Attribute Extraction

8/3/2019 Attribute Extraction

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Attribute extraction:An important application in any detailed30D interpretation study

E. .I. H. RIJKS and J. C. E.M. JAUFFREDShell Internationale Petroleum Maatschappij B. V.llae Hague

The Netherlands

T -.ree dimensionalseism ic s playing an increasingly mportantrole in appraisal nd development lanning. Whereas ,at first, 3 Ddatasetswere aimedat analyzing he structural tyle and he mapping of complex ields, more recently, 3-D seismicha sproved tsvalue n defining detailsat reservoir evel. P orecontent, eservoirfaciesand very small faults which may impede luid m ovementcan often be mapp ed onfidentlywith 3-D seismic.The developmentn 3-D seismicmaging s seenby She lls topmanagement s the single most mportantbreakthroughn tech-nology n many years. Hence in Shell, the role of 3-D seismic smassiveand still increasing n explorationand production se eE.O . Nea tvold, lhe use of 3-D seismic n exploration, appraisalandfielddevelopment, ProceedingsTwel&h WorM Petroleum Con-gress,1987;and77re seof 3-D seismic n explorationandproduc-tion, Energy IndustriesCouncil-Oil, Gas & Petrochemical emi-nar, Baghdad, raq, O ctober 14-16, 1989). In 1990 more than16 000 km* of 3-D seismicwas acquiredby S hell outsideNorthAmerica, exceeding he 1989 total of 10 880 ti by nearly 50percent Figure 1). Many of these urveyswere acquiredon land,in relatively easydesert ermins, in the complexurbansettings fmajor cities and industrial area s n Europ e, and in jungles andswampsn Africa and the Far Ea st.

The importance f the use of horizon attributedisplays n thedetectionof subtle structuraland stratigraphiceatureshas beendemonstrated any imeswithin the Shell Group seeApplicationsof 3-D seismic to &tailed reservoir delineation,by R. Buchananet al., Societyof PetroleumEngineers,SPE 17561-5, 1988; andi%e use of dip and azimuth displays or 3-D seismic intelpreta-tion. by G.J.H. Schoelcher t al., ProceedingsNPS GeophysicalConference, Kristiansand, Norway, March 5-7, 1990). Becauseof the improvement n seismic esolutionand the spec tacularn-formationobtained,attributestudies re consideredndispensibleand are now routinely used n all Shell 3-D interpretations. hispaperdemonstrates, ith a numberof examples, he crucial roleplayedby attributeanalysis n defining subsu rfaceargets.

I nterpretationstrategy. In order to extract attributes n anoptimalway, every singleseismic race n the datase tmustbe in-terpreted.The interpretation pproach sed s called he horizon-oriented pproa ch, nd s summarizedn Figure 2. Once a horizonha sbeenchosen, t is interactively nterpreted n the workstationalong a grid of selectedines. The number of lines forming thisgrid (usuallycalled he control grid) depends n the quality of theseismicdata and on the stratigraphic nd structural ettingof thearea. The controlgrid is then usedas input to an automatic rack-ing programwhich extends he interpretation f the horizon overthe entire data set so that every CD P is picke d.Batchautom atic racking, as it is called (Figure 2), limits theamount of actual interactive nterpretation,but also ensures hatth epicksare phase onsistentrom ine to line in a way not feasiblewith conventional nterpretationon a line to line basis. Seve raliterations re often required. The results,which consistof a gridof two-way ime values or every point of the nterpreted ataset,must be carefully inspected nd edited. Thesevaluesare used ocalculate , n turn, the dip and the azimuthof the horizon at everypoint of the dataset.Dip and azimuthmay be displayed eparatelyor in combination nd he timevalues hem selves reused o createso-called haded elief displays.Additionally, time valuesar eusedas reference or the extractionof other importantattributes, ikereflectionamplitude.Theseare the attributeswhich are discussedin this pape r.Gooddataquality s a prerequisiteor reliableautotracking ndsubseq uent ttributeextraction.Considerable ttentionhas to begiven to the color coding of dip, azimuth, and other attributevalues, and dedicated olor schem es a ve been found to be criti-cal for displayingattributes n an optimal way.D- pnd azimuth. R.M. Dalley et al, of Shell have describedthe basic mathematics ehind dip and azimuth displays or 3-Dseismic nterpretation seeDip and azimuth displays or 3-D seis-mic interpretation, First Break, July 1989 ). The principle of dip

GEOPHYSICS:THE LEADING EDGE OF EXPLORATION SEPTEMBER1991 11


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w MARINE 16600 LAND 16,100 KM

Figure 1. Three-dimensional seismicacquisition n Shell (out-side North America).

