Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola

31 Pages·2016·24.06 MB·English

EAGE
Basin Research (2016) 1–31, doi: 10.1111/bre.12169
Subsurface fluid flow in the deep-water Kwanza
Basin, offshore Angola
Christophe Serie(cid:1),*Mads Huuse,†NielsH. Schødt,‡JamesM. Brooks§andAlan Williams¶
*GlobalNewVenturesExploration,ConocoPhillipsCompany,Houston,TX, USA
†BasinStudiesandPetroleumGeoscienceResearchGroup,SEAES,UniversityofManchester,Manchester,
UK
‡ExplorationDepartment,MaerskOil,CopenhagenK,Denmark
§TDI-BrooksInternational,CollegeStation,TX, USA
¶CGGNPALtd,Edenbridge,Kent,UK
ABSTRACT
Integrated analysis of high-quality three-dimensional (3D) seismic, seabed geochemistry, and
satellite-based surface slick data fromthe deep-water Kwanza Basin documents the widespread
occurrence of past and present fluid flow associated with dewatering processes and hydrocarbon
migration. Seismic scale fluid flowphenomena are defined by seep-related seafloor features includ-
ing pockmarks, mud or asphalt volcanoes, gas hydrate pingoes, as well as shallow subsurface fea-
tures such as palaeo-pockmarks, directhydrocarbon indicators (DHIs), pipes and bottom-
simulating reflections (BSRs). BSR-derived shallow geothermal gradients show elevated tempera-
tures attributed to fluid advection along inclined stratigraphic carrier beds around salt structures
in addition to elevated shallow thermal anomalies above highly conductive salt bodies. Seabed evi-
dences of migrated thermogenic hydrocarbons and surface slicks are used to differentiate thermo-
genic hydrocarbon migration fromfluid flow processes such as dewatering and biogenic gas
migration. The analysis constrains the fluid plumbing system defined by the three-dimensional
distribution of stratigraphic carriers and seal bypass systemsthrough time.Detailed integration
and iterative interpretation have confirmed the presence of mature source rock and effective
migration pathways with significant implications for petroleum prospectivity in thepost-salt inter-
val. Integration of seismic, seabed geochemistry and satellite data represents a robust method to
document and interpret fluid flow phenomena along continental margins, and highlights the
importance of integrated fluid flow studies with regard to petroleum exploration, submarine geo-
hazards, marine ecosystems and climate change.
pretation of shallow fluid flow phenomena over large
INTRODUCTION
areas (Cowley & O’Brien, 2000; Van Rensbergen et al.,
Past and present fluid flow phenomena along continental 2003; Lucazeau et al., 2004; Ligtenberg, 2005; Cart-
margins reflect the dynamic evolution of sedimentary wright et al., 2007; Løseth et al., 2009; Huuse et al.,
basins with numerous implications for energy resources, 2010). Seafloor fluid flow features and associated over-
marine ecosystems, submarine geohazards and climate burden anomalies have helped to define the nature, tim-
change (Milkov & Sassen, 2002; Sultan et al., 2004; ing and intensity of fluid migration in relation to
Berndt, 2005; Cartwright, 2007; Judd & Hovland, 2007; depositional, structural and diagenetic processes includ-
Huuse et al., 2010; Maslin et al., 2010; Anka et al., ing hydrocarbon generation, migration and seepage.
2012).Understandingsubsurfacefluidflowprocesseshas However, direct calibration with seabed geochemistry
recentlybeenamajorareaofinterestinpetroleumexplo- and satellite-based surface slick are limited to a handful
rationfollowingagrowingdemandforenergyandassoci- of studies (Abrams, 1992; Hood et al., 2002; Williams &
ated shift towards challenging exploration settings along Lawrence, 2002; O’Brien et al., 2005; Garcia-Pineda
deep-water continental margins (White et al., 2003). et al.,2010;Loganet al.,2010).
