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GEOPHYSICS, VOL. 64, NO. 2 (MARCH-APRIL 1999); P. 609–620, 15 FIGS.

Tutorial

3-D seismic stratal-surface concepts applied to theinterpretation of a fluvial channel system depositedin a high-accomodation environment

Bob A. Hardage∗ and Randy L. Remington∗

ABSTRACT

A fundamental thesis of seismic stratigraphy is thatseismic reflections follow impedance contrasts that co-incide with stratal surfaces, which are surfaces where de-positional processes occur at a fixed moment in geologictime. This stratal-surface concept is used in this paperto image a narrow (width∼300 ft or 90 m), thin, fluvialchannel system that is embedded within a seismic reflec-tion peak that reflects from a large (about 2× 2 mi or3.2× 3.2 km) area of nonchannel facies that dominatethe waveshape of the reflection peak.

The targeted channel facies are confined to an in-terval that vertically spans less than 30 ft. Accordingto principles of seismic stratigraphy, four conformableseismic stratal surfaces that pass through the interiorof this channel sequence were constructed across a2× 2-mi (3.2× 3.2-km) (approximately) area of a 3-Dseismic-data volume. The channel images portrayed onthese seismic horizons, which were spaced at verticalincrements of 2 ms, illustrate the principle that seis-mic attributes viewed on seismic stratal surfaces provide

valuable images of facies distributions within thin-bedsequences and help seismic interpreters segregate chan-nel facies from nonchannel facies.

These stratal-surface interpretations of the fluvial sys-tem were then integrated to create a thin (8-ms-thick),stratal-bounded seismic-data window that spans theshort geologic time period during which the targetedfluvial depositional system was active. Seismic-attributecalculations within this stratal-bounded analysis windowimprove parts of the channel image by integrating the fa-cies information from all stratal surfaces into a unifiedseismic attribute that vertically spans the total deposi-

tional sequence.A comparison is made between channel images on

seismic stratal surfaces that are conformable to two dif-ferent reference surfaces, one reference surface beingpositioned below the targeted fluvial system and the sec-ond referencesurface beingabove the thin-bed channels.This comparison supports the premisethat seismic inter-preters should extrapolate stratal surfaces both upwardand downward across a thin-bed target to optimize theimage of that target.

INTRODUCTION

The term accommodation space refers to that volume of a depositional basin that lies below base level—a surfaceabove which erosion occurs and below which deposition oc-curs. A high-accommodation depositional environment is abasin setting in which the space available for sediment to ac-cumulate, whether because of basin subsidence or sea-levelrise, always balances or exceeds the volume of sediment in-

Manuscript received by the Editor July 24, 1998; revised manuscript received October 22, 1998.∗Bureau of EconomicGeology, TheUniversityof Texasat Austin, University Station,Box X, Austin, Texas78713-7508.E-mail: [email protected], [email protected] 1999 Society of Exploration Geophysicists. All rights reserved.

put. High-accommodation sediments, which are rarely sub- jected to erosion, therefore, tend to provide excellent records

of depositionalprocesses and environments because once sedi-ment layers are deposited in such an environment, they tendto be altered and modified only by normal compaction anddiagenetic processes. Holocene- to Miocene-age rocks of theGulf of Mexico are excellent examples of high-accommodationdepositional environments. The 3-D seismic-data examples il-lustrated in this paper come from this very basin setting, the

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610 Hardage and Remington

3-D survey area being located only a short distance northwestof Corpus Christi, Texas.

A stratal surface is a depositional bedding plane, that is, adepositional surface that defines a fixed geologic time. Anysiliciclastic rock deposited in a high-accommodation environ-ment contains numerous vertically stacked stratal surfaces. Afundamental thesis of seismic stratigraphy is that a seismic re-flection event followsthe impedancecontrast associatedwith astratal surface, that is, a surface that represents a fixed point ingeologic time (Mitchum et al., 1977; Vail and Mitchum, 1977).Because lithology varies across the area spanned by a largedepositional surface, the implication is that an areally per-vasive seismic reflection event does not necessarily mark animpedance contrast boundary between two fixed rock types asthe reflection traverses a prospect area. The application of thisfundamental concept about the genetic origin of seismic reflec-tions to seismic interpretation is referred to as stratal-surface

seismic interpretation.

