detecting overpressure using porosity … · estimate pore pressure in shales in the carnarvon...

APPEA JOURNAL 2002—559

P.J. van Ruth1, R.R. Hillis1 and R.E. Swarbrick2

1National Centre for Petroleum Geology and GeophysicsThebarton CampusAdelaide University SA 50052Department of Geological SciencesUniversity of Durham,South Road Durham DH1 3LE [email protected]@[email protected]

ABSTRACT

Overpressure has been encountered in many wellsdrilled in the Carnarvon Basin. Sonic logs are used toestimate pore pressure in shales in the Carnarvon Basinusing the Eaton and equivalent depth methods ofestimating pore pressure from velocity data withreference to a normal compaction trend. The crux of porepressure estimation from the sonic log lies in thedetermination of the normal compaction trend, i.e. theacoustic travel time (∆t)/depth (z) trend for normallypressured sediments. The normal compaction trend forshales in the Carnarvon Basin was established by fittingan Athy-type exponential relationship to edited sonic logdata, and is:

Vertical stress estimates are also needed for the Eatonand equivalent depth methods used herein. A verticalstress (σv) relationship was obtained by fitting a regressionline to vertical stress estimates from the density log, andis:

The Eaton and equivalent depth methods yield similarpressure estimates. However, the equivalent depthmethod can only be applied over a limited range ofacoustic travel times, a limitation that does not apply tothe Eaton method.

The pressure estimates from the Eaton method werecompared to pressures measured by direct pressure testsin adjacent permeable units. There is a good correlationbetween Eaton and test pressures in normally pressuredintervals in three wells and overpressured intervals intwo wells. Eaton pressure estimates underestimateoverpressured direct pressure measurements in fourwells by up to 13 MPa. This is consistent with overpressurebeing generated (at least in part) by a fluid expansionmechanism or lateral transfer of overpressure. The Eatonpressures in one well are, on average, 11 MPa lower than

hydrostatic pore pressure recorded in direct pressuremeasurements below the Muderong Shale. The sedimentsin this well appear to be overcompacted due toexhumation.

Mud weights can be used as a proxy for pore pressurein shales where direct pressure measurements are notavailable in the adjacent sandstones. The Eaton pressureestimates are consistent with mud weight in the GearleSiltstone and Muderong Shale in 4 of the 8 wells studied.The Eaton pressures are on average 10 Mpa in excess ofmud weight in the Muderong Shale and Gearle Siltstonein three wells. It is unclear whether the predicted Eatonpressures in these three wells accurately reflect porepressure (i.e. the mud weights do not accurately reflectpore pressure), or whether they are influenced by changesin shale mineralogy (because the gamma ray filter doesnot differentiate between shale mineralogy). Severalkicks have been recorded in adjacent wells within theMuderong Shale and Gearle Siltstone, and this intervalis overlain by significant sediment thickness in thesethree wells. These observations are consistent with theexistence of overpressure due to rapid burial-relateddisequilibrium compaction in the Muderong Shale andGearle Siltstone.

KEYWORDS

Overpressure, Carnarvon Basin, vertical stress, Eatonmethod, equivalent depth method, normal compactiontrend, undercompaction.

INTRODUCTION

The Carnarvon Basin is located along the WesternAustralia coastline (Fig. 1), and overpressure has beenencountered in many wells drilled in the basin. Anunderstanding of the distribution of overpressuredsediments in a sedimentary basin is of importance bothin terms of drilling cost and safety, and in terms ofunderstanding fluid flow in the basin. In particular,understanding the relationship between pore pressureand velocity provides a framework in which pore pressurecan be estimated pre-drill using seismic processingvelocities.

