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The Petroleum Geology of the Ganga Basin, Himalayan Foredeep, Northern India

Mathur, Sudhir; Naidu, Bodapati N.; Adhikari, Subhrashis; Mohapatra, Pinakadhar; Burley, Stuart
Cairn India Ltd., Gurgaon, India.

The present day Ganges Valley in northern India is underlain by one of the largest sedimentary basins in the world, covering an area of approximately 600,000km2, and containing over 8km thickness of proven sedimentary rocks. However, the basin is poorly known and there is little information published on the petroleum geology of the basin. This study investigates the petroleum system of this frontier basin.

There are three temporarily distinct stratigraphic packages in the basin separated by sequence boundaries representing major unconformities. These packages are placed between the Palaeo- and Meso-Proterozoic, the Neo-Proterozoic and lowermost Cambrian, and between the Miocene and Recent. A Cambrian to Ordovician date previously identified for the Neo-Proterozoic to lowermost Cambrian section, based on Acritarch assemblages, is not supported by palaeontological evidence presented in this study. There is, however, evidence of remnant Paleogene sediments below the Neogene Siwaliks.

Although evidence exists of a previously working petroleum system within the Proterozoic, basin modeling indicates that the onset of hydrocarbon generation in the Neo-Proterozoic to Cambrian sediments took place in the early Ordovician (c. 450Ma) and ceased in the early Devonian around 400Ma as indicated by a Transformation Ratio (TR) of 100%. This demonstrates that the main source rock is over mature and was exhausted of hydrocarbons for some 400 million years before present. As a result of the late Palaeozoic uplift the potential for successful trapping hydrocarbons in any Palaeozoic structures is extremely low and that no significant hydrocarbons remain in these sediments. Mineralogical data indicates that the pre-Tertiary sediments are also currently at a low grade of metamorphism and potential reservoir lithologies are characterised by very low remaining porosities.

The Tertiary basin is dominated by continental sediments (the so-called Siwaliks) which extend over the vast majority of the basin, and although these sediments have excellent reservoir properties they have no local source rock potential and few traps. Older sequences are found in the north as the Tertiary wedge thickens. These sequences become marine as the last remnants of the Tethyan ocean prior to Himalayan collision and could contain source rocks, but long distance migration would be required.

The Ganga Basin therefore remains a frontier basin of high exploration risk.


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