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A V O Analysis of a Pennsylvanian AgeChannel Sandstone in theArkoma Basin, Oklahomaby Er ic Kubera , Univer s i ty o f Oklahoma

A B STR A C TA seismic line whic h images a gasproducing channel sand was acquiredin September, 1994, in th e Arkom abasin. The dataset wa s processed toretain relative seism ic reflectionamplitudes and was subsequentlyanalyzed for anomalous amplitudebehavior.Statistical dete rmina tion of severalamplitude attributes was performed,and the results were displayed as'attribute plots'. The attribute plotswere exam ined for information thatwo uld describe the nature of theamplitude anomalies.The reflection ev ent interpre ted torepresent the top of the productivechannel stands out from the rest of thesection, and show s significantamp litude variation. The Am plitud eVariation wi th Offset (AVO) signatureof the channel sand is de termined to bea positive n orma l incidenc e reflectionof mod erate relative am plitude ,display ing a strong increase inamp litude w ith increasing offset.

I N T R O D U C T I O NThe Hartshorne Sandstone is a gasproducing sandstone in the ArkomaBasin, Oklahoma. The Hartshorn eproducing trend extends over seventymiles, across Coal, Haskell, Hughes,and Pittsburg C ounties. Althou ghman y of the delta facies produce in thattrend, the cha nnel sands are of notableimportance. Produ ction from wells inthe channel sands can be from 2 to 10BCFG per we ll (the channels can be upto 300 feet thick and about a mile wide,and can extend linearly for many miles

(J. Ham ilton, personal comm unication,1995-96)).A seismic line which imag ed aproductive Hartshorne channel wasrecorded by Pathfinder Group LLC,Norm an, Oklahoma. The dataset wasload ed into Promax 2-D Seismic DataProcessingpackage at theUniversity of ]Oklahoma to [be processed for relativeamplitudes and analyzed for anAVO signature. Two main goalsof this project are to performAVO analysis to d etermin e thepresence of amplitudeanomalies, and to characterizeany anomalies found.

G E O L O G YStudy Area and Explora t ion Targe tThe seismic line used in this study w assho t in September, 1994, alon g a sectionroad in T.4 N., R. 12 E., PittsburgCounty, Oklahoma. Figure 1 shows thestud y area and the location of the

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MttsburoCountyIt was d ecide d that a processingscheme that contained minimal ~ . P ~

manipulation of the data w ould suffice umm|Ljto gain a first order determination of 7 ~ l xthe presence of an AVO signature. At / C 0 1 d \ !the end of the processing scheme, the / _ ~ . Atnke\ Idata was put through a flow - / I ; ,OilnIy Coulltywhich created amplitude ~ N,,attribute stacks. These ~ Nstacks are analyzed , ~ PitllBbur0 ""and correlated to County / ~ ] \t / . . ~ . ~_ t'4 l - - . , , Y ' ,2 O " 1 .. Jdetermine thepresence andnature of anyamplitudeanomalies.Information thatsuggests an AVOsignatttre isscrutinized, andconclusions aredrawn about theinformationderived from theattribute stacks. Figure I - Map shoun'ng he study area and the location of the seismic ine.

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BeggyFormationSavannaFormatlm

Mok, mtor For,mstionHertahorneFormation

Atoka FermaUonSpire Formation

WapanuokaFommtm

Figure 2 - Regional stratigraphic column for thestudy area.

seismic line. The prim ary target was aPennsylvanian age Hartshorne channelsand, which produces natural gas froma dep th of appro xima tely 3600 feet. Thegas wells produc ing in this areaconstitute the Ashland Field,discovered in 1976 by the C onoco Lane#1-28 we ll.