Tipper (1993) illustrated and discussed situations in which athin-bed seismic reflection can be eitherchronostratigraphic ordiachronous, depending on (1) the vertical spacings betweenbeds, (2) the lateral discontinuity between diachronous beds,and (3) bed thickness. The conclusion that a thin-bed seismicreflection is chronostratigraphic or diachronous thus needs tobe reached with caution because the answer depends on thelocal stratigraphy, the seismic bandwidth, and the horizontaland vertical resolution of the seismic data.

If two seismic reflection events, A and B, are separated byan appreciable seismic-time interval (say a few hundred mil-liseconds) yet are conformable to each other, then the uniformseismic-time thickness between these two events representsa constant and fixed period of geologic time throughout theseismic image space spanned by reflectors A and B. An impli-cation of seismic stratigraphy that can be invoked in such aninstance is that any seismic surface intermediate to A and B,which is also conformable to A and B, is also a stratal surface.The purpose of this paper is to illustratethat this stratal-surfaceseismic-interpretation concept becomes valid when the princi-pleis appliedto the interpretation of a complex channel systemdeposited in a high-accommodation environment.

The channel system used to illustrate the stratal-surfaceconcept of seismic interpretation is a shallow Miocene flu-vial system at a depth of approximately 2200 ft (670 m). The3-D seismic acquisition geometry was designed to produce55× 55-ft (17× 17-m) stacking bins and a stacking fold of 12at this target depth. The signal-to-noise character of theserather low-fold 3-D data is quite good, and the stratal-surfaceseismic-interpretation approach described in this paper pro-duces excellent images of the channel complex.

COLORBARUSEDFOR DATADISPLAYS

A four-tone (black, dark gray, light gray, and white) colorbar is used to display seismic reflection amplitudes in this pa-per. The best way to define the relationships between seismicreflectionamplitudevaluesand thecolor tones is to refer to thewiggle-trace displays in Figures 2 through 9. In these displays,large positive reflection amplitudes are black, small positiveamplitudes are dark gray, small negative amplitudes are lightgray, and large negative amplitudes are white. This same four-tone color bar is used to display seismic reflection amplitudesin map view in Figures 1 and 10 through 14.

HORIZONVIEWOF TARGETEDFLUVIAL SYSTEM

A display of seismic reflection-amplitude strength acrossthetargeted stratal surface is shown in Figure 1. This image corre-sponds to the geologic time during which the targeted Miocenefluvial system was active, and the meander patterns revealed inthe lower-right and upper-right quadrants of the image definethe geometric shapes and spatial positions of the channel com-

plexes that are to be analyzed.The procedureused to constructthis attribute display willbe explained later.Forthe time being,this seismic image will be used only to define the coordinates of seismic profiles that cross selected features of the channel sys-tems. These profiles can then, in turn, be used to illustrate thesubtle changes in reflectionwaveshape that mark thetransitionfrom nonchannel to channel facies. Crossline profiles 174, 200,and 222 (Figure 1) are the key vertical slices through the 3-Dseismic volume that will be used to illustrate the principles of stratal-surface interpretation.

The east-west-trending ripplelikepattern in the south half of the image is not caused by a depositional process. Rather, it isthe result of the seismic analysis window cutting through low-amplitude, zero-crossing regions of the reflection waveforms

in a way that causes the reflection amplitude to shift systemati-cally from a small positive value to a small negative value. Theripple effect can be altered by adjusting the four-tone color barused to display the data.

VERTICALVIEWS OF CHANNELANDNONCHANNEL

SEISMIC FACIES

Crosslines 174, 200, and 222 (Figure 1) will be used to showthe subtle changes in reflection waveform that distinguish

FIG. 1. 3-D seismic image of the targeted thin-bed Miocenefluvial channel systems. This image is a display of reflectionamplitudes on a stratal surface 26 ms below, and conformableto, reference surface 2 defined in Figure 15.



Stratal-Surface Interpretation 611

seismic channel facies from seismic nonchannel facies in thisparticular prospect area. These seismic facies will be viewedby examining the reflection-amplitude response across a se-ries of closely stacked stratal surfaces that collectively span thedepositional time during which the channel system was active.

