deep pore pressures and seafloor venting in the auger basin, gulf

Deep pore pressures and seafloor venting in theAuger Basin,Gulf of MexicoMatthew J. Reillyn1 and Peter B. FlemingswnDepartment of Geosciences,The Pennsylvania State University, University Park, PA, USAwJackson School of Geosciences,The University of Texas at Austin, University Station, Austin,TX, USA

ABSTRACT

Pore £uid overpressures in four reservoir sandstones in the Auger Basin, deepwater Gulf ofMexico,are similar across the basin, suggesting that these sandstones are hydraulically connected overdistances420 km.Small overpressure gradientswithin them suggest upward £ow rates between1and20mmyear�1. At the crest of these sandstones, pore pressure equals or exceeds the least principalstress, andwe interpret that high £uid pressure is fracturing the caprock and driving £ow vertically.Awell drilled into the crest of the Auger sandstones con¢rmed the presence of extreme overpressuresthat converge on both the least principal stress and the overburden stress. Above these zones,spectacular mudvolcanoes are venting £uids today.Overpressured aquiferswith signi¢cant structuralrelief may drive £uid vents and mud volcanoes around the world.

INTRODUCTION

Natural oil and gas seeps are widespread across theGulf ofMexico slope,West Africa and other proli¢c hydrocarbonregions worldwide (Graue, 2000; Hood et al., 2002; Davies& Stewart, 2005).They are often associated with mud vol-canoes and hydrocarbon accumulations, they can causedrilling hazards, they support extraordinary life formsand they may impact climate (Brooks etal.,1987;Gaarenst-room et al., 1993; MacDonald et al., 2002). Seeps pinpoint£uid migration pathways (Abrams & Boettcher, 2000;Hood etal., 2002) and provide information about hydrocar-bon source type and the organic maturity of the trappedhydrocarbons (Abrams, 2005; Whelan et al., 2005).Although seeps and mud volcanoes have been character-ized in extraordinary detail, there has been relatively littlestudy of the processes that drive the spectacular ventingthat is observed.

Overpressures (pressures greater than hydrostatic pres-sure) have been recognized for decades.Themost commonmechanism for overpressure is rapid sedimentation. De-position occurs more rapidly than the rate at which £uidcan be expelled during consolidation, and, as a result, the£uids bear some of the overlying load (Harrison & Sum-ma, 1991; Gaarenstroom et al., 1993; Gordon & Flemings,1998; Swarbrick et al., 2000). Recent work has emphasizedthat when low-permeability sediments rapidly load apermeable aquifer, £ow is focused along the aquifer fromareas of high overburden to low overburden (Flemings

et al., 2002).These aquifers are conduits for large volumesof £uid £ow and the major pathway by which low-permeability mudstone is drained.Where the overburdenis thin, pore pressures can converge on the least principlestress and dilate fractures in the caprock, driving £uid ex-pulsion through the caprock (Finkbeiner etal., 2001; Flem-ings et al., 2002). Seldon & Flemings (2005) illustrated how£ow focusing within deep (43000m) overpressured aqui-fers drives £uid venting at the sea£oor today in the deep-water Gulf of Mexico. This provided a physical model bywhich sea£oor seeps andmudvolcanoes are driven by largeoverpressured aquifers with signi¢cant structural relief.

TheAugerBasin, in the deepwaterGulf ofMexico, is anextraordinary example of this type of coupled system. Ex-tensive hydrocarbon exploration over the last 20 years al-lows us to describe this system in great detail.We mappedthe stratigraphy, pressure, stress and sea£oor morphologyat Auger.We show that pore £uid overpressures in four re-servoir sandstones (the P, Q , R and S) are similar acrossthe basin, suggesting that these sandstones are hydrauli-cally connected over distances 420 km. At the crest ofthese sandstones, pore pressure equals or exceeds the leastprincipal stress, andwe interpret that high £uid pressure isfracturing the caprock and driving £ow vertically. Abovethese zones, spectacular mud volcanoes are venting £uidstoday.

REGIONAL SETTING ANDSTRATIGRAPHYOF THE AUGER BASIN

TheAuger Basin lies 345 km (215mi) southwest ofNewOr-leans in �1000m (3280 ft) water depth (Fig. 1a); it isbounded by tabular salt bodies, which are topographically

EAGE

1Present address: Hess Corporation,One Allen Center, 500DallasStreet, Houston,TX 77002, USA. E-mail: [email protected]: Peter B. Flemings, Jackson School of Geos-ciences, The University of Texas at Austin, University Station,Austin,TX 78713, USA. E-mail: p£[email protected]

BasinResearch (2010) 22, 380–397, doi: 10.1111/j.1365-2117.2010.00481.x

r 2010 The AuthorsJournal Compilationr Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists380

10.1029/2008JB006159

mailto:[email protected]

higher than the basin sediments (Fig.1a). A salt ridge sepa-rates the Auger Basin from the Andros Basin to the eastand the Tampa Basin to the south (Fig. 1a) (Booth et al.,2003). The salt dome in the centre of the ¢eld was em-placed subsequent to the deposition of Auger Basin reser-voirs. This study interprets data from three producing¢elds in theAuger Basin: Auger, Cardamom andMacaroni(Fig.1).

Auger ¢eld lies to the north, against the south £ank ofthe East Auger Salt Ridge; Cardamom is located on thenorth £ank of the East Auger Salt Ridge andMacaroni liesto the south, on the £ank of the basin-bounding salt ridge(Fig. 1a) (Booth et al., 2003). The hydrocarbons in Augerand Cardamom ¢elds are trapped around the Auger saltdome (Figs 1^3). At Macaroni ¢eld, 19 km (12m) to thesouth, hydrocarbons are trapped against the southeast£ank of the surrounding salt. At Auger, the producing in-tervals are between 4.2 km (14 000 ft) and 6 km (20 000 ft)deep (Figs 1^3). Macaroni ¢eld is deeper and has produ-cing intervals from 6.4 km (21000 ft) to 7.3 km (24 000 ft)(Figs 1^3). Structural crests of the sandstones are all lo-cated on the west margin of the basin. At the sea£oor, di-

rectly above the sandstone crests, are several mudvolcanoes that line the edge of the basin (stars, Figs1, 3).

The seven main reservoirs in Auger Basin are Pliocene-to Pleistocene-aged turbidite deposits. They are termedtheT, S, R, Q , P,O andN sandstones at Auger, CardamomandMacaroni.TheQ ,R and P sandstones are produced atAuger andMacaroni.The S and Tsandstones extend fromAuger toMacaroni but are not produced atMacaroni (Figs1^3).TheT, S, R, Q and Pdeepen and thicken to the southtowards Macaroni ¢eld and to the northeast into AndrosBasin (Figs1and 2), whereas theO andN sandstones covera lesser aerial extent.

