Interpretation and practical applications of 4C-3D seismic ...

Rapid advancements in multicomponent acquisition meth-ods and processing techniques have led to numerous appli-cations for converted wave (C-wave) data that are increasinglyused for exploration and exploitation of oil and gas. However,the increased prevalence of multicomponent seismic applica-tions means that interpreters must face many difficult chal-lenges: how to register P-wave time to C-wave time, determinethe best methodology for interpreting C-wave data, and/orhow to apply the C-wave interpretation in assessing the risksof exploration and exploitation prospects. This paper addressesthese issues and attempts to guide the interpreter through themulticomponent data interpretive process with numerousexamples from two East Cameron gas fields in the Gulf ofMexico.

In the fall of 1999, PGS acquired the first commercial mul-ticomponent 3D survey in the Gulf of Mexico. BP licensed thedata in early 2000. The survey, approximately 80 km2 full fold,covers BP’s East Cameron 261 and East Cameron 265 fields.The primary purpose of the survey was to address data degra-dation due to the presence of gas in the shallow sediments.Seismic imaging over these fields, which have more than 30gas-bearing reservoirs in depths of less than 1000 ft to over 15000 ft, has been hampered by severe velocity push-downs andgas cloud effects in the conventional P-wave data. The new4C data, particularly the C-wave data, have unquestionablyimproved imaging in and below gas cloud areas. The fieldswere subsequently re-evaluated, and interpretation of newdata resulted in a much more improved understanding of thehydrocarbon-bearing reservoirs which led to five consecutivesuccessful exploitation wells in East Cameron 261.

Acquisition and processing. While it is not the intent of thispaper to go into the details of acquisition and processing, thereader may be interested in how the survey was acquired andprocessed in order to compare to other multicomponent datawith different acquisition designs and/or processing flows.The survey was acquired with four-component ocean-bottomcables, which were deployed without tension and not dragged.A wide-azimuth patch was used with shot-lines orthogonalto cables. Specific key acquisition parameters included:

• Dual cable orthogonal patch acquisition• Receiver direction = north-south• Shot direction = east-west• Group interval = 50 m • Receiver line interval = 437.5 m (EC 265), 550 m (EC 261)• Source interval = 37.5 m• Source line interval = 420 m (EC 265), 540 m (EC 261)• Source depth = 7.5 m (EC 265), 5 m (EC 261)• Record length = 11 s• 25 � 25 m bin (acquired)

It is important to note that two of the four blocks wereacquired with narrower receiver and source line intervals.The results reveal that: (1) there is better shallow C-wave datacoverage (smaller notches) within the narrower receiver and

source line interval acquisition area as expected, and (2) theoverall C-wave data quality improved noticeably with betterreflection continuity and greater resolution. Examples fromboth areas will be pointed out later in the paper.

In seismic data processing, the following general sequenceof preprocessing steps was first applied to all components:reformatting of field data, geometry assignment, designatureof the air-gun array wavelet and instrument response, anddatuming. On the P-wave data, hydrophone and vertical geo-phone components were combined to reduce water-bottomreverberations. Next gain recovery, deconvolution, and resid-ual statics were applied. They were followed with prestacktime migration for both PZ and PS data.

Background field information. East Cameron 261 Field (100%BP W.I.) and East Cameron 265 Field (50% BP W.I.) are off-

300 THE LEADING EDGE APRIL 2003 APRIL 2003 THE LEADING EDGE 0000

Figure 1. 3D view of East Cameron 261 showing Lenticulina andValvulina (H) structure maps with P-wave amplitude overlays.

Figure 2. 3D view of East Cameron 265 displaying a number of structuremaps with P-wave amplitude overlays, including the 3200-ft sand and the9000-ft sand discussed in this paper.

Interpretation and practical applications of 4C-3D seismic data,East Cameron gas fields, Gulf of MexicoJAY W. NAHM and MIKE P. DUHON, BP America, Houston, Texas, U.S.

