of ocean surface capillary waves by films of oils (slicks), natural film from sea surface micro-layer, biological material, or
physical processes such as current flow or wind will produce an anomalously low backscatter.
A field test program was undertaken to test a range of different sampling methods to determine the best way to ground truth ocean
surface natural hydrocarbon seepage over a range of different sites and examine geochemical changes related to slick formation and
aging. The field program was undertaken at the Coal Oil seep field using several ocean surface sampling methods; conventional
headspace gas (C 1 to C 5 ) and gasoline plus range solid phase microextraction on ocean surface water sample in sealed containers; and
surface contact methods which include the General Oceanics, Shell, and Gore samplers.
Our calibration studies have determined two sampling methods are required to properly collect both the lower boiling point (light
hydrocarbons and gasoline plus range) and high molecular weight (C 12 plus) hydrocarbons. The Gore Sorber slick sampler is a
modified version of the soil sampling module which works best with fresh seepage that contains light hydrocarbon (C 2 to C 26 )
fractions. The General Oceanics Oil Sampler with a DCM solvent extraction works best for the higher molecular weight hydrocarbon
(C 15 plus) fraction including petroleum derived biomarkers.
Copyright © AAPG. Serial rights given by author. For all other rights contact author directly.
References
Abrams, M.A., N.F. Dahdah, and E. Francu, 2009, Development of methods to collect and analyze gasoline plus range (C 5 to C 12 )
hydrocarbons from seabed sediments as indicators of subsurface hydrocarbon generation and entrapment: Applied Geochemistry, v.
24/10, p. 1951-1970.
Jones, A.T., G.A. Logan, J.M. Kennard, and N. Rollet, 2005, Reassessing potential origins of Synthetic Aperture Radar (SAR) slicks
from the Timor Sea region of the North West Shelf on the basis of field and ancillary data: AAPEA Journal, p. 311-331.
AAPG 2010 Annual Conventional & Exhibition
New Orleans, Louisiana 11-14 April 2010
Geochemical evaluation of ocean surface slick
methods to ground truth satellite seepage
anomalies for seepage detection
Michael A. Abrams
Graham A. Logan
Apache Corporation
Geoscience Australia
Australian Government
Geoscience Australia
ACKNOWLEDGEMENTS
Energy & Geoscience Institute at the University of Utah for
permission to present this study, SGC (Surface
Geochemistry Calibration) industry sponsors, and
contributing researchers Nick Dahdah (EGI) and
Emmanuelle Grosjean (GA). Special thanks to Dr. Ira Leifer
University of California SB in the field collection program,
Dr. Andy Bishop (Shell), and Gore & Associates for
providing Gore Sorber technology and field staff.
Surface Geochemistry Calibration Sponsors
Energy & Geoscience Institute
at the University of Utah
AAPG 2010 Annual Conventional & Exhibition
New Orleans, Louisiana 11-14 April 2010
Geochemical evaluation of ocean surface slick
methods to ground truth satellite seepage
anomalies for seepage detection
Goal: Develop collection and analysis protocols to ground
truth ocean surface natural hydrocarbon seepage over a range
of different seep zones and to examine geochemical changes
related to hydrocarbon seepage slick formation and aging.
•
•
•
•
•
Talk Outline
Ocean surface slick satellite anomalies.
Coal Oil Point seep field slick test program.
Collection and analytical methods examined.
Results from Coal Oil Point calibration survey.
Summary and recommendations slick sampling.
Ocean Surface Slick Satellite Anomalies
Microwave backscatter from ocean surface is reduced in areas where
capillary waves are dampened by hydrocarbon seepage or other non
petroleum event providing a dark response in a brighter background.
SAR seepage analysis routine
method to evaluate HC seepage
Contractors recognize not all wave
dampening features are related to
petroleum seepage since radar
does not detect oil directly.
To overcome, several criteria are
used to rank satellite anomaly;
• repeatability:
⇒ temporal repeatability
• shape (dog-leg, blob, or tadpole).
• dimensions (size).
• relationship to subsurface and
near-surface geological features.
Indirect measurement → SAR measures wave dampening NOT hydrocarbons.
What are ocean surface slicks , cont.
SAR survey was undertaken to validate earlier anomalies clustered around seabed bathymetric
edge and approximate edge of effective regional seal ⇒ concluded SAR anomalies related to
petroleum leakage from seal failure. Later work demonstrated many of the SAR anomalies
identified as high ranking were due to tidal current flows and coral spawning event.