Booth etal. (2000) recognized two main styles of deposi-tion atAuger Basin: ponded sheet sandstones and channe-lized sandstones. Ponded deposition produces aeriallyextensive sheet sandstones that o¡er excellent pressurecommunication (Booth et al., 2000). During channelizeddeposition, the sand is con¢ned to fans and channels andmay also be amalgamated, eroded or shaled-out, resultingin poor pressure communication across large distances(Kendrick, 2000). AtAuger andMacaroni, reservoirs tran-sition upward from ponded sheet sandstones to increas-

B

A’

Contour Interval: 1km

Seis

mic

Am

plitu

de

Pore

Pre

ssur

e Ra

tio

(a) (b)

Fig.1. (a) Sea£oor amplitudes in the Auger Basin with shaded structural relief and depth contours in meters. Positive amplitudes(yellow-red) record £ows emanating from the mud volcanoes; the vents on the west £ank have negative amplitudes (blue). Contourinterval is100m. Inset: theAugerBasin lies 345 km (215mi) southwest ofNewOrleans (red star) in �1000mwater depth. (b)Colour mapillustrates the ratio of the Q-sandstone overpressure to the hydrostatic e¡ective stress [P�w=ðsv � PhÞ]; 1km depth contours (metersbelow sea level). Pressure at the crest of the Q sandstone equals overburden stress at the crest of the structure (red zone) [P�w=ðsv � PhÞ ¼ 1].Vent locations are shown as stars. Location of cross- section shown in Fig. 2 is annotated A^A0^B. Location of Fig. 5 isshownwith black box.The 381#1well is shownwith the red circle. Production facilities are shownwith red squares.The distance fromthe Auger tension leg platform to theMacaroni subsea development is 18 km. Seismic image courtesy of CGGVeritas, Houston,TX.

r 2010 The AuthorsJournal Compilationr Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 381

Pore pressures and venting in theAuger Basin

ingly more channelized deposits (McGee etal.,1994;Boothetal., 2000, 2003).TheTand S sandstones are sheet depos-its with excellent aerial continuity, whereas the O and Nsandstones are channel and fan deposits.The R, Q and Psandstones each record transitional deposition: their basesare deposited as sheet sandstones, and they are capped bymore channelized deposits. Sheet sandstone deposits ofthe R, Q and P sandstones are thickest towards the centreof the basin and thin outward. In general, sheet sandstonesare thicker andmore common atMacaroni,whereas atAu-ger there are more channelized deposits (McGee et al.,1994; Booth et al., 2000, 2003). The sheet sandstones (P,Q ,R,S), in particular, are inferred to thin stratigraphicallyat the salt-bound basin margins. Either very thin sandabut directly against salt or, more likely, these sands mergelaterally into low-permeability mudstone.

AUGER BASIN PLUMBING

Seven aquifers (N, O, P, Q , R, S, and T) extend more orless across the Auger Basin from the structural crest tothe northwest to the synclinal lows in the south and east

(Fig. 2). We delineated the spatial distribution of four ofthese sandstones (P, Q , R and S) by mapping the mini-mum amplitude on zero-phase seismic data (Fig. 3). Itwas challenging to map the horizons near the structuralcrests. In this region, there is often a gas wipe-out zone(GWZ) that obscures the seismic data. In addition, sand-stones thin towards the crest and are di⁄cult to resolve(Fig. 2b).The sandstone crests may therefore extend higherthan where we could interpret them on seismic data.Although we do not present maps of the N, O orTsand-stones, we do delineate their travel times and estimateddepths at their crestal position at Auger and Macaroni(Table 1).

Travel times were converted to depth using a dual layervelocity model (Appendix A).To account for the change insea£oor topography we calculated depth to the sea£oorusing a constant water velocity. For each seismic horizon,we calculated average sediment velocity between the sea-£oor and the horizon from a vertical seismic pro¢le at Au-ger (Tables 1 and 3).We then used these average velocitiesto convert from the time-mapped horizon to depth.Thisapproach did not account for spatial variation in averagevelocity between the sea£oor and the mapped horizon.

Auger Macaroni

Salt

Gas Wipe-OutZone

Sub-Salt

A A’ B

S

R Q

Salt

WellPath

Gas Wipe-OutZone

P O

N

T

Legend

20 35

Overpressure (MPa)

GWZ

S

R

QP

O

(a)

(b)

two-

way

trav

el ti

me

Fig. 2. (a) Cross-section of Auger Basin illustrating connectivity and geometry of sandstones across the basin from structural crest tobase. Cross-section A^A^B is located in Fig.1b. Sandstones are coloured by their overpressure (P�w); the P, Q , R and S sandstones havealmost identical overpressure.Tick marks on right-hand side are separated by1s of two-way travel time. (b) At the crest of the structure,sandstones converge onto a transparent seismic zone, the gaswipe-out zone (GWZ). It is di⁄cult to map the updip termination of thesesandstones because the re£ectors thin and because the GWZmakes them di⁄cult to image. Seismic images courtesy of CGGVeritas,Houston,TX.


M. J. Reilly and P. B. Flemings

In fact, average velocity to any reservoir horizon is �15%higher at Macaroni than at Auger; this di¡erence is notsurprising because the horizons are �2 km deeper atMa-caroni than at Auger and, hence, subject to greater over-burden stress.We compared our constant velocity modelwith one that accounted for this velocity variation (Appen-dix A).The result is dramatic; if a spatially varying velocityis used instead of a constant velocity, the crestal depth is765m shallower in the P sandstone, and it is 1200m shal-lower in the S sandstone (Table1). A more appropriate ap-proach would be to use interval velocities derived fromseismic processing to determine depth at Auger. However,these data were not available to us. For the remainder ofthis paper, we use the velocity model that does not accountfor lateral variation in velocity. However, the range of errorillustrated in Table 1 illustrates the potential error in themapped structural depth.

We summarize here the major characteristics of the re-servoir sandstones at Auger, working from the deepest (T)to the shallowest (O).TheTsandstones consist of stackedsheet deposits that are similar to those of the overlyingS sand. TheT sandstone seismic horizon thickens to the

south towards the centre of the basin but does not extendto Macaroni ¢eld.TheTsandstones have much less reliefthan the overlying S sandstones.

The S, R, Q and P sandstones were the easiest to mapbecause they are aerially extensive sheet sandstones.Theyhave similar structural relief, their crests are to the north-west, and theyhave two synclinal lows to the south and thenortheast (Fig. 3).TheS sandstone seismic re£ection, con-tinuous across the basin, is thickest to the south, in thecentre of the basin (Figs 2 and 3).The S sandstones con-tinue to Macaroni, where they have been penetrated bythe Mac3STwell (Fig. 4). The R sandstone can be tracedon seismic across much of the basin. At Auger, CardamomandMacaroni, theR sandstone is composed of laterally ex-tensive sheet sandstones. The sheet sandstones of theQ sandstone series (Q2 and 3) also extend across the basin.Macaroni contains an additional set of sandstones fromthe Q interval that are not found at Auger, [sequences9 and 10 of Booth et al. (2003)]. At Auger ¢eld, theP sandstone consists of three thin channel sandstones,whereas at Macaroni, the P sandstone is a thick section ofsheet sandstones that grade upward into amalgamated

S-Sand

Contour Interval: 0.5km Contour Interval: 0.5km

Contour Interval: 0.5km Contour Interval: 0.5km

Q-Sand

R-Sand

N

N

N

N

P-Sand

Fig. 3. Depth structure maps of the Q , P, S and R sandstones with 0.5 km contours. Sea£oor vent locations are shownwith blue stars.Crests of the Q and P sandstones lie near the 381#1well, whereas the S and R sandstone crests are broader.This is the same mappedarea shown in Fig.1a and b.



sheet and channel sands.The P sandstone is more di⁄cultto map than the underlyingS,R andQ sandstone intervalson seismic data; these thinner, wet sandstones di¡er littlein acoustic impedance from their surrounding shales.

