lee000966 [depth conversion of tangguh gas fields](autosaved)

8/17/2019 LEE000966 [Depth Conversion of Tangguh Gas Fields](Autosaved)

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The Tangguh gas fields are offshore Irian Jaya in easternIndonesia. The depth conversion approach described herewas used to locate and provide the depth predictions forthe last seven certification wells drilled on the Vorwatastructure (Figure 1). Two seismic events were picked on theseismic data and were converted to depth—the Top Kais andthe Base Cretaceous (Figure 2). The Plio-Pleistocene clasticsection overlies the Miocene Kais carbonate. Both carbon-ates and clastics are present between the Top Kais and theBase Cretaceous. The Base Cretaceous horizon is the top of the Late Jurassic shale, which lies immediately above theRoabiba reservoir sand.

A two-layer model was created using the average veloc-ity from the surface to the Top Kais, and the interval veloc-ity from the Top Kais to the Base Cretaceous. Velocityvariation above the Top Kais is almost entirely due to com-paction. The velocity below the Top Kais is strongly influ-enced by lithology variation.

Two depth maps were created from this model, the TopKais depth map and the Base Cretaceous depth map. Addingthe well-derived Late Jurassic shale isopach to the seismi-cally derived Base Cretaceous depth map created the TopRoabiba Reservoir Sand depth map. This approach wasused because the top of the Roabiba Sand is a weak seismicevent, which cannot be picked over the entire area. This prob-lem is aggravated by the fact that peak frequencies at the

Jurassic level are 12-15 Hz.This depth conversion approach was adopted after com-

pletion of well V-4 and was used to locate and provide thedepth predictions for wells V-5 through V-11. The final cer-tification depth maps for the entire Tangguh area were alsogenerated using this method.

In our analysis we use the model, Vavg = V0 + kz. Marsdenet al. (1995) discuss the differences between the averageinterval velocity, which we use in this analysis, and instan-taneous interval velocity. They also discuss the pros and consof holding k constant and varying V0, or vice versa. Velocitiesin our analysis are interpreted as related to compaction andlateral lithology variation. Japsen (1998) provides a detailedanalysis of velocities related to geopressure and uplift in theNorth Sea. He includes many references on depth conver-sion.

Methodology for map construction. The Top Kais horizonis relatively flat and dips to the east as seen in a west-to-east seismic traverse (Figure 2) and in the Top Kais time map(not shown). The variation in average velocity from the sur-face to the Top Kais is almost entirely due to compactionand is modeled very well by the linear function,

Vavg = V0 + kz,where z is the depth to the Top Kais.

The average velocity, Vavg, is computed using the log pickfor the Top Kais and the seismic time pick. To avoid timepick errors due to different vintages of data, we only usethe wells within the 3D marine volume for determining theslope, k (Figure 3).

To create the Vavg map, we replace z in the linear func-tion with VavgT (T = traveltime to the Top Kais) and (after

966 T HE L EADING EDGE OCTOBER 2002 OCTOBER 2002 T HE L EADING EDGE 00

Depth conversion of Tangguh gas fields

T IM K EHO , Saudi Aramco, Dhahran, Saudi Arabia

D HARMAWAN S AMSU , BP, Houston, Texas, U.S.

INTERPRETER’S CORNER

Coordinated by Rocky Roden

Figure 1. Map of Tangguh Gas Fields.

Figure 2. West to east seismic traverse. The Top Kais is a relatively flathorizon dipping gently from west to east. The average velocity to thishorizon is expected to be controlled primarily by compaction. The sec-ond layer is more complex. Notice the additional section below the BaseKais unconformity in the syncline east of the Roabiba-1 well. Lateralvariation in lithology is expected to have a significant impact on theinterval velocity for this layer.

Figure 3. Crossplot—surface to Top Kais. As expected, the average veloc-ity to the Top Kais is primarily controlled by compaction and is a linear

function of depth.


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some algebra) generate the following expression for Vavg:Vavg = V0/(1 - kT).Vavg is computed by grid operations in ZMAP where V0 andT are grids, and k is the constant determined by the linearregression in Figure 3. The T grid is the Top Kais time struc-ture map grid. The V0 map is generated by contouring the V0data computed at each well, V0 = Vavg - kz, where k is the samefor all wells. The V0 and Vavg maps are shown in Figures 4 and5. The resulting depth map is shown in Figure 6.