Crossline 174

Crossline 174 crosses two separate Miocene channel systems,a narrow feature spanning inline coordinates 240–245 and awider feature between inline coordinates 64–80 (Figure 1). Be-cause the stacking bins created by this 3-D recording geometrymeasure 55× 55 ft (17× 17 m), these coordinate ranges meanthat the smaller channel system is about 300 ft (90 m) wideand the larger channel complex is about 900 ft (275 m) wideat the point where crossline 174 crosses the feature. That partof crossline 174 that images the narrow channel is displayed inFigure 2; the part that traverses the wider channel is shown inFigure 3.

FIG. 2. Data window from crossline 174 centered on the northern, smaller channel system. Refer to Figure 1 forlocation of this seismic profile. The targeted fluvial system is embedded in the reflection peak that occurs at 0.70 sat inline coordinate 270. The horizon labeled “reference surface” is the reference seismic stratal surface that isused to construct additional stratal surfaces that pass through the targeted thin-bed interval.

Only 200 ms of data centered about the targeted chan-nel system are displayed in these figures, as will be the casefor crosslines 200 and 222 that follow. These wiggle-trace dis-plays are shown in a highly magnified format to allow subtlevariations in reflection waveshape to be seen easily. Every sec-ond trace is plotted to avoid trace overlap so that individual re-flection waveshapes can be inspected independently. Becauseof this trace decimation, thedata appearto have a trace spacingof 110 ft (34 m), whereas the actual trace spacing in the 3-Ddata volume was 55 ft (17 m). The wiggle-trace data are shownwith a high display gain to ensure that low-amplitude featurescan be seen; some high-amplitude troughs are consequentlyclipped.

A good-quality reflection peak, the “reference surface,” islabeled in each display. This particular reflection peak satisfiesthe fundamental criteria required of a reference stratal surfaceused to study thin-bed sequences such as this fluvial channelsystem, namely (1) the event extends over the total 3-D imagespace and has a high signal-to-noise character, (2) the eventis reasonably close to the targeted thin-bed sequences (within




100 ms in this example), and (3) the event is conformable tothe targeted thin-bed sequence.

Criterion3 is themostimportant requirement forany seismicstratal surface that is to be used as a reference surface—onefrom whichconstant-depositional-time surfaces are to be madethat span targeted thin-bed sequences.

Because the surface labeled “reference surface” follows theapex of an areallycontinuous reflectionpeak,the basic premiseof seismic stratigraphy is that this reference surface follows animpedance contrast that coincides with a stratal surface.

The displays of crossline 174 are repeated in Figures 4 and 5,with four conformable surfaces, A, B, C, and D, that passthrough the targeted thin-bed interval added to the profiles.These four surfaces are, respectively, 92, 90, 88, and 86 msabove—and conformable to—the reference surface. Visual in-spection of the reflection events above and below surfaces A,B, C, and D shows that all of these reflection peaks and/ortroughs are reasonably conformable to the reference surfaceevent.SurfacesA, B, C,andD can thusbe assumed tobe stratalsurfaces, or constant-depositional-time surfaces, because theyare conformable to a known stratal surface (the reference sur-face) and are embedded in a seismic window in which all re-

FIG. 3. Data window from crossline 174 centered on the southern, larger channel system. Refer to Figure 1 forlocation of this profile. The targeted fluvial system is embedded in the reflection peak that occurs at 0.73 s atinline coordinate 120. The horizon labeled “reference surface” is the reference seismic stratal surface that is usedto construct additional stratal surfaces that pass through the targeted thin-bed interval.

flection events are approximately conformable to the selectedreference surface.

The circled features in Figures 4 and 5 identify locationswhere stratal surfaces A, B, C, and D intersect obvious varia-tions in reflection waveform. These waveshape changes are thecritical seismic attributes that distinguish channel facies fromnonchannel facies, as can be verified by comparing the inlinecoordinates spanned by the circled features with the inline co-ordinates where crossline 174 intersects channel features inFigure 1.

Crossline 200

Crossline 200 traverses the northern, smaller channel sys-tem at a rather oblique angle and then transects at least threeloops of the southern, larger channel system (Figure 1). A 200-ms-wide window extending along this crossline profile is dis-played in Figure 6. The event labeled“reference surface” is thesame reference surface labeled on crossline 174 (Figures 2 and3) when that surface is extended to crossline 200. Ideally a ref-erence surface should be smooth. The ripples on this particularsurface, such as the three peaklike distortions between inline




coordinates 125 and 145, are vertical displacements of only onetime sample point (2 ms). The expanded time scale used in thiswiggle-trace display magnifies these small irregularities.