AQUIFER PRESSURES

Multiple repeat formation tests (RFTs) were taken to de-termine formation pressures (Figs 4 and 5).To ensure thatthe pressures measured were the initial pressures and notchanged by production, we used only RFTmeasurementstaken before production. Production began in 1994 at Au-ger, in1999 atMacaroni and in 2000 at Cardamom.We de-termined the water phase pressure at a speci¢c depth foreach sandstone (Fig. 5a and Table 2).

The simplest illustration of this approach is shown inFig. 4c, where multiple RFTmeasurements delineate twodistinct £uid pressure gradients: in the upper, hydrocar-bon-charged zone, the pore £uid pressure gradient is lessthan in the lower, water-saturated zone.We interpret thehydrocarbon^water contact to lie at the intersection ofthese two lines (horizontal dashed line, Fig. 4c). In mostcases, the hydrocarbon^water contact is not delineated di-rectly from the RFTmeasurements because the hydrocar-bon^water contact was not penetrated in the well (Fig. 4aand d). In these cases, the £uid contact was determinedfrom seismic data, and observed pressures were extrapo-lated to this depth (Fig. 5). At the hydrocarbon^water con-tact, hydrocarbon phase pressure (PO) is assumed to equalwater phase pressure (Pw). For this assumption to be cor-rect, capillary entry pressure of the sandstone must below, which is consistent with the 0.11-MPa (15-psi) capil-lary entry pressure observed from core analysis. In sand-stones where no hydrocarbons were present, weestablished water phase pressure from RFT measure-ments made in the aquifer at an arbitrary reference depth(Table 2).

Water phase overpressure P�w� �

is absolute water pres-sure (Pw) less hydrostatic pressure (Ph):

P�w ¼ Pw � Ph ð1Þ

Ph, is calculated by integrating pore £uid density (rf )from sea surface to depth of measurement(Ph ¼

Rrf gdz). A small change in pore water density

changes hydrostatic pressure,Ph, and can result in signi¢ -cant changes in overpressure P�w

� �.To calculate Ph, we as-

sumed a constant seawater density of 1025 kgm� 3,equivalent to a salinity of 35 000p.p.m.This value equatesto a vertical pressure gradient of 10.045MPa km�1

(0.444 psi ft�1).Within the sediment, we assumed a con-stant pore water density of 1043.8 kg m� 3, equivalent to asalinity of 93 000p.p.m. at 75 1C, which is the salinity inthe O sandstone reservoir at Auger.This value equates toa pressure gradient of 10.229MPa km�1 (0.4523psi ft�1).We calculated P�w

� �for each location (Table 2).T

able1.

E¡ectofsedimentvelocity

oncresth

eight

Sand

OWTsea£oor

toho

rizon

atAug

er(s)

OWTsea£oor

toho

rizonat

Macaron

i(s)

OWTsea£oor

toho

rizon

atVent

(s)

Depth

tosand

ston

eatAuger

(mbsf)

Depth

tosand

ston

eat

Macaron

i(m

bsf)

Macaron

isediment

velocity

(m/s)

Aug

ersediment

velocity(m

/s)

Mod

elsediment

velocityat

crest(m/s)

Depth

tocrest(mbsf)

(Auger

velocities)

Depth

tocrest(mbsf)

(mod

elledvelocities)

O1.7

991.864

^3985

4671

2506

2214

^P

1.827

2.107

1.357

4132

5460

2580

2260

1695

3066

2301

Q1.920

2.153

1.402

4407

5932

2755

2294

1704

3216

2390

R2.021

2.252

1.620

4672

5997

2683

2312

1683

3745

2727

S2.099

2.453

1.692

4904

6152

2508

2366

1691

4003

2862

T2.101

2.64

1.855

5054

6695

2546

2404

1792

4460

3207

Com

parisonofvelocitymod

elthatdo

esno

tvaryspatially

(Augersedimentvelocity)withon

ethataccoun

tsforvariation

inaveragev

elocity

withdepth(modelsedimentvelocity)(App

endixA).Ifweprojectthe

velocitychange

observed

betweenMacaron

iand

Auger

tothecrestlocations,the

predicteddepths

aremuchshallower

than

ifweusejusttheAugersedimentvelocities[compare‘D

epth

toCrest(m

odelledvelocities)’with‘D

epth

toCrest(Auger

velocities)’in

therighttwocolumns].

OWT,

one-way

traveltime;mbsf,metersb

elow

sea£oor.



At both Auger and Macaroni, absolute water pressures(Pw) increase at successively deeper stratigraphic levels(Figs 6a and 7). Similarly, pore pressure in any particularsandstone is greater at Macaroni than at Auger becauseMacaroni is deeper (Figs 6a, 7 and 8). Overpressures intheQ ,R andS all increase with deeper stratigraphic levels(Fig. 6b). Overpressures within the P, Q , R and S sand-stones are very similar, lying between 28 and 32MPa (Figs6b and 7 andTable 2).Within the P,Q ,R andS sandstones,pressure is slightly less at Auger than in the equivalentsand at Macaroni (Fig. 6b). The O sandstone pressure atMacaroni is very similar to the underlying Q sandstonepressure atMacaroni and is much higher than theO sand-stone pressure at Auger (Figs 6b and 8).

For theO,P,Q ,R andS sands,we calculated the verticaloverpressure gradient between Macaroni and Auger (Fig.6c andTable 2). At the O level, there is a large overpressuregradient ( �3.1MPa km�1) between the Macaroni O

sandstone and the Auger O sandstone (Fig. 6c), which re-£ects the large di¡erence in overpressure between the twolocations at the O level (Fig. 6b). In contrast, overpressuregradients in the underlying P, Q and R sandstones aresmall (Fig. 6c): the Q sandstone has the lowest gradient,�1% of the hydrostatic gradient (Fig. 6d), whereas the Psandstone has an overpressure gradient of �8%of the hy-drostatic gradient (Fig. 6d). The underlying S sandstonehas a hydraulic gradient as high as 15% of the hydrostaticgradient (Fig. 6d).

We extrapolate the pressure at Auger along the hydro-static gradient over the entire depth of each sandstone(Fig. 8a and b). The shallowest sand, the O, is overpres-sured by 22.4MPa at Auger and by 28.6MPa at Macaroni(Fig. 8a,Table 2).TheO sandstone is di⁄cult to map; it ap-pears to be pinched out between Auger andMacaroni, andit is di⁄cult to image near the structural crest (Fig. 2). Pre-vious work suggests that this sandstone has limited aerial

Macaroni 2STBP P-SandAuger A12 P-Sand Macaroni 2STBP Q-SandAuger 426-1 Q-Sand(b)(a)

Macaroni 3STBP R-Sand Macaroni 3STBP S-SandAuger A18 ST R-Sand Auger 471-1ST S-Sand(c) (d)

Fig.4. Type logs of the P, Q , R and S sandstones in the Auger Basin. Gamma ray (GR) log, resistivity (RES) log and pressure (P) areshownwithAuger ¢eld on the left andMacaroni ¢eld on the right. (a) The P sandstone is thicker and has better quality atMacaroni thanat Auger. (b) The Q sandstone is thick andwell developed at both locations. (c) The R sandstone is much thicker at Auger than atMacaroni.The oil^water contact is imaged at the intersection of the two pressure gradients (dashed line). (d)TheS sandstone atAuger isthicker and of higher quality than atMacaroni.



extent (McGee et al., 1994; Booth et al., 2000, 2003) and weinterpret that the O sandstone is not hydraulically con-nected between Auger and Macaroni. In contrast, at Ma-caroni, the O may be connected to the underlying Qsandstone because they have almost identical pressures(Figs 6b and 8). For the O sandstone, we do not extend thepressure lines very high on structure because we could notimage the sandstone in this location (Fig. 8).TheT, S, R, Qand P sandstones could be mapped across the basin, andwe extrapolated the pressures measured at Auger overtheir structural range. TheT sandstone (Fig. 8a, Table 2),which is not penetrated atMacaroni, has the highest over-pressure (32MPa).