system that prograded from east towes t into the Arkom a Basin. It hasconform able contacts wit h thepredom inantly marine stra ta of theunderly ing Atoka (Houseknecht andothers, 1983) and the coal bearing strataof the overlyin g McAlester (Gossling,1994).The Hartshorne is divided into twogeograp hically separate areas of theArk om a Basin. The division is based onthe numbe r of Hartshorne sandstonesand associated coals. In the northport ion of the basin, the Hartshorneconsists of a single sand unit an d a noverlying single coal unit. In thesou thern pa rt of the basin, there exists aclaystone bed (greater than one footthick) that separates a L owerHartshorne sand and coal, and anUpp er Hartshorne sand and coal(Friedman, 1982).In the field area of this study, the coalunit is missing over the to p of thechannel tha t the seismic line images.Differential compactio n of thesedim ents in the area left the thickdistributary channel as a topographichigh, causing the coal forming marsh

m

400

ago

2110

areas to collect on the flanks of thechannel (J. Forgotson, per sonalcom mun icatio n, 1994-96). From logs inthe area, the isolated Hartshornechannel in this study has beeninterpreted to be approximately onemile wi de and 220 to 230 feet thick (J.Hamilton, personal communicat ion,1995-96)

Production H i s t o r yDesmoinesian sands were one of theearliest drilling targets in the Arko maBasin because of the shallow d epths.The early drilling rigs could reach the1,400 to 3,600 foot target d epth s w ithlittle trouble. Gas production fromDesmoinesian reservoirs was reportedin Allen County, K ansas, in 1873.However, discoveries in the Arko maBasin, Oklahoma, did not occur unti lthe discovery of the Poteau-Gilmorefield in 1910. Figure 3 is a prod uctio ntime l ine showing the Hartshorne gasfields and associated discove ry dates.The Hartshorne sands produce naturalgas in a trend that extends over 70miles, across Haskell, Hughes, a ndPit tsburg Counties. The main trappin g

Regional Stratigraphy and heH artshorn e Depositi onal SystemThe Hartshorne Fo rmation is a mem berof the Krebs Group. This group w asdeposi ted early in the DesmoinesianSeries of the M iddle Pennsylv anianSystem, and includes the youn gerMcAlester, Savanna, and BoggyFormations. Figure 2 represents theregional stra t igraphic colum n for theArko ma Basin in the area of therecorded seismic data and sho wssignificant formations above and belowthe zone o f interest.The Hartshorn e Form ation wasdeposi ted in a t idal ly influenced del ta8 Strata SHAKER JULY-AUGUSSr 997

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log

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1118O U 1119 I 1970 g I

IIIt lF I I D I c a Lg i E~ at rabna~dDJHormI m OIstrktPatun dnmra

Figure 3 - Production time line shoun'ng the discovery of Hartshorne gas )qelds. (After Brown and Parham,1993.)

OCGS - The Shale Shaker


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mechanis m for the Hartshom e channelsis structure. The dominan t drivemechanism is pressure depletion.Recovery estimates are in th e typicalpressure-depletion drive range;approx imated to be 80%. Porosities ofpay zones rang e from 10 to 18%, wit hpermeabilities averaging 6 md. Watersaturations are typically 30%. TheHarts hom e reservoirs have acumu lative prod uction of 655 Bcf. Thistotal is more gas than a ny otherreservoir in the Desm oinesian play.Figure 4 depicts the cumu lativeproduction from the Hartshorne ascompared to other major producingDesmoinesian formations in theArko ma Basin and Eastern Kansas.

A V O E X P L A I N E DD e f i n i t io n o f A V O A n a l y s i sAVO (Am plitude Variation with Offset,or Am plitud e Versus Offset) analysis isthe investigation of seismic P-wavereflection data in search o f acharacteristic change in the am plitudeof a reflection even t with an increase insource-receiver offset and associated

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BRuin Dartluslge AH,, I I - .Oklaboma Kauas reserwelrsFigure 4 Cumula tive and relativegas production rom the Desmoinesian sandstone reservoirs in the ArkomBasin and Eastern Kansas. (After Brown and Parham, 1993.)angle of incidence. Figure 5 depicts therelationship betw een the source-receiver offset and the angle at wh ichthe seismic ray strikes the interface(angle of incidence). The source-

receiver offset associated with shot #6and receiver #6 is significantly largerthan th e offset for shot #3 and receiv#3. It is clear that the angle assoc iatedwi th shot-r eceiv er pair #6 (Os6) is also