Recent advances in seismic imaging have greatly Thispaperpresentsthefirstintegratedfluidflowstudy
enhanced the detailed observation, mapping and inter- in the deep-water Kwanza Basin based on high-quality
three-dimensional(3D)seismic,seabedgeochemistryand
satellite-based surface slick data. Detailed seismic inter-
Correspondence: Christophe Seri(cid:1)e, Global New Ventures
pretationshowsthepresenceofnumerousfluidflowphe-
Exploration, ConocoPhillips Company, 600 North Dairy
nomena including seep-related seafloor features,
Ashford,Houston,TX77079,USA.E-mail:christophe.serie@
conocophillips.com overburden seismic anomalies and associated plumbing
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C. Serie et al.
systems. Direct comparison with surface geochemistry salt sedimentary succession is marked by an initial rapid
and satellite-based surface slick data is used to confirm marine transgression dominated by the development of
thepresenceofseismicallywell-definedseepsandvalidate carbonate platforms (Albian) followed by open marine
ambiguous seep-related features. Seabed geochemistry conditions characterized by clastic progradation and
wasalsousedtodeterminetheoriginofmigratedthermo- deep-water sedimentation (Cenomanian–Oligocene)
genichydrocarbonsandconfirmpotentialmigrationpath- (Schollnberger, 2001; Goldhammer et al., 2002). Subse-
ways along networks of stratigraphic carriers and seal quent tectonic uplift coupled with eustatic lowering
bypass systems. Shallow geothermal gradients derived resultedinerosionontheshelfanddownslopedeposition
from BSR modelling suggest relative contributions from of turbidite fan systems within salt minibasins (Lunde
enhanced conduction through salt and advection from et al., 1992; Jackson et al., 2005; Hay, 2012). Pliocene to
fluid migration. Finally, the importance of integrated present-day is dominated by hemipelagic silts and clays
studies and iterative interpretation is discussed with apart from coarser sediments associated with channels
regard to fluid flow processes and petroleum exploration and fan lobes constrained to river entry points (Pufahl
infrontiersedimentarybasins. et al., 1998). The post salt interval presents numerous
organic-rich source rock intervals (Binga, Itombe,
N’Golome, Teba, Rio Dande, Cunga fms.) (Danforth,
1997;Burwood,1999;Hartwiget al.,2012)andreservoirs
GEOLOGICALSETTING
ranging from Albian shelf carbonates to Tertiary deep-
The Angolan continental margin is known as a world- water fans (Danforth, 1997; Brownfield & Charpentier,
classpetroleumprovincecomprisingaseriesofsedimen- 2006). The Pliocene to Holocene succession, along
tary basins (Katz & Mello, 2000), including the Lower with the predominant mudstone intervals of the
Congo,Kwanza,BenguelaandNamibebasinsformedasa Oligocene–Miocene succession, are therefore commonly
result of Late Jurassic to Early Cretaceous breakup of considered as competent seals for the prolific Oligocene-
GondwanaandsubsequentopeningoftheSouthAtlantic Miocene turbidite reservoirs (Brownfield & Charpentier,
Ocean (Guiraud & Maurin, 1992; Karner & Driscoll, 2006).
1999; Fig. 1). The tectono-stratigraphic evolution of the
KwanzaBasinresultedinthreemajorsequences:pre-salt
(LateProterozoic–Barremian),salt(Aptian)andpost-salt
DATABASE
(Albian–Holocene) (Ala & Selley, 1997; Schollnberger,
2001),allofwhichcomprisekeyelementsoftheAngolan Thisstudy is primarily based on a high-quality, prestack
petroleumsystems(Danforth,1997;Brownfield&Charp- time-migrated, three-dimensional (3D) seismic volume
entier, 2006; Beglinger et al., 2012; Fig. 2). The pre-salt acquired in 2009 by Maersk Oil, in addition to regional
sequence is defined by a series of intracratonic basins two-dimensional (2D) seismic lines and 12 exploration
dominated by continental deposits (Brice et al., 1982). wells (Fig. 3). The 3D volume covers an area of
Followingthecontinentalbreak-up,initialriftingformed 3000 km2inwaterdepthsrangingfrom640 mto1630 m
a series of lacustrine basins within structural troughs with a line spacing of 12.5 m and sampling interval of
trending parallel to the present-day coastline and domi- 4 mstwo-waytraveltime(TWT).Thestudiedintervalis
natedbysiliciclasticandcarbonatedeposits(Unciniet al., imagedwithadominantfrequencyof40 Hz,whichgives
1998).Theprincipalsourcerockforthepre-saltsequence a vertical resolution (k/4) of (cid:1)11–16 m and horizontal
consists of the regionally extensive and prolific Cuvo resolution (k/2)of (cid:1)22–31 m, assumingseismic veloci-
lacustrine shale associated with hydrocarbon accumula- ties between 1800 m/s and 2500 m/s (Sheriff, 1985).