SAR anomalies which correlate to subsurface geology may not always be related to petroleum
seepage since seabed morphology will also affect surface micro-layers, distribution of ocean
surface biological material, and current or wind flow which will also produce an anomalously
low backscatter.
Not all SAR anomalies classified as high ranking related to HC seepage.
Jones et al. (2005) Reassessing potential origins of Synthetic Aperture Radar (SAR) slicks from the Timor
Sea region of the North West Shelf on the basis of field and ancillary data. AAPEA Journal p. 311-331.
Coal Oil Point Ocean Surface Slick Test Program
GOAL
Evaluate sampling methods to ground
truth SAR surface slick anomalies.
Study Area:
• Coal Oil Point seep field one of the most
prolific and heavily studied offshore seep
zones (well mapped seismic & sonar).
• known hydrocarbon origin (Monterey).
• full range seepage activity (fresh → relict).
• approximately 1.5 x 105 m3 per day leaks
from the sea floor to the ocean surface.
• located along linear trends above faults or
fractured anticlines.
N
Sampling Program:
• Sample different seepage activity.
• Background sample location.
RV Spirit of Santa
Barbara
and field research staff – February 2007
Holly
Oil slick
C-Band Synthetic Aperture Radar (SAR) Spatial Resolution 30 m
Coal Oil Point Ocean Surface Slick Test Program: Sample Locations
Goal: sample different types and stages of ocean surface HC seep slicks.
Shane Seep
Tar balls
Trilogy Seep
Seep Tents
2 km
Site
Location
Site Comment
1
Shane Seep
Point source of gas, no obvious surface oil, strong smell of oil
2
Shane Seep edge
Aiming for fresh oil
3
Shane Seep -edge
Aged thin sheen few spots of rainbow effect pancakes
4
Trilogy Seep
Fresh oil breaking at surface, strong oil smell
5
Seep tent site
Very large bubble plumes, oil breaking out of bubbles, very fresh oil
5A
Near tent site
older slicks at edge of seep tent area
6
Patch Seep
low level seepage, fresh oil, very thin oil film over entire area, diffuse bubbles
7
Patch Seep
older oil thin film covers surface, wavy rainbow slicks over surface
8
Patch Seep area
background around seep, thin film of oil covers surface, no color
8A
Patch Seep area
rippled water for background transition from thin oil surface to 'clean' water
Background
background 'clean water', well rippled surface no obvious oil
9
Patch Seep Area
Background
Location
Collection and Analytical Methods Examined
Multiple methods were examined to evaluate ocean surface light to
high molecular weigh hydrocarbons sampling and analysis methods.
Gore Sorber Slick Sampler®:
• GORE-TEX ePTFE (polytetrafluoroethylene) and
sorbent filled collectors.
• ePTFE chemically inert microporous hydrophobic
structure allows HC vapor transfer and not water.
• Gore Sorber placed in ocean surface slick then placed
sealed special glass container for shipping.
• analyzed by thermal desorption coupled with GCMS for
C2 to C28+ hydrocarbons.
Equilon-SEPCO Sheen Sorbent:
• Shell sheen/slick sampler chemically treated
tetrafluoroethylene polypropylene strips.
• wrapped in destructible wrap (dissolves) then exposed
to slick with maximum absorbent surface area
• oiled slick sheen sorbent placed in special container,
frozen in field, then shipped for analysis.
• solvent washed with CH and MCH remove sorbed HC.
• extract analysis program:
- whole extract GC-FID (gas chromatography)
- GC-MS full-scan with ion chromatograms to identify
components along with mass spectra.
CH = cyclohexane MCH = methylcyclohexane
Collection and Analytical Methods Examined, cont.
General Oceanics Oil Sampling Net Kit:
Disrupter Container
• drag net through ocean surface slick.
• method used by USCG for oil spill studies.
- place oiled net in jar ship to laboratory.
- add cyclohexane to jar and thoroughly shake.
- transfer to test tube and remove water.
- concentrated extract to GC and GCMS analysis.
• modified SGC analysis method:
- place oiled net in disrupter container without blades
- freeze onboard and ship to laboratory for analysis
- analytical procedures for GO sampler net:
1. disrupter syringe HS light hydrocarbons (C1-C5).
2. disrupter HS SPME gasoline range HC (C5-C10).
3. solvent (CH & MCH) extraction HMW HC (C12+).
Surface water sample:
• collect surface water sample with disrupter container
without internal blades plus chemical grade salt to
prevent bacterial activity
→ 2/3 water sample and 1/3 air headspace
• seal container, freeze, then ship for laboratory analysis
• analytical procedures undertaken on disrupter sample
1. disrupter syringe HS light hydrocarbons (C1-C5).