VERTICAL STRESS (sV) AND LEASTPRINCIPAL STRESS (sH)

We estimated the vertical stress (sv) and the least principalstress across the basin. In sedimentary basins, it is com-monly assumed that the overburden (sv) is the maximumprinciple stress and, thus, the least principle stress is or-iented horizontally (Turcotte & Schubert, 1982).We there-fore term the least principal stress theminimum horizontal stress(sh).The overburden stress (sv) was estimated by integrat-ing the bulk density from wireline density logs at Auger.No density data were available for the ¢rst 1500m belowthe sea£oor, and in this zone densities were extrapolatedfrom deeper measurements using an exponential function(Seldon & Flemings, 2005; Reilly, 2008).We assumed thatthe change in vertical stress with depth below sea£oor ob-served at Auger was representative of the change in verti-cal stresswith depth anywhere in theAugerBasin. Becausethe water deepens from 520m at the vent location to 876m

P* = 27.97 MPa

80 85 90 95 100

11.6 12.1 12.6 13.1 13.6 14.1 14.6

6

54

3

45

6

7

8

471-1

426-1

A11

OWC at 5343m

N

17.6

18.0

17.2

16.8

16.4

Depth T

VD

SS

(kft)Dep

th T

VD

SS

(km

)

5.0

5.1

5.2

5.3

5.4

Pressure (MPa)

Pressure (KPSI)

(a)

(b)

Fig. 5. (a) Structure map (depth below sea level) of Auger ¢eld attheQ horizon,with the hydrocarbon zone delineated in dark grey(1km contours).Map is located in Fig.1b. (b) Pressure-depth plotillustrating estimation ofwater-phase sandstone pressure whendirect pressure measurements are in the hydrocarbon column.

Table 2. Pressures at Auger andMacaroni ¢elds

Sand LocationDepth to pressuremeasurement (mbsl) Pw (MPa) P�w(MPa) dPw=dz (MPa/km) dP�w=dz (MPa/km)

O Auger 4938.000 72.800 22.449 13.450 3.221Macaroni 6848.000 98.490 28.647

P Auger 5158.000 80.740 28.139 10.950 0.721Macaroni 7010.000 101.020 29.520

Q Auger 5343.000 82.614 28.121Macaroni 7095.000 100.663 28.293 10.302 0.072

R Auger North 5929.000 88.710 28.222 10.680 0.451Macaroni 7218.000 102.477 28.850

S Auger 5989.000 89.436 28.334 11.784 1.554Macaroni 7269.000 104.519 30.370

T Auger 6842.000 101.820 31.993

Where a hydrocarbon^water contactwas identi¢ed, depth (mbsl,meters below sea level) of pressure measurement is depth of hydrocarbon^water contact(‘Depth to PressureMeasurement’).Where a hydrocarbon contactwas not identi¢ed, we selected an arbitrary depth in the aquifer, where we had an RFTmeasurement. P�w is calculated fromEqn (1) with a seawater density of1025 kgm� 3 in the water column and a pore water density of1043.8 kg m� 3 withinthe sediment.The vertical pressure gradient (dPw=dz) and the overpressure gradient (dP�w=dz) are calculated assuming: dP

�w

dz ¼PwA�PwMZA�ZM

� dPhdz . PwA and

PwM are absolute pressures at Auger andMacaroni) andZA andZM are vertical depths below sea surface at Auger andMacaroni. dPh/dz is the hydrostaticgradient (10.229MPa km�1).Water depth is1125m atMacaroni, 876m at Auger.RFT, repeat formation test.



at Auger to 1100m at Macaroni, the overburden stress is4MPa less at Auger and 6.4MPa less at Macaroni than atthe vent location (Table 3).

The least principle stress (sh) was estimated from leak-o¡ tests (LOTs). LOTs or pressure integrity tests (PITs)are performed routinely before drilling below a newcasing shoe. In the case of a LOT, borehole pressure israised until £uid is lost through the formation viafractures. In contrast, in the case of a PIT, borehole pres-sure is raised to a predetermined stress, regardless ofwhether the formation is losing £uid. The leak-o¡value is assumed to approximate least principle stress(sh) (Roegiers, 1989).

We used only LOTs to analyse least principle stress be-cause they are a more accurate indicator (Roegiers, 1989).Unfortunately, although there were many PITs at Auger,only four LOTs were available (Table 4).To develop a gen-eral relationship between least principal stress (sh) andoverburden stress (sv), we calculated the e¡ective stress ra-tio (K) (Pilkington, 1978) for each leak-o¡ measurement,where

K ¼ ðsh � PwÞðsv � PwÞ

ð2Þ

K de¢nes where the least principal stress will lie as afunction of pressure (Pw) and overburden stress: if K5 0,

then sh equals in situ pressure (Pw), and if K51 then sh

equals overburden stress (sv).We assumed that in situ pres-sure was hydrostatic (Pw5Ph) and found that K rangesfrom 0.76 at a depth of 2400m to 0.97 at a depth of 6101m(Table 4).We interpret thatK increases with depth becausethere is more overpressure with depth (i.e. Pw is not equalto Ph). Seldon & Flemings (2005) and Lupa et al. (2002)found similar behaviour at Popeye (sh/sv5 0.95) andBull-winkle ¢elds (sh/sv5 0.985), respectively.To calculate leastprinciple stress at the vent locations, we used Eqn (2), as-suming hydrostatic pressure (Pw5Ph) andK5 0.9 (dashedline, Fig. 8).

SEAFLOOR VENT COMPLEX

Eight cone-shaped mounds located along the northwest£ank of theAuger minibasin are interpreted to be mudvol-canoes (Figs 1, 9 and 10). Seven of these lie directly alongthe £ank of the basin (#1^#7, Figs 3 and 9), and one liesapproximately 5 km farther west within the upraised £ank(#8, Figs 3 and 9). The largest vent (#1, Fig. 9) is 1170m(3840 ft) in diameter and �30m ( �100 ft) high; the othervents are similar to one another in size and are �840m( �2750 ft) in diameter and 50^75 ft (15^23m) high. A longnormal fault runs along the £ank of Vents 3 through 7 andintersects with a second basin fault that runs along the

Pore Pressure (MPa)70 80 90 100 110

O

P

Q

R

S

T

Pressure (PSI)

11000 12000 13000 14000

Overpressure (MPa)20 22 24 26 28 30 32 34

O

P

Q

R

S

T

Overpressure (PSI)

3000350040004500

Overpressure Gradient (MPa/km)0 1 2 3 4

O

P

Q

R

S

Overpressure Gradient (PSI / ft)

0.00 0.05 0.10 0.15

Ratio of Over pressure Grad. toHydrostatic Grad

0.0 0.1 0.2 0.3 0.4

O

P

Q

R

S

Overpressure Gradient Ratio

(a)

(c)

(b)

(d)

Fig. 6. Pressures in di¡erent sandstones at Auger andMacaroni. (a)Water-phase pressures (Pw) within the Auger Basin. (b)Overpressure(P�w) in theAugerBasin. (c)Overpressure gradient (dP

�w=dz). (d)Ratio of overpressure gradient to hydrostatic gradient. See

Table 2 for how these values were calculated.



south edge of Vents 1 and 2 (Fig. 10a).Vent 8 is on the westside of the tabular salt body and may not be directly tied tothe sandstones in Auger Basin.