tions within the Grey Cuvo sandstones and lacustrine Theseismicdataarezerophasewithapositiveamplitude
Toca carbonates (Danforth, 1997; Uncini et al., 1998; representing a downward increase in acoustic impedance
Burwood, 1999). The onset of restricted marine condi- depictedbyared,orangeoryellowpeak.Negativeampli-
tions led to the deposition of thick evaporites that facili- tudesrepresentdownwarddecreasingacousticimpedance
tatedlatergravity-driventhin-skinneddeformationofthe andaredepictedbyalightbluetrough.Thefocusofthis
post-salt sequence. Regionally, the post-salt sequence is study has been the post-salt interval down to 4 s TWT
characterized by an upper slope extensional domain with due to restricted access to seismic data from the pre-salt
grabens and rafts, as well as a downslope contractional interval.
domain dominated by salt diapirs, salt walls and salt The seabed geochemistry data consists of 55 piston
nappes (Duval et al., 1992; Lundin, 1992; Marton et al., cores analysed by TDI-Brooks International. All piston
2000; von Nicolai, 2011; Quirk et al., 2012). In addition core locations were selected based on regional 2D seis-
to structural deformation and subsequent trap formation mic lines during a number of sampling campaigns
in the post-salt interval, the Loeme salt (Aptian) repre- between 1984 and 1996. The database includes a full
sentsaregionalandeffectivesealforthepre-saltsequence set of geochemical analyses including Total Scanning
(Danforth, 1997; Brownfield & Charpentier, 2006), apart Fluorescence (TSF), Headspace Gas Analysis (HS),
from locations where salt welds allow pre-salt oil migra- C15+ Hydrocarbon Analysis, as well as piston core
tionintopost-saltreservoirs(Danforth,1997).Thepost- descriptions (Fig. 3).
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2 BasinResearch©2015JohnWiley&SonsLtd,EuropeanAssociationofGeoscientists&EngineersandInternationalAssociationofSedimentologists
Subsurface fluid flow in the Kwanza Basin
Fig. 1. (a)SimplifiedtectonicmapoftheAngolancontinentalmarginincludingmainsub-basins,majortectonicelements(Marton
et al.,2000)andhistoricalreviewoffluidflowstudies(Seri(cid:1)e,2013).(b)Geologicalcross-sectionillustratingtheregionaltectono-strati-
graphicframeworkthroughtheKwanzaBasin(Martonet al.,2000;Quirket al.,2012).
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C. Serie et al.
Fig. 2. KwanzaBasinchrono-stratigraphicchartsummarizingtectono-stratigraphicframeworkandpetroleumsystemevents(Brice
et al.,1982;Haqet al.,1987;Duvalet al.,1992;Guiraud&Maurin,1992;Lundeet al.,1992;Lundin,1992;Ala&Selley,1997;Dan-
forth,1997;Pufahlet al.,1998;Unciniet al.,1998;Burwood,1999;Karner&Driscoll,1999;Martonet al.,2000;Schollnberger,
2001;Goldhammeret al.,2002;Jacksonet al.,2005;Brownfield&Charpentier,2006;vonNicolai,2011;Beglingeret al.,2012;Hay,
2012;Quirket al.,2012).
Satellite-based surface slick interpretations provided facies (Mitchum et al., 1977). The post-salt sequence is
byFugroNPAformpartofaregionalstudybasedon15 dividedintochronostratigraphicintervalsbasedonexist-
synthetic-aperture radar (SAR) scenes in the Kwanza ing regional seismic interpretations calibrated to nearby
Basin, and include 12 SAR scenes over the study area exploration wells (Fig. 3a) (Schollnberger, 2001; Hudec
(Fig. 3). Seep interpretation includes slick emission &Jackson, 2004; vonNicolai,2011),includingdatafrom
points,slickoutlines,clusteredslicksandslickcategories. a recent exploration well drilled within the study area
(Maersk pers. comm., 2012). In addition to the standard
seismicstratigraphictechniques,structuralinterpretation
METHODOLOGY mainly linked with salt tectonics is defined using seismic
facies, variance attributes and geometrical comparison
3Dseismic
with known salt-related structures including salt diapirs,
The tectono-stratigraphic framework within the study salt walls, salt nappes and associated structural deforma-
area is interpreted using standard seismic stratigraphic tion(Alsop,1996;Hudec&Jackson,2007;Dooleyet al.,
techniques based on reflection terminations and seismic 2012). Detailed interpretation of chronostratigraphic
©2015TheAuthors
4 BasinResearch©2015JohnWiley&SonsLtd,EuropeanAssociationofGeoscientists&EngineersandInternationalAssociationofSedimentologists
Subsurface fluid flow in the Kwanza Basin
(b)
(a)
Fig. 3. (a)MapoftheKwanzaBasinalongtheAngolancontinentalmarginshowingthelocationofthestudyareaandextentofavail-
abledataincludingregionaltwo-dimensional(2D)seismiclines,three-dimensional(3D)seismicvolume,explorationwells,satellite
seepdataandseabedgeochemicalsurvey.(b)Mapofthestudyareawithavailabledataincluding3Dseismicvolumecovering
3000 km2,55pistonscoressolelyselectedonregional2Dseismiclinesand12synthetic-apertureradar(SAR)scenes.
