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THE USE OF ADVANCED SEISMIC TECHNIQUES TO STUDYCARBONATE RESERVOIRS
G.T.F.R. Maureau* and D.H. van Wijhe,Nederlandse Aardolie Maatschappij B .V. Schepersmaat2 Assen Netherlands; and F. R. van Veen, c o Shell Internationale Petroleum Maatschappij P .O .Box 162 The Hague Netherlands
Abstract. The geometry of carbo nate reservoirs is controlled by the original environment of deposition, changes resultingfrom diagenetic processes and tectonic disturbances. U nde r very favourable circumstances it may be possible to predictthe lateral extent of such reservoirs, but often the overprinting of diagenetic processes renders this very difficult. Rapidlateral changes in reservoir characteristics are common and generally no detailed geological model is available, certainlynot in th e early stages of field development. Recent advances in seismic technology, however, may provide th e reservoirengineer with new techniques for predicting the lateral extent of the reservoir, its quality porosity), thickness and even,perhaps, porefill. In this paper we describe a technique that was used to predict in a qualitative fashion the lateralextension and reservoir quality porosity) in a gas field in the Netherlands. T his was a multi-disciplinary study combininggeological, geophysical an d petrophysical know-how.Rsom. Dune manire gnrale la gometrie tri-dimensionnelle des rservoirs carbonats est le rsultat de lacombinaison: du milieu de dpt originel; de la diagnse; et de la tectonique. Dans certains cas particulirementfavorables, il peut tre possible de prvoir lextension latrale d e ces rservoirs, toutefois linfluence de la d iagnse estgnralement si im porta nte quune telle prvision est en fait trs difficile. Les variations latrales des caractristques d eces rservoirs sont souvent rapides, et dan s la plupart des cas il ny pas de modles geologiques dtailles auxquels il soitpossible de se reporter, du moins pendant les premires phases du dvelopement dun nouveau champ. De rcentsprogrs dan s le domaine de linterprtation sismique permettent toutefois ingnieur de rservoir de prdire lextensionlatrale dun rservoir, ses variations de porosit et dpaisseur et mme dan s certains cas favorables, le remplissage despores. Dans cet article les auteurs dcrivent une technique qui a t utilise, pour la prvision qualitative de lextensionlatrale et de la porosit des rservoirs carbonats dun c ham p d e gaz des Pays Bas. Cette technique st le rsultat dunetude pluridisciplinaire faisant appel aux connaissances spcialises dune quipe comprenant des gologues, desgophysiciens et des ptrophysiciens.
1 INTRODUCTIONReservoir engineers familiar with the appraisal and
development of carbonate reservoirs are aware of thepossible complexities in the geometry of these fields.Although the porosity distribution may have beenrelatively regular and predictable in the original environ-ment of deposition, later diagenetic and tectonic proc-esses may so distort this geometry that geologic consid-erations alone cannot be used in the siting of appraisaland development wells. For these reasons the use ofseismic data for optimizing the siting of appraisal anddevelopment wells has been considered.In the past decade considerable effort has beenexpended by the geophysical industry to predict facies,porosity and/or porefill using seismic amplitude measure-ments, inverted seismograms or pseudo-velocity logs
Now with Petro-Canada Calgary Canada
and stratigraphic modelling., 3 6 7 l 2 In general,these studies involved clastic or simple carbonate se-quences where the effectsof rock bulk density or seismicreflection data were usually neglected.