The most reliable pressure measurements insedimentary basins are drill stem tests (DSTs) andwireline formation interval tests (WFITs). Many authorshave described the distribution of overpressure in thehigh permeability aquifers of the Carnarvon Basin usingdirect pressure measurements (DSTs/RFTs) and/ordrilling data (Nyein et al, 1977; Horstman, 1988;Zaunbrecher, 1994; van Ruth et al, 2000; Tingate et al,2001; Otto et al, 2001). DST and WFIT tests, however,cannot be performed in low permeability sediments,

DETECTING OVERPRESSURE USING POROSITY-BASEDTECHNIQUES IN THE CARNARVON BASIN, AUSTRALIA

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560—APPEA JOURNAL 2002

P.J. van Ruth, R.R. Hillis and R.E. Swarbrick

where mud weights are unreliable indicators of porepressure (van Ruth et al, 2000). Few authors haveattempted to describe the distribution of overpressure inthe less permeable claystone sequences of the CarnarvonBasin (Vear, 1998; Tingate et al, 2001). Low permeabilitysediments strongly influence the movement of fluids insedimentary basins, and it is within low permeabilitysediments that overpressure is generated (Aplin et al,1995; Otto et al. 2001).

Overpressure is commonly associated withundercompacted (higher than normal porosity)sediments. In a constant lithology, zones of anomalousacoustic travel time may be interpreted as overpressuredwith respect to a normal compaction trend (Pennebaker,1968). The aim of this paper is to estimate the porepressure in the low permeability lithologies in theCarnarvon Basin using acoustic travel time from thesonic log in 38 wells (Fig. 1). This paper firstly outlines amethodology for performing quantitative pressureanalysis in the Carnarvon Basin and compares the Eaton(1972) and equivalent depth methods of estimating porepressure from velocity data with reference to a normal

compaction trend. Secondly, the distribution ofoverpressured shales in the wells analysed, as determinedby the Eaton (1972) method, is discussed and comparedto direct pressure measurements.

QUANTITATIVE PORE PRESSUREDETERMINATION FROM THE SONIC LOG

The origin of overpressure in the Carnarvon Basin isthought to be predominantly disequilibrium compaction,with minor contributions from hydrocarbon generationand horizontal stress (Vear, 1998; Swarbrick and Hillis,1999; Tingate et al, 2001). Sediments compact duringburial due to an increase in the mean effective stress(Goulty, 1998). If fluid loss from compacting sediments isimpeded, some or all of any additional increase in meanstress is borne by the fluid, retarding compaction andcausing overpressure. Thus, sediments that have becomeoverpressured via disequilibrium compaction have ahigher porosity than sediments at a similar burial depththat are subject to hydrostatic pore pressure. In thisstudy overpressure was evaluated in shale sequences

Figure 1. Well location map. The wells have been classified according to direct pressure measurements below the Muderong Shale. Wellsthat have overpressured direct pressure measurements (>11 MPa/km) are shaded red. Wells with no overpressure direct pressuremeasurements are shaded black.


Detecting overpressure using porosity-based techniques in the Carnarvon Basin, Australia

using acoustic travel time data by detectingundercompaction (abnormally high porosity) withreference to a normal compaction trend. Sediments thathave become overpressured from fluid expansionmechanisms (e.g. hydrocarbon generation, lateraltransfer) may not be associated with a significant porosityanomaly (Bowers, 1994; Teige et al, 1999). Hence, thetechniques presented in this paper under-estimateoverpressure generated by such mechanisms.

Quantitative pressure analysis using sonic logs is basedon determining the difference between an observedacoustic travel time and the acoustic travel time valuefor a normally compacted sediment. In this study theEaton (1972) method and the equivalent depth methodswere used for quantitative pressure estimation.

Sediments compact as the mean effective stress actingon the rock matrix increases (Goulty, 1998). However,mean effective stress is difficult to determine, mainlydue to uncertainty in the maximum horizontal stressmagnitude. Vertical effective stress, however, can becalculated from the density log and the lithostatic stressprofile does not vary significantly in the CarnarvonBasin. In this study vertical stress has been used as anapproximation of mean stress. Hence, the effects ofchanges in horizontal stress on the generation ofoverpressure have not been considered in this study.

