Petroleum Systems of the Mumbai Offshore Basin , India *

Abstract

Mumbai Offshore Basin, a pericratonic rift basin in the western continental shelf of India, covers about 148,000 km2 from coast to 200 m isobath. The basin is divided into six tectonic blocks (Tapti-Daman, Diu, Heera-Panna-Bassein, Mumbai high-Deep Continental Shelf [DCS], Ratnagiri, and Shelf Margin), and the sedimentary fill ranges from 1100-5000 m. Several large oil and gas fields have been discovered in this basin, and the presence of hydrocarbons has been established in the multiple pay zones belonging to L-III limestone reservoir of Miocene age (only in Mumbai high), Mukta (early Oligocene), Bassein (middle Eocene), Panna (Paleocene to early Eocene) reservoirs, and Daman (early Miocene-late Oligocene) and Mahuva (early Oligocene) formations in Tapti Daman block. This work presents the detailed geochemical evaluation on more than 200 rock extracts and 250 oil samples and maturity modeling in key generative depressions of all the tectonic blocks of the basin. Studies indicate that Paleocene to early Eocene Panna Formation has good to excellent source-rock characteristics in the basinal part of every block in the entire basin. The middle of Panna Formation attained the maturity of 0.6% VRo equivalent at about 12-20 Ma. The Panna source rocks have predominantly Type III kerogen in lower layers and Type II and III mixed kerogen in upper layers. However, few source-rock layers in Mahim graben and Deep Continental Shelf (DCS) area contain Type I kerogen. Based on the variations in organofacies, the oils have been placed into two groups. The first group oils, having only terrestrial source input, correlate with the source-rock extracts of lower layers, whereas the second group oils, with terrestrial and marine mixed input, correlate with the source-rock extracts of upper layers of the Panna Formation. Based on the reservoir and source combinations, major petroleum systems in the basin are: Panna-L-III, Panna-Mukta, Panna-Bassein, and Panna-Panna.