INTERPRETER’S CORNERCoordinated by Rebecca B. Latimer

shore Louisiana in 160 ft of water, approximately 175 milessoutheast of Houston, Texas. These gas fields were discoveredin the 1960s, and the majority of the known reserves were pro-duced during the 1970s-early 1990s. Recent production hadfallen off to levels where economic viability of the fields wasin question. Successful recent exploitation drilling has signif-icantly increased reserves and the production rate whileextending the life of East Cameron 261. The fields combinedhave produced over 650 BCFG to date.

Field pays range in depth from 1000-15 000 ft (with morethan 30 producing reservoir sands), and they are typically rep-resented by bright amplitudes on the P-wave seismic data. Thereservoirs are Plio-Pleistocene in age and were deposited ina near-shore environment in predominately prograding deltaicsequences. The traps are usually faulted three-way or four-way structures in a growth fault environment. Hydrocarbon-bearing stratigraphic traps, although not common, are alsopresent. Typical porosities range from 18% to slightly over 30%.Permeability is generally high (100s of millidarcies). Most pro-duction is in normally pressured sediments, with only thedeepest 15 000 ft sand being overpressured. Figures 1 and 2show 3D views of East Cameron 261 and East Cameron 265.

P-wave to C-wave time registration. Because of the lack ofworkstation software tools designed specifically for C-waveinterpretation and industry’s general inexperience in C-waveinterpretation, geophysicists often face many challenges withmulticomponent seismic data. The first and possibly the mostimportant hurdle is caused by the slower arrival times for C-

waves than for P-waves for the same subsurface horizon. Theprocess of correlating P-wave reflectors to C-wave reflectorsfrom identical subsurface horizons, called registration, is crit-ical for interpreting multicomponent data. For example, if theaverage VP/VS ratio of sediments down to a reflector is 3, theC-wave time for that reflector will be twice that of P-wave time.

In this study, numerous attempts were made to visuallycorrelate packages of reflectors from P to C after compressingthe C-wave time to approximately match the P-wave time.More often than not, visual correlations were misleadingbecause of differences in reflectivity responses from P to C.Also, a PS synthetic seismogram was created using dipole sonicand density log data, and an attempt was made to tie the PSsynthetic seismogram to a C-wave section. This method alsowas unsuccessful because of the limited depth and poor qual-ity of the dipole sonic log. Additionally, a “pseudo-PS syn-thetic seismogram” was created with velocities calculatedusing regression equations determined from P and S log dataelsewhere on the Shelf Gulf of Mexico. This result also did notprovide an adequate synthetic tie to allow us to confidentlyregister P to C.

After much frustration in trying various methods to reg-ister the time from P to C, a new method was developed andsuccessfully implemented into the project. This new techniqueutilizes fault planes to register the time; hence, it is called “thefault plane registration method.” The entire procedure wasdone on the workstation interactively using Landmark’sSeisWorks and TDQ applications. Other similar software canjust as easily be utilized. Described below is a detailed step-


Figure 4. C-wave line (right) compared to its corresponding P-wave line(left) after proper squeezing was applied. Note that the P-wave fault andhorizon interpretations are directly overlain onto the C-wave line.

Figure 5. C-wave line (left) and its instantaneous phase section (right)demonstrating the use of one attribute display to further refine the faultpicks. For example, should the fault be picked a bit lower in the circledarea?

Figure 6. Representative P-wave (left) and its corresponding C-wavenorth-south line (right) across the gas cloud area. Note reflection continu-ity and overall imaging improvement in shallow sections on C-wave andcompare, in particular, with the circled area on P-wave data.

Figure 3. P-wave (left) and corresponding C-wave line (right) showingthe matching of P-wave to C-wave time via “the fault plane registration”method. Red circles represent sample locations of P-wave and C-wavetimes at the purple fault plane used to derive a squeeze function.

by-step explanation of this technique.

1) Load P-wave and C-wave seismic data (2D or 3D) into aworkstation. Ideally, the C-wave data should already haveundergone some initial squeezing to enable approximatevisual correlation of seismic features from P to C; however,this is not imperative.