Eaton Method

The Eaton (1972) method requires determination ofthe ratio of the normally compacted acoustic travel timeto the observed acoustic travel time in the zone ofinvestigation (Equation 1). The method is based on theprinciple that the relationship between this ratio andpore pressure depends on changes in the vertical stressgradient (Eaton, 1972; Mouchet and Mitchell, 1989).Pore pressure is calculated using the following equation:

(1)

Where;Pp = pore pressure (MPa);Phyd = hydrostatic pore pressure (MPa);σv = vertical stress (MPa);∆tnorm = normal compaction trend (µs/m), and;∆tobs = acoustic travel time (µs/m).

Equivalent depth method

The equivalent depth method relates pore pressure tothe difference in vertical stress between the observationpoint and the equivalent depth (Equation 2). Theequivalent depth is the depth at which the normallycompacted acoustic travel time is the same as that in thezone of investigation (Fig. 2; Mouchet and Mitchell,1989). Pore pressure is calculated using the followingequation:

Pp = Peq + (σv - σB) (2)

Where;Peq = hydrostatic pore pressure at the equivalentdepth (MPa), and;σB = vertical stress at the equivalent depth (MPa).

The accuracy of the Eaton (1972) method and theequivalent depth method is dependent on the extent towhich the acoustic travel time anomalies are related tooverpressure via changes in porosity. Hence it is importantto remove effects from the sonic log that are not relatedto changes in porosity.

Log editing/lithology filter

The sonic log measures acoustic travel time in aformation. Acoustic travel time in a formation isdependent on a number of variables such as hole conditionand lithology. In this analysis an attempt is made tonormalise the sonic log in order to make it dependentupon porosity alone. The most common source of noise inthe sonic log is cycle skipping (Rider, 1991). Cycle skippingis caused by poor borehole conditions and results in noisespikes (Rider, 1991). A despike filter was applied to thesonic log to remove noise associated with cycle skipping.Intervals with poor borehole conditions were identifiedusing the DRHO and caliper logs where available, andthen removed from the sonic log.

De

pth

Acoustic travel time

zeq

z

Norm

al C

om

pact

ion

Tre

nd

Figure 2. The equivalent depth is the depth at which the normallycompacted acoustic travel time is the same at that in the zone ofinvestigation. (Modified after Mouchet and Mitchell, 1989).



Acoustic travel time also varies with lithology. Thesonic and gamma ray logs were smoothed and resampledto minimise small-scale lithological effects associatedwith thin beds. Coals were removed by applying a soniclog cut-off filter of 330 µs/m. Only shales wereinvestigated. They were isolated for analysis using gammaray cut-off filters.

The Triassic to Lower Cretaceous sequence in thestudy wells consists of interbedded siliciclastic shalesand sandstones (Fig. 3). The Upper Cretaceous – Tertiarysequence is a mixed carbonate/siliciclastic sequence thatprogressively changes to pure carbonate. The sedimentsare dominantly carbonate in the Upper Cretaceous andtotally carbonate in the Oligocene (Young et al, 2001;Fig. 3). Both the carbonates and sandstones have lowgamma ray responses and thus a gamma ray filter isgenerally effective in differentiating them from shales.However, shales above the Muderong Shale havesignificant kaolinite (low K) clay content and exhibitrelatively low gamma ray values. Hence, in sedimentsabove the Muderong Shale a lower gamma ray cut-off wasrequired so that shales were not filtered out of theanalysis. The gamma ray filters for the Carnarvon Basinwere devised by comparing cuttings lithology with thegamma ray log. The gamma ray filters used to identifyshales in this study are:• Surface to Muderong Shale: GR>40 API, and;• Muderong Shale to total depth: GR>80 API.

Normal compaction trend for shale acoustictravel time

The normal compaction trend is the average evolutionof acoustic travel time with depth under hydrostatic porepressure conditions. An accurate normal compactiontrend is vital to determining overpressure, as overpressuremight be identified by deviations from the normalcompaction trend. Errors in the normal compaction trendeither mask or over-emphasise overpressure.

