Parts of Godavary and Krishna deltas covering an area of about 70,000 Sq.km. is found to be prolific with gas resource. More than half of this area lies in the offshore region covering shallow water zone (upto 400m water depth) and deep-water zone (upto 2000m water depth). The total gas reserves so far identified in the K-G basin come to more than 2 trillion cu.m. Oil reserves are insignificant. Reliance Company is planning to take out most of the reserves rapidly with peak output lasting for 8 years commencing from 2008 and completely by 2022.Similar production rates may be followed by the other organisations also. The reservoir rocks of this resource are sandstones of Mio-Pliocene age of Tertiary period (5 to 24 Million years old) and of Upper Cretaceous age (67 to 144 m.y).

Oil and gas fields located in the Tertiary sandstones elsewhere in the world have shown subsidence of land because of their compaction as a consequence of withdrawal of these hydrocarbons. Some examples are California, Texas and Louisiana in U.S.A, Venezuela, Netherlands, and Italy etc. Landsubsidence was reported to be in the range of 1.5 to 8m with rates varying from 2 to 35 cm per year.

Even Cretaceous formations in the K-G basin seem to be prone to subsidence problem as they are found to be relatively soft.

Water injection was found to be successful in the Wilmington oil field of California to raise the depressed land and to reclaim it. However, this operation is said to be expensive and reduce profit margins of the oil. Huge amounts are spent on reclamations of the affected areas in Louisiana and other parts of U.S.A. There are very few oil fields in the K-G basin.

In the exclusive gas fields, there are no reports of mitigation measures. Probably they are not feasible economically. In the problematic gas fields of Wadden Sea, Netherlands, Venice, and Italy offshore fields, detailed studies have been carried out to predict the extent of subsidence in the coastal regions and exploitation is planned under controlled conditions. The Army Corps of Engineers and the State regulate oil and gas development in coastal Louisiana.

In the light of above case studies, a note was prepared on the imminent danger of land subsidence in the Krishna -Godavary deltas, and communicated to the concerned authorities in the state and central governments between September, 1995 and may, 1997 for information and necessary precautions. This information was widely published by the newspapers.

Recently, natural gas fields in Indonesia were shut down due to land subsidence.
GPS studies in the Gas fields in Gujarat recorded a maximum subsidence of 13 cm in two years from 2004 to 2006.There is good correlation between gas produced and the subsidence of land.

The first precaution to be taken up by the exploration organizations in any prospecting areas is to establish a network of benchmarks and their continuous monitoring to recognize any changes in land surface. In the recent years, Remote sensing technique using SAR (synthetic Aperture Radar) data collected by the ERS -1&2 (European Space Agency Remote Sensing) has become handy for fast evaluation.

The O.N.G.C has taken steps to develop subsidence prediction model for oil and gas fields of Gujarat in 1997. Software for computer modeling on land subsidence is available now in the market.

Director General of Hydrocarbons has initiated studies for probable land subsidence in production of shallow gas in Gujarat.

What is the status of precautions taken in the K-G basin?

The geological formations which are alternate sequences of sandstones and clays (claystones and shales) are sloping towards the sea at more than 10 degrees in some horizons. The basalt trap which is a hard rock with a thickness of 10 to 250 m lies in the midst of these sediments is inclining towards the sea. In similar geological settings along the coast in Louisiana, a slow continuing horizontal creeping was noticed. New Orleans is said to be at the tip of a gigantic, slow moving landslide. It is said that Southern Louisiana is not only sinking, it is sliding.

The deep-water reservoirs in the K-G basin are located along the continental slope part at a distance of 30 to 50 Km from the coast. But still, the effects of gas withdrawal can extend on to the delta land as the radius of influence of such fields in East Texas is found to be up to 100km and beyond. In fact, in such truncated reservoirs, the pressure drop would be nearly double to that of normal reservoirs with infinite aerial extents.

Minor earthquakes of 3 to 4 magnitude on Richters scale are very common in oil and gas fields due to slumping of chunks of earth and reactivation of faults. There are minor growth faults in the sediments and major faults in the basement rocks of the K-G basin. Therefore, seismic activity of any magnitude is possible in this area also. The increase in frequency of minor earthquakes in Prakasam and Guntur districts in A.P and in parts of Gujarat need detailed studies from this angle. Such tremors in the basin accentuate the process of sliding of the landmass into the deep sea. Submarine mass movements are reported in the offshore oil and gas fields in the North Sea and adjoining areas.

The Krishna-Godavary deltas are not godforsaken places like other oil and gas fields of the world, which are wetlands or wastelands. It is a rice bowl of the country with dense population. In the initial stage of subsidence in the area, the rivers would change their courses or form in to lakes and with reversal of gradients; they may turn into backwater channels with the seawater. These effects may surface out as a rule of thumb, once the production reaches its peak i.e. within a decade. The delta land, which is 1 to 3m above mean sea level, may ultimately sink and submerged by the sea in most of its extent. While the fossil fuels are going to be exhausted in a few decades, and when efforts are under way for alternate fuels, is it wise to lose such a valuable land leaving about a crore of people in lurch for the sake of short-term gains.