2. disrupter HS SPME gasoline range HC (C5-C10).
HS = headspace SPME = solid phase microextraction
Collection and Analytical Methods Examined, cont.
Headspace Solid Phase Microextraction (HSPME)
HSPME relies on an “equilibrium” between sample,
container
headspace,
and
fiber
providing
a
representation of slick gasoline plus range (C5-C10)
hydrocarbon relative concentrations.
→ Disrupter water and GO net sample
Evaluated for sediment gasoline range analysis but
untested to examine effectiveness in the detection
ocean surface light hydrocarbons (less than C10).
FID1 A, (SBSLICKS\SB3NET-S.D)
FID1 A, (SBSLICKS\SB4NET-S.D)
Site 3 Shane Seep (edge) – aged thin sheen
pA
Site 4 Trilogy seep – fresh oil breaking at surface
pA
45
GO Net in Disrupter SPME GC
SPME area: 118
C1-C5 gas: 7 ppm
Wetness: 2.7 %
CO2: 71,989 ppm
11
10
9
8
40
35
GO Net in Disrupter SPME GC
SPME area: 3,361
C1-C5 gas: 43 ppm
Wetness: 11.3 %
CO2: 14,524 ppm
30
25
20
7
15
6
10
5
5
0
5
10
15
20
25
30
35
40
min
0
FID1 A, (SBSLICKS\SB3W-S.D)
5
10
15
20
25
30
35
40
min
30
35
40
min
FID1 A, (SBSLICKS\SB4W-S.D)
pA
pA
18
11
Water in Disrupter SPME GC
SPME area: 81
C1-C5 gas: 325 ppm
Wetness: 0 %
CO2: 950 ppm
10
9
8
7
16
Water in Disrupter SPME GC
14
SPME area: 679
C1-C5 gas: 1,289 ppm
Wetness: 9.4 %
CO2: 944 ppm
12
10
8
6
6
5
0
5
10
15
20
25
30
35
40
min
0
5
10
15
20
25
Abrams et al. (2009), Development of methods to collect and analyze gasoline plus range (C5 to C12) hydrocarbons from seabed
sediments as indicators of subsurface hydrocarbon generation and entrapment, Applied Geochemistry, 24 p. 1951-1970.
Collection and Analytical Methods Examined, cont.
Sampling Method versus Hydrocarbon Range
Each collection and analysis method examines different boiling point range.
HC Gas
Gasoline Plus
High Molecular Weight
C25+ Biomarkers
C1 to C5
C5 to C12
C12 plus
Headspace
[water sample & GO net]
HSPME (disrupter)
[water sample & GO net]
Gore Sorber Slick®
[thermal desorption]
General Oceanics Net
[CH and MCH solvent extraction]
[extract GC and GCMS]
Equilon-SEPCO Sheen
[CH and MCH solvent extraction]
[extract GC and GCMS]
Next: compare results from different methods on different types of slicks.
Changes in Slicks Signal With Time: Stage 1
Fresh seep Volatiles Intact
Directly over fresh seeps gas and gasoline
range hydrocarbons can be detected using.
160
(ng)
Concentration (ng)
140
Background 'clean water'
120
Fresh oil breaking surf ace 2
100
NOTE: volatiles detected Gore Sorber & HSPME only.
80
60
40
20
0
C5
C4
C7
C6
C8
C9 C10 C11 C12 C13 C14 C15
General Oceanics Net Sampler
Seep Tents – Large bubble plume,
oil breaking out of bubbles,
very f resh oil
16
EGI-5
5
18
24
Seep signal
Trilogy Seep
Fresh oil breaking surface 2
Gore Sorber 526262
m/z 43
15 16
Pr
Ph 19
28
26
30
HH
20
n-alkane
32
standard
H
n-acid
H
n-alcohol
plasticiser
i5
H
i6
4
6
UCM: unresolved complex mixture
i4
i7ai
7
i8
ai
ai
1.00
2.00
3.00
4.00
Hopane
5.00
6.00
8
i9
7.00
8.00
9.00
25.00
30.00
35.00
40.00
Fresh Surface oil
Near Trilogy Seep
45.00
50.00
55.00
60.00
Fresh Surface oil
Near Trilogy Seep
1m
Gas plume
1m
65.00
Changes in Slicks Signal With Time: Stage 2
Volatile Loss (very quickly)
Gore Sorber Slick Sampler®
400
Seep Tents
Concentration (ng)
350
300
250
Background 'clean water'
Bubble plume, oil breaking surface
Older slick thick, rainbow effect
Thick oil, rainbow effect
200
150
100
As ocean surface seeps age, they are
subject to greater volatile loss,
concentration of heavier hydrocarbons
(C12 plus), and become relatively thin:
- very strong petroleum odor noted
during sampling from volatile loss.