A normal compaction trend for shale acoustic traveltime in the Carnarvon Basin was established by fitting anexponential relationship to averaged acoustic travel timesfrom the 16 normally pressured wells analysed in thisstudy. Wells were selected as normally pressured basedon direct pressure measurements in sandstones whereavailable, and mud weights where no direct pressuremeasurements were available. The sonic logs, however,were edited to remove the sandstones where the directpressure measurements were undertaken. Therefore it isassumed that the low permeability lithologies in thenormally pressured wells are in pressure equilibriumwith the adjacent sandstones and can be used to establisha shale normal compaction trend. An absence of lateralfacies variation is also assumed within the units analysed.

The normal compaction trend established in this studyis based on the Athy (1930) relationship (Equation 3):

Figure 3. Simplified stratigraphy of the Carnarvon Basin (Modifiedafter Hocking, 1988; Woodside, 1988; Stagg and Cowell, 1994;Mildren 1997).

(3)

Where;φ = porosity;φo = initial porosity;C = compaction coefficient (m-1), and;z = depth (m).



To establish a normal compaction trend for the soniclog the Athy (1930) relationship was expressed in acoustictravel time using the time average equation suggested byWyllie et al (1956). The Wyllie et al (1956) time averageequation is:

(4)

Where;∆tma = matrix travel time;∆tf = fluid travel time, and;Cp = Wyllie correction factor.

By substituting Equation 4 into Equation 3 the followingrelationship between acoustic travel time and depth isobtained (Bulat and Stoker, 1987):

(5)

Values for Cp, ∆tma, ∆tf, φ0 and C were obtained byfitting Equation 5 to shale acoustic travel time valuesfrom the 16 normally pressured wells (Fig. 4).

The values obtained for the Carnarvon Basin are Cp =1.65, ∆tma = 225 µs/m, ∆tf = 620 µs/m, φ0 = 0.6 and C =0.00103. Substituting these values into Equation 5 yieldsEquation 6, the normal compaction trend for acoustictravel time in shales used in this study:

(6)

Determination of equivalent depth

The equivalent depth is the depth at which the normallycompacted acoustic travel time is the same as that in thezone of investigation. The equivalent depth relationship(Equation 7) was obtained by solving Equation 6 fordepth. Equivalent depth was then calculated fromacoustic travel time using the following equation:

(7)

Where;zeq = equivalent depth (m).

Determination of vertical stress

Mildren (1997) calculated vertical stress in 20 wells inthe Carnarvon Basin by integrating the density log. Avertical stress relationship was calculated (Equation 8)by fitting a regression line to these vertical stressestimates (Fig. 5).

(8)

Figure 4: Normal compaction trend for shale acoustic travel time.The normal compaction trend in the Carnarvon Basin was obtainedby fitting an exponential relationship to the edited sonic log datafrom 16 normally pressured wells.

Where;σv = applied vertical stress below sea floor (MPa).

The excellent fit of the regression line in Figure 5 tothe vertical stress estimates (R2 = 0.996) suggests thatthere is little variation in vertical stress across the 20wells. Hence the vertical stress relationship defined bythis regression line (Equation 8) is considered to be agood estimate of the vertical stress in wells where thereis either no density log, or the vertical stress has not beenpreviously calculated. There is a significant differencebetween the vertical stress relationship for the CarnarvonBasin presented in this paper and the 1 psi/ft (22.6 MPa/km) assumption that is commonly used (Mouchet andMitchell, 1989). The Carnarvon-specific relationship, forexample, yields a vertical stress 10% less at 1 km depth(20.4 MPa as opposed to 22.6 MPa).



