Convolution of Technology and Concepts Entails Enticing Deepwater Opportunities, India

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1 P Convolution of Technology and Concepts Entails Enticing Deepwater Opportunities, India Duggirala Moses Nathaniel, Rabi Bastia, Srinivas Tenepalli, Kenneth D Silva, Ravi Verma Petroleum Business (E&P), Reliance Industries Limited, India dm.nathaniel@ril.com Summary Skepticism is an essential part of hydrocarbon exploration as it is not possible to proclaim a law of nature until the idea has survived the exploration drill test. The evidences as recorded by exploration tools and their subsequent interpretations must constitute a proof on the likelihood of striking subsurface hydrocarbon pools. Consequently, a review and synthesis of bold ideas and technology involving long offset seismics, gravity and magnetics, within the frame-work of plate tectonic concepts, permit the identification of deep- and ultra-deepwater opportunities having substantial potential to act as additional petroleum provinces. They include the 85 E and the sediment localizations of the Bengal Fan in the eastern waters, large-scale geological structures of inversion origin in the south western waters (off Kerala) and Laxmi and Laxmi basin in the northwest waters (off Mumbai) of India. Introduction A vast petroleum reserve potential exists for the Indian offshore while only a minuscule has been established till date. Historical analysis of exploratory efforts so far indicated that Dirubhai Field in the eastern waters, next major find to the giant Bombay-High, happened only after a period of 28 long years. This is either due to a confinedexploratory-thought-process to look for obvious prospects or due to the anesthetizing belief that chances of exploring giants in the offshore basins are rather bleak. Conversely, a revisit to the concepts of plate-tectonics in conjunction with bold ideas and innovative technology holds great promise for deep and ultra-deepwater opportunities. Hydrocarbon Habitats Krishna-Godavari basin and situated in between the eastern margin of India and the 85 E ridge with more than 8.0km thick sediments at its depocentre; the central basin, between the two ridges, with 22 km thick depocentre in the north at the delta of the Ganges-Brahmmaputra river system; and the eastern basin, with an average thickness of only 2 km, between the 90 E ridge and the Andaman and Nicobar island arc group(fig.1). Long offset seismic, gravity and magnetic characterization of major tectonic elements such as 85 E ridge, Vishnu fracture zone, Laxmi ridge and Indus and Bengal fans enhanced the understanding of the megaregional tectonostratigraphic frame-work and thereby sector-wise hydrocarbon habitats. Eastern Sector: Bay of Bengal is the sediment repository of Bengal Fan and pre-collision sediments, characterized by its three extended arms with decreasing thicknesses stretching up to 7 S. Two tectonic elements, 90 E and 85 E ridges, positioned longitudinally, divide the sediment apron into three basins: the western basin, juxtaposing the Owen Fractur e Zone Carlsberg Laxmi Chagos Laccadieve Central Indian Vishnu Fracture Zone 85ºE 90oE Figure 1: Map view of Bengal and Indus fans along with morpho-tectonic elements 90 o E, 85 o E, Vishnu fracture Zone, Laxmi and Chagos Laccadieve (Modified after Christian et. al., 2000) (Black lines represent the morpho-tectonic features and Red lines represents seismic sections included in the text)

2 Although, debate on the origin of the distinct gravity low, characteristic of the 85 E ridge is still inconclusive, a common view is that it was evolved during the Late Cretaceous period (M.V.Ramana et.al, 1997). Recent long offset seismics proposed an entirely new paradigm for 85 E ridge as a continental-rift-fragment of Cretaceous age, formed due to a jump of the oceanic spreading centre towards east (Steve Henry and Al Danforth, 2007), indicating a high prospectivity for the ridge itself which is km in width and km in length (fig.2). Either way, the elongated 85 E ridge acted as a barrier to facilitate restricted conditions for organic enrichment of sediments of the western basin. The super-thick depocentre of the central basin, as a result of deep burial related exposure to elevated temperatures is favorable not only to yield hydrocarbons of thermal origin, but also for the development of biogenic gas in commercial quantities. However when curie isotherms (Subramanyam et.al, 2004) were utilized as a basis for envisaging heat flow regimes, the 8 km depocentre of western basin appears to be a favorable situation for thermogenic hydrocarbons and 22km depocentre of the central basin, for biogenic hydrocarbons predominantly. Possible hydrocarbon traps include drapeovers on the existing highs of 85 E ridge, closures against faults, compaction relief stratigraphic features and channel-levee complexes. In essence, the abyssal plains of Bengal Ocean provide an optimistic preview of petroleum precursors for exploration. Southwestern Sector: The episodic rifting of the Gondwana during the period of the Jurassic till the end of the Cretaceous, involving separation of continents from each other viz. Africa, Madagascar, India, Sri Lanka and Antarctica, facilitated megaregional rift-drift structural superpositions in the southern- tip of India i.e., deepwater Kerala basin. Plate tectonic reconstruction of Late Jurassic to Early Cretaceous (fig. 3), demonstrates that the basin was situated on the north-eastern part of Proto- Mozambique Ocean, with Antarctica as the major provenance of sediment supply under favorable oceanic and climatic conditions for organic productivity. A synthesis of regional seismics, gravity and crustal-scale seismic velocity (fig. 4A), magnetics (fig. 4B) and electromagnetic induction (fig. 4C) qualify the southern tip of India as a triple junction (fig. 4D) (Arora et. al., 2002) with coast parallel and coast transverse extensional/shear type fault/fracture zone systems (Raval et. al., 2003). Structuration in the study area is controlled by the Vishnu Fracture Zone (VFZ), a N-S trending regional tectonic element identifiable on gravity in conjunction with regional ocean floor bathymetry and magnetic anomaly maps. 2a Depth (km) NW 0 10 SE 20 2b Figure 2a. Free-Air Gravity Anomaly Map showing the subsurface manifestation of 85 o East, 2b. Seismic illustration of 85 o East (red) (After Steve Henry and Al Danforth, 2007). Figure 3: Plate-tectonic reconstruction indicating longitudional ocean adjoining southern and western sectors nearing the geological period of Late Jurassic Early Cretaceous. (after Dariusz Jablonski, 2006)

