However, the resulting additional payments will lower the headroom under OIL's 'BBB-' standalone credit assessment, Fitch said in a statement.
The government last week announced that state-owned upstream oil producers must pay state royalties on the gross value of crude oil produced domestically instead of the previous method of using the net price after discounts to state-run refiners.
The new formula applies retrospectively from February 2014, resulting in a back-payment to cover the period up to 2015-16.
This, along with with payments for acquisition of a share in Taas Yuriakh and Vankor from Russia's national oil company, Rosneft, is expected to weaken OIL's leverage beyond what is comfortable for its standalone credit assessment.
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"However, Fitch believes OIL's leverage will improve in 2017-18 in the absence of large M&A even though the higher royalty payments will reduce the company's netback," the statement said.
Netback is the revenue after all the costs for bringing one unit of oil to the market.
This compares with a netback of around USD 33-34 per bbl at that level after discounts and existing statutory levies.
"We also estimate royalty charges to rise by around USD 0.5 per bbl and USD 0.7 per bbl at higher crude oil prices of USD 55 and USD 60 per bbl, respectively, compared with the previous formula," Fitch said.
"However, if prices are hiked regularly, this could reduce under-recoveries, as kerosene accounted for over 40 per cent of total petroleum subsidies in 2015-16. Nevertheless, it remains uncertain how the Indian government will approach subsidies at higher crude prices, especially prices above USD 60 per bbl," it added.