2) Carefully interpret the most obvious and visible faults, keyhorizons, and any peculiar features on a representative P-wave line.

3) Display the same interpretation on the corresponding C-wave line. Depending on how well the initial squeeze wasdone, there may be some noticeable discrepancies on faultplacements and horizon interpretation on the C-wave line.The discrepancies should typically be on the order of a fewmilliseconds to a couple of hundred milliseconds verticallyhigh or low if a reasonable initial squeeze was applied tothe original C-wave data. Aside from the inherent expecteddifferences, such as those resulting from gas cloud areas,the discrepancies are likely attributable to the imprecisionassociated with the initial squeezing of the C-wave data.

4) Choose a fault, preferably one that cuts from shallow todeep, and a fault that is well imaged on both P and C. Byplacing the cursor on the P-wave fault plane (note the P-wave time) and moving the cursor up or down along thesame trace to the C-wave fault plane (note the C-wavetime), one can accurately determine the amount of “mis-registration” on C. Repeat this step from shallow to deepalong the fault plane to get representative P and C time pairs.

5) Repeat step 4 at other locations (along other fault planes orany other events that can be easily correlated). In a smallsurvey area, one function may work fine, but in large sur-vey areas or areas where the geology is very complex, morefunctions will probably produce better results.

6) Apply the derived P and C time pairs to “depth convert”the C-wave data. Now, run a reverse operation (depth totime) using a one-to-one function to bring the C-wave databack to time.

One key advantage of the fault plane registration methodis that an interpreter can do all the steps using a workstationinteractively and immediately be able to view the results. Thequality control is also done quickly by displaying the P-waveinterpretation on the C-wave data and by noting the discrep-ancies of fault placements and interpreted horizons.Oftentimes, if the time registration is not done properly, P-wave interpreted horizons will not follow the C-wave reflec-tors. In those cases, the applied function needs to be revisedand rerun. Figures 3 and 4 show application of this techniqueto the East Cameron 4C data set. Figure 5 demonstrates anapplication of a C-wave instantaneous phase section to fur-ther adjust the fault interpretation.

Interpretation methodology. Delivery of the final processedseismic volumes from PGS occurred in early January 2000, amonth prior to BP’s drilling program within East Cameron261. Obviously, there was not sufficient time for interpreta-tion of the data before drilling. There were, however, somekey factors that allowed us to drill a well with such a shorttime for interpretation and data analyses. First, comprehen-sive work had been done on previously acquired streamer 3Ddata in the area. Secondly, the P-wave volume of the new 4Cdata was processed first and delivered to us earlier, and fur-ther work was done using this new P-wave data prior to thedelivery of the C-wave data in early January. Most of Januarywas spent on the P to C time registration described earlier andinterpretation of the C-wave data.

Careful planning and streamlined interpretation of the C-wave data was imperative due to the drilling schedule. Somekey interpretation strategies that enabled us to meet the tightschedule were: (1) Most of the faults previously interpretedusing the P-wave data were directly used for the C-wave datainterpretation. This was possible because of successful P to Ctime registration. (2) All P-wave based interpreted horizonswere directly displayed in the C-wave volume and were prop-


Figure 7. P-wave (left) and C-wave (middle) time structure maps of3200-ft sand and the P and C time difference map (right). The C-wavemap illustrates the true structural configuration, and the time differencemap shows the general area of the gas cloud area. The outlined zones onthe C-wave map represent the gas charged areas.

Figure 8. P-wave (left) and C-wave (right) amplitude extraction maps ofthe 3200-ft sand. Gas-charged areas are outlined.