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Figure 5- Schematic showing the relationship between shot-receiver of~et and the associated angle of incidence at the reflector. Note that an increase in offset results inincrease in the incidence angle.JULY-AuGuST1997 / SHALESHAK



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larger than the an gle associated withshot- receiv er pa ir #3 (0s3). In o therwords, the angle of the incident rays a tthe bound ary increases with sourcereceiver offset.It is necessary to under stand how theampli tud es observed in the seismicdata are related to the reflectioncoeffiden ts of the target reflectors. Thereflection coefficient directly conn ectsthe physical properties of the rockswi th the amoun t of ene rgy t ransmi ttedand reflected. The am pli tude of aneven t in the trace data is arepresen tation of the reflectioncoefficient, bu t is no t eq uivalen t to it.This is because seismic frequencies andampli tu de information are lost due tothe physical interaction of the seismicwave w i th the rocks through which i ttravels. All effects on am plit ude d ue toprocesses othe r th an reflection itself arecommonly grouped ' together andreferred to as the earth filter. It is thegoal of re la tive amp li tude p rocessing torecover the am pli tudes lost to earthfiltering so that the trace amplitudeinformation can be used to representthe re flection coefficient directly.Consider Figure 6.Figure 6a represents the seismic energytravel path and angle of incidence for anear offset trace. Figure 6b re presentsthe travel path and angle of incidencefor a far offset trace. Note th at thetravel pat h fo r the far offset trace islarger tha n th at of the near offset trace.

Figure 6a shows an incident and areflected ray f rom a relatively shortoffset (small angle of inciden ce = 0).The equation which defines thereflection coefficient for the inter face is:RCi AR1 Equation 1A nwhere An is the incident ampli tude, andA a is the reflected amp litud e for th enear offset case. The equation w hichrepresents the reflection coefficient forthe far offset case (large angle ofincidence = ) in Figure6b is:RC2 AR2 Equation 2A i2where Ai2 is the incident ampli tude,and A/2 is the reflected amplitu de forthe far of fset case.

The earth acts as an attenua ting filteron the energy passing through. One ofthe a t tenuation mechanisms, sphericaldivergence, causes the am pli tude todeca y as 1 (distance traveled) (Yilmaz,1987). For near offset traces, theamp litud e striking the interface islarger than the ampli tude striking theinterface from far offset traces, ow ing tothe distance traveled through thesubsurface. The seismic wav e for ashort offset rayp ath has trav eled ashorter distance thro ugh the subsurfaceand, therefore, has more energy thanthe seismic wav e traveling the longoffset raypath. The difference inrecorded ampli tude between the former

[a] Hear O ffset Ibi Far lffsetI I I I

Figure 6 (a) - Seismic energy travel path and angle of incidence fo r a near offset trace.Figure 6 (b) - Travel path an d angle of incidence for afar offset trace.10 SHALESHAKER JULY-AuGusT1997