intervals was used to create a seafloor geological map ciated with the occurrence of denser sediments, hard-
highlightingthedistributionandextentofexposedstrati- grounds, precipitated authigenic carbonates, layer of
graphicintervalsatthepresentdayseafloor. shell debris or surface gas hydrates (Roberts, 2001;
Seismic scale fluid flow phenomena are defined by Judd & Hovland, 2007).
the occurrence of seep-related features on the seafloor Overburden seismic anomalies are defined by detailed
and associated seismic anomalies in the shallow subsur- mapping of reflection amplitudes, shapes and spatial dis-
face. The bathymetry within the study area is derived tribution.Theoccurrenceofpastandpresentfluidflowis
from time-depth conversion of the interpreted seafloor characterizedbythepresenceofdirecthydrocarbonindi-
TWT-structure map and assuming a seawater velocity cators (DHIs), bottom-simulating reflections (BSRs),
of 1500 m/s. Detailed observations and interpretations hydrocarbon-related diagenetic zones (HRDZs), pipes,
of seep-related fluid flow features were fine-tuned using polygonal faults, diagenetic fronts, sediment injections
seafloor seismic reflectivity, discontinuity and dip attri- and volcanic intrusions (Shipley et al., 1979; Løseth
butes characterizing both morphological and geophysi- et al., 2009; Huuse et al., 2010). Bottom-simulating
cal signatures of particular fluid flow features such as reflections (BSRs) are usually considered as the seismic
pockmarks, mud volcanoes, mud mounds, gas hydrate expression of free gas below hydrate-bearing sediment
pingoes, carbonate mounds and possible asphalt volca- (Singh et al., 1993; Berndt et al., 2004). BSRs are com-
noes (Roberts, 2001; Roberts et al., 2006; Evans et al., monlycharacterizedbyastrong,reversed-polarityreflec-
2007; Judd & Hovland, 2007; Jones et al., 2014). Nega- tion generally following the seafloor topography and
tive amplitude anomalies are interpreted as the result occasionally crosscutting stratal reflections in areas of
of free gas in surface sediments or unconsolidated gas complexstructure(Shipleyet al.,1979).Recentadvances
bearing sediment extrusions (Evans et al., 2007), as in three-dimensional seismic imaging have revealed the
opposed to positive amplitude anomalies generally asso- common occurrence of more subtle and complex BSR
©2015TheAuthors
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C. Serie et al.
typesincludingdiscontinuousandplumingBSRs(Shedd and very light oil that quickly disperse and evaporate.
et al.,2012). Thehorizontaldisplacementofsurfaceslicksfromitssea-
floor vents will depend on the water column and surface
currents where seeps can be found in a circumference of
Seabedgeochemistry
up to five times the water depth, and occasionally up to
Seabedgeochemistryanalysesarebasedonsedimentsam- tensofkilometresduetothehorizontallayeringofthesea
ples from piston cores up to 6 m in length with location (Garcia-Pinedaet al.,2010).