The carbonate reservoir in which we performed theprediction of the optimum appraisal well location wasthe Zechstein 2 Carbonate, a member of the PermianZechstein evaporite sequence. We were aware that inevaporite sequences the effects of rock bulk density couldnot be neglected and soour study concentrated on the useof acoustic impedance logs combined with acousticimpedance seismic sections. Acoustic impedance logs arecreated by simply multiplying velocity from appropri-ately depth-matched bore-hole-compensated BHC)sonic logs with compensated formation density FDC)
Acoustic impedance seismic sections are createdby the numerical integration of standard seismic reflec-tion sections.In the sections to follow we describe how acousticimpedance logs were obtained and subsequently filtered
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DEVELOPMENTS I N RESERVOIR EN GIN R ING PO l O 1 )08The second question has been dealt with in the sectionon the geological model. There we noted that the mainarea of interest was the platform/upper slope environ-ment and that diagenesis, although unpredictable, gen-erally increased rather than decreased porosity. In thegeophysical modelling we attempted to translate ageological situation into the analogue geophysicalexpres-sion. We began, however, by first determining whatparameter(s) is (are) most important in causing theseismic response. By studying both well log and core datafrom wells drilled in the various environments ofdeposition, we concluded that porosity was perhaps thesingle most important parameter which controlled theseismic response. In Fig. 4 we see two plots involvingdata from Well-1, the key well in the study. The first plot,Fig.4 (top), is a plot of core porosity versus sonic transit
WELL-CORE POROSITY
VSSONIC TRANSIT TIMES
i
time, where it is clear that porosity strongly affects thesonic velocity. Furthermore, we noted that the Wyllietime average equation14 fitted the data set very well,certainly within the standard deviation limits. Figure 4(bottom) shows a plot of reflection coefficient versusporosity for water saturations S , ) o loo ,50 and 0 .The reflection coefficient is that of anhydrite versuscarbonate, recalling, of course, that the 2 2 Carbonate isboth overlain and underlain by anhydrite. It is clear thatthe effectof porefill (gas versus water) is relatively small;in fact, because of the depth of the reservoir, 14 mwe know from the Gassman-Biot thatporefill has little or no effect on seismic velocity. Wefurther note from Fig. 4 (bottom) that the cut-off porosityfor seismic recognition is about 5 .Having established that porosity was apparently the
100000 PPM NGCL
LINEAR REGRESSION
SONIC TRANSIT TIME KSEC /FTy:
I35 30 - 2 5 20 -I5 10 -5 O t5 10
REFLECTION COEFFICIENT 1 )
Fig. top) Core porosity versus son ic transit time in well 1 Note that porositystrongly affects velocity. Wyllies equation fits data very w ell for h igh-salinity fluids.bottom) R eflection coefficient of the Carbonate-Anhydrite interfac e as a funct ionof porosity and water saturation.
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PD l O 1 ) DEVELOPMENTS IN RESERVOIR ENG1NE ERI NG 209dominant factor in determining the acoustic impedancecontrast between anhydrite and carbonates, we launchedinto a full-scale modelling exercise. The seismic acousticresponse of some 12 different models was made. Eachmodel represented a specific environment of deposition,and a broad range of different physical parametersvelocity, density and thicknesses) was attributed to each
of the units in the models. After construction of thesemodel acoustic impedance logs, the logs were filtered byuse of zero-phase band-pass filters in the seismic band-width and the response was analysed. The conclusions ofthis study were that reservoir thickness and porositydistribution determined whether or not the 22 Carbonatecould be resolved from anhydrite by use of seismic data.
For example, a single thin bed of carbonate some 15 mthick and with a 15 porosity produces a clear seismicresponse. A series of thin (5 m), high-porosity (15 )carbonate beds interspersed in tight carbonate alsoproduces a clear seismic response.
Perhaps the sum total of our modelling experience isbest shown by means of Fig. 5. The model, Fig. 5 top),shows the physical details of the porosity distribution ofthe 22 Carbonate-note that the normal Zechsteinevaporate sequence, although not shown in detail,
overlies the 2 2 Carbonate. It is underlain by the Z1Anhydrite. This model, which we called the patchydevelopment, is a simulation of the porosity distributionas one might expect it on the platform/upper slopeenvironment or in turbidites. We note a pod of high-porosity carbonate encased in much tighter carbonatewith thin, high-porosity beds extending into the tightzones.
Figure 5 bottom), shown at the same horizontal scale,i s he geophysical realization of the model, here displayedas a synthetic acoustic impedance section. The modelwas filtered using a typical seismic band-pass filter.Clearly the highly porous carbonate in the centreproduces a strong seismic response. At the left-hand sideof the section, in the tighter zone with porosity of 5 , theresponse, although visible, is much weaker. On the right-hand side, in the tightest zone porosity 373 thecarbonate was indistinguishable from the anhydriteabove and below, the response of total anhydrite/carbonate body being a strong relatively low-frequencysignal. Note that the thin, high-porosity beds extendinginto the tight material on the right-hand side produceda noticeable seismic response.
