High resolution satellite geoids/gravity over the western Indian offshore for tectonics and hydrocarbon exploration

Indian Journal of Marine Sciences Vol. 38(1), March 2009, pp. 116-125 High resolution satellite geoids/gravity over the western Indian offshore for tectonics and hydrocarbon exploration R. Bhattacharyya 1, P. K. Verma 2 and T. J. Majumdar 1 * 1 Earth Sciences and Hydrology Division, Marine and Earth Sciences Group, Remote Sensing Applications Area Space Applications Centre (ISRO), Ahmedabad, India 2 School of Studies in Earth Sciences, Vikram University, Ujjain, India [E-mail: tjmajumdar@sac.isro.gov.in] Received 22 May 2008; revised 15 September 2008 The present study consists of various satellite geoid/gravity maps of the western Indian offshore region and correlated with known tectonic features such as Bombay High, Chagos Laccadive ridge complex, Laxmi ridge. The satellite-derived gravity maps have been compared with those of ship-borne gravity for validation purpose. Spectral analyses of gravity data over the study area brings out various components of interest, which could be correlated with subsurface features. The interpreted results indicate a positive correlation between the known geological elements and gravity field. [Keywords: Satellite altimetry, Geoid and gravity anomaly data, Hydrocarbon prospects, Bombay High, Laxmi Ridge, Carlsberg Ridge, Spectral analysis] Introduction Satellite altimetry has recently emerged as an efficient alternative for expensive ship-borne gravity survey 1,2 and powerful reconnaissance tool for exploration of offshore region. With the advent of more and more altimetric mission it is possible to develop a high-resolution gravity database of spatial resolution ~3.33 km. The averaged sea surface height over the Indian offshore ( 10 to 108m) as obtained from satellite altimeter is a good approximation to the classical geoid. The undulations of the geoidal surface can be directly interpreted in terms of subsurface geological features such as sedimentary basins, basement highs and lows etc. 2. The geoidal anomalies can be converted to free-air gravity anomalies which are useful in the deep sea where ship-borne geophysical data is unavailable or scanty. Rapp 3 had developed a method for prediction of the gravity anomaly using spherical harmonic coefficients up to degree and order 30 and above. Sandwell and Smith 4 have generated marine gravity field from Geosat and ERS-1 satellite altimetry. Majumdar et al. 2 have developed a brief methodology for delineation of offshore structural features using altimeter data. The western continental margin of India (WCMI) has been described as divergent type by Thompson 5 *Corresponding author and as passive by Pratsch 6. Numerous oceanic islands and seamounts characterize the seafloor. Most of our knowledge about the ocean comes from ship data acquired during International Indian Ocean Expedition (IIOE) 7. The ship-borne geophysical data along the WCMI brought out several tectonic features which improved our understanding about the oceans. The western Indian sub-continent is fringed by the Arabian Sea. The offshore region is characterized by Kutch, Saurastra, Bombay and Konkan-Kerala sedimentary basins which are evolved through the rift and drift phases in a passive divergent margin set up. The initial phase of rifting resulted in a system of NNW-SSE trending horsts and grabens parallel to the continental margin. In the northern Arabian Sea, the sea bottom topography is rugged up to about 3,200 m due to the influence of the Indus Fan; thereafter the gradient decreases gradually. Off Bombay, the continental shelf is about 300 km wide which is narrower to the south. The western Indian offshore is geologically a complex region with remarkable hydrocarbon potential (e.g. Bombay High). The present study consists (1) Generation of the high resolution classical and residual geoids and related gravity anomalies in ARCGIS environment over a part of the western Indian offshore, (2) validation of high resolution satellite gravity with ship-borne gravity, (3) study the correlation between