selected on regional 2D seismic lines, or where available
on 3D seismic and swath bathymetry data. Three sedi-
BSR-derivedshallowgeothermalgradient
ment sections near the bottom of the core are generally
sampledtodeterminenear-surfaceindicationofmigrated Commonly observed along the continental slopes, BSRs
thermogenic hydrocarbonsbymeasuring TotalScanning are generally associated with the base of the gas hydrate
Fluorescence(TSF)intensitiesofsedimentextracts,light stability zone (GHSZ) defining the subsurface depth
hydrocarbons by headspace extraction and C hydro- below which natural gas hydrates are no longer stable
15+
carbons by gas chromatography in sediment extracts (Sloan&Koh,2008).Basedonnaturalgashydratestabil-
(Abrams,2013).TSFdetectsandmeasuresorganiccom- ity conditions, water bottom temperature and thermal
poundscontainingoneormorearomaticrings;increasing conductivity,BSRdepthscanbeusedtoestimateshallow
TSF intensity corresponds to increasing amount of aro- geothermal gradients and associated surface heat flow
matic compounds in sediment extract characterizing the (Yamano et al., 1982; Grevemeyer & Villinger, 2001). In
presence of migrated thermogenic hydrocarbon in near addition to the conventional surface heat flow measure-
surfacesediments(Abrams,2005).HeadspaceGasAnaly- ments and with relatively good consistency (Lucazeau
sis determines the concentration of light hydrocarbon et al.,2004),thisapproachhasbeenappliedtobothsingle
gasesincludingmethane,ethane,propene,propane,bute- reflection profiles and three-dimensional reflection data
nes, iso-butane, n-butane, neopentane, iso-pentane and sets. Widespread BSRs are used to estimate shallow
n-pentane. Light hydrocarbons are used to evaluate geothermalgradientoverlargeareaswhileprovidingsig-
anomalous hydrocarbon sources (bacterial vs. thermo- nificantconstraintstounderstandingprocessesalongcon-
genic)(Brookset al.,1983;Abrams,1992).C15+Hydro- tinental margin scale such as local thermal anomalies
carbon analysis determines the concentration of high associated with fluid flow (Grevemeyer et al., 2004;
molecularweighthydrocarbons(C )basedonsediment Hornbachet al.,2012),contrastsinthermalconductivity
15+
extract gas chromatography. C analysis provide gas between sedimentary formations (Lucazeau et al., 2004)
15+
chromatograms and C to C n-alkane, pristane, phy- and surface processes including erosion and sedimenta-
15 34
taneandunresolvedcomplexmixture(UCM)concentra- tion(Martinet al.,2004).
tions for sediment extracts in order to evaluate the Bottom-simulating reflection-derived shallow geother-
presence of migrated thermogenic hydrocarbons mal gradient estimations correspond to the temperature
(Abrams,1992). difference between bottom-water temperature at the
seafloor and estimated temperature at BSR-depth. Bot-
tom-water temperatures of the Kwanza Basin (between
Satellite-basedsurfaceslicks
latitudes 8°S to 12°S and longitudes 10°E to 14°E) were
Surface slick distribution and intensity are interpreted derived from seawater temperature data compiled from
fromsatellite-basedremotesensingdata.SyntheticAper- the NOAA-NODC World Ocean Atlas (Locarnini et al.,
ture Radar (SAR) has proven to be highly successful in 2010). Temperatures at BSR-depth were estimated from
delineating natural hydrocarbon seepages from oil slicks empirical gas hydrate stability law for seawater salinity
on the sea surface (Garcia-Pineda et al., 2010; Logan andmethanesystem(Lu&Sultan,2008):
et al., 2010). SAR sensors send out a microwave signal, PðT;SÞ¼expððCs:SþDsÞ:TÞ:expðEs:SÞ:Fs;
typically with a wavelength of several centimetres and
build an image from the radiation reflected back to the where(PishydrostaticpressureinkPa,Tistemperature
satellite(Lawrenceet al.,1998).SARslicksareareasthat at the base of the GHSZ in K, S is salinity in pore
have anomalously low backscatter values compared to water (0 < S ≤ 0.050 mol NaCl/mol H O) and
2
backgroundlevelasaresultofdampenedcapillarywaves coefficients Cs = 0.1711726397, Ds = 0.1046133676,
on the ocean surface. Dampening of the capillary waves Es = (cid:3)34.14836102andFs = 1.010769956 9 10(cid:3)9).