The same profile is shown in Figure 7, with stratal surfaces A,B, C, and D included in the display. Again these surfaces are,respectively, 92, 90, 88, and 86 ms above, and conformable to,the reference surface. The circled data windows highlight thechanges in reflection waveform that correspond to the channelfacies that are traversed in the southern channel loops. (Referto Figure 1 for the channel positions on crossline 200.) Two of these channel-facies intervals are included in the circular datawindow that spans inline coordinates 110–130; only one chan-nel facies occurs in the window centered on inline coordinate80. As stated in the discussion of crossline profile 174, surfacesA, B, C, and D are stratal surfaces that show areal distributionsof channel and nonchannel facies at four distinct, constant de-positional times during the genesis of the channel system.

FIG. 4. Data window from crossline 174 showing stratal surfaces that traverse the northern, smaller channelsystem. This data window is the same as the one shown in Figure 2. Surfaces A, B, C, and D are reasonableapproximations of surfaces of constant depositional time because (1) they are conformable to the referencesurface that follows a seismic stratal surface and (2) they are embedded in a 200-ms data window in whichall seismic reflection events are approximately conformable. The highlighted data window encircles the subtlechanges in reflection waveform that identify the seismic channel facies. The inline coordinates at the center of this circular data window correspond to the position of the channel image in Figure 1.

Crossline 222

Crossline 222 extends across two point-bar segments of thesouthern fluvial channel system (Figure 1). Part of the profilethat is centered on these two fluvial features is displayed inFigure 8 to show the position of the reference surface whenthat seismic surface is extended to crossline 222. Followingthe previous format used for crosslines 174 and 200, the sub-sequent figure (Figure 9) shows the positions of stratal sur-faces A, B, C, and D that are conformable to, and 92, 90,88, and 86 ms above, respectively, this reference surface. Thetwo highlighted data windows in this display emphasize thevariations in reflection waveform that occur when the pro-file crosses the two point-bar complexes. The inline coordi-nates on which these circular data windows are centered corre-spond to the coordinate positions of the point-bar meanders inFigure 1.




STRATAL-SURFACE IMAGESOF CHANNELSYSTEMS

The reflection-amplitude behavior across stratal surfaces A,B, C, and D is shown as Figures 10, 11, 12, and 13, respectively.The vertical time separation between these successive stratalsurfaces is 2 ms, which corresponds to a vertical depth separa-tion of only 6–7 ft (1.8–2.1 m) for the low-velocity rocks wherethis fluvial deposition is preserved. As these amplitude-based

imagesare viewedin sequence, startingwith surface A andend-ing with surface D, the effect is approximately the equivalentof stripping offsuccessive layersof deposition, each layer 6–7 ft(1.8–2.1 m) thick.The total isopach interval defined by the fourstratal surfaces is approximately 25–30 ft (7.5–9 m) thick.

That the two channel systems (the northern one and thesouthern one) are not completely imaged on any of these fourhorizons implies that none of the four surfaces is a perfectstratal surface. That considerable portions of the channels ap-pear on each surface, however, is evidence that each horizon isa reasonably good approximation of a stratal surface.

Surface B (Figure 11) is probably the best approximation of a stratal surface that coincides with the deposition time duringwhich both channel systems were active. Surface A (Figure 10)

FIG. 5. Data windowfrom crossline174 showing stratal surfaces that traverse thesouthern, largerchannel system.This data windowis thesame as theone shown in Figure 3. Surfaces A, B, C,and D arereasonable approximationsof surfaces of constant depositional time, as explained in Figure 4. The inline coordinates at the center of thecircular data window correspond to the position of the channel in Figure 1.

loses some of the detail that is imaged on surface B. Althoughsurface C (Figure 12) fails to image parts of the prominentmeander loops of the southern system, it shows new channelscoming into the complex from the southeast corner of the im-age space. Surface D is the best image of the downstream partof the southern channel (and splays?) that extends southwestfrom the meander-loop area. In summary, each surface—A, B,C, and D—shows a slightly different geometric shape and spa-tial position of selected parts of the channel complex, in effectshowing thegenetic growthand lateral movement of the fluvialenvironment as approximately 25–30 ft (7.5–9 m) of sedimentwas deposited.

