Bangalore: Vengannapalli, a nondescript village in Anantapur district of Andhra Pradesh, may be sitting on a reservoir of gas or oil, say scientists who have been able to make the discovery with the help of soil bacteria that live exclusively on a diet of hydrocarbons like methane, ethane and propane.

A high concentration of these bacteria is an indication that gaseous hydrocarbons are seeping out to the surface from oil or gas reservoirs below the ground, says Anurodh Dayal, a scientist at the National Geophysical Research Institute (NGRI) in Hyderabad.

A large presence of these bacteria at two zones in the Ganga basin and another zone in Sagar district of Madhya Pradesh helped NGRI scientists predict that the areas are rich in hydrocarbons.

In Vengannapalli, NGRI scientists led by Dayal launched a study last year following newspaper reports of seepage of natural gas from a bore well in the village.

The scientists collected 30 soil samples from the village from one meter below ground. Analysis of the samples showed the presence of methane, ethane, propane and butane gases and "high counts of bacteria" that utilise these hydrocarbons as the only carbon source for their growth.

"We call this microbial prospecting," Dayal told sources. "This method, coupled with adsorbed soil gas and carbon isotope studies, suggests that hydrocarbon micro-seepage of subsurface origin is present in the area and indicates that the area is worth visiting for conventional petroleum exploration."

According to the NGRI scientists, the presence of methane eating bacteria is usually an indication of gas fields, while ethane, propane and butane are assumed to be indicators of oil in deep reservoirs below ground.

Dayal and other members of his team - M.A. Rasheed, M. Veena Prasanna, T. Satish Kumar, and D.J. Patil - have reported their findings in the latest issue of the Current Science journal, published by the Indian Academy of Sciences, Bangalore.

"We plan to carry out detailed soil sampling in the region after the rains," Dayal told sources.
The NGRI finding is likely to prompt the Oil and Natural Gas Commission (ONGC) to undertake exploratory drilling in that area. Dayal said ONGC officials were aware of the finding but had not yet reacted officially.

Microbial anomalies have been proved to be reliable indicators of oil and gas with the reported success rate of 90 percent but this method is not used alone.

"It can be integrated with geological, geochemical and geophysical methods to evaluate the hydrocarbon prospect of an area and to prioritize the drilling locations, thereby reducing drilling risks," Dayal said.

The NGRI study area in Andhra Pradesh is located in Tadpatri Formation of the Cuddapah Basin, consisting of shale, quartzite and dolomite rocks with maximum thickness of about 4,600 metres. The entire Cuddapah Basin covers an area of 44,500 sq km with aggregate thickness of 12 km.

"The geological and geophysical studies reveal that Tadpatri shales can be a potential source rocks for hydrocarbons, having the essential prerequisites such as their undisturbed nature favourable for the generation and accumulation of hydrocarbons," the scientists said in their report.

Cuddapah is not the only basin where the NGRI scientists are employing the microbes to help in oil search.

According to Dayal, his team has recently carried out microbial survey of soil in the Ganga basin. He said the results "are very interesting and we have been able to discover two zones in the basin rich with hydrocarbon".

He said that using the same technique his team has also located a "hydrocarbon enriched zone" in Sagar district of Madhya Pradesh, details of which will be announced in two months' time. IANS