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INTRODUCTION

Post stack multi attribute analysis and SpectralDecomposition have been successfully used to delineate theextension of sandstone reservoirs in North Sarbhan field ofSouth Cambay Basin. The field produces oil from MiddleEocene sandstone reservoirs, GS- 8 & GS-9 of the HazadMember of Ankleshwar Formation. 3D seismic data acquiredover this field during 1999-2000 has been used for the analysis.

Based on the interpretation of 2D seismic data, thefirst discovery well was drilled during the year 1999-2000.The well produced oil & gas from GS-8 & GS-9 sands ofHazad Member of Middle Eocene. Subsequent drilling of twomore wells B&C produced hydrocarbons from the samereservoirs. In order to delineate the extension of pay sands,3D survey was conducted over this prospect. Based on thesubsurface data of wells, the seismic data was calibrated andinterpreted. Multi-attribute (amplitude and phase) studies andspectral decomposition analysis were carried-out within thewindow in which GS-8 and GS-9 sands are situated. The studyhelped in reconstructing the paleodepositional models andareal distribution of these reservoir sands of Hazad Member.

GEOLOGICAL SETTING

The study area (Fig.1) lies in the Jambusar-BroachBlock of Cambay Basin and forms part of NE rising flank ofBroach depression. In the basinal part, Tertiary and Quaternarysedimentary sequence of about 5500m thicknessunconformably overlies the Deccan Trap and thins out to2500m towards east. In the early phase of sedimentation,

Trapwash /conglomerates /claystone facies of OlpadFormation were deposited under fluvial environment. Alluvialfan facies deposited along the flanks of paleohighs whichgrade laterally to clay/claystone facies. After the depositionof Olpad Formation, wide spread marine transgression coveredthe Basin and thick Cambay Shale was deposited during UpperPaleocene to Early Eocene period. This was followed byvarious regressive and transgressive cycles. The sandy facies

Detection of Thin Sandstone Reservoirs Using Multi AttributeAnalysis And Spectral Decomposition On Post Stack

3D Seismic Data, North Sarbhan Oil Field,South Cambay Basin, Gujarat, India

P.H. Rao, G. L. Hansa, Sangeeta Savanur, S. Mangal, B. Ramegowda,Laxmi Shanker & S.P. Painuly

Western Onshore, Basin, ONGC, Baroda

ABSTRACT : Conventional sand models with their inherent limitation of available well data are the commonly used tools fordelineation and development of oil and gas fields. With the tremendous advancements in seismic technology, the detection of thinsands has become much easier. Multi attribute analysis when combined with Spectral Decomposition provides a better and moremeaningful solution for delineation of thin sands. By adopting this approach for the present study, detection of thin sands andtransformation of geophysical data into meaningful depositional models in North Sarbhan oil field was carried out.

5th Conference & Exposition on Petroleum Geophysics, Hyderabad-2004, India PP 752-759

Figure 1 : Map of Cambay Basin showing study area.

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Thin Sandstone Reservoirs Using Multi Attribute Analysis And Spectral Decomposition

of Hazad, Ardol, Dadhar, Tarkeshwar and Babaguru weredeposited in regressive phases. These regressive cycles wereinterrupted by transgressive cycles giving rise to the depositionof shaly facies of Kanwa, Telwa and intervening shales ofTarkeshwar and Babaguru Formations.

The Cambay Shale deposited during the firsttransgression in Upper Paleocene to Early Eocene is a wellestablished source rock in the entire South Cambay Basin.The main reservoir rocks in this area belong to Hazad Memberof Middle Eocene age. The thick Kanwa Shale overlying theHazad Member is present throughout the area and acts as acap rock. The generalized stratigraphy is shown in Fig 2.

STRUCTURE

Isochron map on top of GS-9 (Fig 5) brings out thestructural configuration of the area. The North Sarbhan wellsare falling on a NNE-SSW trending nosal feature. Two sets offaults trending EW and NS are mapped in the area. Howeverdistribution of oil/ gas indicates that the EW trending faultsdo not act as barriers. Similar nosing features are seen in theeastern part of the area.