STRATAL-BOUNDED SEISMICANALYSISWINDOWS

The channel-system images shown in Figures 10–13 aresurface-based images; that is, the seismic attribute that is dis-played is limited to a data window that vertically spans onlyone data sample. When a 1-point-thick data window is agood approximation of a stratal surface that passes throughthe interior of a targeted thin-bed sequence, then seismic at-tributes defined on that surface can be important depictions




FIG. 6. Data window from crossline 200. Refer to Figure 1 for location of this profile. The targeted fluvial systemsare embedded in the reflection peak that occurs at 0.72 s at inline coordinate 200. The horizon labeled “referencesurface” is the reference seismic stratal surface that is used to construct additional stratal surfaces that passthrough the targeted thin-bed interval.

FIG. 7. Data window from crossline 200 showing stratal surfaces that traverse channel facies. This profile is thesame as the one shown in Figure 6. Surfaces A,B, C,and D are reasonable approximations of surfaces of constantdepositional time because (1) they are conformable to the reference surface that follows a seismic stratal surfaceand (2) they are embedded in a 200-ms data window in which all seismic reflection events are approximatelyconformable. The highlighted data windows encircle the subtle changes in reflection waveform that identify theseismic channel facies. The inline coordinates inside these circular data windows correspond to the positions of the meander loops in Figure 1.




of facies distributions within the sequence, as the images inFigures 10–13 confirm.

An alternate, and usually more rigorous, way of determiningfacies distributions within a thin-bed sequence is to calculateseismic attributes in a data window that spans several datapoints vertically yet is still confined (approximately) to onlythe thin-bed interval. The bottom stratal surface of this datawindow must reasonably coincide with the onset depositionaltime of the sequence, and the top stratal surface must be agood approximation of the shutoff depositional time for thesequence. Such a data window is a stratal-bounded seismic

analysis window.

StratalsurfacesAandDshowninsectionviewinFigures4,5,7,and 9 and thenin map view inFigures10 and 13 are examplesof surfaces that define a stratal-bounded data analysis windowthat spans a targeted thin-bed sequence, specifically the thin-bed fluvial channel system that is the interpretation objectiveof this study. In this instance, the analysis window is 4 datapoints (8 ms) thick. As previously stated in the discussions of Figures 10–13, surfaces A, B, C, and D are good-quality stratalsurfaces because each horizon images a significant part of thethin-bed fluvialsystem.Because eachof thesefour seismichori-zons is a good approximation of a constant-depositional-timesurface,collectively the four surfaces are a good representation

FIG. 8. Data windowfrom crossline 222 centered on selected point-barfacies. Refer to Figure 1 forlocation of thisprofile. The targeted fluvial system is embedded in the reflection peak that occurs at 0.72 s at inline coordinate150. The horizon labeled “reference surface” is the reference seismic stratal surface that is used to constructadditional stratal surfaces that pass through the targeted thin-bed interval.

of thefacies distributionwithin thetotalthin-bedsequencethatthey span.

One way to evaluate facies-sensitive seismic informationspanned by surfaces A and D is to calculate some type of averaged reflection-amplitude attribute in each stacking bin of the 4-point-thick data analysis window bounded by horizonsA and D. For example, the average peak amplitude between Aand D is displayed in Figure 14 and shows an alternate imageof the total channel system. Specifically, the image shows moredetail about the meandering channel in the extreme southeastcorner of the image space, identifying how that channel trendsto the northwest and blends into the larger fluvial system. Thissoutheast part of the channel system is not obvious on stratalsurfaces A or B (Figures 10 and 11). Although the systemin the southeast corner of the image is detectable on stratalsurfaces C and D (Figures 12 and 13), it is not seen in the samedegree of detail as in Figure 14. An improved image of thisparticular part of the thin-bed system appears in Figure 14because the stratal-bounded data window defined by surfacesA and D is a good approximation of the depositional-timeinterval when that part of the fluvial system was active,whereas no individual stratal surface—A, B, C, or D—is agood approximation of a depositional-time surface that passesthrough this particular part of the thin-bed sequence.




