Oil and Natural Gas Corporation cannot start the exploratory drilling process in the highly prospective Mahanadi basin, as the environment ministry has not granted it clearance.

The environment ministry had earlier decided that ONGC could carry on drilling in the MN-DWN-98/3 block during May-October to avoid any harm to the Olive Ridley turtles that breed in the area.

But, the six-month window period ended without any clearance even though the file had been cleared by the environment secretary's office because Environment Minister A Raja was out of town, said ONGC executives.

"We kept a rig waiting for four days for which we were paying Rs 250,000 daily. The rig was finally moved to the Krishna Godavari region," said a senior ONGC executive, adding that now the weather window did not permit drilling.

Reliance Industries also did not get the clearance though the environment ministry had earlier told the companies that they could drill during the non-nesting season.

Other exploration blocks in the area apart from GAIL-Gazprom block NEC-OSN-97/1 are ONGC and GAIL India's MN-OSN-97/3."These blocks do not fall in the way of turtles. So, clearances for them was not a problem," said an executive.

Turtles nest annually during January-March on the beaches of Gahirmatha in the Bhitarkanika Wildlife Sanctuary (Kendrapada district), the mouth of the river Rushikulya (Ganjam district) and the mouth of the river Devi (Puri district).

Two Reliance exploration blocks, NEC-OSN-97/2 and MN-DWN-98/2, were awaiting environmental clearances due to migration of the threatened turtles.

A time-restricted clearance was earlier granted to Reliance, but was withdrawn later for the MN-DWN-98/2 block. The block falls on the return path of migratory turtles, which travel long distances to nest along the Orissa coast.

A turtles tracking study was also being carried out by the Dehradun-based Wildlife Institute of India to assess the impact of drilling on turtles.