The mud volcanoes have £at tops and negative seismicamplitudes that record a large decrease in impedance atthe water^sea£oor interface (blue, Figs 9 and10c).The ne-gative seismic amplitudes may result from free gas and li-quid mud present at craters of the mud volcanoes asdescribed byMacDonald et al. (2000) and Kohl & Roberts(1994).

Flow-like features, oriented down the topographic gra-dient, emanate from the mudvolcanoes (Figs 9, 10a and b).These features have positive seismic amplitudes that aremore positive than those of the adjacent, undeformed,sea£oor (Figs 9 and10).The higher amplitudes of the £owsmay be caused by a greater degree of consolidation causedby deposition as a debris £ow (Seldon & Flemings, 2005;Sawyer et al., 2009). Flows fromVents 1 and 4 have a totalarea of 15 and10 km2, respectively.

Crests of the S, R, Q and P sandstones are located be-neath a series of cylindrical GWZs (Figs 2, 10 and 11).Thesea£oor vents overlie these GWZs, and we infer that theGWZs feed the sea£oor vents. An alternative interpreta-tion is that the GWZ at depth may result from attenuationnear the sea£oor due to the presence of gas. The chaoticseismic signature of the GWZs may result from £uids de-stroying depositional layering and removing any acousticimpedance contrasts as they travel upward (Kohl & Ro-berts, 1994; Graue, 2000). Expelled gasses may also be-come trapped in small pockets, further compounding theproblemwith seismic imaging (Graue, 2000). Seismic hor-izons decrease in amplitude and become increasinglyharder to image with closer proximity to the GWZ (Figs 2and 10c). BeneathVent 4, the GWZ is, in sections,42 kmwide, and many horizons fade into it (Fig.10c).

The P and Q sandstones have structural crests beneathVent 4 (Figs 3a and b, 9, 11).The S and R sandstones havea much broader crest, with high points below Vent 1 (Figs

5

80 90 100

6

Dep

th (

km) D

epth

(kft)

Pressure (kpsi)

Pressure (MPa)

P* = 28.14 MPa

24

7

20

16

11 12

(a)

(c)

(b)

(d)

13 14 15

P-sand

Car A11Car A11ST

Aug A12

Mac 3Mac 3ST

Aug A13

16

20

11 12 13 14 15

Aug Q1 471-1ST1Aug Q2 471-1ST1

Aug Q3 471-1ST1Aug Q1 426-1

Aug Q2 426-1

Car A11STCar A11

5

80 90 100

6

Dep

th (

km) D

epth

(kft)

Pressure (kpsi)

P* = 28.12 MPa

OWC at 5343m

Q-sand

24

7

Mac 2STBP1Mac 4ST

Pressure (MPa)

16

20

12 13 14

5

80 90 100

6

24

7

15

R-sand

Dep

th (

km) D

epth

(kft)

Pressure (kpsi)

Pressure (MPa)

Dep

th (

km)

Dep

th (kft)

Pressure (kpsi)

Pressure (MPa)

P* = 28.33 MPaP* = 28.22 MPa

OWC at 5989m

S-sand

Mac 2STMac 4ST

Aug A18STMac 3

12 1614 18

18

20

22

24

26

80 100 120

6

7

8

Aug 426-1Mac 03ST

Aug S1 471-1Aug S2 471-1

V

V

V

V

Fig.7. Graphical comparisons of pore pressures atAuger,Macaroni andCardamom for the P,Q ,R andS sands. Symbols (e.g., trianglesor squares) represent individual repeat formation test (RFT)measurements in particular wells. Hydrostatic pressure (dashed line) iscalculated at the Auger location (Ph) assuming a seawater density of 1025 kg m� 3 in the water column and a pore water density of1043.8 kg m� 3.The solid line represents the extrapolation of absolute pressure at Auger (Table 2, column 3) along a hydrostatic pressuregradient of10.23MPa km�1. Overburden stress (dash^dot) represents overburden stress calculated at Auger (sV).



Pressure /Stress (MPa)

0 40 80 120

Dep

th (

km)

0

2.0

4.0

6.0

8.0

10.0

Pressure (KPSI)

0 5.0 10.0 15.0 20.0

Depth (kft)

10.0

20.0

30.0

0


55 60 65 70 75 80

Dep

th (

m)

3000

3500

4000

4500

Pressure (psi)

8000 9000 10000 11000

Depth (ft)

10000

11000

12000

13000

14000

15000

(b)

(a)

Fig. 8. (a) Reservoirpressures and principlestresses in the Auger Basin.T, S, R, Q , P and Osandstone pressures (Pw) areplotted over their structurallimits. Pore pressures aremeasured at Auger and thenextrapolated along ahydrostatic gradient(10.23MPa km�1) from thislocation to the crest and baseof the sandstones (asmeasured in depth below sealevel). For each sandstone,the base and crest of thesandstone is markedwith asolid symbol, and location ofthe pressure measurement ismarkedwith an open symbol.(b) Expanded view atstructural crest. Overburdenstress (sv) is calculated atAuger and then shifted toaccount for the shallowersea£oor at the crest (see text).Least principle stress (sh) isbased on leak-o¡measurements made atAuger [see text and Eqn (2)].



3b and c, 9).We conjecture that the deeper sandstones inthe Auger Basin are fuelling Vent 1 whereas the shallowerQ and P sandstones are fuelling Vent 4.

The vents are long lived.We compared seismic ampli-tudes at the sea£oor (Fig.9) with those16ms and 56m be-low the sea£oor (Fig. 10a and b). At 16ms below thesea£oor,Vent #1 is imaged, but £ows associated with theother vents are not (Fig. 10a). At this depth, the GWZ(Fig. 10c) follows the fault strike. At 56ms below the sea-£oor, an older expulsion event is imaged throughout theregion (Fig. 10b). Finally, mud £ows adjacent to a volcanoare imaged as deep as 250ms below the sea£oor (Fig.10c).

Kohl & Roberts (1994) explored Vent 1 using mannedsubmersibles (Fig. 9). They observed methane and mudventing and found that 65% of the foraminifera recoveredwere of Miocene age. They interpreted that pressurized£uids entrained clays from depth.On the basis of thinningof hemipelagic sediment across the uppermost £ows thatemanate fromVent 1, Roberts & Carney (1997) suggestedthat the mud£ows postdate the sea level lowstand at 17 ka.MacDonald et al. (2000) placed a temperature probe in themud lake ofVent 4 and used satellite imagery to track slickscreated from eruption events. Rapidly £uctuating tem-peratures coincidedwith the occurrence of large oil slicksin the area. MacDonald et al. (2000) interpreted from themagnitude of the temperature £uctuations that £uids re-leased during the study must have originated from at least2310m (7575 ft) beneath the sea£oor, which is approxi-mately the depth of the P sandstone crest (Fig. 3). Aharon(2003) found long chained hydrocarbons at the mudvolca-noes that match the 2% of high alkane (oC11), a ¢nger-print of produced hydrocarbons at Auger ¢eld.