BHATTACHARYYA et al.: HIGH RESOLUTION SATELLITE GEOIDS FOR HYDROCARBON EXPLORATION 117 the gravity and the sediment thickness, and (4) delineation of different structural features that can be used to infer probable hydrocarbon prospects. Materials and Methods The study area has been chosen over a part of the western Indian offshore under Lat. 12-22 N and Lon. 67-77 E (Fig. 1). High-resolution gravity database generated from Seasat, Geosat GM, ERS-1/2 and TOPEX / POSPEIDON altimeters data of the Arabian Sea have been used in the study. The data sets are more accurate and detailed (off-track resolution: about 3.33 km and grid size: about 4 km). The assimilation of gravity data has been done by Hwang et al 8. The National Geophysical Data Centre sediment thickness data have been utilized for the present study. Other geological data have been obtained from published reports of ONGC. The free-air gravity anomaly map has been generated from the high resolution satellite gravity data over the western Indian offshore region (lat. 10 S-25 N and long.55-77 E) to identify and locating the morphological features as described below while describing Tectonic scenario of the western continental margin. The high resolution gravity data is able to detect all the geomorphological features (Fig. 2). High resolution satellite-derived gravity data have been compared with ship-borne gravity over the Bombay High for detailed validation (Figs 3a and b). The sea surface height (SSH) with respect to the reference ellipsoid is computed for instrumental bias and atmospheric propagation delays. It is a fundamental geophysical parameter used for various oceanographic and geophysical studies 2,3. Sea surface height (SSH) = Orbit height (H) Corrected altimetric range (h ) (1) The SSH observed by the altimeter is only an instantaneous sea surface topography and deviations Fig. 1 Morphological map of the western Indian offshore (MR: Murray Ridge, OFZ: Owen Fracture Zone, GA: Gulf of Aden, LR: Laxmi Ridge, GO: Gulf of Oman, CR: Carlsberg Ridge, CIR: Central Indian Ridge, SWIR: South West Indian Ridge; SEIR: South East Indian Ridge; RTJ: Rodriguez Triple Junction) 7. The detail study area has been marked by the box Fig. 2 Free-air gravity anomaly image generated over the western Indian offshore using high resolution satellite gravity data. Locations of the seamounts/ridges mentioned are after Bhattacharya and Chaubey (2001) 7

118 INDIAN J. MAR. SCI., VOL. 38, No. 1, MARCH 2009 anomaly and geoid undulation which is represented as: F(Δg) = g o k F(N) (2) Fig. 3 Comparison of free-air gravity (satellite-derived and ship-borne) over the Bombay High region of SSH from the geoid are due to various dynamic variabilities e.g. ocean tide, solid tide, electromagnetic bias, inverse barometric pressure effect etc. The classical geoid is an equipotential surface which approximates well to the mean sea surface. The residual geoid is obtained after removing long wavelength components, caused mainly due to the deeper earth effects, from the classical geoid; thus it contains information due to bathymetry as well as lithospheric anomalies. Fast Fourier Transform (FFT) approach uses geoid for computation of free-air gravity anomaly 9. This approach is based on a flat earth approximation and is derived from the two fundamental equations 10,11. Chapman 9 established a relation between gravity where, F(Δg) = Fourier transform of free-air gravity anomaly F(N) = Fourier transform of geoid undulation g o = Normal gravity k = one-dimensional wave number associated with wavelength λ Details of the methodology to obtain geoid and gravity from altimeter-derived sea surface height have also been discussed elsewhere 2,4,8,12. Different spectral components of geoid and free-air gravity anomalies may correspond to geological features lying at different depth levels. Accordingly the residual geoid and gravity anomaly data of the Bay of Bengal are segmented into intermediate and long-wavelength spectral components 13. Longwavelength component (200-500 km) mainly reflects the undulations at crust-mantle boundary, whereas intermediate-wavelength component (100-400 km) is utilized to reflect the lateral variations within the basement topography and continental-rise sediments. The information thus deduced/inferred can be used to outline the tectonic trends are ultimately to understand the isostatic compensation of several geological structures at different depths 14. Thus the coefficients of different harmonics were computed from geoid and gravity values and then coefficients corresponding to different wavelengths 100-400 and 200-500 km, were combined and inverse Fourier transformed to calculate intermediate and long wavelength components. Tectonic scenario of the western continental margin The major tectonic lineaments of the western Indian offshore are the extension of three Precambrian orogenic trends the NNW-SSE Dharwar trend, the NE-SW Aravalli trend and the ENE-WSW Satpura trend which dominate the structural fabric of Western India as well as its offshore region. The Precambrian trends have a direct bearing on the north to south sequential rifting of the Indian subcontinent during the break-up of Gondwalaland 15-17 and later broke up from Madagascar in the middle Cretaceous and followed by rifting from Seychelles microcontinent at the end of the Cretaceous from India 18,19. Hence, although temporally these rifts seem to be the result of Gondwanian break-up, they were controlled spatially by Precambrian trends.

