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Geodynamically, Melandong Field in Indonesias north-west Java Basin (Figure 1) resides in an active margin areacontrolled by the subduction of the Eurasia plate in thenorth and beneath the India-Australia plate in the south.During the Tertiary period, the basin underwent two tec-tonic deformation phasesa tensional stress regime duringthe Paleogene and a compressional stress regime during theNeogene.

The depositional process was also affected by these twotectonic phases and resulted in two main sediment groups:Paleogene synrift deposits represented by the Talang AkarFormation (TAF), and Neogene postrift deposits represented

by the Baturaja, Cibulakan, Parigi, and Cisubuh formations(Ascaria et al., 2000). Major producing reservoirs in the basinare the Talang Akar and Baturaja.

The Paleogene synrift Talang Akar Formation depositedin a fluvial environment in the older, lower interval whichgradually changed to fluviodeltaic and shallow marine envi-ronments in the younger, upper interval. It consists mainlyof interbedded quartz sand, lithic arenite, and shale withcoal streaks.

The early Neogene postrift sequences are interbeddedargillaceous sands with thin limestones in the lower sectionwhich gradually change to the pure limestone of BaturajaFormation in the upper part. The postrift sediments are

characterized by karstification of Baturaja Formation over-lain by transgressive sequences consisting mainly of fine-to-medium grained glauconitic sands passing upward intothe pure limestone of Lower Cibulakan Formation. Figure2 illustrates the characteristics of prerift, synrift, and postriftdeposits in the study area. Figure 3 gives the complete strati-graphic column of the Northwest Java Basin.

Integrating seismic attributes for reservoir characterizationin Melandong Field, Indonesia

SIGITSUKMONOand DJOKOSANTOSO, Institute of Technology Bandung, Indonesia

ARISAMODRA, WALLYWALUYO, and SARDJITOTJIPTOHARSONO, Pertamina Upstream, Jakarta, Indonesia

INTERPRETERS CORNER

Coordinated by Rebecca B. Latimer

532 T HE LEADING EDGE MAY2006

Figure 1. Location of the study area.

Figure 2. Illustration of the prerift, synrift, and postrift deposits in the study area.

Downloaded 25 Sep 2012 to 114.79.55.229. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/

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The main objective of the study was to complete a seis-mic reservoir characterization on the fluviodeltaic channelsof Talang Akar Formation and the carbonates of Batu RajaFormation (BRF).

Seismic reservoir characterization is a process to describequalitatively and/or quantitatively the detailed character ofthe reservoir using primarily seismic data. The process gen-erally has three main steps: delineation, description, andmonitoring.

Reservoir delineation is the determination of a reservoirsgeometry, including the faults and facies changes which canaffect production. Reservoir description defines the reservoirsphysical properties (porosity, permeability, water saturation,pore fluid, etc.). Reservoir monitoringmainly is associated withidentifying physical property changes during the productionof hydrocarbons. In this case, it is mainly porosity that willaffect the reservoir production performance. The integratedresults were subsequently used to further the explorationand development of the TAF and BRF reservoirs.

Methodology. Seismic attribute analysis is generally definedas a derivative of the basic seismic measurement. All avail-able horizon and formation attributes are not independentof each other. The differences are in the analysis of the basic

information contained in the seismic wavelet and their result-ing seismic properties. For example, the acoustic impedanceresulting from seismic trace inversion will give informationon the product of velocity and density, while the instanta-neous phase (from the complex trace) will highlight theinformation of wavelet phase.

The attributes used in charac-terizing TAF and BRF reservoirswere:

1) complex attributes for the topsof the TAF and BRF surfaces,BRF facies identification, andmapping direct hydrocarbonindicators (DHI)

2) an rms amplitude varianceattribute for paleogeographyand facies analysis, and sandthickness mapping