DISCUSSION

The Auger Basin is a coupled hydrodynamic system:high pore pressures generated by rapid loading of low-permeability mudstone are dissipating by lateral £owwith-in dipping permeable sandstone bodies. At the crest ofthese sandstones, pore £uid pressure converges on the leastprinciple stress, elevated pore pressures create fracturepermeability in the caprock and £uid is vented vertically.Today these £uids are expelled through mud volcanoes atthe sea£oor. This £ow system has been proposed and de-scribed previously. Dickinson (1953) and Rubey & Hubbert(1959) recognized that in overpressured strata, the pressuregradient is less in permeable sandstones than in thebounding low-permeability mudstone, and England &others (1987), Mann & Mackenzie (1990) and Yardley &Swarbrick (2000) suggested that under these conditions fo-cused £ow results.We have described the relationship be-tween elevated pore pressures at the crest of structuresand elevated permeability (Finkbeineretal., 2001;Flemingsetal., 2002; Lupa etal., 2002).Here we have presented an ex-traordinary documentation of this process by integratingdetailed mapping with pore pressure analysis.

Three pressure regimes are present at Auger (Fig. 8aand Table 2): (1) the T sandstone is deepest and has thehighest overpressure; (2) the P, Q , R and S sandstones areat intermediate depths and have similar overpressures thatare less than that of theTsandstone (Fig. 8b) and (3) the Osandstone is the shallowest and has the lowest overpres-sure.These pressure zones re£ect the stratigraphic archi-tecture. The T is separated from the S by a thick shaleassociatedwith amajor £ooding event. In turn, theP sand-

Table 3. Pressure, stress and depth in the Auger Basin

Sand LocationSandstonebase (mbsl) Pw at base (MPa)

Sandstonecrest (mbsl) Pw at crest (MPa) sv at crest (MPa) sh at crest (MPa)

O Auger 6968 ^ ^ ^ ^ ^P Auger 8450 114.41 3006 58.72 57.32 53.83Q Auger 9300 123.09 3216 60.86 60.62 58.18R Auger North 9600 126.27 3745 66.38 72.45 69.38S Auger 9800 128.42 4003 69.12 78.54 75.12T Auger 10 800 142.31 4460 77.46 89.75 85.68

Estimated sandstone pressures, stresses and depths (mbsl, meters below sea level) at crestal locations and deepest locations (base) mapped in the AugerBasin (Fig. 3). o¤ h is calculated from Eqn (2) assumingK5 0.9 and pore pressure (Pf) equal to hydrostatic pressure.

Table 4. Leak-o¡ measurements (LOT)made during drilling at Auger

Sub sea depth (m) LOT (sh) (MPa) sv (MPa) Ph (MPa) K ¼ sh � Phð Þsv � Phð Þ

2400 36.25 39.9 24.389 0.763741 64.4 68.9 38.106 0.854720 87.2 93.06 48.121 0.876101 125.3 127.3 62.247 0.97



stone is separated from the overlyingO by a thick shale in-terval associatedwith sea-level rise at the Plio^Pleistoceneboundary (McGee et al., 1994; Booth et al., 2003).

At Auger, overpressures within the P^Q^R^S intervalare almost identical (between 28.1 and 28.3MPa; Figs 6band 8b and Table 2). This similarity implies that thesesandstones are hydraulically interconnected eventhough the individual reservoirs have verydi¡erent hydro-carbon^water contacts. This similarity in water phaseoverpressure in conjunction with very di¡erent hydro-carbon^water contacts suggests that the hydraulic connec-tions are downdip from the crests, perhaps due toerosional amalgamation ordue to juxtaposition of di¡erentsandstones across faults that increase in displacementdowndip.

Pressures were measured at Auger andMacaroni in theO, P, Q , R and S sandstones (but not theT). In all cases,overpressure is greater at Macaroni than in the equivalentsandstone at Auger (Table 2). For the P to S interval, pres-sure di¡erences are small: 1.381, 0.172, 0.628 and 2.036MPafor the P, Q , R and S, respectively (Table 2). Similar over-pressures in the same sandstones 20 km apart suggest thatthese thick, high-permeability, sandstones are hydrauli-

cally connected between Auger and Macaroni. The largeroverpressure di¡erences in the S and the P suggest thatthese sandstones are less connected between Auger andMacaroni than are the Q and R sandstones (Fig. 5 andTable 2).

Seismic mapping and reservoir production history alsosuggest that the P, Q , R and S sandstones are connectedacross the Auger Basin. Many studies have concluded thattheP,Q ,R,S andTsandstones are laterally extensive sheetsandstone deposits, and our interpretation of the well logsis in agreement with this work (McGee et al., 1994; Boothet al., 2000, 2003). Finally, reservoir simulations in the Ssandstone require an aquifer volume 53 times the volumeof the ¢eld, roughly the entire basin, to support observedproduction rates (Kendrick, 2000).

Because the O sandstone overpressure is much higherat Macaroni than it is at Auger (Fig. 6b), we interpret thatit is not connected between Auger andMacaroni.This in-terpretation is supported by studies showing that the Oand N sandstones contain erosional channel sequences¢lled with interbedded mudstones and sandstones thatare inferred to have low permeability (McGee et al., 1994;Booth et al., 2000, 2003).

12

3

4

5

6

8

AugerSalt Dom

e

Macaroni

Auger Field Car

dam

omFi

eld

7

–28000–20000

–10000

2800020000

10000

0

N

Sei

smic

Am

plitu

de

Fig.9. 3-DAmplitude map of sea£oor in theAugerBasinwith100m depth contours overlain. Flows emanate frommudvolcanoes (redsand yellows). Locations of mud volcanoes are numbered. Negative amplitudes (blues) at £at tops of the mud volcanoes may record freegas or oil in the craters. Seismic image courtesy of CGGVeritas, Houston,TX.



The upward £ow velocity within the P,Q ,R andS aqui-fers can be estimated using Darcy’s Law:

u ¼ �km

HP� ð3Þ

where u is Darcy £ow velocity, k is permeability and m isviscosity. At Auger, reservoir permeability in the O sand-

stone is 3.94 � 10� 13m2 (400md), andviscosity is approxi-mately 0.5 � 10� 3 Pa � s. Given the vertical pressuregradients in theP,Q ,R andS sandstones (Table 2), verticalDarcy £ow velocities are between 1 and 20mmyear�1

[Eqn. (3)]. The permeabilities may be very di¡erent thaninferred here; however, this estimate of the upward £owrate within individual Auger sandstones emphasizes thatvery small overpressure gradients (only a few percent of

–128

128

0

16ms Below Seafloor

–128

128

0

–128

128

0

GWZ

West East

C C’

56ms Below SeafloorT

ime (m

s)

Am

plitu

de

Am

plitu

de

Am

plitu

de

(a) (b)

(c)

Fig.10. (a)Flattened time slice from16ms below sea£oor.Gaswipeout zone (GWZ)underlies mudvolcanoes 3^7.Tops of previous £owsfrom mud volcanoes1and 2 are imaged as grey £ow-like features oriented to the southeast. (b) Flattened time slice from 56ms belowsea£oor. Flows emanated fromvolcanoes 3 to 7 down the £ank of the basin towardsMacaroni. (c) Shallow time cross-section of mudvolcano 4. Previous extruded £ows are delineated. GWZ is large vertical section of chaotic and nearly transparent re£ectors beneath themud volcanoes.Vertical axis is in two-way travel time: tick marks are separated by 0.5 s. Seismic data courtesy of CGG-Veritas. Seismicimage courtesy of CGGVeritas, Houston,TX.



the hydrostatic gradient, Fig. 6d) can drive signi¢cant £owrates.