This particular model was extremely useful in our laterMODEL
SALT 5
BESAL ANHYDRITE 5
v 3vvv
Z 1 ANHYDRITE
ACOUSTIC IMPEDANCE SECTION FIL TER 5-10-45-68
22
O 5 MFig. top) Typical geological model showing physical details of the 22 Carbonate in the platform-upper slope environment.bottom) Seismic acoustic impedance representationof above model. Note the changes in characterof the Carbonate event
s a function of porosity and porosity distribution.
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PO l O 1 )1 EVELOPMENTS I N RESERVOIR ENGINEERINGprediction because it provided a qualitative basis forinterpreting observed seismic anomalies. On Fig. 3, forexample, a number of the seismic expressions areobserved that are visible on the model shown in Fig.(bot om)
Well log studiesTo round o he feasibility phase we studied a suite of
acoustic impedance logs derived from actual downholemeasurements. These data, representative of the mainenvironments of deposition, were also filtered to simulateseismic acoustic impedance data and studied to deter-mine seismic resolution. We could, in this fashion, seewhat the detrimental effects of geological noise were onthe seismic resolution. Although no examples are shownhere, our conclusions were essentially hose as determinedin the geophysical modelling phase, namely that thecarbonate reservoir thickness and porosity distributionwere the main contributors to the seismic response. Wecan summarize the results of the feasibility study bysaying:(1) It is important to establish whether or not the
reservoir can be observed on seismic data and lateralchanges associated with i t. This is done by appropri-ately matching filtered acoustic impedance well logswith neighbouring seismic acoustic impedance data.
(2) From geological/geophysicalmodelling and well datastudies one determines those basic parameters af-fecting seismic response of the reservoir. Theseparameters include reservoir thickness, internal po-rosity distribution, porefill and surrounding lithology,to name the most important.
Having established the basic criteria for reservoirporosity/porefill prediction, one can then tackle actualseismic data using all the experience learnt from thefeasibility phase. In our case we felt we had sufficientencouragement to continue into the prediction phase.
Phase II Prediction and verificationAs Wells 1 and 2 were connected with a good-qualityseismic line, this seemed to be an ideal location to make
a first attempt to predict reservoir porosity. We recallthat in Well 1 we had encountered some 58m of thismember developed predominantly as an olitic carbonatewith an average porosity of 12 .Although the seismic processing sequence of this lineis both interesting and important, suffice it to say thathigh-fidelity processing was performed followed by
wavelet derivation and phase zeroing, wave equationmigration and finally integration to obtain an acousticimpedance seismic section. Recalling Figure 3, whichshows a portion of the line around the W-1 location, wenote in general the correspondence between the well andseismic acoustic impedance data is good. Although the
2 2 Carbonate is generally easy to recognize, on theseismic section its signal is discontinuous, which is inagreement with the patchynature we had expected fromthe modelling studies.
Using the experience gained from the modellingexperiments and the interpretation in the W-1 area, wemoved our attention to W-2. We compared the BasalZechstein portion of the primary synthetic seismogramof W-1 with the seismic data in the neighbourhood of theW-2 location. We concluded that W-2 should be deviatedto the west to penetrate the best reservoir development(Fig. 6).
WaL 2 CURFACE
O ZOOM
Fig. 6. Migrated zero-phase reflection section around desireddownhole target area with the filtered synthetic o wellinserted. Note again relatively goo match o well and seismicdata and clarity and discontinuityo Carbonate event.
Needless to say, we were more than delighted whencore and log data confirmedourprediction. Olite streaksand abundant bioclastic material found in the coreconfirmed the shallow marine environment (Fig. 7 . Anaverage porosity of 6.5 with streaks of 14 or more wasdetermined from logs.