BHATTACHARYYA et al.: HIGH RESOLUTION SATELLITE GEOIDS FOR HYDROCARBON EXPLORATION 119 The older NNW-SSE trending tectonic elements parallel to the Dharwarian trend is one of the dominant features, which has influenced the depositional pattern 20-22. Three major faults in the western Indian landmass are continued offshore. The Bombay High field is a large doubly plunging anticline (Paleohigh), bounded by NNW-SSE trending basement controlled faults 21 ; and towards west, it grades into monoclinal deeper continental shelf. The western continental shelf region is characterized by a series of longitudinal extension faults in parallel sets which gives rise to a series of horst and graben structures. The present continental margin consists of NW-SE trending structural features such as Laccadive ridge, Laxmi ridge, Prathap ridge complex, Panikkar ridge and are approximately parallel to the west coast of India. Apart from the above features, there are also several NE-SW tectonic features like the Kathiawar horst, Surat graben, Bombay horst, Ratnagiri graben, Vengurla horst, Konkan graben, Tellicherry horst and are prominent upto the western continental margin 15,18-20. The Laccadive and Laxmi ridges divide the continental margin of western India and the adjoining Arabian Sea into two basins (Figs. 1 and 2). The Eastern basin lies in the east of these two ridges and the western basin lies between the Carlsberg ridge and these two ridges. Isolated topographic highs are present on the continental slope rising between 8 N and 16 N, which is known as Prathap Ridge Complex 20. The western Indian ocean in the study area consists of a few deep-sea basins and ridges. Some of these were originated by the present system of mid-oceanic spreading centre of Carlsberg ridge in the southwest, while others are the remnants of continental break-up stage tectonics. Names of the major basins, seamounts and the ridges of the western Indian ocean including the study area are shown 7 Fig. 1. Results and discussions We have used satellite geoid/gravity data (Figs 3 and 4) to infer sedimentary basins. These basins are associated with gravity lows. Sediments become more compact and denser as the depth of their burial increases. Consequently when folding produces an anticline, denser rock are brought closer to the surface along its axis and results in a linear positive gravity anomaly within a regional low formed by the whole basin. Conversely, because of the low density of salt, salt dome gives gravity low 23. Since, the hydrocarbons are confined in the sediments they are also reflected by gravity low signatures. Figure 4 Tectonic blocks showing hydrocarbon sectors (after Sreenivasan and Khar 26 ) superimposed over the free-air gravity anomaly map Hydrocarbon prospecting in the western offshore Up to 200 m isobath, the pericratonic Mumbai offshore basin covers an area of over 148000 km 2 of western Indian continental shelf. The basis is divided into six tectonic blocks with the presence of hydrocarbons. The Pleistocene sedimentation rates in this area vary from 4-8 cm/1,000 year off Saurashtra 24 to 5 14 cm/1,000 year off Bombay and ~ 50 cm/1,000 year off Mangalore 24,25. The most promising oil fields of India are situated in this region 26. The oil/gas fields could be demarcated in different major blocks, viz. (1) Tapti-Daman block, (2) Panna block, (3) Bombay High block, (4) Ratnagiri block, (5) Diu block, and (6) Shelf margin block. These blocks and corresponding oil and gas fields have been superimposed over the gravity map to delineate the signatures of the hydrocarbon-bearing zones. The details of these blocks have been given in Fig. 4.

