Stratigraphy and Sedimentation of Bombay Offshore Basin

Correlation of seven chronostratigraphic sequences in over 300 wells drilled in the six structural blocks constituting the Bombay Offshore Basin has helped in defining the stratigraphy, paleogeography and facies distribution and suggesting a model for shelf carbonate deposition. The sequences correlated are: Paleocene-Early Eocene, Middle-Upper Eocene, Lower Oligocene, Upper Oligocene, Lower Miocene, Middle Miocene and Middle Miocene-Holocene.

The most significant observations arc 1) Surat depression and its southward extension through other grabens up to Ratnagiri block contain a restricted marine, paralic and continental shale, sand and coal facies of 500-3000 m. thickness, deposited during syn-rift. Paleocene-Early Eocene phase. This is the principal source rock in the basin. 2) The main reservoirs are the Miocene limestones in Bombay High and Middle Eocene and Lower Oligocene limestones in other areas. 3) The main sandstone reservoirs are in the Oligocene sequences in Daman-Tapti area of Surat depression and in the Paleocene-Lower Eocene sequence in Heera-Bassein and Ratnagiri blocks. 4) By tracing the clastic dispersal.pattern, it is shown that Saurashtra basin and Shelf Margin basin with their excessive Upper Oligocene-Holoccne fill are less favourable than the rest of the basin in hydrocarbon potential.

A homoclinal ramp model is suggested for the shallow shelf carbonate deposition. The suggested carbonate depositional model indicates that the platfonn areas are susceptible to shale incursions from the east which could affect the reservoir properties of limestone in many areas. Almost all the major carbonate pay zones are associated with unconformities and diastems and the principal porosity is telogenetic.