In contrast, the average peak reflection amplitude in thestratal-bounded window (Figure 14) does not image the north-ern channel system as well as do stratal surfaces A, B, and C(Figures 10–12). Other attributes, such as the average instan-taneous frequency, could be calculated in this stratal-boundedwindow and evaluated as options for imaging the channel sys-tem. These alternate imaging attributes will not be used herebecause our purpose is simply to illustrate that the stratal-surface concept and the stratal-bounded window techniquehave relative advantages and disadvantages and should alwaysbe used in a thin-bed interpretation, their results being inte-grated into a unified model of the depositional system.

COMBININGUPWARDANDDOWNWARD

EXTRAPOLATIONSOF SEISMIC STRATAL SURFACES TO

IMPROVE THIN-BED INTERPRETATION

The channel images shown up to this point were created byinterpreting a referencestratal surface below thetargeted thin-bed sequence and then extending conformable stratal surfacesupward until they passed through the targeted channel system.In this instance, the reference surface is approximately 90 msbelow the channel sequence that is to be interpreted (Figures4, 5, 7, 9).

In challenging thin-bed interpretations such as the fluvialchannel system considered here, it is important to define two

FIG. 9. Data from crossline 222 showing stratal surfaces that traverse point-bar facies. This data window is thesame as the one shown in Figure 8. Surfaces A,B, C,and D are reasonable approximations of surfaces of constantdepositional time, as explained in Figures 4 and 7. The inline coordinates at the centers of these circular windowscorrespond to the position of point bars in Figure 1.

seismic reference surfaces that bracket the thin-bed systemthat is to be interpreted, one reference surface being belowthe interpretation target and the second reference surfacebeing above the target. By creating conformable referencestratal surfaces above and below a thin-bed system, we canextrapolate conformable seismic stratal surfaces from two di-rections to sweep across a thin-bed target. One set of seis-mic stratal surfaces is commonly a better approximation of constant-depositional-time surfaces within the targeted thin-bed sequence than the other set is and produces more accurateimages of facies patterns within the thin-bed unit.

To illustrate the advantage of this opposite-direction conver-gence of seismic stratal surfaces onto a thin-bed target, a sec-ond reference surface was interpreted above (and, in this case,closer to) the targeted fluvial system. Specifically, this secondstratal reference surface followed the apex of the reflectiontroughs immediately above the thin-bed channels (Figure 15).The fluvial system is about 24–30 ms below this second refer-ence surface.

The reflection-amplitude response across the channel sys-tems observed on a stratal surface 26 ms below and con-formable to this overlying reference surface is displayed inFigure 1. The improved channel image in this case occurs be-cause stratal surfaces that are conformable to the overlyingseismic stratal surface are better approximations of constant-depositional-time surfaces for this channel system than are




FIG. 10. Reflection-amplitude behavior on stratal surface A,which is 92 ms above, and conformable to, the selected refer-ence surface. A four-tone color bar is used to display reflec-tion amplitude and polarity (see text). Locations of crosslines174, 200, and 222 are indicated. That a large part of thetwo fluvial systems is depicted on this horizon is evidencethat surface A is a reasonably good approximation of a con-stant-depositional-time surface.

FIG. 11. Reflection-amplitude behavior on stratal surface B,which is 90 ms above, and conformable to, the selected refer-ence surface. This surface, approximately 6 ft (2 m) below, andconformable to, stratal surface A (Figure 10), shows a slightlyimproved image of the channel systems. A four-tone color baris used to display reflection amplitude and polarity (see text).

FIG. 12. Reflection-amplitude behavior on stratal surface C,which is 88 ms above, and conformable to, the selected refer-ence surface. This surface, approximately 6 ft (2 m) below, andconformable to, stratal surface B (Figure 11), shows new chan-nel systems in the extreme lower-right quadrant of the imagespace.

FIG. 13. Reflection-amplitude behavior on stratal surface D,which is 86 ms above, and conformable to, the selected refer-ence surface. This surface, approximately 6 ft (2 m) below, andconformable to, stratal surface C (Figure 12), provides a bet-ter image of the fluvial system extending toward the lower-leftcorner of the image space.



FIG. 14. Average amplitude of reflection peaks in a thin,stratal-bounded window spanning the targeted Miocene flu-vial depositional sequence. The top boundary of the window isstratal surface A; the bottom boundary is stratal surface D.