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/ D I P = LO C AL t im e G R AD IE N TFigure 3. Principle of dip and azimuth calculation.

1 INTERPRETATION STRATEGY 1

1 PICK HORIZON ON SELECTE D GRID OF LINES 1+I BATCH AUTOW TIC PICKING(EVERY TRACE INTERPRETED) I

1HORIZON AlTRIBUTE PROCESSING: 1

l DIPl AZIMUTHl COMBINED DIP/AZIMUTHl SHADED RELIEFl AMPLITUDE

Figure 2. Horizon-oriented interpretation strategy.

Figure 4. Tie map.12 GEOPHYSICS: THE LEADING EDGE OF EXPLORATION SEPTEMBER 1991


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andazimuthcalculationss simple Figure 3). The dip and azimuthparameters re. espectively he magnitude nd the direction, roma local reference,of the time gradientvector, calculated t eachsampleof the interpretedhorizon. The computa tions re carriedout by fitting a plane h roughadjacentdatapointsand posting hecalculated alues t thecentraldatapoint (Figure 3). The dip valuecan be expresse dn degreesor radians, or more com monly, inmilliseconds er meter.Dip and azimuth values are normally displayedon separatemaps.Thesemapsshould e studiedndependently, ecauseaults,which affect the mappe dhorizon, do not necessarily how upequally clearly.This is illustrated y the first exam ple.Figure4 shows he imemap of a horizon representinghe top of a reservoir,at a depthofabout 3000 m, in a southernNorth Sea field. Figures Sa and 5bshow he corresponding ip and azimuthdisplays.These revealatype and degreeof faulting which was never observed efore nthis ield. When compa ring othdisplays, he azimuthmapclearlyprovides betterdefinition of the faulting than the dip m ap.The ex planation or the difference n the clarity of faulting be-tween he two attributess simpleand s illustrated n Figure 6. Afault will be clearly defined on the azimuth map when the dipdirectionof t he fault plane is oppo site o the dip directionof thebeds. t will be poorly expre ssed hen the fault plane dip direc-tion is similar to tha tof the horizon.A fault will be bestexpressedon the dip map when the dip angle of the fault plane is notably

different rom the horizon dip, and will be poorlyexpressed henthe dip angle is close o that of the horizon.

Figure 5a. Dip map display calculated from time vah~es ofFigure 4 (arrows outline faulting).

In the exa mple, the subtle faulting revealedby the dip andazimuthmapsha da great mpacton the field development trateg y.These aults, whichhave throwsof less han 10 m, are strike-sliprelatedand coincidewith zonesof tigh t reservoir.They alsocom-partmentalizehe reservoir, eadilyexplaining hedifferenthydro-carbon-water ontacts ncounteredn some wells. As a result ofthisdetailed nterpretation nd new structural oncept, he originalfield development lan was drastically evised.Anotherexam ple llustrating hepowerof dip andazimuth ech-niques s from a m ajor onshore ield in t he Niger delta. The 3-Dinterpretation f the Nun River field has been discuss ed y J.D.Bouvieret al. in Three-dimensional interpretation of th eNun Riverfield, N igeria, AAPG Bulletin 73, November 11, 1 989. Despitedifficult terrain conditionsnecessitatingonsiderable xpenditure,seismicdataof goodquality was acquired n this area (Figure 7).The data quality deteriorate s little on the crestof the structuredue to the presenceof near-surface ayers characterizedbyanomalous elocities.The time map (Figure 8), of one of the reservoirs t a depthof about 1500 m, shows he severe aulting which affects hishorizon. However, this faulting is far better expressed n the dipand azimuthdisplays,which providea thorou gh nderstanding fthe complex ault patternaffecting he ield (Figure9). Here again,both displays have to be studied, as some faults (indicatedbyarrows n Figures9a and 9b) are well expres sed n one display,but poorly detectable n the other, and vice versa.

Figure 5b. Azimuth display calculated from time values ofFigure 4.