Figure 9. Plot of P-wave versus C-wave amplitude values of 3200-ft sand.Selected points that are bright on P and dim on C (in color) represent gasas seen on the map view to the right.

erly utilized. This allowed immediate identification of misin-terpreted zones such as those in the gas cloud area. Even incorrectly interpreted areas, the P-wave horizons did not alignperfectly on C-wave peaks, troughs, or zero-crossings. Somecauses for this misalignment are attributed to the inherent dif-ferences in P and C acoustic impedances, phase changes, lat-eral velocity variations, and imprecision of P to C timeregistration. Depending on the specific area and the cause ofmisalignment, some horizons were reinterpreted using the P-wave interpretation as a guide while others were tracked formaximum peaks, troughs, or zero-crossings using a smalltime analysis window to recreate new horizons that honor theC-wave data. (3) Finally, using the C-wave horizons producedfrom the work discussed above, various attribute maps werecreated which were further analyzed via appropriate cross-plots.

With the interpretation methodology described above, we


Figure 10. Plot of P-wave versus C-wave amplitude values of the 3200-ftsand. Selected points that are bright on P and also bright on C (in color)represent “false” bright spots as seen on the map view to the right.

Figure 11. Comparison of P-wave (left) and C-wave (right) time structuremaps of the 9000-ft (B2) sand. Note the differences of the highs, particu-larly in the areas circled in yellow.

Figure 12. Comparison of P-wave (left) and C-wave (right) time slices at2344 ms. Boxed area represents approximately the area of the 9000-ft sandmap.

were able to eliminate much duplication of effort and meetthe tight deadlines imposed on our drilling program.

Acquiring new data at the late stage of prospect genera-tion may perhaps be viewed as a huge burden, particularly ifthere is a set drilling schedule. However, in this project, theincorporation of the new 4C data into our interpretationsundoubtedly contributed to the success of our drilling pro-gram.

Application: 3200-ft sand. It is a common knowledge that P-wave energy can be scattered and attenuated when travelingthrough gas-saturated reservoirs. This is particularly a prob-lem when the objective zones lie below shallow gas sedi-ments, which degrade imaging of deeper features. The 3200-ftsand is a typical Class III sand and it is fairly continuous reser-voir, generally 20 ft thick. Approximately 40 billion ft3 of gashas been produced from three wells in two simple three-wayfault traps. This reservoir lies beneath gas sands at 1000 ft, 1700ft, and 2800 ft. The P-wave imaging of this reservoir is verypoor because of distortion caused by the overlying gas sedi-ments and overall background gas in the vicinity. The dis-continuity of reflectors, velocity push-downs, and generalstructural ambiguity are quite obvious on the P-wave data.The 3200-ft sand P-wave time structure map also reveals prob-lematic areas where there are lows in the middle of whatshould be an overall structural high trend. The C-wave imag-ing of this reservoir, on the other hand, is excellent. The gascloud is not present and, instead, crisp continuous reflectors


Figure 14. Log of Texaco well EC265 A5 showing the pay zone of the9000-ft sand.

Figure 15. Comparison of P-wave (left) and C-wave (right) seismicresponses of the 9000-ft sand. Location of this seismic profile (A-A’) isindicated on Figure 11.

Figure 13. Comparison of P-wave (left) and C-wave (right) amplitudeextraction maps of the 9000-ft (B2) sand.

Figure 16. Comparison of P-wave (left) and C-wave (right) seismicresponses of the 9000-ft sand. Location of this seismic profile (B-B’) isindicated on Figure 11.

Figure 17. AVO modeling illustrating PP and PS responses of wet andgas-charged 9000-ft (B2) sand.

Figure 18. Plot of P-wave versus C-wave amplitude values of 9000-ftsand. Selected points that are bright on P and dim on C (in color) repre-sent known gas area and prospects as seen on the map view to the right.

are in place along with clearly defined faults previously notwell imaged on the P-wave data (Figure 6). Anew 3200-ft sandmap constructed using the C-wave data shows the three-wayfault trap configuration very distinctly (Figure 7).

An additional step was taken to more precisely outline thegas cloud area that had an impact on this reservoir. A newmap was created by taking the difference between the P andC (postregistration) two-way times from the 3200-ft sand timestructure maps (Figure 7). The map shows that the time deltaincreases drastically from a just few milliseconds in the back-ground to a few tens of milliseconds in the gas cloud area.Thus, the amount of velocity push-down on the P-wave vol-ume due to shallow gas can be estimated using this map.