COMPARISON OF PORE PRESSUREPREDICTED BY THE EATON (1972) AND

EQUIVALENT DEPTH METHODS

The Eaton (1972) and equivalent depth methods yieldsimilar pore pressure estimates in all wells. An exampleof this is shown in Figure 6. The Eaton (1972) methodestimates are more scattered than the pressure estimatesfrom the equivalent depth method. This scatter isinterpreted as residual noise in the edited sonic lograther than actual pressure variations. Therefore theequivalent depth method is considered to be lesssusceptible to noise than the Eaton (1972) method.However, the relationship used to calculate equivalentdepth (Equation 7) is only defined for acoustic traveltime values between 225 and 616 µs/m. Hence, theequivalent depth method can only be used where theacoustic travel time data fall inside this interval.Additionally, the equivalent depth method exaggeratesdeviations towards underpressure below 2,500 m. Due tothese limitations we have chosen to use the pore pressureestimates calculated using the Eaton (1972) methodwhen discussing the distribution of pore pressure in theCarnarvon Basin.

DISTRIBUTION OF PORE PRESSURE IN THECARNARVON BASIN

The Carnarvon Basin has been separated into threestratigraphic groupings based on lithology and the amountof pressure information available from direct pressuremeasurements. These intervals are:• Upper Cretaceous to Recent (Toolonga Calcilutite to

Recent);• Lower to Upper Cretaceous (Gearle Siltstone/

Muderong Shale), and;• Total Depth to Lower Cretaceous (Total Depth to Base

Muderong Shale).The Eaton (1972) pore pressure estimates from eight

wells chosen to represent the different pore pressure regimesin the Carnarvon Basin are shown in Figure 7. Directpressure measurements (DST, WFIT) and mud weights arealso shown for comparison. When comparing direct pressuremeasurements in the sandstones with the pressure estimatesin the shales it is necessary to assume that the shales are inpressure equilibrium with the adjacent sandstones. This isnot always the case, however. The extent to which thisassumption is true in the Carnarvon Basin, and potentialsources of discrepancies between sandstone and shalepressure estimates, are discussed below.

Upper Cretaceous to Recent (ToolongaCalcilutite to Recent).

The interval above the Top Gearle Siltstone containsno direct pressure measurements, and is usually drilledwith low mud weights. Kicks were taken, however, in theToolonga Calcilutite below 2800 mbsl in Fisher–1,Haycock–1 and West Dixon–1 (Tingate et al, 2001). Thehigh permeability sediments in this interval are usuallynormally pressured, with the exception of theaforementioned kicks. Only Wells D, E, H and G containedited sonic log data in this interval. The pressureestimates are normally pressured in all these wells withthe exception of Well H (Fig. 7). The pressure estimatesin this interval in Well H range from overpressuredimmediately above the Gearle Siltstone tounderpressured around 2000 mbsf. The depth of theoverpressured interval estimated in Well H correlateswith the kicks observed in this interval in other wells.However, the estimated pressure anomaly may not bereflecting actual changes in pore pressure. As a gammaray filter of 40 API was used in this interval, it is possiblethat the edited sonic log values represent mixed lithologysediments. Hence, the non-hydrostatic pressure estimatesin this interval may reflect changes in lithology ratherthan actual changes in pore pressure.

Lower to Upper Cretaceous (Gearle Siltstone/Muderong Shale).

This interval comprises the thick shale sequences ofthe Gearle Siltstone and the Muderong Shale (Fig. 7).

Figure 5. Vertical stress relationship. The vertical stress relationshipwas established by fitting a power relationship to the vertical stressestimates of Mildren(1997). The vertical stress estimates werecorrected for water depth.



There are no direct pressure measurements in thisinterval. Kicks have been recorded, however, in theMuderong Shale in Venture–1 and West Tryal Rocks–1(Tingate et al, 2001). Mud weights are usually raisedwhen drilling this interval to combat drilling problems.Normal pressures are estimated in Wells A, C, D and E.In Wells F, G and H, pressure estimates indicate highoverpressure, even in excess of the mud weight usedwhile drilling the formations (Fig. 7). This suggests thateither the pressure estimates are overestimating thepore pressure, or that the Muderong Shale and GearleSiltstone were drilled under balanced in these wells.