mayoccurthroughavarietyofcauseswhichincludenatu- PressureatBSRdepthsarebasedonhydrostaticpres-
ral film slicks, pollution slicks and seepage slicks. Seep sure encompassing water column height and BSR thick-
originsaredefinedbasedonparameterssuchassize,loca- ness. BSR thicknesses are determined from time-depth
tion,morphology,flowdirection,repetition,edgecharac- conversion using seismic velocity analysis with velocity
teristics,oceanfeaturesandgeologicalcontext(Lawrence ranging between 1500 and 2400 m/s due to lateral vari-
et al., 1998; Williams & Lawrence, 2002). Slick occur- ability within sedimentary rock outcropping at the sea-
rencesarealsocontrolledbythetypeofleakinghydrocar- floor. In the absence of fluid composition data, a simple
bonsandcommonlyassociatedwithoil,asopposedtogas seawater salinity-pure methane system was used while
©2015TheAuthors
6 BasinResearch©2015JohnWiley&SonsLtd,EuropeanAssociationofGeoscientists&EngineersandInternationalAssociationofSedimentologists
Subsurface fluid flow in the Kwanza Basin
-
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atigraphicNotethep7,and8).
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©2015TheAuthors
BasinResearch©2015JohnWiley&SonsLtd,EuropeanAssociationofGeoscientists&EngineersandInternationalAssociationofSedimentologists 7
(cid:1)
C. Serie et al.
Fig. 5. Post-saltisochronmap(msTWT)highlightingpost-saltbasingeometrywiththepresenceofminibasin(mb),saltdiapir(sd),
saltnappe(sn),saltwall(sw)andregionalstructuralelements.
Fig. 6. Seafloorgeologicalmaphighlightingthedistributionofoutcroppingstratigraphicunits(Aptian–Eocene,Eocene–Pliocene,
Quaternary)asaresultofstructuraldeformationassociatedwithintensesalttectonics.
©2015TheAuthors
8 BasinResearch©2015JohnWiley&SonsLtd,EuropeanAssociationofGeoscientists&EngineersandInternationalAssociationofSedimentologists
Subsurface fluid flow in the Kwanza Basin
consideringuncertaintyassociatedwithfluidcomposition flanksandredepositionwithinminibasins.TheMiocene–
and pore fluid salinity (Holder et al., 1987; Dickens & Pliocene section is defined by continuous, conformable,
Quinby–Hunt, 1994; Sloan & Koh, 2008; Minshull & moderate to high amplitude reflections, as opposed to
Keddie,2010). numerous fan/channelcomplexesobservedinthenorth-
ernpartoftheKwanzaBasin(Hay,2012)aswellashighly
polygonally faulted interval documented in the Lower
RESULTS CongoBasin(Gayet al.,2004;Andresen&Huuse,2011).
Nonetheless, the Miocene–Pliocene section is character-
3Dseismic ized by the presence of bowl-shaped structures up to
300 ms TWT in depth and 4 km in diameter (Fig. 7).
Tectono-stratigraphic framework
Bowl-shapedfeaturesareinterpretedastime-transgressive
Three-dimensional seismic interpretation based on the bedforms formed by bottom current scour around
regional tectono-stratigraphic framework indicates the bathymetric highs (Seri(cid:1)e, 2013), however, features with
presence of a thick (>5500 ms TWT) post-salt interval similar size have also been interpreted as funnel-shaped
strongly affected by salt tectonics (Figs 4 and 5). The collapselinkedtogashydratedissociation(Hoet al.,2012;
base of the post-salt interval is defined by the top salt Imbert & Ho, 2012). The Quaternary section is defined
reflection characterized by a strong positive amplitude by moderate to high amplitude reflections associated
in the shallow section, as opposed to a negative ampli- with the presence of a deep-water fan complex originat-
tude in the deeper section due to overlying Albian car- ingfromthesoutheastcornerofthestudyarea(Fig. 7).
