understanding of basement configuration in deep offshore …...understanding of basement...

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Understanding of basement configuration in deep offshore area of Western offshore basin ofIndia through gravity data .D Saha1*, Romi Ganju2 and Puneet Gaba3

GPS, A&A Basin, ONGC, Jorhat* [email protected], 3 Geophysics Group, KDMIPE, ONGC, DehradunKeywordsCOB, Residual.

IntroductionThe present architecture of Western Continental Margin ofIndia (WCMI) has mainly evolved due to rifting betweenMadagascar and India during the late Cretaceous followedby short span of rifting between Seychelles-Laxmi Ridgeand India during K-T boundary and finally rifting betweenSeychelles and Laxmi Ridge during Early Tertiary (Nortonand Sclater1979; Courtillot et al 1988; White andMcKenzie1989; Bhattacharya et al 1994; Storey 1995;Storey et al 1995; Collier et al 2008; Minshul et al 2008).

The present study has been carried out to have betterunderstanding of basement configuration and crustalarchitecture in western offshore basin. Satellite altimetryand Satellite free air gravity data has been used for thisstudy. The objective of the study is to mainly delineate theCOB (Continental to Oceanic boundary), delineation ofbasement depth and presence of Mesozoic sedimentationalong the modeled seismic profiles.

Fig-2 Tectonic map of the Assam & Assam Arakan basin

Fig-1:

Gravity anomaly of Western continental margin of India

Fig-2:Regional Gravity anomaly of Western continental margin

of India

Discussion of Gravity Map :

Gravity map (Fig.1 and 2) over the Western continental,which shows shelf edge gravity high all along the coast,then parallel alternate corridors of gravity low and high.Thelow next to shelf edge high which runs in SSE- NNW ismainly related to Western offshore basinal low. Asignificant negative gravity anomaly was observed overLaxmi Ridge. Regional gravity anomaly map (Fig.2) whichwas prepared through wave length filtering of gravity datakeeping wavelength more than 200km also complementsthe gravity anomaly map except major discontinuities atsome places over major gravity high indicating that seriesof isolated gravity highs are shallow events and it broughtout gravity low over whole western basin confirming thatwestern basin gravity high is the impact of shallowcausatives. Mumbai high gravity low has been more

11th Biennial International Conference & Exposition

Understanding of basement configuration in deep offshore area of Western offshore basin of India through gravity data

prononced in regional gravity anomaly map because ofMoho depression.

Residual gravity anomaly map for western offshore basinwas prepared which brought out major tectonic elements ofthe region understandably. The map indicates two majorNNW-SSE and N-S gravity trends corresponding to easternbasin (East of Laxmi ridge) and series of plumes whichresulted as a consequence of cumulative effect of rift, driftand subsequent tectonic evolution. Gravity low over Laxmiridge has been interpreted due to presence of continentalsliver. Discontinuous disturbed pattern less gravitysignatures over western basin may be the signatures ofshear zone off sea floor spreading accounting to separationof India and Madagascar. (Fig-3)

Fig-3:Residual gravity anomaly map of Western continental

margin of India

Gravity Modeling:

Seismic and wells constrained Gravity modeling have beenattempted along GXT seismic lines GXT-3000, 4000, 5000,

7000, SE-1 and SE-2 . Water depth, sedimentary sequencesand top of Basement have been taken from seismic marker.Densities used for gravity modeling have been taken fromdensity logs of various wells falling in the study area listedin table-1 and table-2.

Table-1

SeismicProfile

WellName

DrilledDepth(DD)(meter)

Formation at DD

GXT-3000

A 4624 Trap

B 6050 Trap

C 4573 Jakhau (Paleocene)

D 3606 Jakhau (Paleocene)

E 5720 Jakhau (Paleocene)

GXT-4000

F 3985 Devgarh Formation

G 3059 Weathered Basalt

H 3340 Deccan Trap

I 2580 Archean Basement

GXT-5000

J 3250 Trap

K 3283 Cretaceous sediments

L 3037 Oligocene sediments

M 2230 Basalt

SE-1 N 4053 Basalt

O 3912 Basalt

P 3552 Bhuwangiri formation

Table-2

Density (g/cc)

Sea water 1.03Sediments 2.4-2.45ContinentalCrust

2.7

Oceanic Crust 2.85Mantle 3.05Deccan Trap 2.75-2.755Intrusive 3.3MesozoicSediments