The amplitude extraction of the reservoir sand was cre-ated using both the P-wave and C-wave volumes. The P-wave amplitude extraction map (Figure 8) displays severalbright anomalies that are indicative of gas. It is also apparentfrom well penetrations and volumetric calculation of the reser-voir that a sizable portion of the gas-charged area does notappear bright on the P-wave amplitude map because of datadeterioration due to shallow gas. The C-wave amplitudeextraction map (Figure 8), however, shows most of the gas-charged area to be noticeably dim. In fact, the outline of three-way fault traps on the C-wave time structure map matchesvery well with the outline of dim areas on the C-wave ampli-tude extraction map. This observation is one of many exam-ples cited that demonstrates the pattern of bright on P anddim on C responses for gas sands.

A plot of amplitude values from the P-wave and C-waveamplitude extraction maps was generated for further analy-ses. The entire mapped area was used as input to distinguishanomalous areas from the background. A zone was selectedon the plot that corresponds to a cluster of points that repre-sents bright amplitudes on P and dim amplitudes on C. Thesechosen points were then transferred into map view, whichrevealed that most of the points lie on the gas-charged area.(Figure 9). To make a contrast, a different zone was selected;only those points that are bright on P and C were highlighted.The corresponding map view exhibited that these may in factrepresent “false” bright spots caused by lithologic anomaliesand not by fluid (Figure 10). The key learning point here isthat in this normally pressured Class III environment, gassands have a characteristic seismic response of bright on P anddim on C.

Application: 9000-ft sand. The 9000-ft sand (Class II), locallycalled the B2 sand, is typically 80-130 ft in thickness but, inplaces, can exceed 250 ft. The sand is deposited as part of themassive, thick deltaic sequences seen throughout the region.The traps for this productive sand are highly faulted three-way and four-way closures. The structure is set up by a seriesof antithetic faults to a major down-to-the-south growth fault.There are several gas-charged traps for this sand, but the arealextent of each is much smaller in comparison to that of the3200-ft sand discussed earlier.

The top of the B2 sand was independently mapped usingboth the P-wave and C-wave volumes (Figure 11). In con-structing these time structure maps, faults interpreted fromthe P-wave data were primarily used. The overall appearanceof both structure maps is very similar; however, there are sub-tle but very significant differences. Notice the shifts of the struc-tural highs from one map to the other (areas circled in yellowon Figure 11). Well ties to these maps reveal that the C-wavemap is, in fact, more accurate than the P-wave map. Moreover,with the anticlinal axis of the overall structural trend in mind,the C-wave map makes much more geologic sense.Representative time slices at this depth using P-wave and C-wave volumes (Figures 12) succinctly illustrate the trap con-

figuration. (Note that the boxed southeastern corner corre-sponds to the B2 map area.) It is also observed that while theoverall structure of the mapped area may be better definedon the C-wave time slice, the P-wave time slice visibly con-tains more details with better resolution. The C-wave data at


Figure 19. Comparison of P-wave (left) and C-wave (right) seismicresponses of the Valvulina (H) reservoirs. Notice the flat spot representingthe gas-water contact on the P-wave line is absent on the C-wave line.

Figure 20. Comparison of P-wave (left) and C-wave (right) seismicresponses of the Valvulina (H) reservoirs. Note arrows pointing to thegas-water contact zone of the Valvulina (H)-4 reservoir. At this location,the P-wave reflector changes from bright to dim and the C-wave reflectorchanges from dim to bright.

Figure 21. Comparison of P-wave (left) and C-wave (right) seismicresponses of the Valvulina (H) reservoirs.

Figure 22. Comparison of P-wave (left) and C-wave (right) time slicesrepresenting the Lenticulina gas sand.

this depth have narrower frequency bandwidth and, there-fore, subtle faults and fine details are sometimes not very wellilluminated.