short offset raypath, and the latter lonoffset raypath , is in the AIx term. It isassum ed that after the applicat ion of specific gain correction to the d atadurin g processing, the Aix terms havebeen set equal (the gain, and theman ner in wh ich i t is used w il l bediscussed whe n i t is presented in theprocessing scheme). In addit ion, theampli tu de differences caused by thetravel path length for the upg oing rayare equalized in the processing. It isthen possible , through Equations I an2, to a l low the observed ampli tud e(ARx) values to rep resent the reflectiocoefficient values. M ore precisely, theexact magnitud e of the reflect ioncoefficient is not as im por tant as itsrelative change with offset.It is kno wn that the reflectioncoefficient of an interface is a functio nof the physical properties of the rockunits involved and the angle at whichthe seismic energ y strikes the interfacWhen w e assume that the observedseismic amp li tudes correct ly representhe reflection coefficient for a b ound arangle/offset dep ende nt varia tions inampli tu de indicate angle/offsetdep ende nt varia t ions in the reflect ioncoefficient. Therefore, if a processingscheme retains the relative amplitudeof the seismic data after correcting forthe ampli tude at tenuation of the earthfilter, the variations of the observ edampli tudes are directly re lated throu gthe reflection coefficients, to thephysical properties of the rockformation. A basic idea that mak esAVO analysis applicable to oil and gaexploration is that a change o f thephysical proper ties of the rock iscaused by the introduction of gas intothe por e fluids. It is this effect thatsuggests an analysis of the present dafrom the Ashland Field may reveal anAVO response.

RESULTSInterpretation of Attribute PlotsAttribute #1 (Promax Manual) is theIntercept. This value represents the



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CLIP#298 314 330 346 362 378 393 409 425 441 457 473 489

TiitilliSl2nn

Intercept(+)

i - )

0.933

H O T C O L O R SS I N 2 0

C O L D C O L O R S

0.292-0.061

-0.886

'Zero-Offset' or Norm al Incidence (N.I.)reflection amplitu de. T he zero offsetamp litude is directly related to thereflection coefficient calculated fro mthe sonic and density logs because at anincidence angle of zero there is no lossof P-wave energy to the creation ofmod e converted waves.

The Intercept plot for this data set isshow n in Figure 7. On the color scale,red and y ellow correspond to positivenorm al incidence reflection coefficients,and w hite and blue correspond tonegative norm al incidence coefficients.Notice able reflectors at 300 ms, 500 ms,and 600 ms on this plot correspo nd to

12 St u ~ SJoa~n~ JurY-AuGusr1997

Figure 7 - Plot of the Intercept attribute. Note thebright reflectors at 300, 500, and 6 00 ms. T heHartshorne Reflection occurs at approximately 6 80ms. under CM P #419.

strong reflection events o n the brutestack. No geologic correlation has beenmade to identify the formations



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CDP#298 31,4. 330 346 362 378 393 469 425 441 457 4,73 489

Time200

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800

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associated with these high normalincidence reflections.The literature su rr ou nding theHar tshome channel sands suggests thatthe channel shou ld be a high-impeda nce bou ndary (Rutherford andWilliam s, 1989). The sonic log

associated wi th the Lane 1-28 show edthe interpreted top of the channel tohave a significantly large increase i n P-wav e velocity. However , the attributedisplay does not show a significantlyhigh intercept valu e for the Hartshornereflection (Approx. 680 ms). A possibleexplanation for this apparent

Fi gure 8 Plot of he Sign (In tercept) * Gradientattri bute. Since this plot uses the value of heintercept as well as the sign, the notation or heoperation woul d be better written as (Signed)I ntercept * Gradient. Not ice he Har tshornedisplayin g hot colors (peaks). See Table l for plotparameters.

JULY-AUGUSt 997 / SHALESHAKER1



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discrepancy is that the reflection fromthe Hartshorne sand could be of lowerrelative impedance whe n compared tothe reflections at 300 ms , 500 ms, and60 0 ms, and therefore does not show asan ano malous ly bright reflection in theIntercept plot. This plot is useful forbeginn ing to identify and characterizethe Hartshorn e reflection event in termsof ampl itude behavior.The second attribute ana lyzed is theSign (intercept) * Gradient. It is themathematic al product of the sign of theintercept and the g radient. Thisattribute retains the magnitu de of thegradient, but its polarity varies with thecombin ed polarities of the gradien t andintercept. A strong gradien t willprovide a strong response.

(+) intercept '* (+) gradient = peak(-) intercept * (-) gradient = peak

(-) intercept * (+) gradient = tro ugh(+) intercept * (-) gradient = troug h

Table 1 Parameters used in the calculation of the Sign (Intercept) * G radientplot and the corresponding result.