Total depth to Lower Cretaceous (total depth tobase Muderong Shale)

There are many direct pore pressure measurements inthe interval from the Base Muderong Shale to totaldepth. The sediments immediately below the MuderongShale are generally normally pressured with the exceptionof Well H. An upper normally pressured compartment,underlain by an overpressured compartment wasencountered in this interval in Wells A,B and F. In theMungaroo Formation in Well G the WFIT measurementsform a gradient steeper than hydrostatic gradient defininga pressure transition zone. This implies that the intervalswhere the measurements were undertaken are not inpressure communication, and are therefore

compartmentalised. There are several hydraulicallyisolated gas gradients in Well H giving further evidenceof pressure compartmentalisation. The pressure estimatescorrelate with normally pressured direct pressuremeasurements in Wells A, E and H, and withoverpressured direct pressure measurements in Wells Gand H (Fig. 7). The pressure estimates are up to 13 MPalower than overpressured direct pore pressuremeasurements in Wells A, B, F and G. Thisunderestimation of pore pressure is consistent withoverpressure generated (at least partly) by fluidexpansion or chemical processes which reduce porosity(Bowers, 1994; Teige et al, 1999). In Well C, the pressureestimates are underpressured, while the direct pressuremeasurements are normally pressured.

ACCURACY OF POROSITY-BASED POREPRESSURE PREDICTIONS

The Eaton (1972) pressure estimates presented in thispaper correlate with the pressure estimates from directpressure measurements in adjacent sandstones in somewells, and are different in others. The difference betweenshale estimates and sandstone measurements suggestseither that the pressure estimates are inaccurate, or thatthe sandstones and shales in these wells are not inpressure equilibrium. Several potential explanations forthe differences between the pressure estimates and valuesfrom direct pressure measurements are outlined below.

If the pressure estimates are indeed correct thenobserved differences in pressure between shales andadjacent sandstones indicate that these shales andsandstones are not in pressure equilibrium. Such apressure regime can occur when there is lateral transferof pore pressure in permeable units that is not reflectedin adjacent shales, or when the generation of abnormalpressure is isolated within certain lithologies (e.g. thegeneration of hydrocarbons in source rocks). Well G is apotential example of lateral transfer: in the upperMungaroo Formation in Well G the RFT pressuremeasurements indicate near normal pressures whereasthe pressure estimates indicate approximately 13 MPaof overpressure (Fig. 7). The RFT pressure measurementsdefine a transition zone in the middle MungarooFormation with moderate overpressure at the base of theMungaroo Formation. A possible explanation for thedifference in pore pressure estimates in the upperMungaroo Formation is that the permeable units in thisinterval have good lateral connectivity allowing excesspressure to bleed off. Lateral transfer may also act toincrease pore pressure in permeable units relative to thenon-permeable strata where the rocks become pressuredfrom adjacent highly overpressured sediments. This couldbe validated by investigating the lateral distribution ofhigh-resolution seismic interval velocities estimated fromtomographic inversion.

The presence of any exhumed sediments (sedimentsnot at their maximum burial depth) in the CarnarvonBasin is another potential source of error in the Eaton

0

500

1000

1500

2000

2500

3000

3500

0 5 10 15 20 25 30 35

pressure gradient (MPa/km)

depth

(mbsf)

Hydrostatic Pressure

Mud Weight

DST

RFT

Equivalent depth pressure estimates

Eaton (1972) pressure estimates

Figure 6. Eaton (1972) pressure estimate gradients versusEquivalent depth pressure estimate gradients. Direct pressuremeasurements, hydrostatic gradient and mud weights are alsoshown for comparison.



Figure 7. Pressure profiles established using the Eaton method. Direct pressure measurements, hydrostatic gradient and mud weights arealso shown for comparison. DST measurements were only used where pore pressures have been extrapolated.



