TECTONIC ROLE IN STRUCTURE FORMATION AND SEDIMENTATION IN MUMBAI OFFSHORE BASIN

Abstract

The Mumbai or Bombay Offshore Basin located off the west coast of India is known for the supergiant oil/gas field Bombay High, the largest oil/gas field discovered in the country. The field was discovered in the year 1974. The major part of the country’s production of oil comes from this field, and from a number of other smaller oil and gas fields discovered subsequently in this basin. This pericratonic basin bordering the Indian continent initiated as a rift basin in the Late Cretaceous- Early Paleocene times in response to the magmatic upwelling caused by the Reunion plume moving under the Indian Plate. It is preserved within the Indian Craton, and floored by the Deccan Trap basalts or in some cases by the Middle Proterozoic granite/ granite gneiss. A subsequent northward drift in the Early Eocene resulted in the separation of the Indian landmass from that of Seychelles, and a marine transgression. The Indian plate collided with the Eurasian plate from Early Oligocene period onwards, leading to the rise of the Himalayan mountain system, and westward tilting of the Mumbai Offshore Basin. The Paleocene, Early Eocene rift system consists primarily of the Bombay Rift, and the Murud, Kori depressions developed successively to its west. All these rifts trend roughly NNW-SSE. The rifts widen northwards, tilts north and also towards east. The rifts are segmented with each of the segments separated by NE-SW to ENE-WSW trending paleo-highs/ transfer zones oblique to the rift geometry. There is an offsetting relationship of the rift segments across such faults. The rift geometry is masked by the massive influx of clastics during the Miocene times. The rift geometry of horsts and grabens controlled the sedimentation through periodic reactivation along the graben bounding faults. Based on lithologic association and palaeography the basin is broadly divided into Tapti-Daman block, Heera-Panna-Bassein block, BH-DCS block, and Ratnagiri Block. The Tapti Daman block, located north of Mahim-Daman cross-trend, is highly clastic prone being fed by the fluvial systems originating within the Indian landmass. The remaining blocks to the south of this cross-trend are located in palaeo-highs, and thus in an area of sediment bypass. The sediments within the paleo-grabens are the volcano-clastic sediments of the synrift stage, and clastic sediments of Early Eocene age overlying either the Deccan Trap basalts or the granite gneiss. In the platform area carbonates of Middle Miocene age occur, whereas in the Tapti-Daman block continuous sedimentation of clastics is recorded without any significant geologic breaks. The studied structures, North Tapti and B-12 fall in the Tapti-Daman block, and the third structure Bassein fall in the Heera-Panna-Bassein sector. All these structures are gas producers, with the Bassein and B-12 field contributing a substantial production of gas. In terms of size, N. Tapti has an area of about 45 km2, Bassein has an area of about 200 km2 and B-12 structure has an area of about 600-700 km2. The prospects studied utilised roughly 2500 km2 of 3-D seismic ii data volume, well logs, analysis of well data gathered from the well completion reports, unpublished in-house interpretation reports, and reports and articles available in public domain. The North Tapti structure is a doubly plunging ENE-WSW trending anticline at Tapti, Mahim, Daman, Lower Mahuva and Mahuva Formation levels overlying a basinal low. It is one of the many ENE-WSW trending structures, bound to the south by a similarly trending fault occurring to the south of the main Narmada lineament forming a part of the Narmada wrench system. The B-12 structure is an elongated dome with NE perturbation, overlying a basement high and a depositional low in the east. The Bassein structure an elongated NNW-SSE high, is located on a horst block flanked to the east by the Mahim Graben. A faulted anticlinal high occurs in the central part of the over the horst, with the eastern extension of the structure occurring over the low of the Mahim graben. The structures located at widely separated areas in the basin, and associated with different lithological associations / rock properties were studied to understand to get an idea about the extent of tectonic deformation in different parts of basin. The study was also done to establish the definitive fold geometry, fault fold relationship, and kinematically viable structural evolution. The interpretation included preparation of structure contour maps, preparation of isopachs, structural overlays on isopachs and palinspastic restoration using 2D Move (Midland Valley). Fault pattern study and kinematic studies were done both regionally within the structures that have been studied. Analysis of the structural balancing, show a maximum shortening of 18% in the North Tapti Structure, 8% in the B-12 structure and 8% in the Bassein structure for the oldest stratigraphic surface from the present day length. Shortening is an indicator of deformation intensity, and the extent of deformation across various structures does indicate that the dominant lithology or the rock properties do have some relationship in the deformation process. Structural overlays on isopachs and a graphical plot of the shortening percentages of stratigraphic surfaces in each of the structures have defined main deformational phases at end Late Oligocene (B-12), Early Miocene (Bassein), and Late Miocene (N. Tapti). Fault orientation and kinematic analysis bring northeast rotation of the rift related faults in each of the rift segments indicating a right lateral movement south of each of these transfer faults. The strike slips resulted in bending of the rift related faults towards east. Structure formation is possibly related to the plastic deformation resulting in a bulge of the less competent shales and claystones brought out by the bending of these related faults, which has uparched of the more competent formations overlying it. Riedal shears along some of the rift related faults is also identified. Partitioning of the stressed zones in different segments separated by transfer faults is also seen