C ombmed dip and azimuth. To overcome he problemof thedifference n detectabilitybetweenone display and another, dipand azimuthcan be displayed n combination.Figure 10 show scombined ip and azimuthmap of the reservoir eferred o above.The different colors n the w heel represent he azimuth valueswhile th e color intensity is an indication of the dip angle (thesteeper he dip, the darker he color). Synthetic aults hrowing othe southwest how n dark blue, and antithetic aults hrowing othe northeast re in deep orange. The northeastward nd south-westwarddipping flanks of the roll-over structureare clearlyrecognizable.Within the box outlined n Figure 10, the faultsar ediscontinuous nd change irectionover a shortdistance.A setofinteractingsyntheticand antithetic aults can be ob servedwhichresults n a triangular, northeasterly ipping fault block protrud-ing into the oppositelank. Suchsmall details,whichmay indicatefault block connectivity,would be easily misse d n a convention-al interpretation f the vertical sections. Figure 6. Fault expression on dipkuhnuth displays.GEOPHYSICS:THE LEADING EDGE OF EXPLORATION SEPTEMBER1991 13


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Figure 7. Nun River field seismic line.

Figure 8. Nun River field time map.

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Figure 10. Nun River field combined dip/azimuth display (outlined area shows northeast dipping fault block w edging into oppositeflank).

ILLlJMlNATlON

-A B-

SHADED RELIEF MAPS--+A a-+-

M A X I M U MILLUMINATION- +- EUMINATION

Figure 11. Shaded relief-artificial illumination principle. Figure 12. Annerveen field, The Netherlands, northwest-southeast seismic line.

GEOPHYSICS: THE LEADING EDGE OF EXPLORATION SEPTEMBER 1991 17


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Figure 13 . Annerveen field shadedrelief map.

S ade d relief (artitlcial illumination). In a shad ed elief dis-play a grid is producedn which he valuesare proportional o thebrightness f th e reflectionwhen illuminated by a light source.The principle s som ewhat imilar to looking at the map of a seis-mic m arker which would be illuminatedby th e sun shining roma certain direction at a given elevation. Figure 11 illustrates heprinciple on a sketched aulted geologicsurface.Dependingonthe directionof illumination, fault F will app ear s a dark (A) ora bright (B) line-up.The exam ple llustrating he shaded elief display s from theAnnerveengas ield situatedust southof the giant Groningenga sfield in The Netherlands.The completeattributestudy has beenpresented y G. Hoetz and D.G. Watters seeSeismichorizon at-tribute mapping or the Annerveen gas jeld, 1990 EAEG con-ference, o be published n First Break in 1991). The main reser-voir, as n the Groningen ield, is the Rotliegend andstoneFigure12).Figure 13 showsa shaded elief display of the top reservoirhorizon with, in this case, llumination from the northwes t.Th emap reveals many structural features and, in particular, thepresence f a small graben black arrow) in the southeasternar tof the field. The southeastw ardading a ult of this graben s notilluminatedand appears s a black ine-up. The other ault plane,hading n the oppos ite irection, s brightly lluminatedan dappearsas a white line-up. The throwsof these aults are very small, infact, less han 1 0 m. The grabenshowsonly as two faultson thedip map (Figure 1 4) which lacks he relief effect.Other differences an be seenbetweenshaded elief and dipdisplays.The fault trend in th e westernpart of the area, whichappearso be a continuous venton the dip map , has n fact a dis-tinct en-echelon onfigurationon the shaded elief m ap (Figures13 and 14, the w hite arrow). This observations of great nterestas it hints at fault block comm unication.The useof the shaded elief technique houldbe optimizedbystudying displays with various directions of illumination andvariouselevationangles.A fault directionparallel o the directionof illumination will not be apparent n this type of display.Am plitude. More or less successful mplitudeand bright spotstudies ave beencarriedout on 2-D data for years. But 3-D seis-mic, with the help of modem interpretation echniques,nowprovidescontinuous mplitudemapsof suchhigh resolution hathydrocarbons nd reservoirdelineation an oftenbe interpretednspectacular etail.In the Niger delta, experience ackedup by many studies asestablished clear relationshipbetween hydrocarbons nd am-plitude anoma lies.The am plitudemap in Figure 15 is from thesame ield as discussed arlier, but from a deeper eservoir evelthan the level illustrated n F igures8- 10 . High values dark red)are indicativeof the presence f hydrocarbons.n the upperpartof themap, a clearanomaly,conformablewith structural ontours,outlinesan accumulationn a w ell defined ault block. Similarly,amplitudeanomalies re present n other fault blocks in the mid-dle and ower partsof the map. Mostnotably, heamplitudes howa clear communication etween hese ault blocks.

Figure 14. Annerveen field dip m ap.

Figure 15. Nun River field amplitude map (dark red = hii-amplitude values).