Because the velocity function is linear, the contours of the velocity map, Vavg, look similar to the time map. For thissame reason, the depth map also looks similar to the timemap.

The Base Cretaceous time map is shown in Figure 7.

Judging by the amount of structure on this map we expectthe interval velocity for this layer to be more complicatedthan the simple compaction model used for the first layer.This expectation is reinforced by the seismic data. The addi-tional section just below 1500 ms between wells Roa-1 andV-3 on the seismic traverse (Figure 2) clearly indicates thatwe should expect lateral velocity variations due to changes

in lithology.For simplicity, however, we again proceed by using alinear model. The interval velocity, Vint, from the Top Kaisto the Base Cretaceous is modeled as,

Vint = V0 + kZmid,where Zmid is the depth to the midpoint of the Top Kais toBase Cretaceous layer. Zmid represents the average depth of the second layer.

We could have chosen the top, Ztop, or the base, Z base, but neither of these represents the depth of layer 2 as accu-rately as the average depth, Zmid. One can imagine a layer2 scenario where Ztop is constant and Z base is dipping. Vintin this case will increase where layer 2 is thickening as the

base gets deeper. If you plot V int versus Ztop, you will notsee a correlation because Ztop is constant. Similarly, if Ztopwere dipping and Z base were constant, you would not finda correlation between Vint and Z base. In both cases, however,you would find a correlation with Zmid, which is the aver-age depth (Ztop + Z base)/2.

In general, neither Ztop nor Z base are constant. This is whatseparates layer 2 from layer 1. Ztop for layer 1 is constant.Therefore, for layer 1, plotting Vint (Vint = Vavg for layer 1)versus Zmid is no different than plotting versus Z base. For layer1, Zmid = (0 + Z base)/2 = Z base/2. So Zmid is just a factor of 2different than Z base. This means the correlation would be thesame whether you used Zmid or Z base. We chose to use Z base,which is much more commonly used for the first layer.

Figure 8 shows the regression for the data within the 3Dsurvey. Notice that the scatter is much larger than for the

Top Kais average velocity (Figure 3). Clearly, the variationin interval velocity for the second layer cannot be explained by compaction alone.

Returning to the expression for interval velocity, becauseZmid is not known away from the wells, we use Zmid =ZKais+VintT/2, and rewrite Vint as:

Vint = (V0 + k ZKais)/(1 - kT/2),where ZKais is the Top Kais Depth grid, T is the Top Kaisto Base Cretaceous isochron grid, and the V0 grid is com-puted in the same manner as described previously for theTop Kais depth map. The gradient, k, is the constant deter-mined by the linear regression in Figure 8.

Figure 9 shows the V0 map for the second layer. Asexpected, this map shows considerably more variation than


Figure 4. Top Kais V 0. If the linear model fit the data exactly, V 0 wouldbe constant. As shown here, V 0 is almost constant, indicating that thelinear compaction model is quite accurate.

Figure 5. Top Kais V avg. The V avg map looks similar to the time map.Velocity increases to the east as the Top Kais gets deeper.

Figure 6. Top Kais depth. Because there is essentially no lateral velocityvariation due to lithology, the depth map looks very similar to the timemap.

Figure 7. Base Cretaceous time. The Kalitami, Wiriagar, Ofaweri,Roabiba, and Vorwata structures are dominant features on the BaseCretaceous time horizon.


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the V0 map for the first layer. Because we are using a linearmodel for interval velocity, the effect of lateral variation inlithology will show up in the V0 map. The interval velocityis composed of two components, V0, which is a function of x and y, and kZmid, which is a function of depth. Therefore,in general, lateral lithology variation is modeled by the V0term, and compaction is modeled by the kz term.

To see if this V0 map makes geologic sense, we can exam-ine the isochrons of the geologic intervals that compose thesecond layer. Figure 10 shows a seismic traverse flattenedon the Base Cretaceous. Figure 11 is the isochron for layer2, Top Kais to Base Cretaceous. All present day BaseCretaceous structural highs show up as thins on this

isochron. The thick carbonate sections occur in the synclines between the structures.