3 4a 4c 4b 4d Figure 4a. Map view of the P wave velocities at the depth interval of, indicating low-velocity zones centered at the southern tip of peninsula, western India (Cambay) and NE India (modified from Kennet and Widiyantoro, 1999), 4b. Magnetization distribution map for Indian sub-continent, 4c. The spatial variation of conductance (logarithmic scale) in and around peninsular India, deduced from thin sheet model covering an area of 0 20 N and E. derived from MAGSAT vertical field anomalies (after Singh et al., 1991)., 4d. Major rift basins of India and possible triple junction locations derived by integrating geophysical data (Veeraswamy et al., 2004)

4 Long offset seismic characterization of VFZ (fig. 5A & B) suggests that it is a transform fault, responsible for structural inversion of the pre-existing rift-graben architecture. Large-scale geological structures of inversion origin are identified on the eastern flank of the VFZ, wherein adequate heat-flux and vertical migratory pathways are expected to facilitate the hydrocarbon generation-migration-entrapment paradigm. basement of Laxmi and Laxmi basin as having numerous volcanic intrusions, which incidentally were interpreted as corresponding to seafloor spreading anomalies by earlier workers. However, recent deep seismic imagery brought out a clear picture of the crust below Laxmi and thus indicated it as a constituent of the failed rift of Late Cretaceous age, in agreement with the interpretation of Krishna et.al, a 6a Laxmi Vishnu Fracture Zone W E West East Cape Comorine High Vishnu Fracture Zone Laxmi Laxmi Basin 5b 6b MOHO Figure 5a: Free-Air Gravity Anomaly Map showing the subsurface manifestation of Vishnu Fracture Zone and 5b. Seismic illustration of Vishnu Fracture Zone uplifted rift graben. Western Sector: Laxmi, situated to the west of Laxmi Basin, is a significant morpho-tectonic feature along the northwestern continental margin of India and has been qualified as one of the continental slivers dispersed in the Arabian Sea (Fig. 6A & B)(ref). Analysis of various geophysical datasets helped decipher the nature of Figure 6a: Free-Air Gravity Anomaly Map showing the subsurface manifestation of Laxmi and 6b. Seismic illustration of Laxmi (After Steve Henry and Al Danforth, 2007) Based upon the structural configuration, it was further proposed that, the point of confluence of Laxmi, Cambay and Narmada basins is a triple junction analogous to the Afar-Triple-Junction of Red Sea (Steve Henry & Al Danforth 2007). Expected heat flow in view of continental

5 genesis, additional heat from volcanic intrusions and its proximity to Indus and Narmada systems, make Laxmi basin a favorable sink for organically enriched sedimentation with adequate maturity thresholds to generate hydrocarbons. Conclusions New frontiers for additional hydrocarbon reserves exist in the deepwater sectors of Indian continental margins as demonstrated by innovative geophysical technology in conjunction with bold geological concepts. Exploration of these enticing exploration targets are expected to contribute substantial additional reserves in tune with Hydrocarbon Vision-2025 of the country. References Arora, B. R. and Subba Rao, P. B. V., 2002, Integrated modeling of EM response functions from Peninsular India and Bay of Bengal; Earth Planets Space, 54, Al Danforth, Steve Henry, 2007, Plate-Tectonics and Hydrocarbon Potential along Indian Continental Margins: Interpretation from regional GXT Profiles, Project Report, GX Technologies. Dariusz Jablonski, 2006, Paleogeographic maps and lithofacies distribution deposited during indicated time slice, Finder Exploration. Krishna. K. S., Gopala Rao. D. and Sar, D., 2006, Nature of the crust in the Laxmi basin (14 o to 20 o N), western continental margin of India; Vol 25, TC1006, DOI /2004TC Ramana, M. V., Subramanyam, V., Chaubey, A. K., Ramprasad, T., Sarma, K.V.L.N.S., Krishna, K.S., Maria Desa and Murty, G.P.S., 1997, Structure and origin of the 85 o E ; J. Geophysical Res., 102, No. B8, Raval, U. and Veeraswamy. K., 2003, Modification of geological and geophysical regimes due to interaction of mantle plume with Indian lithospehere; J. of the Virtual Explorer, V. 12, Subramanyam, M., Rao, N.V. and Rao, T.K.S.P., 2004, Delineation of Basement and Curie Isotherm of Bay of Bengal from Spectral Analysis of Magnetic Data; J. Ind. Geophys. Union, Vol. 8, No. 2, Christian, F.L., Spiess, V., Peter, M. and Joseph, R. Curray., 2000, Summary on the Bengal Fan An Introduction to a drilling proposal.