The P-wave and C-wave amplitude extraction maps of theB2 sand (Figure 13) reveal a number of interesting anomalies.The areas outlined in black on the P-wave amplitude extrac-tion map are known gas areas that are characterized by brightamplitude anomalies. In contrast, the same areas on the C-wave amplitude extraction map show that they have consis-tently dim amplitude anomalies identical to the conclusionsderived from the 3200-ft sand study. Applying the same con-cept, several new prospects were generated, two of which areoutlined in red.

A representative P-wave and the corresponding C-waveseismic line over Texaco well EC265 A5 (Figure 14), a B2 sandgas producer, are shown in Figure 15. Additionally, anotherP-wave and its C-wave equivalent seismic profile over Shellwell EC266 1, a B2 wet sand penetration, are presented inFigure 16. Notice the consistent pattern of a bright on P anddim on C combination for the gas-charged B2 sand and a dimon P and bright on C combination for the wet B2 sand at therespective well locations. With a careful observation of the B2reflector in both P and C (Figure 16), one can distinctly see thesame pattern from fault block to fault block. (Known gas andprospect fault blocks are bright on P and dim on C; other faultblocks are exhibiting just the opposite behavior indicatingwet sand as proven by the EC266 1 penetration.) A detailedstudy of additional pay zones throughout the field resultedin similar conclusions.

AVO modeling of the B2 sand was performed and sub-stantiated that the amplitude strength of the PS reflector atthe shale/sand boundary is weakened with the introductionof gas into the sand (Figure 17). It is presumed that the slopeof this converted P-wave to S-wave AVO is primarily depen-dent on the S-wave impedance contrast at the interface. It isconcluded that the rock properties in the study area is suchthat, even in a normally pressured Class II environment, thegas sands have bright on P and dim on C signatures.

Figure 18 shows a plot designed to separate gas sands fromwet sands. The area that represents bright amplitude on P anddim amplitude on C is selected on this plot, and those pointsare once again transferred into map view. In contrast to the3200-ft sand plot, the y-axis now measures the differences ofP and C amplitude values. This type of plot exhibits a morelinear trend of points, which facilitates choosing an analysiswindow. The obvious observation from Figure 18 is yet againthat only the gas-bearing anomalies and identified prospectsare highlighted on the map view when bright on P and dimon C amplitude points are selected from the plot.

Other applications. More examples of interpreted and unin-terpreted P-wave and C-wave seismic lines and time slices arepresented here. Figures 19-22 are from data with widersource/receiver spacing; Figures 24-26 are from narrowersource/receiver spacing. As mentioned, it is quite apparentthat the overall quality of the C-wave data is noticeablyimproved on data with enhanced acquisition parameters. Alsothe reflectors are more continuous with broader bandwidthand higher resolution.

Some interpreters may just glance at the inferior qualityof the C-wave data and decide to only use the P-wave datafor prospecting; there is additional valuable information to befound in the C-wave data. Careful mapping and keen obser-vation of the differences in P and C responses are absolutelyessential in maximizing the uses of the multicomponent data.

As an example, let us analyze well EC261 A5ST2, one ofthe five recent discovery wells. Figure 19 is a P-wave and C-wave comparison of a line across this well. The well wasdrilled for multiple targets, with the primary target being theValvulina (H)-4 sand. While the quality of the C-wave datamay not allow accurate structural mapping, the overall C-waveresponse is dim for this reservoir. Incidentally, a distinct flatspot is seen on P-wave at the main part of the field pay, whichis noticeably absent on C-wave. The strong P-wave flat spotresponse is primarily due to the large P-wave velocity con-trast between gas and water-saturated sands. C-wave reflec-tivity, on the other hand, is due to contrasts in S-wave velocityand density. The density contrast alone is not sufficient in thiscase to produce an observable flat spot on the C-wave. An inter-esting seismic expression of the gas-water contact zone is also


Figure 23. 3D view of the Valvulina (H)-4 structure with a P-waveamplitude overlay. The gas/water contact seen here corresponds to the flatspot seen on Figure 19.