Figure 2 : Generalized stratigraphy of Broach Block, Cambay Basin.

3D DATA ANALYSIS

Calibration

The seismic data was calibrated with the help of logsand synthetic trace (Fig 3) generated at well ‘A’. As the 3Dvolume is a minimum phase data, zero crossing of the eventsare correlated with litho boundaries. Based on the calibration,reflections close to top of GS-9 and GS-(6+7) are correlatedthrough out the area. The calibration indicates that the positiveamplitude (Shown as black in Fig 4) developed within thispack is due to the presence of GS-9 sands.

Figure 3 : Synthetic seismic trace from well A

Figure 4 : Inline seismic section showing calibration at well A.

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ATTRIBUTE ANALYSIS

The positive amplitude developed between thecorrelated events, top of GS-9 and GS-(6+7), correspond topay sands as established in the calibration. This event is patchybut well resolved throughout the area. Using the STRATAMPprogramme, amplitude attribute maps (Fig 6) brought out thelateral distribution of this sand in the entire area. The depthcontours superimposed on the maximum positive amplitudeindicate that the distribution of this sand is along the nosalfeature and over the terrace. The log correlation (Fig 7) depictsthat the GS-8 and GS-9 sands are thinning towards the east ofthe area and ultimately merge beyond the limit of seismicresolution, producing tuning effects. To isolate the effects oftuning, frequency weighted amplitude map (Fig 8)corresponding to GS-9 was prepared. This has defined theareal extension of these sands better. To further substantiatethis observation, horizon slicing at every 2 ms was carriedout in the window 70 ms above and below the top of GS-(6+7) on the flattened volume. Clear depiction of developmentof GS-9 sand can be observed on these slices. The Horizonslices 18 to 26 ms. above GS - (6+7) are shown in Fig 9.

The Horizon slices 2ms to 6ms below (Fig 10) thetop of GS-(6+7) brought out a meandering channel trendingnorth to southeast, along the strike direction. The channelprobably corresponds to the GS -(6+7). As no drilled wellfalls on this channel, the feature could not be calibrated. TheHorizon slice 16ms below GS-(6+7) very close to the bottomof Hazad Member (Fig 11), indicates yet another welldeveloped channel feature. This is clearly seen on the inlinesand crosslines. Wells A and D are located over this channelfeature and show development of silty facies. However,

Figure 5 : Time structure map at a reflector close to GS-9. Figure 6 : Maximum positive amplitude attribute map correspondingto GS-9

Figure 7 : Electrolog correlation through well A, B and C indicatingthinning of Hazad sequences towards east.

Figure 8 : Frequency weighted amplitude map of GS-9 indicatingdistribution of reservoir facies.

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Figure 9: Horizon slices A, B and C corresponding to 18, 22 & 26ms above GS-(6+7).

Figure 10: Meandering channel seen on horizon slice 4ms belowGS-(6+7).

development of sandy facies in the up dip direction can notbe ruled out, as high reflection amplitudes are observed withinthe channel in the area.

To delineate these channel features, RMS amplitudemap (Fig 12) was prepared between the reflection of GS-(6+7)

Figure 11: Horizon slice 16 ms below GS-6+7) close to Hazad Baseindicating well developed another channel feature.

Figure12: RMS amplitude of the pack between bottom of the GS-(6+7) and Y marker indicating channel.

and bottom of Hazad. As RMS amplitude is very sensitive tochanges in the amplitude, the attribute clearly brought out thechannel configuration. High amplitude positive signature inthe eastern part, within the channel suggests channel beingsand filled. To corroborate this observation SpectralDecomposition was carried out over the data.