FIG. 15. Location of reference surface 2 on crossline 200. Reference surface 1 is the horizon labeled “referencesurface” in the displays of crossline 200 in Figures 6 and 7. Reference surface 2 is an alternate seismic stratalsurface positioned above the thin-bed target. The positions of the thin-bed channels on this profile are definedby the circular data windows in Figure 7.

stratal surfaces that are conformable to the deeper referencesurface. This result illustrates that upward and downward ex-trapolations of conformable stratal surfaces across a thin-bedtarget are a recommended interpretation procedure, especiallyin those instances when valid stratal reference surfaces can beinterpreted both above and below the targeted thin-bed

CONCLUSIONS

The interpretation of thin-bed reservoirs in 3-D seismic datavolumes can be achieved by (1) interpreting a reference sur-face that is conformable to the areal geometry of the thin-bedsequence and (2) creating seismic stratal surfaces conformableto this reference surface that pass through the thin-bed tar-get. If the seismic stratal surfaces constructed according to thislogic are satisfactory approximations of constant-depositional-timesurfaces that existed during the deposition of the thin-bedsequence, then seismic attributes across these stratal surfacescan be valuable indicators of facies distributions within thesequence. Evidence that this stratal-surface approach to 3-Dseismic thin-bed analysis is a robust interpretation technique isprovided by the fluvial channel images described in this paper.A previous publicationsupportsthe same conclusion(Hardageet al., 1994).

An expanded application of this stratal-surface concept is tocalculate seismic attributes inside a thin, stratal-bounded anal-ysis window that is centered vertically on the thin-bed target.Facies-sensitive attributes extracted from carefully constructedstratal-bounded windows are commonly better indicators of facies distributions within a thin-bed target than are attributesthat are restricted to a 1-point-thick stratal surface that passesthrough the target. This fact implies that the geologic timeinterval during which a thin-bed sequence is deposited cansometimes be more accurately portrayed by a stratal-bounded




data window than a fixed geologic time during the thin-beddeposition can be approximated by a 1-point-thick seismicstratalsurface.Thefluvialchannelimagesincludedinthispaperdemonstrate that a rigorous 3-D seismic thin-bed interpreta-tion should be based on attributes extracted from both stratalsurfaces and stratal-bounded windows.

Thistwo-pronged approach to thin-bed interpretation can befurther strengthened by extrapolating seismic stratal surfacesand stratal-bounded windows ontothe thin-bed targetfrom op-posite directions, that is, from both below and above the thin-bed target. The logic in this dual-direction extrapolation is thatone of the seismic reference surfaces is generally more con-formable to the thin-bed sequence than is the other referencesurface, and this improved conformability leads to improvedattribute imaging of facies distributions within the thin bed.

ACKNOWLEDGMENTS

The current investigation was done as a part of the indus-try funded research project, Seismic Vector-Wavefield Char-

acterization of Complex Reservoirs, to illustrate seismic-interpretation principles that need to be implemented in orderto characterize complex thin-bed reservoirs. The seismic in-terpretation was done using software technology provided byLandmark Graphics Corporation under its University GrantProgram.

REFERENCES

Hardage, B. A., Levey, R. A., Pendleton, V., Simmons, J., and Edson,R., 1994, A 3-D seismic case history evaluating fluvially depositedthin-bed reservoirs in a gas-producing property: Geophysics, 59,1650–1665.

Mitchum, R. M., Jr., Vail, P. R., and Thompson, S., III, 1977, Seis-mic stratigraphy and global changes in sea level, part 2, the de-positional sequence as a basic unit for stratigraphic analysis, inPayton, C. E., ed., Seismic stratigraphy—Applications to hydrocar-bon exploration: Am. Assn. Petr. Geol. Memoir 26, 53–62.

Tipper, J. C., 1993, Do seismic reflections necessarily have chronos-tratigraphic significance?: Geology, 130, 47–55.

Vail, P. R., and Mitchum, R. M., Jr., 1977, Seismic stratigraphy andglobal changes of sea level, part 1, overview, in Payton, C. E., ed.,Seismic stratigraphy—Applications to hydrocarbon exploration:Am. Assn. Petr. Geol. Memoir 26, 51–52.

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