HR: 0800h
AN: S51B-07    [Abstracts]
TI: An example of extruded sandstones resulting from hydrothermal venting associated with igneous intrusions
AU: * King, S
EM: stephen.king@bg-group.com
AF: BG Group, 11A Osborne Road Ikoyi, Lagos, Nigeria
AB: The presence of anomalous sands within deep water Palaeogene sediments offshore south east India can be explained by the effects of hydrothermal venting. The sands are found within a deep marine sequence of pelagic shales, with no obvious lateral sedimentary provenance, they are believed to have been extruded at the sea bed via hydrothermal venting. Seismic examples along with well data of these unusual phenomena are presented which support this origin for their presence within this pelagic section. The Mannar sub basin, which lies off the south east coast of India, has its origin in Mid Cretaceous rifting. It is one of a series of SW-NE trending rift basins which together are collectively referred to as the Cauvery Basin. Sediments in the syn-rift section are in excess of 3000m are comprised of a series of both continental and marine clastics. Late Cretaceous and early Palaeogene drift sediments are comprised of deep marine pelagic shales. The Period from the Maastrichtian to the Early Eocene was one of limited sediment input into the Mannar basin, resulting from a westward tilt of the Indian plate, a time of widespread deposition on the western margin of the sub continent. Within this predominantly shale prone interval there are encountered isolated sandstone intervals. These sands exhibit unusual petrographic and sedimentary characteristics which indicate that they have been transported relatively short distances and not long distance from the shelf edge which in this basin may have been several hundred kilometres to the North and West. Along the entire margin of western and south eastern India there is widespread evidence of igneous activity, the most notable being the Deccan Traps of Central and Western India. This igneous activity has been dated from Late Cretaceous up to Early Eocene. The igneous activity in the Mannar basin is postulated to be Eocene in age. In the Mannar Sub basin the igneous activity can be recognised on 3D seismic data and exhibits the increasingly familiar geometries seen in other similar settings such as the NW margin of the European plate. Within the area being presented one particularly well imaged igneous complex is observed beneath an anomalous high amplitude set of seismic reflections which are interpreted to be extruded clastic sediments, brought to the sea bed be as a result of hydrothermal venting, triggered by the igneous intrusions. Two wells have been drilled which penetrated intervals with similar seismic responses to those illustrated. These wells penetrated sands with very high GR readings and cores taken in the wells indicate that they are debris flows with many typical debris flow characteristics such as fluid escape features. The lateral extent and thickness of the extruded section can cover several square kilometres and range in thickness up to 50m. These dimensions indicate that these "extrudites" could well be of economic importance as hydrocarbon bearing reservoirs
DE: 0900 EXPLORATION GEOPHYSICS
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3022 Marine sediments: processes and transport
DE: 3075 Submarine tectonics and volcanism
DE: 3642 Intrusive structures and rocks
SC: Seismology [S]
MN: 2007 Joint Assembly