- changes in compound distribution
from fresh to older slicks.
50
0
160
C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15
Trilogy Seep
Concentration (ng)
140
120
Background 'clean water'
fresh oil breaking surface 1
100
80
fresh oil breaking surface 2
fresh oil
As HC seep related slicks become thin, it is
increasingly difficult to collect and
analyse the petroleum related slick:
- Gore Sorber slick sampler is most
sensitive in detection older slicks.
Near Trilogy Seep
Thin aged slicks
60
40
20
1m
0
C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15
Fresh thicker slicks
Changes in Slicks Signal With Time: Stage 3
Slicks become very thin and alteration occurs.
Near Trilogy Seep
Older Surface oil
Near Shane Seep
Moose/tar Slicks
Old oil
1m
1m
Fresh thicker slicks
Fresh, thick,
brown slick
Thinning,
rainbow slick
Aged, thin,
grey slick
18
Tar blobs
16
General Oceanics Net Sampler
Patch Seep – older oil f ilm covers surf ace, wavy
rainbow slicks
n-alkane
standard
n-acid
n-alcohol
Ph
H
Aging Slicks
Hopane
Pr
Hydrocarbon
leakage
UCM: unresolved complex mixture
nPr
Gas bubbles
with oil skin
25.00
Biodegraded oil
Mobile hydrocarbon
Phase
30.00
35.00
40.00
45.00
50.00
55.00
60.00
65.00
NOTE: Some oil slicks appear heavily
biodegraded; where does this
occur → within ocean surface or
near surface sediments ?
Changes in Slicks Signal With Time
Gas loss
Volatile loss,
Concentration
of C15+
Dissolution,
Volatile loss,
Elevated C20+
Concentration of
HMW biomarkers
Aging Slicks
Gas and oil
in bubble
Provides
UCM
Gas bubbles
with oil skin
Slick sampling method versus
changes in slick composition.
Biodegraded oil
Mobile Hydrocarbon
Phase
NOTE: Potential pick up of UCM
compounds during migration
from subsurface to near-surface.
AAPG 2010 Annual Conventional & Exhibition
New Orleans, Louisiana 11-14 April 2010
Geochemical evaluation of ocean surface slick methods to
ground truth satellite seepage anomalies for seepage detection
Important Observations
Detection Methods:
• Gore Sorber and HSPME (via GO nets and water) detect gasoline range
hydrocarbons directly over active fresh hydrocarbon seepage/slicks.
• Only Gore Sorber detected hydrocarbons in thin/aged slicks with lower
concentration light and HMW hydrocarbons or gas seepage only.
• GO Net and Equilon-SEPCO Sheen Sorbent with solvent extraction detect
HMW hydrocarbons in fresh and less altered (thin) surface slicks.
NOTE: problems as the slick becomes aged and very thin.
• GO Net enhances UCM signal (collection capture more polar compounds ?).
• GCMS analysis of extracted material from GO Net and Equilon-SEPCO Sheen
Sorbent does provide usable results.
• No GCMS data from Gore Sorber and HSPME sample and removal process.
• Ocean surface slick water collection not an effective method to obtain slick
hydrocarbon sample for geochemical analysis.
Changes in slick with time:
• As seep slicks age, loose volatiles and concentrate heavier hydrocarbons.
• Thin aged slicks are difficult to detect/analyse (Gore Sorber most sensitive).
• Some oil slicks appear heavily biodegraded (occurs subsurface).
AAPG 2010 Annual Conventional & Exhibition
New Orleans, Louisiana 11-14 April 2010
Geochemical evaluation of ocean surface slick methods to ground truth satellite seepage anomalies for seepage detection, cont.
Recommendations
All ocean surface satellite anomalies MUST be ground truth no matter how
high your confidence level is (correct shape, repeatability, or association
with geological features such as migration pathways and/or seal failure).
We recommend two ocean surface hydrocarbon slick sampling
methods to capture two different types/ranges of hydrocarbons:
 Light hydrocarbons – Gore Sorber Slick Sampler®
 High molecular weight HC*: General Oceanics Oil Sampling Net
* with DCM solvent extraction to include biomarkers.
Michael Abrams, University of Utah
Graham Logan, Geoscience Australia
Thank you,
Michael Abrams and Graham Logan
Andrew Bishop, Shell E&P
Rowland Rincon, Gore and Associates
Ira Leifer, University of California SB