Our interpretation that there is an overpressure gradi-ent between Auger and Macaroni for the P, Q , R and Ssandstones depends on pore £uid density [Eqn (1)]. If thepore £uid density is lower thanwe assumed, then the over-pressure gradient between Macaroni and Auger will behigher, and vice versa.We assumed a pore water salinity of93 000p.p.m. at an average temperature of 74 1C, which re-sults in a pore £uid density of 1043.8 kg m� 3. A 10%change in temperature or salinity results in 0.06% and0.04% change, respectively, in bulk density at these condi-tions. Thus, if salinity in the Q sandstone were increasedfrom 70 000 to100 000 p.p.m., there would be no overpres-sure gradient in this sandstone.We infer that overpressuregradients that are41% of the hydrostatic gradient recordtrue overpressure di¡erences, not inaccuracies in assumed£uid density.

When pore pressures in the P and Q sandstones are ex-trapolated along the hydrostatic gradient from Auger tothe crest, we ¢nd that the P sandstone pressure exceedsoverburden stress, whereas the Q sandstone pressureequals overburden stress (Fig. 8b). In this crestal location,water phase pressure (Pw) converges on least principlestress (sh): horizontal e¡ective stress (s0h ¼ sh � Pw) goesto 0 and even becomes negative.We infer that at this loca-tion, permeability is elevated because fractures are openedin the caprock. Increased permeability may occur because

optimally oriented faults are critically stressed at this loca-tion or because pore pressure has truly exceeded the leastprincipal stress and hydraulic fracturing has occurred(Finkbeiner et al., 2001; Flemings et al., 2002)

The S and R sandstones have the same pore pressure asthe P and Q , but they do not extend high enough for theirpore pressures to cause caprock failure (Fig. 8).We inferthat pore pressures in the R and S are the same as in the Pand Q because they are hydraulically connected.We sug-gest that pore pressure in the entire P, Q , R and S intervalis controlled by the leak point at the P and Q sandstones.The underlyingTsandstone and the overlyingO sandstonehave pore pressures at their structural crest that are lessthan the least principle stress, and their pressures are notcontrolled by failure at their crest (Fig. 8b).

Our estimates of pressure, elevation, and least principlestress at the crestal location are uncertain. At the Q level, avelocity model that accounts for spatial variation in velo-city predicts that the crest of the Q sandstone would be600m shallower than presented in Fig. 8 (Table 1), result-ing in an overburden stress that is less than the pore pres-sure.We have also assumed that least principle stresses arede¢ned by the e¡ective stress ratio, K, de¢ned at Augerand that pore £uid pressure is hydrostatic in this relation-ship [Eqn (2)]. In fact, Eqn (2) implies that as overpressurerises, least principal stress converges on overburden stress.Thus, at the crest of Auger, where pore pressures are veryhigh, least principal stress may converge on overburden

Table5. Nomenclature

Variable Description Dimensions

g Gravitational acceleration LT�1

k Permeability L2

K E¡ective stress ratio ^th Time to horizon Ttsf Time to sea£oor Tm Viscosity MLT�1

OWT Seismic one-way travel time TPh Hydrostatic pressure ML�1T� 2

Po Hydrocarbon-phase pressure ML�1T� 2

Pw Water-phase pressure ML�1T� 2

P�w Water-phase overpressure ML�1T� 2

u Darcy £ow velocity LT�1

Va Sediment velocity at Auger LT�1

Vm Sediment velocity atMacaroni LT�1

Vsw Velocity of sound in seawater LT�1

Vz Sediment velocity LT�1

Z Depth from sea surface Lrb Bulk density ML� 3

rf Fluid density ML� 3

sh Minimum horizontal stress (assumed to equal least principle stress) ML�1T� 2

s0h Minimum horizontal e¡ective stress ML�1T� 2

sv Overburden stress or maximum principle stress ML�1T� 2

s0vh Hydrostatic e¡ective stress ML�1T� 2

f Porosity ^dT Macaroni ¢eld time ^ Auger ¢eld time TDV Di¡erence in velocity LT�1



stress. Ultimately, what can be surmised is that pore pres-sures at the Q and P level more or less equal overburdenstress at the crestal location and that pore pressures inthe other reservoirs do not reach the least principalstress.

Fluid venting in the Auger Basin has been recognizedfor years. It has been related to sea-surface oil slicks (Mac-Donald et al., 2000), gas hydrate accumulations (MacDo-nald et al., 2002), mud venting (Kohl & Roberts, 1994),chemosynthetic communities (MacDonald et al., 2000;Aharon, 2003) and radioactive barite deposits (Aharon,2003). Paleovent complexes have also been discovereddeep in the subsurface at Auger (Shew et al., 1993).We in-terpret that the mudvolcanoes venting material at the sea-£oor today record expulsion of £uid from the P and Qsandstones (Fig. 9). These mud volcanoes are underlainby cylindrical GWZs that are the £uid migration pathway(Figs 2 and10). It is remarkable that these migrations occurfrom the reservoir depth to the sea£oor.These migrationpathways are termed £uidization pipes or diatremes (Stewart&Davies, 2006).

The entire hydrodynamic system is illustrated with ageological cross-section (Fig.11).The deep overpressuredP, Q , R and S sandstones pinch latterly towards the salt£ank low where accommodation was low during deposi-tion: they are sealed vertically and laterally by either low-permeability mudstone (most likely) or by the salt itself.Focused £ow elevates pore pressure at the crest to litho-static pressures and these pressures dilate fractures in theoverlying caprock, driving £uid upward. Lithostatic £uidpressures con¢rm that these sands are not connected di-rectly to the sea£oor or the pore pressures would be ap-proximately hydrostatic. Fluids migrate vertically throughthe caprock for more than 2500m before ultimately being

expelled through vents that have diameters of a kilometrewith relief as great as 100m.

Mud volcanoes gained international attention becauseof the 2006 eruption of theLusi mudvolcano, Java Indone-sia. As of August 2008, Lusi covered an area of 7 km2, haddisplaced 30 000 people, and continues to expand at ratesof 5000^180 000m3day�1 (Davies et al., 2007, 2008; Maz-zini et al., 2007;Tingay et al., 2008).The processes drivingsubsurface £uid £ow and sea£oor venting in theAugerBa-sin, Gulf of Mexico, are very similar to those arti¢ciallycreated in Java. Davies et al. (2008) described how at theLusi mud volcano, drillers inadvertently drilled into anoverpressured zone having a low mud weight. The resultwas that severely overpressured £uids moved up throughthe open borehole to shallower depths.The borehole itselfprovided the permeable pathway that allowed the deepoverpressures to project upward along the hydrostatic gra-dient until the pore pressure exceeded the least principlestress at shallower depths.

The concept that pore £uid pressure can be extrapo-lated along the hydrostatic gradient throughout a basinprovides a conceptual approach to predicting where £uidventing will occur. In Fig. 1b, the ratio of the Q sandstoneoverpressure to the hydrostatic e¡ective stress(P�w=sv � Ph ) is plotted in colour.When this ratio equals1, pore pressure equals least principle stress (red) and ca-prock failure is predicted (Fig. 1b). Note that zones at ornear failure are directly beneath the mud volcanoes (redzones, Fig. 1b). A practical consequence of this under-standing is that drilling in the red zones in Fig. 1b wouldboth be very di⁄cult (because the pore pressure convergeson the least principal stress) and no hydrocarbons wouldbe trapped in this location (Flemings et al., 2002; Seldon& Flemings, 2005).