Figure 24. Comparison of interpreted P-wave (left) and C-wave (right)dip lines across the gas cloud area.

Figure 25. Comparison of uninterpreted P-wave (left) and C-wave (right)strike lines across the gas cloud area.

Figure 26. Comparison of interpreted P-wave (left) and C-wave (right)strike lines outside the gas cloud area.

seen in Figure 20. Using careful observation, one can see thatthe P-wave Valvulina (H)-4 reflector goes from bright to dimand the corresponding C-wave reflector does just the oppo-site at the contact zone. Figures 21 and 22 show two moreexamples of P and C gas sand signature comparisons. Figure21 compares P-wave and C-wave seismic profiles across theValvulina (H) reservoirs; and Figure 22 is a representativetime slice comparison of the Lenticulina gas sand. A 3D viewof the Valvulina (H)-4 structure with a P-wave amplitudeoverlay is shown in Figure 23. The gas/water contact seen herecorresponds to the flat spot on Figure 19.

A marked improvement of the C-wave data quality isquite evident with narrower source and receiver line spacing.A dip line over the gas cloud area, a strike line over the gascloud area, and a dip line outside the gas cloud area are shownin Figures 24, 25, and 26, respectively. The superiority of C-wave imaging in the gas cloud area is quite obvious, allow-ing the interpreters to accurately pick faults, construct maps,and tie wells. Good quality C-wave data, such as these, notonly allow better mapping of areas that are poorly imaged inP-wave, but also enable more technical analyses like thosedescribed in this paper. The authors firmly believe that con-siderable value was attained through the incorporation ofmulticomponent seismic interpretation in our exploration andexploitation efforts. Perhaps with even better C-wave dataquality, the industry will begin to employ wider and moresophisticated applications of multicomponent data in thefuture.

Conclusions. Several conclusions are made from this study.Using the fault plane registration method, the original C-wavetimes are squeezed successfully to match the P-wave times.Without a doubt, the C-wave data within and below the gascloud area yield far superior subsurface images than the P-

wave data. This allows for more accurate well ties and hencebetter structure maps. Additionally, a comparison of the P-wave and C-wave amplitude extraction maps reveals that gassands in the study area (both Class II and Class III) are con-sistently bright on P-wave and dim on C-wave. This dimmingobserved on C-wave may be an important criterion for dis-tinguishing gas sand reflectors verses “false” bright spots.

Suggested reading. “Useful approximations for converted-waveAVO” by Ramos and Castagna (GEOPHYSICS, 2001). “Imagingthrough gas clouds with converted waves” by Cafarelli et al.(World Oil, 2001). “Anisotropic 3D prestack depth imaging of theDonald Field with converted waves” by Nolte et al. (SEG 2000Expanded Abstracts). “Characterizing reservoir by using jointly P-and S-wave AVO analyses” by Jin (SEG 1999 Expanded Abstracts).Combining P-wave and S-wave seismic data to improve prospectevaluation by Hardage (Bureau of Economic Geology, TheUniversity of Texas at Austin, Report of Investigations 237, 1996).“Amplitude-versus-offset variations in gas sands” by Rutherfordand Williams (GEOPHYSICS, 1989). TLE

Acknowledgments: The authors thank BP management for allowing publi-cation of this paper. We are grateful for the enthusiasm and the strong sup-port for the project given by Steve Decatur, the asset manager at the time ofthe study. We are indebted to the members of BP’s Upstream TechnologyGroup, particularly Hans Sugianto, Dan Ebrom, and Jerry Beaudoin fortheir technical assistance, constructive comments, and discussions.Additionally, sincere appreciation is given to BP’s Wendy Kurek for helppreparing the figures. Finally, we thank PGS for its great work in the acqui-sition and processing of these 4C data and its cooperation in delivering thedata early enough to have an impact on the drilling program.

Corresponding author: [email protected]


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