(1972) method. Log-based overpressure analysis inexhumed basins is complicated because overpressureand exhumation each affect porosity / depth relationshipsin a different manner. Overpressure is commonlyassociated with under-compacted (higher than normalporosity) sediments. Exhumation results, however, insediments being over-compacted with respect to theircurrent burial depth. Densley et al, (2000) estimated thatup to approximately 900 m of exhumation has occurredin the Carnarvon Basin since the Late Cretaceous. Thisestimate is inferred from sonic anomalies in the MuderongShale, Gearle Siltstone/Haycock Marl and the ToolongaCalcilutite. Kaiko and Tingate (1996) concluded fromtemperature data, however, that the majority of wells inthe Carnarvon Basin are currently at maximumtemperature (and hence maximum burial depth), withonly the wells along the eastern margin of the basin beingexhumed. The section below the Muderong Shale in WellC may be overcompacted, possibly accounting for aninterval where the pressure estimates are underpressuredwhile the direct pressure measurements are normallypressured (Fig. 7).

Changes in shale mineralogy, in particular in thesmectite content, can cause significant changes in theacoustic properties of shales (Tingate et al, 2001). Thegamma ray log lithology filters used in this study do notdifferentiate changes in shale mineralogy (with theexception of kaolinite). The pressure estimates suggestthat the Gearle Siltstone and Muderong Shale areoverpressured in Wells F, G and H. However, the onlyactual evidence of overpressure in the Muderong Shaleand Gearle Siltstone are kicks taken in Venture 1 andWest Tryal Rocks–1 (Tingate et al, 2001). Hence, it isunclear whether the sonic log anomaly witnessed inWells F, G and H is related to overpressure viaundercompaction, or changes in shale mineralogy. Thesonic log response in the Muderong Shale and GearleSiltstone correlates with smectite content (Tingate et al.2001; Dewhurst et al, this volume). The pressure anomaliesin Wells F, G and H, however, correspond to kicks takenin adjacent wells suggesting that the Muderong Shale isindeed overpressured in this region. Additionally, themagnitude of the pressure anomaly in the MuderongShale and Gearle Siltstone increases with increasingdepth to top Gearle Siltstone, i.e. the largest sonic loganomalies in the Muderong Shale and Gearle Siltstoneare overlain by the thickest sequence of sediments andhence, have been subjected to the largest cumulativeburial. This observation is consistent with the existenceof overpressure due to rapid burial-related disequilibriumcompaction in the Muderong Shale and Gearle Siltstone.Moreover, the sonic log anomaly would be expected todecrease with depth if smectite content was the primarycontrol on the sonic log response. This will occur assmectite content decreases with burial (increasing stressand temperature) when smectite transforms to illite.Nonetheless, it is likely that the sonic log anomaly in theMuderong Shale and the Gearle Siltstone is related toboth overpressure and shale mineralogy. More

sophisticated analysis might involve detailed lithologicaldetermination of clay mineralogy types and the creationof normal compaction relationships for shales withdifferent clay mineral composition.

CONCLUSIONS

1 The normal compaction trend for acoustic traveltime in shales in the Carnarvon Basin, established byfitting an exponential relationship to sonic log datafrom 16 normally pressured wells is:

2 The vertical stress relationship for the CarnarvonBasin was defined by fitting a regression line throughvertical stress estimates in 20 wells, and is:

This vertical stress relationship for the CarnarvonBasin is preferable to the 1 psi/ft gradient, which isoften used.

3 Pressure estimates using the Eaton (1972) method inshales correlate with direct pressure measurementsin adjacent sandstones for normally pressuredintervals in three wells and overpressured intervalsin two wells. Pressure estimates, however,underestimate overpressure from direct pressuremeasurements in four wells by up to 13 MPa. This isconsistent with overpressure generated (at least inpart) by a fluid expansion mechanism, or lateraltransfer of overpressure. The pressure estimates inone well are on average of 11 MPa lower than thehydrostatic pore pressures recorded in direct pressuremeasurements below the Muderong Shale. Thesediments in this well appear overcompacted due toexhumation.

