Abstract

The paper reviews the exploration activities in the Andaman forearc basin. Structural analysis reveals various stages of evolution with corresponding sedimentation cycles. Subduction complex comprises accretionary prisms and forms the floor of the ponded Neogene sediments in the Upper trench slope and also the base of the young accretionary basins in the west of the Outer high. Unconformity overlying subduction complex is much younger in the Lower trench slope as compared to the east. Reliable and continuous reflections are absent within subduction complex. Favorable lithofacies trends vis-a-vis structural evolution of the area have been outlined. This has enabled to identify broadly the future hydrocarbon prospects in the region.

Introduction

The Andaman forearc basin lies in the eastern part of Bay of Bengal, having an arcuate trend, with Sumatran Archipelago and Indo-Burmese ranges in its south-southeast and north-northeast, respectively. The Andaman-Nicobar group of islands constitute a part of the island arc system formed by converging plate motions.

Bathymetry of the Andaman basin and the adjoining areas is complex as evident from Figure 1. The map is based on bathymetry and physiographic studies carried out in detail by Rodolfo (1969). The chain of Andaman-Nicobar islands are dissected by a number of deep sea channels. The Great Passage to the south of Great Nicobar is the deepest one attaining a depth of about 2000 m. Equally conspicuous is the Ten Degree channel. The East Andaman shelf abruptly reaches a depth of about 2000 m and again rises giving prominence to the intervening valleys. One such valley is between South Andaman and Invisible Bank. The western shelf up to 2000 m. bathymetry is relatively wider than that of the east and the associated slope is also less steeper though it touches basin floor around 3000 m. The Java Trench occurring in the west of Sumatra, looses its manifestation towards north near Nicobar islands. Presence of volcanic islands and sea mounts is no exception viz. Barren and Narcondom islands, and the Sewell sea mounts. Another prominent shallow feature is the Invisible Bank.

Investigations till 1977 mainly included geophysical surveys restricted to a few seismic profiles in the offshore as a part of reconnaissance surveys conducted by the Indian Ocean Expedition (Burns, 1964 and Peter et. al., 1966) and geological surveys in the Andaman-Nicobar Outer high (also a Structural high and a non-volcanic Outer ridge). The Oil and Natural Gas Commission accelerated its exploration programme in the basin by acquiring 6370 line kms. of seismic and magnetic data through contract in the year 1977.

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