Interce pt plot, the reflection of interestwas sh own to be a peak. It follows thatwe have evidence for an AVO responsecharacterized by a peak which becomesstronger in amplitude with increasingoffset.

CONCLUSIONSUsing the tw o attribute stacks createdby an amplitud e sensitive processingscheme, the top of the Hartshornechannel sand w as positively identifiedas a reflection from a kn own gas-bearing interval with distinctiveAmpl itude Variation with Offset (AVO)characteristics. The response ischaracterized as a positive normalincidence reflection of moderat eamplitu de whic h increases in

amplitude withincreasin g offset.This AVO signaturecould be helpful intracking the targethorizon on astacked section,assuming that thediscontinuousresponse is not afunction of thesignal/noise ratioof the trace gathersbeing analyzed.

(The Sign (Intercept) * Gradient plot forthe Hartshorne data is shown in Figure8. The zone of interest shows relativelyclear but discontinuo us events in red.According to the parameters of the plot(shown in the table above), thisindicates our target reflection is either astrong peak wh ich increases inmagn itude w ith offset, or a strongtrough which increases in magn itudewit h offset. Referring back to the

A C K N O W L E D G M E N T SI wou ld like to thank the PathfinderGroup for allowing me to use this dataset. I wo uld also like to thank theOCGS for their support towa rds thecompletion of m y M asters project.

LITERATURE CITEDBrown , R.; and Parha ra, K., 1993,

"Desmoinesian Fluvial-DeltaicSandstone Arkorna Basin, Oklah omand Eastern Kansas": in Bebout, Dan d Gra smick, M. (eds.), Atlas ofMajor Mid-Continen t GasReservoirs, Coordin ated by theTexas Burea u of Econo mic GeologyAusti n, Texas, p.36-39Friedman, S.A., 1982, Determin ation ofReserves of Methan e From CoalBeds for Use in Rural Com munitiein Eastern Oklahoma: OklahomaGeolog ical Survey, SpecialPublica tions 1982-1983, 32 p.Gossling, J., 1994, Coa lbed Met han e ofthe Hartshorn e Coals in Parts ofHask ell' Latimer, LeFlore, McIntosand Pittsburg Counties, OklahomaUniversity of Oklahoma, Norm an,M. S. ThesisHousek necht, D. and Others, 1983,Tectonic-Sedimentary Evolution ofthe Arkoma Basin and Guide Bookto Deltaic Facies, HartshorneSandstone: SEPM Mid-Con tinentSection, v.1, 119 p.Promax, Operatin g M anual, Version 6.Adva nce Geop hysical' Denver, 199Ruthe rford, S.; and Williams, 1989,Am plit ude Versus Offset Variationsin Gas Sands: Geophysics, v.54, no.Yilmaz, O., 1987, Seismic D ata

Processing: Investigation inGeoph ysics, SEG, v.2

About the Author: Eric Kubera was bornin D unkirk, NY. He received his B.S. inGeophysics from SUNY at Fredonia in1993 and a M.S. in Geoph ysics from theUniversity of Oklaho ma in December,1996. He is now in Hou ston wi th theExploration Technology Group of BHPPetroleum (Americas).

e'l~.D.. m . ~.t. m 11m m ~ EKol oot loaC o rp . o m m ~ o n "Pmimoay ~ ~ I m , i l ~ u i o mD e m o p m , ~ ~ t t o e ~ ~ r i e ,~ m

ROBERT F . EHINGER

C, n ~ ~o~ogla13509O r e en C e dm " a n e O K C (403) 47~-7J74Okla lwma City , OK 7JIJ ! TuI~ (918) 627-5021

14 SHALeSHAXER JULY-AuGUST 997

~ Karl W. EberhartTerritory Manager12913 BuckboardRd.

l andmark Tel: (405) 399-2810Fax: (405) 399-2813A Hallibureon Compxny E-Mail: [email protected] : / I w w w . lgc . com


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