3) acoustic impedance for poros-ity mapping

The application of complexattributes. The computation ofcomplex seismic attributes is

basically a transformation to sep-arate the information on ampli-tude and angle (phase and frequency) into distinct displays.Information in the seismic section will be mathematically

manipulated to produce a new display which will enhancethe amplitude or angle component. The term complexdoes not mean that the procedure and its results are com-plex but refers to the computation which assumes that con-ventional seismic trace data are the real part of a complexmathematical function. The imaginary component is ob-tained by applying a Hilbert transform to the real seismictrace. The imaginary trace is identical to the real trace butwith its phase shifted 90. It can be assumed that this rep-resents the potential energy while the real trace representsthe kinetic energy of particles that are oscillating as theyrespond to seismic wave energy. Imaginary trace data areused as a basis for other complex attribute computations.Because the imaginary trace is merely the result of a 90

phase shift from the real data, it does not contain new infor-mation but it can give a new perspective on the interpreta-

tion because it often highlights reflector details which areobscured in real data.In this study, the main types of complex attributes used

were instantaneous phase and the cosine of the phase.We know that phase will always have values between

180 and +180and that, in general, as the real trace changesfrom the peak to the trough, the instantaneous phase changesfrom 0 to +180. On the trough, the instantaneous phase issharply wrapped from +180 to -180. Therefore an instan-taneous phase display is like a discontinuous sawtooth dueto the sharp change in the phase. To obtain a more nor-mal display, the cosine of phase was used in this project.

Independent of the magnitude of peak or trough ampli-tudes, the magnitudes of instantaneous phase are always

MAY2006 THE LEADING EDGE 53

Figure 3. Stratigraphy of Northwest Java Basin. The studied reservoirswere Talang Akar Formation and Batu Raja Formation (BRF).

Figure 4. Flattened instantaneous-phase section showing that the development of the TAF delta-plainchannel was mostly controlled by the paleohigh normal growth faults.


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Figure 5. Cosine of phase section showing the BRF facies. The top of BRF is identified by the green line and the top of TAF by the magenta line.The red line shows the position of a 2000-ms time slice in the variance cube.

Figure 6.A closeup of cosine of phase showing the DHI anomaly as a flat spot and reversals of the sign of the cosine phase in a structure northwest ofwell Melandong-1. Red is positive and blue is negative. The extent of the flat spot (dashed yellow line) can be roughly mapped and is indicated by redrings in Figure 13.


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the same (0 for peak amplitude, 180for troughs). Because of this, the instan-taneous phase tends to balance the weakand strong reflectors. Therefore whenanalyzing instantaneous phase, it is eas-ier to interpret the weak coherent reflec-tors such as unconformities or sequence

boundaries (SBs) and reflectors with dif-ferent dips (onlaps and downlaps) asshown in Figure 4.

It has been widely known that thetops of both TAF and BRF were sequence

boundaries (SBs) associated with re-gional relative sea-level drops. In theseismic sections, these two tops werecharacterized by distinct erosional trun-cation and onlapping/downlapping ofseismic reflectors. Therefore for pickingand mapping these two SBs, instanta-neous phase was intensively applied.

Using seismic stratigraphic conceptswhich mostly rely on the geometry of thereflectors, the BRF carbonate facies can be grouped into themain reef, backreef, and forereef environments. To enhance

the reflectors geometric differences for these three facies, thecosine of phase displays were used (Figure 5). The backreeffacies is characterized by parallel, horizontal, strong contin-uous reflectors with high similarity. The main reef hasmounded, discontinuous reflectors with low similarity. Theforereef has sigmoid discontinuous reflectors with low sim-ilarity. As these three facies are also characterized by a degreeof similarity, variance was used to map their distribution.

DHI signature analysis was performed to locate ampli-tude anomalies such as flat spots, bright spots, dim spots,and polarity reversals which could be associated with thepresence of hydrocarbons. Phase attributes again wereapplied intensively in this analysis because the phaseattribute enhances the weak reflectors such as the flat spotsor the cosine of phase reversal in the top of gas column and

gas-fluid contact. In Figure 6, for example, the DHI signa-ture is clearly displayed as cosine phase changes from pos-itive to negative at the top of the reservoir and from negativeto positive at the flat spot showing the gas-water contact.

The application of variance and rms amplitude attributes.The TAF section was deposited in the environment from thedelta plain to the delta slope (Priambodo et al., 2001). Furtheranalysis using well and seismic data revealed that, in thedelta plain, the TAF fluvial channels developed under con-trols of synsedimentary faults and the presence of paleo-highs. To map the channel geometry and thickness of thesand in the channels, a combination of rms amplitude andvariance was used (Figure 7).