120 INDIAN J. MAR. SCI., VOL. 38, No. 1, MARCH 2009 Several large oil/gas fields are present in this region. The presence of hydrocarbon belongs to L-III limestone reservoir of Miocene age (only in the Bombay High), Mukta (Early Oligocene), Bassein (Middle Eocene), and Panna (Paleocene to early Eocene) reservoirs26. The time rock stratigraphy of the Bombay offshore basin along with markings on the hydrocarbon-bearing formations is given elsewhere26,27. The Paleogene (Paleocene Eocene Oligocene) section of the Surat depression is considered as a major source and the multiplicity of hydrocarbon generation centers remains conspicuous features of this basin. Organic matter is dominantly Kerogen type III. Commercial oil/gas accumulations have been discovered by ONGC in this region: Bombay High (1974), Heera / South Heera (1977), Neelam (1987), Panna and Mukta (1987)26. The NGDC (National Geophysical Data Centre) data28 are gridded with a spacing of 5 5. Sediment thickness data were complied from three principle sources e.g. published isopach maps, ocean drilling results (ODP and DSDP) and seismic reflection profiles achieved at NGDC. The average sediment thickness over the study area is around 3 km (Fig. 5). The sediment thickness appears to be very less (~0.5 km), over the Laccadive region which gradually increases towards the coast. Over a large portion of the deeper part, north of 18 N, the sediment thickness Fig. 5 Sediment thickness map over the study area (after NGDC28) varies between 2 and 3 km mainly due to the discharge from Indus and westerly flowing rivers of India like Narmada, Sone and Tapti. Geoid and gravity for offshore exploration High resolution satellite-derived gravity data have been compared with ship-borne gravity over the Bombay High for detailed validation (Figs. 3 a and b). The overall trend of the contour patterns matches strikingly well in both the cases. In addition, few new structures could also be identified. RMS error of the high resolution satellite gravity is around 2.7 mgal. The classical and the residual geoid imageries generated over the western Indian offshore (67-77 E and 12-22 N) are shown in Figs. 6 and 7 respectively. Figure 6 shows that the major trends of the continental margins are along NW-SE and NNW-SSE with a southeasterly dip from Gujarat offshore to Mangalore offshore. Additionally, a subtle trend could be observed along NE-SW. The classical geoid anomaly is one of the very basic input which gives information of the subsurface below till the core of the solid earth. Generalized trends of lithosphere and beyond will be reflected in the classical geoid image, some of which are demonstrated in Fig. 6. Figure 7 is the residual geoid image generated after removal of deeper earth effects29,30, which provides primary information on the existence of various geological features. The Bombay High, Laxmi basin and ridge became prominent features in this image. The western margin of India is characterized by presence of horst Figure 6 Classical geoid anomaly map over the study area

BHATTACHARYYA et al.: HIGH RESOLUTION SATELLITE GEOIDS FOR HYDROCARBON EXPLORATION 121 Fig. 7 Residual geoid anomaly map over the study area and/or graben like features, besides NW-SE trending regional faults ridges, basins and a NW-SE trending basement high off Bombay (Fig. 8a). A number of NE-SW tectonic trends, e.g. the Kathiawar horst, Surat graben, Bombay horst, Ratnagiri graben, Vengurla horst, Konkan graben and Tellicherry horst have also been demarcated in the gravity image/interpretation with alternate gravity highs and lows from Kathiawar horst totellicherry horst (Figs. 8 a and b). The major faults in the western offshore region are superimposed over the gravity image (Fig. 8b). A 3D gravity image has been generated over the study area to obtain a better perspective view of the region (Fig. 9). Two gravity anomaly crosssections AB and CD of the gravity image have been studied at northern and southern portions respectively showing the gravity undulations for different structures. The continental slopes and basins show gravity lows, while the gravity highs are associated with ridges and continental boundary (Figs 10 a and b). Spectral analyses of the gravity image over the study area (Lat. 12-22 N, Long. 67-77 E) are shown into two components, namely, 100-400 and 200-500 km (Figs 11 a and b). The Laxmi and Pratap Ridges show different basement structures while the continental area shows least variation. The free-air gravity values in the region vary from 86 to +55 mgal. The gravity anomaly gradually increases with Fig. 8a Free-air gravity anomaly map over the study area from high resolution satellite gravity data Fig. 8b Free-air gravity anomaly map over the study area showing offshore structures