bonates with relatively higher velocity and density
compared to underlying salt (Fig. 4). The salt is char-
Seafloor fluid flow phenomena
acterized by reflection-free to chaotic reflections defin-
ing the presence of numerous salt related structures Detailed 3D seismic interpretation reveals a wide variety
including salt diapirs, salt walls and salt nappes (Figs 4 of fluid flow-related features on the seafloor including
and 5). The present-day structural framework is domi- pockmarks and mounds of variable sizes and rugosities
nated by salt-cored anticlines trending NW-SE and (Figs 7 and 8). The seafloor dip map highlights the
associated elongated salt diapirs to the north, as presence of pockmarks with varying degree of intensity
opposed to salt walls and salt nappes to the south with randomly distributed pockmarks in minibasins,
(Fig. 5). In a regional context, the occurrence of NW- curvilinear pockmark trends following the edge of salt
SE trending salt-cored anticlines is linked to the more nappes, pockmarks along fault scarps, linear pockmark
extensive diapiric fold belt documented to the north trends following the axes of Quaternary minibasin
(Marton et al., 2000; von Nicolai, 2011). In addition to depocentres, pockmarks controlled by seabed sediments
Tertiary salt tectonics, the Albian interval is character- and the presence of bowl-shaped structures (Fig. 7). In
ized by the presence of NE–SW-oriented buckle folds addition to the predominant occurrence of pockmarks,
and thrusts interpreted to represent an early phase of distinct mounded features above topographic highs are
syn-sedimentary salt deformation and comprises diver- also documented based on dip attribute and seismic
gent stratigraphic intervals related to the development reflectivity seafloor maps (Figs 7 and 8). Figure 8b
of rim-synclines linked to early salt diapirism during shows the presence of protruding features defined by
the Late Cretaceous (Quirk et al., 2012). Present-day circulartoellipsoidalmounds of80–300 minextentand
seafloor is characterized by the presence of broad 5–40 m in height. Mound morphologies vary from
bathymetric highs representing the surface expression smooth, well-rounded, steep-sided mounds (9°–16°)
of underlying salt structures, and generally associated (M1, M2, M6) to rough, uneven, gently dipping
with older stratigraphic units outcropping at the sea- mounds (3°–10°) (M4, M5, M7, M8). The mounds are
floor (Fig. 6). characterized by higher reflectivity than the seafloor,
The post-salt interval has been subdivided into seven indeed appearing as high amplitude anomalies. Detailed
seismic-statigraphic units bounded by eight interpreted interpretation suggests that the occurrence of these
seismic horizons tied to the regional stratigraphic frame- mounds is most likely related to the development of gas
work(Fig. 4).TheAlbianintervalisdefinedbycontinu- hydrate pingoes associated with the formation and
ous,conformable,moderatetohighamplitudereflections dissociation of massive gas hydrate in the shallow sub-
and commonly affected by NE–SW-oriented salt-cored surface (Seri(cid:1)e et al., 2012). Figure 8d shows the pres-
bucklefolds.TheCretaceousintervalisdefinedbydiver- ence of another protruding feature defined by a circular
gentstratigraphicintervals,developmentofrim-syclines. central apex about 0.5 km in diameter and up to 50 m
TheEocene–Oligoceneintervalischaracterizedbylowto in height, and elongated branches of 1.5–2.5 km in
moderate amplitude reflections showing overall thinning extent with height up to 25 m trending along faults
towardsalt-inducedstructuralhighsandcharacterizedby bounding the crestal collapse of the underlying salt dia-
the predominance of Oligocene mass transport deposits pir. Away from the central apex, one of the elongated
(MTDs). The MTDs are generally associated with salt branches lines-up directly with afault scarp bordered by
remobilizationandsubsequentinstabilityabovesaltdiapir a series of individual pockmarks (Fig. 8d). This feature
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C. Serie et al.
(a)
(b) (c) (d)
(e) (f)
Fig. 7. (a)SeafloordipmapillustratingthepresenceofnumerousbathymetrichighsmainlydefinedbythedepositionaledgeofQua-
ternarysedimentsandrepresentingthesurfaceexpressionofunderlyingsaltdiapirsandsaltnappes,aswellasseep-relatedseafloor
featuresincludingpockmarks,linearpockmarktrends,extrusivemoundsandgashydratepingoes.(b–f)Two-dimensionalseismicpro-
files(B–B’,C–C’,D–D’,E–E’,andF–F’)illustratingrelationshipbetweenpockmarkoccurrencewithrespecttosaltstructures,faults,
recentsedimentdepocentresandbowlshapefeatures.Note:seemoundedfeaturesontwo-dimensionalseismicprofileonFig. 10.
is also characterized by a chaotic surface with relatively exposedoldersedimentaryrockscroppingoutatthesea-
low amplitudes similar to the surrounding seafloor asso- floor(Figs 6and8d,e).Smallerseafloorfeaturescharac-
ciated with recently deposited sediment and contrasting terized by similar chaotic, low amplitude seafloor
with moderate to high amplitudes associated with reflectionsarealsoobservedonnearbytopographichighs
©2015TheAuthors
10 BasinResearch©2015JohnWiley&SonsLtd,EuropeanAssociationofGeoscientists&EngineersandInternationalAssociationofSedimentologists