Unravelling Mesozoics through Sub-Basalt Imaging: Implications for Hydrocarbon Exploration in Kerala-Konkan Basin, Offshore SW India

Rawat, Sushma; Sivester, Justin M.
Western Offshore Basin, Oil & Natural Gas Corporation Limited, Mumbai, India.

Rift and post-rift architecture of SW margin of India in Kerala-Konkan(KK) basin can be correlated with episodic tectonic events accompanied by extensive magmatic activity in E.Cretaceous (Madagascar CFB equivalent) & L.Cretaceous-E. Paleocene (Deccan Volcanic Province) and pre & syn-rift sequences in Mesozoic & Paleogene associated with rifting of India from Madagascar (93-89Ma) and later separation of Seychelles from India(65Ma).

Initial exploration thrust was on Tertiaries, but poor results led to shift focus to possible Mesozoic sedimentary sequences below Paleocene basalt. Drilled wells data and various seismic campaigns illustrate the variability in aggregate thickness and elastic properties of these layers. Three recent drilled wells were terminated before reaching sub-basalt sediments, as Paleocene basalts were thicker than expected.

Imaging of deeper sections is paramount to better understanding of underlying Mesozoic sequences. Interpretation of seismic data acquired in basalt-covered areas in KK basin is challenging because of poor imaging. Base-basalt is not always reflective and probably a transitional contact, with low acoustic impedance. Underlying Mesozoic strata produce incoherent weak reflections.

New technologies in data API have improved understanding of sub-basalt Mesozoic stratigraphy. Q-Marine technology DiscoverOver-Under & GXT, Long-Offset and processing application (GeoTomo, PSDM Beam Migration), have brought better imaging of sub-basalt Mesozoic strata.

Broad structural framework of Mesozoic rift architecture indicates it to be oblique to Tertiary rifting denoting difference in prevailing stress directions. Large structural closures, are identified below basalts. Kerala Trough is inferred to contain 1-4 km thickness of Mesozoic strata with E-L. Cretaceous lacustrine and marine source rocks, floored either by Santonian basalt or stretched continental crust as basement. L. Cretaceous Cochin Formation sandstones form reservoir facies.

Improved API, with technologies like MMT, CSEM & SBN, along with Multi-component seismic processing, will increase deeper penetration, unravelling extensive Mesozoic depocenters beneath basalt envelope, hence upgrading deep-water prospectivity of KK basin. Integrated interpretation with GM will lead to better basin architecture visualization & provide predictive geologic models.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012