122 INDIAN J. MAR. SCI., VOL. 38, No. 1, MARCH 2009 Fig. 9 3D perspective view of gravity image of the study area (Vertical Exaggeration: 1000) Fig. 11a Spectral analyses (100-400 km) results as obtained over the western Indian offshore from high resolution gravity data Fig. 10a Free-air gravity cross-section across AB Fig. 11b Spectral analyses (200-500 km) results as obtained over the western Indian offshore from high resolution gravity data Fig. 10b Free-air gravity cross-section across CD depth and the range of gravity values became high at very high gravity values ( 178 to +201 mgal) due to denser materials. However, the variation of gravity is less (~ 22 mgal) due to uniform density distribution at higher depths. The satellite geoid/gravity data (Figs 3 and 4) have been used to infer sedimentary basins/hydrocarbon-

BHATTACHARYYA et al.: HIGH RESOLUTION SATELLITE GEOIDS FOR HYDROCARBON EXPLORATION 123 Fig. 12 (a-e) Free-air gravity anomaly vs. sediment thickness for oil/gas producing fields bearing regions associated with the gravity lows. The gravity data have also been used to demarcate the extension of different tectonic blocks producing oil/gas. The sediment thickness is high in the Tapti Daman block (1600-2800 m) due to the high sediments discharge from westerly flowing rivers of India e.g. Tapti, Narmada, Sabarmati. The Panna block, adjacent to Tapti block, shows mostly uniform sediment thickness ranging from 1400 to 1700 m. The Bombay High block, adjacent to the Panna block, showing sediment thickness variation between 1700-2100 m, is characterized by rugged sediment thickness topography. The Diu block, in the north of the Bombay High block, shows a relatively low variation (1400-1500 m) in sediment thickness. Sediment thickness in western offshore is found to be high in NE-SW region (Fig. 5). An attempt has been made to establish a correlation between the gravity and the sediment thickness for oil/gas fields in the reported tectonic blocks of the western Indian offshore (Fig. 12 a-e). Conclusions Prominent trends in the Arabian Sea are NW-SE, and NNW-SSE along with another NE-SW trend. The present continental margin consists of numerous NW- SE and NNW-SSE trending structural features, viz. Laccadive ridge, Laxmi ridge, Pratap ridge complex, Panikkar ridge and is approximately parallel to the western margin of India; and a number of NE-SW

124 INDIAN J. MAR. SCI., VOL. 38, No. 1, MARCH 2009 tectonic trends, from north to south: the Kathiawar horst, Surat graben, Bombay horst, Ratnagiri graben, Vengurla horst, Konkan graben, Tellicherry horst which could be clearly demarcated along with the delineation of their extensions in the satellite-derived high resolution gravity image 31-34. Various spectral components of the gravity data as generated over the study area could be found useful to demarcate the subsurficial features of interest e.g. extensions of the Shelf Margin block, Laxmi ridge and Panikkar ridge in the subsurface. The hydrocarbon-bearing formations in various tectonic blocks have been delineated. Acknowledgements The authors are grateful to Prof. C. Hwang, National Chiao Tung University, Taiwan for providing very high resolution satellite gravity data and related software and to Dr. R. R. Navalgund, Director, SAC, for facilities and keen interest in this study. Ship-borne gravity data received from KDMIPE (ONGC) over Bombay High is thankfully acknowledged. 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