Variance, which measures the degree of dissimilarity, isapplied to enhance lateral changes of seismic data due tochanges of geologic conditions. In the variance cube, themain similarity measurement is in the crosscorrelation,which compares the center trace with neighboring traces bymultiplying samples from the center trace times those of aneighboring trace within a window and normalizing by theautocorrelations of these two traces. Because the TAF flu-vial channels were developed under controls of synsedi-mentary faults and the presence of paleohighs, the similarityof traces crossing the faults or paleohighs is generally asso-ciated with a low autocorrelation.

Faults or paleohighs with significant vertical elevationare generally low-similarity zones which also occur if there

are abrupt contrasts in seismic character due to stratigraphicor lithologic changes (such as in the case of channel sands).To evaluate the validity of this analysis, the well datas grosssand thickness was plotted against rms amplitude vari-ance. A linear relationship was obtained where high vari-ance or low similarity associated with a paleohigh wasrelated to thinner gross sands and the high similarity wasrelated to thicker gross sands of the channel deposit (Figure8). However, since this relationship is based on data from

just three wells, this conclusion is preliminary and must beapplied cautiously.

The application of acoustic impedance. Acoustic imped-ance (AI) is a rock parameter determined by lithology, poros-


Figure 7. 3D visualization of the distribution of TAFs fluviodeltaic deposits using rmsamplitude.

Figure 9. Plot between AI (horizontal axis) and porosity for BRF inter-val. The lower AI is associated with higher porosity.

Figure 8. The attribute rms amplitude variance (horizontal axis)plotted against gross sand (vertical axis) for TAF interval.


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Figure 11. Time slice at 2000 ms of the vari-ance attribute (time slice position is shown inFigure 5). Low variance or high similarity isimaged as a light color; high variance or lowsimilarity is imaged as a dark color. It isapparent that the backreef and main reef linea-ment at well Melandong-1 are controlledmainly by the structure.

Figure 10. 3D model of TAF gross sand distribution using the attribute rms amplitude variance. Low attribute values (blue) are associatedwith thick sand; high attribute values (red) are associated with the thin sand.


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ity, fluid contents, depth, pressure, and temperature. AI willbe controlled mostly by the P-wave velocity. Porosity andgas content have the greatest effect on the value of P-wavevelocity. In this area, high porosity is usually related to lowvelocity and vice versa.

Poststack seismic inversion techniques are generallyused to get quantitative AI values from reflectivity. Theapplied inverse modeling algorithm, basically, is a decon-volution between the seismic record and seismic wave whichthen produces the AI section. The AI section will give thesubsurface geology more detail than the normal seismicsection, because the reflection coefficients on the normal seis-mic section will image the interface boundary while AI willimage the layer itself.

In this study, AI was used to map the porosity of the TAFand BRF. A crossplot between AI and porosity (Figure 9), asexpected, shows a relatively linear relationship with higherporosity associated with low AI and vice versa.

Results. Using the method above, the paleogeography andthe facies of the TAF and BRF reservoirs were modeled andmapped. Figure 10 shows the 3D visualization of the TAFfacies and the paleogeography using the rms amplitude variance attribute. From this visualization, it could be con-cluded that the channel geometry and gross-sand distribu-tion in TAF were very affected by the presence of paleohighsand faulting during TAF deposition. The east-west tensionstress system created paleohighs (horsts) and paleolows(grabens) bounded by NS, NNESSW and NWSE normalfaults, which controlled the channel geometry and gross sanddistribution. The NS and NNESSW faults played a moredominant role than the NWSE faults.

The depositional extent of backreef, main reef, and for-

ereef facies of the BRF could also be deduced from therelated variance attribute model (Figure 11). Two reef sys-tems could be recognized; the Melandong in the southeastand the Wanajaya in the northwest. In both, the backreeffacies, with high similarity, was imaged as a low-variancearea (light color), but the main reef and forereef with lowsimilarity were imaged as high-variance areas (dark color).The lineaments of the backreef, main reef, and forereef faciesin both reef systems were NS, agreeing with the previousdiscussion that NS faults played a dominant role in TAFand BRF facies development. In the open basin betweenthese two reef systems, several oval-shaped features indi-cating pinnacle reefs could be recognized. The linear fea-ture of the barrier reef in the south end of Melandong reefsystem was also observed.