Spectral decomposition provides better focussing ofthe various depositional features on frequency slices thanhorizon based attribute slices in time. For each bed thicknessdepending on the interval velocity of the bed, there exists afundamental frequency that gives tuning effect that maximizesits amplitude response compared to other frequencies.Therefore different depositional features respond for differentfrequencies and are better focussed at these frequencies.


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Keeping in mind this property of spectral decomposition, thetuning cube analysis was carried-out on this 3D volume byselecting a 100 ms window, centered about the interpretedhorizon GS-(6+7). Tuning cube analysis was made forfrequencies varying from 2 Hz to 100 Hz at an interval of 2Hz. Tuning cube analysis transforms the temporal window ofthe zone of interest from time domain to frequency domain.The resulting “Tuning cube” can be viewed in cross sectionalview or in plan view, as a frequency slice. This was obtainedby animating the amplitude vs. frequency through the entirerange of frequencies. Taking a clue from the zone of interesttuning cube, the tuning frequency slices from 17 Hz and 37Hz were drawn from the frequency frozen flattened horizonvolume. Fig 14 and 15 indicate the channel feature withenhanced amplitude.

Figure14: The channel feature 16ms below GS (6+7) from spectraldecomposition frequency cube resolved at 17 Hzfrequency.

Figure15: Meandering channel better resolved by spectraldecomposition at 37 Hz tuning frequency.

These, when compared with the RMS amplitude mapdrawn from the Strat Amp, show better resolution at 17 Hzand 37 Hz frequencies. These channel features are the newexploration plays identified in this area.

GEOLOGICAL MODELING

The multi attribute maps obtained from theinterpreted 3D volume are translated into a geological modelto understand the paleogeography, sand geometry anddepositional environment of GS-(6+7), GS-8 and GS-9 sandsof Hazad Member.

Paleogeographic model of GS-(6+7) sands: Sand isolithmap (Fig 16) of GS-(6+7) derived from the log data andseismic attributes, shows a maximum sand thickness of 7m,4m and 3m in wells A, B and C respectively. Gradual increasein thickness is observed in south and southwest direction. 2mof sand is observed in well D drilled to the south of well A.Towards east, 4 to 6m thick sand bodies are inferred based ontime slice maps and amplitude maps, which indicate itsdeposition in channel/ point bars. Geological model, basedon well data, log motif and seismic data is presented in Fig 17and it shows that the area towards east is fluvial dominatedforming meandering channels and point bars in upper deltaplain environment. Isochronopach map (Fig 18) from Hazad

Figure15: Sand isolith map of GS-(6+7) indicating gooddevelopment of reservoir facies in the western part of thearea. Isolated sand bodies towards east have beenidentified based on attribute analysis and SpectralDecomposition. These sands might have been depositedin point bar environment.

top to Y marker superimposed with meandering channelindicates development of point bars over the terrace feature.

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Figure 17 : Geological model of GS-(6+7) sands, based on log dataand seismic attributes. The wells A and B are falling inthe distributary channel and well C falls in the inter-distributary area of lower delta plain environment. Thearea located to the east of well A have upper delta plainenvironment having meandering channels / point barfacies.

Figure 18 : Geological model superimposed on isochronopachmap between Hazad top to Y marker indicatingmeandering channel passing through the terrace featureobserved in isochronopach map of Hazad Member.

The area towards wells A, B and C falls in lower delta plainhaving distributary channels and intra distributary areas. Paleoenvironment of GS- (6+7) of North Sarbhan and adjacent areais depicted in Fig 17.