By looking at layer 2 in more detail, we see that it is com-posed of an upper carbonate section and a lower clastic sec-tion. The Top Paleocene to Base Cretaceous isochron isshown in Figure 12. The Paleocene clastic wedge is thick overthe onshore part of Wiriagar and thins to the south andsoutheast. The smooth variation in thickness away from thePaleocene depocenter indicates that there was little or nostructure at Base Cretaceous time.

Figure 13 shows the Top Eocene to Top Paleoceneisochron. The Eocene clastic wedge is thin over the onshore


Figure 9. Top Kais to Base Cretaceous V 0. Notice that the V 0 map for thislayer is not almost constant as it was for the Top Kais.

Figure 10. Eocene clastics and Paleocene wedges (seismic traversethrough V-7). By looking at a series of isochrons within the second layer,we can better understand the relationship between the V 0 map and lithol-ogy variation.

Figure 11. Isochron—Top Kais to Base Cretaceous. The Tangguh struc-tures are quite noticeable in this isochron. They formed during the

Miocene prior to the Base Kais unconformity.

Figure 12. Isochron—Top Paleocene to Base Cretaceous. The Paleoceneclastic wedge is thick over the onshore part of Wiriagar and thins to thesouth and southeast. The smooth variation in thickness away from thePaleocene depocenter indicates that there was little or no structure at BaseCretaceous time.

Figure 13. Isochron—Top Eocene clastics to Top Paleocene. The Eoceneand Paleocene clastic wedge is thin over the onshore part of Wiriagar,thickens off the Paleocene shelf edge and thins to the south and southeast.The smooth variation in thickness indicates that there was little or no

structure at the time the Eocene clastics were deposited.

Figure 8. Crossplot—Top Kais to Base Cretaceous. Interval velocity atwell locations is plotted versus the midpoint depth of the second layer.Notice that the velocity for this layer is not a simple linear function of depth.


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part of Wiriagar, thickens off the Paleocene shelf edge andthins to the south and southeast. The smooth variation inthickness indicates that there was little or no structure atthe time the Eocene clastics were deposited.

Adding the previous two isochrons produces the TopEocene to Base Cretaceous isochron (Figure 14). This inter-val contains Eocene and Paleocene clastics and Cretaceouscarbonates. Because the Cretaceous has uniform thickness,these carbonates have little impact on lateral velocity vari-ation. Notice that the clastic section varies smoothly. It is

thick to the north and thins to the south, southeast, andsouthwest.

Because the velocity of the carbonates is higher than thevelocity of the clastics, we can get an idea of the lateral veloc-ity variation due to lithology by constructing a carbonatefraction map. The carbonate fraction (Figure 15) is theisochron of the carbonate interval from the Top Kais to theTop of the Eocene clastics divided by the isochron of theentire interval from the Top Kais to the Base Cretaceous.Notice that the carbonate fraction varies from 0.40 over theonshore portion of Wiriagar Deep to over 0.80 east of Vorwata. As expected, the V0 map is similar to this map, withhigher velocities associated with higher carbonate fraction.

Ideally, having noticed the correlation of velocity and

carbonate fraction, we would prefer to separate the secondlayer into two layers and use a three-layer model.Unfortunately, the Top Eocene clastics horizon cannot be tiedto the wells with sufficient accuracy to create a three-layermodel which is more accurate than the two layer model. Thisis because the Top Eocene clastics do not correspond to adefinitive log marker. Therefore, a three-layer model is notused. Instead, the carbonate fraction map is used qualita-tively to modify the V0 map for the second layer.

Another model investigated for predicting the isochorefor the second layer was a dual regression of the intervalvelocity as a function of both depth and carbonate ratio, Vint= V0 + k1Zmid + k2CF. Because both the carbonate fractionand the depth to the Base Cretaceous increase to the south-east, and increase in the syncline, it is possible to create an

accurate velocity model with less dependence on depth(lower k1), and a stronger correlation with carbonate frac-tion.

This approach was not used initially because the veloc-ities were not high enough on the flanks of Roabiba to closethat structure at the expected spill point. It was neveradopted later on because the results of the single regressionmodel were very accurate.

The interval velocity for the second layer is computedusing the V0 map as described above. The map is shown inFigure 16. The Base Cretaceous depth map is created byadding the Top Kais to Base Cretaceous isochore to the TopKais depth map.

