Gas Hydrate Occurrence in the Krishna–Godavari Offshore Basin Off the East Coast of India
characterise this basin. The continued eastward tilting of the continental blocks resulted in evolution of a Tertiary deltaic system with a south-eastward-prograding shelf edge.12
clastic and carbonates were deposited over the rift-filled sediments, and variable thickness shale–silt–clay and sand beds of the Upper Cretaceous to Recent age10–12
The KG basin
thus developed has an upper Paleozoic to lower Mesozoic non-marine graben fill sequence onshore and an upper Mesozoic to Holocene sequence of marine sediments in the offshore.
The hard collision between the Indian and Eurasian plates during the drifting phase in Miocene resulted in the mountain building of the Himalayas, and the subsequent monsoon activity promoted the rejuvenation of the Ganges, Brahmaputra, Mahanadi, Krishna, Godavari, Cauvery and other minor tributaries, facilitating major transportation of huge sediment influx into the Bay of Bengal.13
This
The high terrestrial input is a rich source of organic carbon, and its quick burial resulted in methane generation both at shallow and deeper levels. An extensive marine transgression took place in the early drift phase with south-easterly tilt in the late Cretaceous, which led to the deposition of Ragavapuram shale.8 Tertiary period is marked by several cycles of transgression and
sediment flow has manifested in the form of deltas and delta-fronts. The Mahanadi, KG and Cauvery delta-fronts on the inner shelf and beyond are some typical examples of this sort. The shelf is incised by headward-eroding point-source canyons, which are now abandoned and filled with clay and re-worked sediments. Several V-shaped canyons flanked by steep faults characterise the upper to middle slope regions, while turbidity channels and levee wedges traverse the shelf and slope.14
The
regression and rapid seaward progradation of deltaic domains. This process led to a narrowing of the shelf and the deposition of sediments associated with growth faults and fan deltas. Geochemical parameters reveal greater terrestrial input in the Paleocene that resulted in occurrences of Tertiary oils offshore. The drilling results confirm the presence of hydrocarbons in the Permo–Triassic to Pliocene. More specifically, the middle/upper Miocene to mid-Pliocene is the promising hydrocarbon prospect off Godavari.8,12
Pliocene
sands, in the combination of either sand–silt–clay or sand–silt–shale sequence, have proved the best pay-off zones of either crude oil or gas. Occasionally, the sand in a stratigraphic channel fan complex of lower Cretaceous age shows the presence of hydrocarbon prospects. From the drilling results it appears that most of the hydrocarbon occurrences onshore are from the Permo–Triassic, whereas offshore lower Cretaceous to Pliocene are potential zones. Abnormal pressures due to advection of gases are observed, especially in the Palaeogene deposits, these being the prospecting zones in the KG basin.9
Since
hydrocarbon generation is taking place in the KG offshore, the upward migration of natural gases can play a vital role in the occurrence and formation of gas hydrate deposits at shallower levels, where the required pressure–temperature conditions prevail.
Materials and Methods
Four multidisciplinary cruises on-board the research vessels ORV Sagar Kanya and AA Sidorenko were undertaken during 2002–2003 by the NIO and acquired geophysical, geological, geochemical and microbial data in the KG offshore. Some important results from the JR cruise are also incorporated towards better understanding and evaluation of the
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