Paleogeographic maps of GS-8 sands: Sand isolith map(Fig 19) based on log and seismic data shows a thickness of 4to 6m in the wells A, B& C. The sand appears to have beendeposited in tidal channels and river mouth tidal environmentaligned in NE-SW direction with tidal influence fromsouthwest. Gamma Ray log character in well A shows a finingupward sequence, which is indicative of tidal channelenvironment, whereas well B shows coarsening upwardsequence indicative of river mouth tidal ridge deposits. RMSamplitude map of GS-8 (Fig 20) suggests the presence of anumber of such channel/ bars to the east of wells A and B.The core at GS-8 level in a well located NW of the well A iscomprised of fine to very fine bioturbated sandstone whichalso suggests tidal influence in the area. Analysis of logs andlaboratory data along with sand geometry maps based onseismic attributes, suggests that the north western part of thearea is affected by tides, where as the area towards southeastappears to be fluvial dominated. Entry of sediments appearsto be from NNE. Paleogeographic map (Fig 21) of GS-8 sandshows the inferred depositional environment.

Figure 19 : Sand isolith map of GS-8 based on seismic attributesand log data indicating presence of number of sandbodies aligned in NE-SW direction having sandthickness varying from 4 to 8m in the drilled wells.The input is inferred from North and affected by tidalinflucence from southwest, giving rise to the discretenature of sand bodies.

Paleogeographic model of GS-9 sands: Sand isolith map(Fig 22) based on seismic attributes and log data shows athickness of 3-4 m in North Sarbhan wells. Sand geometryand log motifs in wells A, B & C suggest its deposition astidal bar deposits trending NNE-SSW direction. RMSamplitude map and frequency weighted amplitude maps (Figs

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6 and 8) show the presence of a number of sand bodiestrending NNE–SSW in the area. Core data in well B at GS-9level indicates the presence of flaser bedding with bioturbation,suggestive of tidal influence in the area. Sand entry appearsto be from north and north east. Paleogeographic map ofGS-9 (Fig 23) depict the environment of deposition at thislevel.

Testing data of wells A, B, C and D indicate thediscrete nature of sand bodies which is also inferred from thesand map based on attribute analysis. Presence of oil in the

well A located structurally up with respect to another gasbearing well B located downdip leads to the conclusion thatthese wells are located over different sand bodies. The pressuredata observed in the wells A and C also suggests that thesewells are hydrodynamically connected and the faults do notact as barriers.

Figure 20 : RMS amplitude map of GS-8 superimposed with depthcontours indicating the presence of high amplitudealong the nostal feature.

Figure 21 : Paleogeographic model of GS-8 sand indicating thepresence of number of sand bodies which are inferredto be deposited in tidal channel / river mouth tidalridge environment.

Figure 22 : Sand isolith map of GS-9 indicating sand geometry ofGS-9 sand. Numbers of sand bodies, aligned in NE-SW direction have been inferred based on well andseismic data. Each body has separate identity givingrise to discrete nature of the sand.

Figure 23 : Paleogeographic model for GS-9 sand indicating that

the sands are deposited in tidal environment forming

tidal bars aligned in NE-SW direction having tidal

influence from southwest direction.

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CONCLUSION

• The post stack 3D Seismic data of North Sarbhan area isof good quality and enabled perfect calibration with thelog data.

• Multi attribute analysis and Spectral Decomposition,together provided a better tool for delineation of thin sandsof Hazad Member.

• Spectral Decomposition clearly brought out the channelfeatures within Hazad and led to identification of newexploration play in this area.

ACKNOWLEDGEMENTS

The authors express their gratitude to Mr. S.K.Mandal, ED-Basin Manager, Western Onshore, Baroda, forproviding an opportunity to work on this project. The authors

also sincerely acknowledge Mr. S. S. Sawkar, Mr. A.K. Sinhaand Mr. M.J. Panchal for their support during the preparationof this paper.

REFERENCES

Mayor. S. et.al 2001 Identification of Future Thrust Areas

through Sediment Distribution Patterns of Hazad Memberaround Gandhar Field.

Murthy Dr. J.V.S.S.N et.al 2001 Report on the software developmentof “Tuning Cube” (Spectral Decomposition of shortwindow Seismic waveforms ).

Partyka, G.A. et.al 1999 Interpretational Applications of Spectral

Decomposition in Reservoir Characterization, TheLeading Edge, vol. 18, No.3, pg 353-360.


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