One problem with the layer approach to depth conver-

sion is artifacts in the depth map due to faults in the shal-lower layers. This is a problem for the Wiriagar Deep area because the higher intensity folding there resulted in sig-nificant faults in the Top Kais horizon. Simply adding thelayer two isochore to the Top Kais depth map results in TopKais faults being visible on the Base Cretaceous depth map.

This problem was addressed by computing the averagevelocity from the surface to Base Cretaceous by dividing theBase Cretaceous depth map by the Base Cretaceous timemap. The average velocity map was smoothed west of thesyncline between Wiriagar Deep and Vorwata to remove theartifacts, and then multiplied by the time map to computethe final Base Cretaceous depth map (Figure 17).

Results. Table 1 compares actual versus predicted BaseCretaceous depths for Vorwata. The predicted depths for V-5 through V-11 were taken from a map generated in the latefall of 1997. The right column shows the distance from thenearest control well. Figures 18 and 19 show, for the Vorwataarea only, the layer 2 interval-velocity map and BaseCretaceous depth map. The control points used for devel-


Figure 14. Adding the previous two isochrons produces the Top Eocene toBase Cretaceous isochron. This represents the clastic portion of the secondlayer.

Figure 15. The carbonate fraction is computed by dividing the carbonateisochron (Top Kais to Top Eocene clastics) by the layer two isochron (TopKais to Base Cretaceous). This is an approximation because the actualcarbonate fraction is a ratio of isochores not isochrons. Notice the similar-ity between the carbonate fraction map and the V 0 map for layer 2 (com-

pare to Figure 9). This similarity is convincing evidence that the variationin V 0 is due to lateral variation in lithology, and, as a result, increases ourconfidence in the V 0 map. The carbonate fraction concept allows us toextend the V 0 map away from well control.

Figure 16. Top Kais to Base Cretaceous V int. The structures are visible inthe interval velocity map due to the Zmid term, which causes V int toincrease with depth.


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oping the depth map are wells V-1 through V-4(red).

The accuracy of the depth map is excellent, par-ticularly considering the narrow bandwidth of theseismic data after propagation through about 6000ft of carbonates. Notice that the errors are less than50 ft at depths of 12 500 ft, for V-5, V-6, V-9, and V-10 which are up to 6.2 km from the nearest control.The errors are twice that for V-8 and V-11, but thesewells are twice as far from the nearest control.

Dolomites, encountered for the first time in V-11, aremost likely the cause for higher than predictedvelocities to the east in V-11 and V-8.

It is important to note that all predicted depthscame from the same map. Predicted depths for wellsin this table were not updated using results fromprevious wells. Three of the wells were drilledsimultaneously. After drilling was completed, a finalmap was generated which exactly ties all wells.

The largest error is at V-7; however, this is a spe-cial case. It was located on the northwestern flankof the Vorwata structure with the goal of encoun-tering the gas/water contact. Therefore, the well waslocated at the presumed spill point of the structure

and aggressively positioned down flank so that theGWC would be encountered at the top of the RoabibaSand (Figure 20). Notice that the well is in the bot-tom of the syncline in the time domain.

The primary control on the velocity at this location isthe predicted GWC from pressure data. The carbonate frac-tion map shows higher carbonate fraction in the syncline

between Wiriagar Deep and Vorwata. Because the V0 mapwas derived from well data only, the syncline did not showup as higher velocity on the V0 map. Therefore, the veloci-ties on the V0 map were increased in the syncline to be con-sistent with the higher carbonate fraction in the syncline.This allowed the depth of the Top Roabiba sand to be pusheddown below the GWC.

As shown in Table 1, the Base Cretaceous came in shal-

lower than predicted. The GWC was not encountered in thewell. In order to explain how the Vorwata structure canclose without encountering water in V-7, we examined therelative thicknesses of the different carbonate sections acrossthe syncline. Because the Kais and Sago limestones haveabout the same velocity but are slower than the underlyingEocene carbonates, the syncline is shallower in depth on theeast than on the west. This is because the higher velocityEocene carbonates make up a larger fraction of layer two atthe west edge of the syncline than at the east edge.

