Abstract

The Mannar Basin lies between south-western Sri Lanka and the Indian coastline, in water depths ranging from 50–3000m. Seismic data indicates that this is a failed rift basin containing over 5 seconds TWT of inferred Late Jurassic-Early Cretaceous to Recent sediments constituting four discrete tectono-stratigraphic packages.

The postulated petroleum system comprises:

• Source – late Jurassic lacustrine or marginal marine shale and coal; early Cretaceous marginal marine or marine shale and coal.

• Reservoir – late Cretaceous marine sandstones, Tertiary turbidites.

• Seal – Paleocene regional shale or intraformational shale.

• Trap – basement-related horst and tilted fault blocks within the synrift and rift and sag megasequences; Neogene compressional structures; three major periods of Cenozoic basin-floor and slope fan deposition provide stacked fan floor and slope deposits up to 1.0 second (TWT) thick and represent a major stratigraphic fairway in an apron of sediments tracking along the basin floor and lower slope parallel to the Sri Lankan margin.

Synthetic burial history modelling indicates that the inferred Late Jurassic to Early Cretaceous synrift source rocks commenced oil generation around 120 Ma, peaking at 105-100 Ma within the uppermost Cretaceous. Modelling also predicts a second, minor generative phase in the Neogene. A currently-active petroleum system is confirmed by a satellite seep study.