A A’ B

Auger MacaroniVent 4

Sea Floor


0 40 80 120

Dep

th (

km)

0

2.0

6.0

10.0

Pressure (KPSI)0 10.0 20.0

Depth (kft)

30

Q S

andstone

0

20

10

Fig.11. (a) Cross-section through the Auger Basin. Cross-section A^A0^B is located in Fig.1b. On the western side of the basin, thecrests of theP,Q ,R andS sandstones thin laterally towards the salt bounded basin margin andunderlie the sea£oor vents.Vertical axis isin depth: tick marks are separated by 2.0 km. (b) Pressure vs. depth of the Q sandstone. At the crest of the Q sandstone, pore pressuresequal the overburden stress (sv).



We learned during ¢nal revisions to this paper that the381#1well had been drilled in the highest part of the Au-ger Basin structure (red circle, Figs1and 3).This well pro-vides striking con¢rmation that pore pressure, leastprincipal stress and overburden stress all converge at thecrestal location as predicted in Fig. 8. During the initialwell, there was circulation loss (grey line, Fig. 12a and b),which occurswhen borehole pressure exceeds least princi-ple stress and £uids are lost to the formation.The well wasthen sidetracked and cased to a deeper depth, and drillingproceeded at a higher mudweight (dark black line, Fig.12aand b). Drilling was di⁄cult, and mud weight was raisedand lowered so as neither to have circulation loss nor for-mation pressure to exceed borehole pressure (Fig. 12b).Least principal stresses, determined byLOTs (red squares)suggest that at the Q sandstone level, least principal stressexceeds overburden stress. This di¡erence could re£ectour underestimation of overburden stress or that leastprincipal stress may exceed overburden stress. Ultimately,the well was abandonedwhen no hydrocarbons were foundand when drilling could not proceed because pore pres-sure had converged on least principle stress.

CONCLUSIONS

We mapped the stratigraphy, pressure, stress and sea£oormorphology in the Auger Basin, o¡shore Gulf of Mexico.Pore £uid overpressures in four reservoir sandstones (theP, Q , R and S) are similar across the basin, suggesting thatthese sandstones are hydraulically connected over dis-tances420 km.Small overpressure gradientswithin themsuggest upward £ow rates between1 and 20mmyear�1. Atthe crest of these sandstones, pore pressure equals or ex-ceeds least principal stress, andwe interpret that high £uid

pressure is fracturing the caprock and driving £ow verti-cally. Above these zones, spectacular mud volcanoes areventing £uids today. A well drilled into this zone con-¢rmed that pore pressure, least principal stress and over-burden stress all converge at this location.

The Auger Basin is a coupled dynamic system in whichdeep and high pore pressures within a large aquifer drivehydraulic fracturing at the reservoir crest and sea£oorventing.Venting will continue until overpressures withinthe Auger Basin mudstones are su⁄ciently dissipated sothat the sandstone pressure drops below the least principalstress at the crest.The presence of sea£oor venting can beused to estimate pressure within deeply buried aquifers.Pore pressure can be predicted by assuming that reservoirpressure equals least principal stress in the caprock at itscrest. Pore pressure can then be predicted throughout a hy-draulically connected reservoir volume, which can be usedto estimate trap integrity and potential column heights andto design safe and economic drilling programs.

ACKNOWLEDGEMENTS

We thank C. Bohn, D. Sawyer, J. Schneider and J. Lupa foruseful discussions and H. Nelson for technical support.Reviews by B. Couzens and two anonymous reviewersgreatly improved the manuscript.We thank Shell Oil foraccess to well log data and pressure measurements. Wethank CGGVeritas, Houston, for access to seismic data.We thank T.Wilson, S.Waters, B. Bohn and C. Arroyo atShell Oil for their support of this project. Landmark Gra-phics Seisworkss 3-D, Paradigm Geologs and the free-ware Generic Mapping Tool program were the primarysoftware tools used in this study. This research was sup-ported by the Penn State GeoSystems Initiative (Shell and

Density (kg /m3)

1000 1200 1400 1600 1800 2000 2200

TV

D (

m)

1000

1500

2000

2500

3000

3500

4000

Mud Weight (lbs /gal)

10.0 12.0 14.0 16.0 18.0

Depth (ft)

4000

6000

8000

10000

12000

(b)

Pressure /Stress (MPa)20 30 40 50 60 70

Dep

th (

m)

1000

2000

3000

4000

Pressure (psi)4000 6000 8000 10000

Depth (ft)

4000

6000

8000

10000

12000

(a)

Fig.12. Drilling results for the 381#1well (red circle in Figs1 and 3) with predicted pore pressure in the Q sandstone (blue line).The¢rstwellwas terminated (grey line), and then a sidetrack (BP2)wellwas drilled at a higher mudweight (black line).The overburden stress(thin solid line) and least principal stress (dashed line) are derived from measurements at Auger ¢eld (also shown in Fig. 8). Ultimately,the well was abandoned as pore pressures converged on least principle stress at the level of the Q sandstone. (a) Pressure^depth plot.Thick grey and black lines record pore pressure calculated from mudweight during drilling. Red squares record leak-o¡ test stresses.The blue line recordsQ sandstone pressure. (b) Pressure gradient plot. In a gradient plot, actual pore pressure is divided by depth at anypoint. It is used by drilling engineers to portray pressure and stress conditions in a borehole. In this example, mud density (density of£uid in borehole) was raised from hydrostatic (1023 kg m� 3) to �1800 kg m� 3 during drilling (thick black and grey lines).The muddensity necessary to equal Q sandstone pressure is shownwith the blue line.



Chevron), the U.T. GeoFluids Consortium and the HellerGeoSystems Endowment at Penn State.

APPENDIX A: DEPTH CONVERSIONVELOCITY MODEL

We use a two-layer velocity model to convert a time-basedstructure map to depth (Reilly, 2008).

Z ¼ Vz th � tSfð Þ½ � þ Vsw � tSf½ � ðA1Þ

whereZ is depth from the sea surface,Vz is the average se-diment velocity to the horizon [Eqn (A2)], th is total one-way seismic travel time to the horizon, tsf is one-way timeto the sea£oor and Vsw is speed of sound in seawater(1500ms�1).

Vz is greater atMacaroni than at Auger, presumably be-cause e¡ective stress is higher at Macaroni than at Auger,resulting in greater compaction.To account for the varia-tion in velocity with burial depth that is observed, we de-veloped the following function:

Vz ¼ Va þ Va � Vmð Þ dtDT

� �ðA2Þ

Vz is sediment velocity at any given depth,Va and Vm areaverage sediment velocities to the horizon being mappedat Auger and Macaroni ¢elds, respectively. DT is total dif-ference in travel time to the mapped stratigraphic horizonbetween Auger andMacaroni. dt is the di¡erence in traveltime at any location on the map and the travel time to thathorizon at Auger ¢eld.

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Manuscript received 25 September 2008;Manuscript accepted 19April 2010.



10.1029/2008JB006159

deep pore pressures and seafloor venting in the auger basin, gulf

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