4. CONCLUSIONSThe combination of well and seismic acoustic imped-ance data has allowed us to successfully predict theZechstein 2 Carbonate reserloir porosity. Such tech-
niques must be applied with caution, however. Instructurally complex areas long-range facies/porosityprediction is fraught with traps for the unwary. To ensureproper identification of the important lithologies, onemust combine a knowledge of the gross lithological andstructural setting, acoustic impedance models and welllogs when interpreting seismic acoustic sections.
Feasibility experiments combining both model andactual downhole da ta were necessary to properly definethose factors important to the determination of porosityin the Carbonate. In general, we found that thismethodology provided a logical and useful frameworkfor assessing the possibility of performing porosityprediction.
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P l O ( 1 ) DEVELOPMENTS IN RESERVOIR ENGINEERINGWELL 1 WELL-
K MGR S L3a GR S L
A Al A nf A
21 1
1 0 0 M
- 5 0 M
0R E D INTERV L Ioo =.D1 S H W MARINE HIGH ENERGY
Fig. 7 . Comparison of ga mm a ray, sonic logs and core dat a of wells 1 and 2 showing similarity of environm entsof deposition.From the above it is clear that close co-operationbetween geologist geophysicist and petrophysicist is
essential to the success of such studies. Throughout theentire project it was our experience that the boundariesbetween their separate technical responsibilities becamequite vague.
4. Gevers, E.C.A. and W atson, S.W. A quantitative analysisof seismic data using well logs. SPE Paper 7439 AnnualConference, Houston 1978.5. G a s m a n , F. Ue ber die Elastizitt porser Medien. Vjschr.Natu . Ges . Zrich, 96 1-23.6. Lavergne, M. and Willm, C. Inversion of seismograms andpseudo-velo city logs. Geophys. Prospect., 1977 25 231-250.7. Lindseth, R.O. Seislog process uses seismic reflectiontraces. Oil Gas J. , 1976 Oct. 25 67-71.8 . Lindsey, J.P., Schram m, Ir. M.W. and Nemeth, L.K. Newseismic technology can guide field development. Wld . Oi l ,cknowledgements
The authors are indebted to the Shell InternationalePetroleum Maatschappij the Nederlandse AardolieMaatschappij Esso Europe Inc. and DSM-AardgasB.V. for granting permission to publish the contents ofthis paper.
REFERENCES1 . Backus, M.M. and Chen, R.L. Flat spot exploration.
Geophys. Prospect., 1975 23 533-577.2. Biot, M.A. Gen eralised theory of acoustic propagation inporous dissipative media. J. Acoust. Soc. A m . , 1962 34,12541264 .3. Dedman, E.V., Lindsey, J.P. and Schramm Ir. M.W.Stratigraphic modelling: A step beyond the bright spot.W l d . Oil, May 1975 61-65.
June 1976 59453.9 . Richter-Bernburg, G . Zur Palogeographie des Zechsteins.In : 1 giacimenti gassiferi dell Europa occidentale Roma,Accademia N azionale Dei Lincei, Vol. 1 pp 87-99.10. Sannem ann, D., Zimdars, J . and Plein, E. Th e BasalZechstein between the Rivers Ems and Weser, NWGermany. Z. dt. geol. Ges., 1978 1291 1 . Sheriff, R.E. Factor s affecting seismic amplitudes. Geophys.Prospect., 1975 23 125-138.12. Sheriff, R.E. Inferring stratigraphy from seismic data. Bull.Am . Ass. Petrol. Geol. 1976 60 No. 4 528-542.13. Taylor, J.C.M. and Colter, V.S. Zechstein of the Englishsector of the Southern North Sea Basin. In: Woodland,
A.W. (Editor), Petroleum and the Continental Shelf of North-West Europe, Vol. I , Geology.14. Wyllie, M.R.J., Gregory, A.R. and Gardner, G.H.F. Anexperimental investigation of factors affecting elastic wavevelocities in porous m edia: Geophysics, 1958 23 459-493.15. Ziegler, P.A. Geology and hydrocarbon provinces of theNorth Sea: G eo . J . 1977 l 1) . 7-32.
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