The combinedstudyof am plitudeand other attributedisplaysresultedn a detailedstructural eassessm entf this ield. Th is hada marked mpacton the appraisal nd development trategy f thefield. In particular, h edrilling of two previouslyplanned ppraisalwells wascancelled,basedon convincingstructural vidence,andresulted n savings asily covering he costof the 3-D survey.The study of amplitudes s also a powerful ool for reservoirdelineation. This is illustratedby the investigationof a channelfeature n the Balingian Province, offshoreSarawa k n the FarEas t. In this oil and gas province, seism ic nterpretation s com -plicated by poor re servoir continuity and intense aulting. Thereservoirs n the main productive nterval, sa ndsdepositedn afluviodeltaic environment,have imited lateral continuityand, asa result, the seismiccharactern this interval is variable.Figure 16a depictsa seismicsection rom a 3-D survey overone of the fields. The objectiveof the study was to map the topof a sand at about 900 ms (approximately1000 m). From log

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Figure 16a. Seismic ine of channel amplitude study, BalingianProvince, Malaysia.

correlation t appearedhat a sand, a few meters hick, in well 6had for lateral equivalenta thick (40 m) sand n well 9 about 1km farthernorth. The charac teristicog shapeof this sandwas n-dicativeof the presence f a channel. On the seismicsection,atreservoir evel, only a vague amplitudevariation in the are a ofwell 9 could be detected Figure 16a). The two seismic oops,correspondingespectively o the top and the baseof the sand n-terval, were interpretedon a coarsegrid of lines, and the inter-pretationwas extendedhrough hedatase tvia an automaticrack-ing program .Amplitudeswere thenextracted or both oops.Theamplitudedisplaysshow a distinct channel eature Figures 16band 16c), the lower loop display clearly revealing he outline ofthe channelbody. The amplitudemap of the upper oop showshigher evel of stratigraphic etail with, in particular, he indica-tion of a possiblecrevasse play system. ntegrationof the welldata w ith the amplitude displays confirmed this interpretation(Figure 16d). Well 9 , locatednear the middle of the channel,ha sgoodsanddevelopment.Well 2 on the edgeof the channel,well6 outside he channel n a proximal overbankarea, and wells 4and 5 in the crevas se play found progressivelyhinner sands.This studydemonstrateshe powerof a horizon-orientednter-pretationstrategy o detectsubtlestratigraphic etail beyond heresolution f conventional nterpretation.

Figure 16b. Amplitude map (basesand).

C onclusions.A horizon-orientednterpretation pproach, om-bining automa tic olum e rackingand subsequentttributeextrac-tion, is thekey to thedetection f subtlestructural ndstratigraphicdetail which is gene rally beyond the resolutionof vertical datainterpretation. hrough hesemethods,t hasbecome pparentha tstructures re considerablymore complex than previously ob-served,as numerous mall faults and fracturedirections an nowbe detected.These detailed nterpretations re pa rticularly vahr-able in the inves tigationof reservoirconnectivityand expectedfluid flow patterns.The ability to delineate es ervoirbodies n alow net-to-grossormationcan lead to optimally selected rillinglocations, i nificantcost eductions, ndgreatly mproved eservecstimates.

Figure 16c . Amplitude map (top sand).

Acknowledgments: he authors thank their colleaguesat Shell orcommentsand assistance n providing material for this article.They are indebted to Shell tntemationale Petroleum Maatschap-pij BV for permission o publish. Special thanks are due to ShellUK E&P, Shell Petroleum DevelopmentCompanyof Nigeria Ltd,the Ministry of Petroleum Resourcesof Nigeria and the NigerianNational Petroleum Corporation, Nederlandse Aardolie Maat-schappij, Sarawak Shell Be&ad, and Petroliam Nasional Berhadof Malaysia for permission o publish materialrom the respectiveareas.

E. J. H. Rijks graduated ngeology rom theUniversity of Leiden, The Netherlands. Hejoined Shell International in 1968 as a geo-physicist and has worked in several capa-cities in both exploration and productiondepartments n Nigeria, Sarawak (Malay-sia), Tunisia, and The Hague head oflces.He is presently headof production seismicwithin the Depattmentof Production Geol-ogy.

J. C.E. M. JaufSfed graduated from theEcole Nationale Superieurede GeologiedeNancy (France). He joined Shell Intema-tional in 1971 as a geophysicistand hasheM variouspositions in the United King-dom, Gabon, The Netherlands, Algeria,and Tunisia. He is presently senior seis-mologist n the Production Seismic groupin Shell head o&es in The Hague.

Figure 16d. Integrated seismic/logs nterpretation.

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