An integrated analysis of the sequences and faciesrevealed that the depositional interval of the preTAF wasrelated to the transgression which ended when sea leveldropped. The depositional interval of TAF was also related

to the transgression process which reached a maximum whenBRF was deposited and also ended with a fall in sea level.Thus overall, the preTalang Akar, Talang Akar, and Baturajasequences represent a transgressive cycle, with a depositionaldirection of NNWSSE and a landward direction to the north.

Mapping porosity and DHI signatures. As discussed pre-viously, the porosity maps for the TAF and BRF intervalswere constructed from the AI derived from seismic inver-sion. Because the seismic inversion was completed foronshore data only, the porosity maps could be constructedonly in these areas. Figure 12 shows a porosity map for a10-ms window below top TAF. Red is associated with lowAI and high porosity; blue, with high AI and low porosity.


Figure 12. 3D visualization of the distribu-tion of the top TAF porosity derived from

AI. The paleohigh was subjected to a moreintense erosional process which in turncontrolled the porosity distribution.


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Based on the porosity maps of the TAF and BRF, it wasapparent that the porosity distribution in these two intervalswas controlled by the NS, NNESSW and NWSE faults.In addition to the fault control, the paleoelevation also playedan important rolei.e., the east has a higher elevation thanthe west and therefore was subjected to a more intensive ero-sional process and appears to have higher porosity.

During the DHI signature mapping, six potential ampli-tude anomalies were observed. The first was in a structurenorthwest of well Melandong-1, the second and third werewest of Melandong-1, the fourth was in the proximity of wellKHB-2, the fifth and sixth were northeast and north of wellWanajaya-1. The observed DHI signatures were mainly evi-dent in the BRF interval as flat spots and as a reversal of thecosine phase (Figure 6). Recent drilling results confirm theexistence of hydrocarbons in the structure northwest of wellMelandong-1 and within the Melandong structures.

This technique for mapping DHI signature allows theextent of the flat spot in each anomaly to be roughly mapped.Figure 13 shows that the flat spots with the largest arealextent (within the red rings) were concentrated in the east-

ern portion of study area. This may be related to the factthat the east experienced a more intensive faulting and ero-sional process which would tend to increase the porosity ofthe rocks in the area.

Conclusions. The integration of phase attributes, rms ampli-tude, variance, and AI has been successfully applied to theinterpretation and characterization of the TAF channels andthe BRF carbonate reservoirs in Melandong Field.

The 3D visualization of the TAF facies and the paleo-geography using an rms amplitude variance attributeconcluded that the channel geometries and gross sand dis-tribution in the TAF were highly affected by the presenceof paleohighs and faulting at the time of the TAF deposi-

tion. The east-west tension stress system created paleohighs(horsts) and paleolows (grabens) bounded by NS, NNE-SSW, and NWSE normal faults. These faults then controlledthe channel geometries and gross sand distribution.

The BRF carbonate facies can be grouped into three cat-egories; main reef, backreef, and forereef. Two reef systemscould be recognized; the Melandong in the southeast andthe Wanajaya reef system in the northwest. The lineamentsof the backreef, main reef, and forereef in both reef systemswere NS, showing that these faults played a dominant rolein controlling the TAF and BRF facies development.

Integrated analysis of the sequences and the faciesshowed that the preTalang Akar, Talang Akar, and Baturajasequences represent a transgressive cycle, with the deposi-tional direction trending NNWSSE with the landwarddirection to the north. The NS and NNESSW faults alsocontrolled the porosity distribution of the TAF and BRF.Paleoelevation also played an important role. The east, withhigher elevation than the west, was subjected to a moreintensive erosional process and thus has higher porosity.Because the east experienced more intensive faulting and

erosion which increased the porosity of the area, the largerprospects were also found to be concentrated in that area.

Suggested reading. Play concept of syn-rift and post-rift sedi-ments of Cipunegara low, northwest Java by Ascaria et al.(Proceedings of Indonesian Petroleum Association AnnualConvention, 2000). Reservoir development in Talang Akarsequence, Cipunegara low, northwest Java by Priambodo et al.(Proceedings of IAGI & GEOSEA-10 Convention, 2001). TLE

Acknowledgments: The authors thank Pertamina Upstream managementfor providing the data, support, and permission to publish this work.

Corresponding author: [email protected]


Figure 13. DHI signatures (red circles) overlain on the depth structure map of top BRF. Larger flat spots were more concentrated in the east ofstudy area, possibly because the eastern area experienced more intense faulting which then increased the porosity of the rocks in this area.

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