2.68



Gravity modeling along profile GXT-3000 (Fig.4) showsthat gravity low over Laxmi ridge is due to presence ofcontinental sliver. Modeling also depicts the extension ofMesozoic sediments below the trap. The model explains thesharp peak in gravity high in shelf region by introducingpossible crustal under-plating near in the region. COB(Continental Oceanic Boundary) has also been demarcatedin the model

Fig4. Gravity model along profile GXT-3000

Gravity modeling of lines GXT-4000 (Fig.5) shows shelfedge high, gravity low over Laxmi ridge because ofpresence of Continental sliver and gravity high in middle ofthe profile was inferred because of possible presence ofintrusive body arising out of abandoned spreading Centre.Deccan Trap was encountered in well H at a depth of3340m which has been considered in this model. COB wasalso delineated from the gravity signature.


Gravity modeling of lines GXT-5000 (Fig.6) suggests thatcontinuity in gravity rise after fall of gravity in thecontinental crust part of the profile has been explainedpartly due to transition in oceanic crust and partly due topossible presence of intrusive body. While delineating theextension of the Trap in this model well data of wells J andM have been considered


Gravity modeling of profile GXT-7000 shows shelf edgehigh. It also explains gravity high over Chagos LeccadiveRidge and gravity high over Pratap Ridge. COB has alsobeen demarcated in the model.


W



Gravity modeling of line SE-1 (Fig.8) shows that gravityhigh in the beginning of the profile (NE) is because ofShallow bathymetry. On the basis of the wells N and O,extension of Deccan Traps has been delineated. Gravitydata along this profile suggests that Mesozoic sedimentsmight be present below the Trap. The whole profile isunderlain by continental crust

Fig8. Gravity model along profile SE-1

Gravity modeling of line SE-2 (Fig.9) shows that thisprofile also lies completely over continental crust.Modeling also suggests the presence of Deccan Traps andpossible presence of Mesozoic sediments below.

Fig9. Gravity model along profile SE-2

Based on the above mentioned gravity modelling resultsconstraining with seismic and well data COB has beenmarked in the basin. (Fig-10)

Fig10. COB interpreted has been marked with dashed line

Conclusion:

This study has brought out the COB (Continental toOceanic boundary) all along WCMI (Western continentalmargins of India). The study also predicts the basementconfiguration over Arabaian sea by depicting totalsedimentary thickness. Depth to Moho has also beendemarcated in the results.This study also explains gravity low over Laxmi Ridgethrough gravity modeling along profile GXT-3000 andGXT-4000 by concluding that the Laxmi ridge iscomprising of continental crust in the form of a sliver

Acknowledgement:

The authors express their gratitude to Mr C. MahapatraExecutive Director-Basin Manager, A & AA Basin,ONGCJorhat and Mr. C .M Jain GM-Head Geophysical servicesJorhat for permission to publish this work.

The authors also express profound gratitude to Dr. D.NSingh , GGM -HOI KDMIPE, ONGC, for his constantinspiration to present the paper and shri A.K Parakh GM



(Geology)- Head Basin Research Group-III , KDMIPE, forhis critical suggestions at every stage of writing this paper.

References

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2.J.Shah,W044,71st Eage Conference, Amsterdam,Hydrocarbon in sub basalt sediments-Basin modelling inKerala konkon basin, Offshore West india.

3. P. Unnikrishnan* and T.S. Balakrishnan ,7th

International Conference and Exposition of PetroleumGeophysics, SPG,2008 Analysis of Crustal Structure inWestern Offshore Area Using Gravity Data.

4. Sandwell, D. T., Smith, W. H. F., 1997, Journal ofGeophysical Research, 102, 10, 039-10, 054 Marinegravity from Geosat and ERS-1 satellite altimetry,5. Norton, I. O., Sclater, J. G., 1979., Journal of theGeophysical Research, 84, 6803-6830. A model for theevolution of the Indian Ocean and the breakup of theGondwanaland.6. White,R. S., McKenzie, D., 1989. Journal of GeophysicalResearch, 94,7685-7729.Magmatism at rift zones: thegeneration of volcanic continetal margins and floodbasalts,

7. Minshull, T.A., White, R.S., Barton, P.J. & Collier, J.S.,1992. Mar. Geol., 104, 265–277Deformation at plate boundaries around the gulf of Oman,.

8. Storey, M., Mahoney, J.J., Saunders, A.D., Duncan, R.A.,Kelley, S.P., Coffin,M.F., 1995. Science 267, 852-855.Timing of hotspot-related volcanism : and -the..breakup of Madagascar and India.

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