4 Pressure estimates match mud weight in the GearleSiltstone and Muderong Shale in four of the eightwells studied. The pressure estimates are on average10 MPa in excess of mud weight in the MuderongShale and Gearle Siltstone in Wells F, G and H. It isunclear whether the pressure estimates in thesethree wells accurately reflect pore pressure (ie. themud weights do not accurately reflect pore pressure),or whether they are influenced by changes in shalemineralogy (because the gamma ray filter does notdifferentiate between shale mineralogy). Severalkicks have been recorded in adjacent wells withinthe Muderong Shale and Gearle Siltstone, and thisinterval is overlain by significant sediment thicknessin these three wells. These observations are consistentwith the existence of overpressure due to rapid burial-related disequilibrium compaction in the MuderongShale and Gearle Siltstone.



ACKNOWLEDGEMENTS

The Australian Society of Exploration Geophysicistsis thanked for providing funding support for this project.Part of the original data upon which this study is based isowned by IHS Energy and forms part of the IH/E NWS ofAustralia pore pressure database. This manuscript hasgreatly benefitted from the advice of Peter Tingate of theNCPGG on overpressure in the Carnarvon Basin, andDave Dewhurst of CSIRO on shale properties. The authorswould also like to thank Scott Mildren of the NCPGG forproviding vertical stress estimates. Satyavan Reymondof Schlumberger and Kevin Dodds of the APCRC arethanked for their thoughtful review comments.

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SWARBRICK, R.E., AND HILLIS, R.R., 1999—The Originand influence of overpressure with reference to theNorth West Shelf, Australia. APPEA Journal 39 (1),64–72.

TEIGE, G.M.G., HERMANRUD L., WENSAAS, L., ANDNORDGARD BOLAS, H.M., 1999—The lack ofrelationship between overpressure and porosity in NorthSea and Haltenbanken shales. Journal of PetroleumTechnology, 16, 321–35.

TINGATE, P.R., KHAKSAR, A., VAN RUTH, P.,DEWHURST, D., RAVEN M., YOUNG, H., HILLIS, R.,AND DODDS, K., 2001—Geological Controls onOverpressure in the Northern Carnarvon Basin. APPEAJourna1, 41 (1), 573–93.

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Peter van Ruth completedhis BSc (Hons) from theUniversity of Adelaide in 1998and commenced studying forhis PhD at the NCPGG in1999. His projectconcentrates on over-pressure in two main studyareas, the Cooper Basin andthe North West Shelf ofAustralia. Member: AAPG,

ASEG, PESA, EAGE and SEG.

Richard Hillis holds theState of South Australia Chairin Petroleum ReservoirProperties/Petrophysics atthe NCPGG. He graduatedBSc (Hons) from ImperialCollege (London, 1985), andPhD from the University ofEdinburgh (1989). Afterseven years at AdelaideUniversity’s Department of

Geology and Geophysics, Richard joined the NCPGG in1999. His main research interests are in petroleumgeomechanics and sedimentary basin tectonics. Member:AAPG, AGU, ASEG, EAGE, GSA, GSL, PESA and SEG.

Richard Swarbrick com-pleted his PhD at CambridgeUniversity in 1979 and spent10 years with Mobil OilCompany on UK and USassignments. In 1989 hejoined the academic staff atDurham University and isnow Reader in PetroleumGeology, with researchinterests in overpressure,

fluid flow and sediment compaction. He has been principalinvestigator for the Geosciences Project into OverPressure(GeoPOP) at Durham, and has published widely onoverpressure in petroleum systems. He is collaboratingwith the NPCGG on an ARC-funded project onoverpressure and contemporary stress. He is an AssistantEditor of Geofluids, Associate Editor for the Bulletin ofAAPG and Fellow of the Geological Society of London.Member: AAPG, PESGB and EAGE.

THE AUTHORS

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detecting overpressure using porosity … · estimate pore pressure in shales in the carnarvon...

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