Conclusion. The depth predictions for four of the final fivecertification wells located by ARCO on the Vorwata struc-ture were accurate within 50 ft of actual tops, at depths of

almost 13 000 ft. These wells were 4-6 km from the nearestwell used for generating the depth map. Two other wellshad about twice the error but were also twice as far (10-11km) from the nearest control well. This success, along withaccurate prediction of reservoir thickness, provided a con-fident determination of bulk volume for certification.

Generating velocity maps, as opposed to using a func-tion to convert directly from time to depth, is a very usefulstep in interpretation and is highly recommended. Bullseyes around wells in the V0 map may indicate errors in theseismic time picks, or errors in the formation tops.Generating preliminary V0 maps while still interpreting theseismic time horizons allows resolution of these problemsearly in the interpretation. Smoother anomalies in V0 may


Table 1. Base Cretaceous depth—actual versus predicted

Well

V-5V-6V-7V-9

V-10V-11V-8

Mean

Actual

12 47012 37112 79112 524

12 52312 87512 396

Predicted

12 42012 38213 03012 481

12 5201274412278

Error

-5011

239-43

-3-131-118

-14

%Error

-0.400.091.87

-0.34

-0.02-1.02-0.95

-0.11

Distanceto nearestwell (km)

5.03.64.26.2

4.511.29.6

Averagedistance to

V-2, V-3, V-4

5.94.89.5

10.1

10.214.211.0

The prognoses for these seven wells were very accurate. It is important to note that allof these prognoses came from a single map constructed after V-4. The prognoses werenot updated after each well was drilled. Ignoring V-7 for a moment, which is a specialcase, notice that wells drilled up to 6 km from the nearest control well were predictedwithin 50 ft at depths over 12 000 ft. Depth errors over 100 ft only occur for V-8 andV-11 which are 10 to 11 km from the nearest control. These two wells came in deeperthan predicted due to faster dolomites which were not encountered in the carbonatesection in the wells to the west. The simple carbonate/clastic model for Layer 2 did nottake into account variation in carbonates. Note that these prognoses are much moreaccurate than the scatter around the linear regression would indicate. This is because

the scatter is not random. The lithology variation, which causes the scatter, has beentaken into account through the V 0 map. Since V 0 is not constant, the interval velocitymodel is no longer a simple linear model.

Figure 17. The Base Cretaceous depth map is computed by multiplyingthe interval velocity and isochron grids for Layer 2 to produce the Layer 2isochore map which is added to the Top Kais depth map.

Figure 18. Kais to Base Cretaceous V int (Vorwata only). This depth con-version method was used to predict the Vorwata wells drilled after V-4.This map shows the locations of the control wells in the Vorwata area:Roabiba-1 and V-1 through V-4. Notice the large variation in intervalvelocity east of V-2. These velocity contours could easily be drawn in amore east-west direction resulting in interval velocities hundreds of feet

per second different than shown. Because Layer 2 is about 6000 ft thick,velocity errors this large would result in depth errors of several hundred

feet.


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indicate areas of uplift or lateral velocity variation due tochanges in lithology. These hypotheses can then be testedusing other data, such as isochrons.

Suggested reading. “Regional velocity-depth anomalies, North

Sea Chalk: A record of overpressure and Neogene uplift anderosion” by Japsen et al. (AAPG Bulletin, 1998). “Analytic veloc-ity functions” by Marsden et al. (TLE, 1995). T L E

Acknowledgments: We thank Benny Yusuf for his contributions and the following companies for permission to publish: Atlantic Richfield Indonesia

(Now BP Indonesia), KG Berau Petroleum, KG Wiriagar Petroleum,Occidental Oil and Gas Corporation, Nippon Oil Exploration, BGInternational Indonesia, CAIRNS, and Indonesia Natural Gas Resources

Muturi. Tim Keho was formerly with BP, Jakarta, Indonesia.

Corresponding author: [email protected]


Figure 19. Base Cretaceous depth (Vorwata only). This is the close-up of the Base Cretaceous depth map showing the prognosed wells, and thecontrol wells.

Figure 20. V-7 was located on the northwestern flank of the Vorwatastructure with the goal of encountering the gas water contact. Because theKais and Sago limestones have about the same velocity but are slowercarbonates than the underlying Eocene carbonates; the syncline is shal-lower in depth on the east than on the west. This is because the highervelocity Eocene carbonates make up a larger fraction of layer two at thewest edge of the syncline than at the east edge.

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