State-owned Oil and Natural Gas Corporation's (ONGC) 64 domestic fields have total reserves of 1.06 billion tonne (Bt) of oil and oil equivalent natural gas, according to independent audits.
ONGC, in preparation for its share sale next month, had engaged Gaffney Cline and Associates (GCA) and DeGolyer & MacNaughton (D&M) to audit 64 domestic fields of the company. GCA prepared audited etimates for Mumbai High field and D&M prepared audited estimates for 63 other fields.
According to the audit estimates, the 64 fields have a 3P (total oil reserve) of 558.40 million tonne and 503.11 million tonne of oil equivalent gas.
The audited estimates are higher than ONGC's self-assessment of 966.18 million tonne of oil and oil equivalent of gas reserves in the 64 fields that includes the giant Mumbai High.
A company official said oil reserves were primarily a measure of geological risk - of the probability of oil existing and being producible under current economic conditions using current technology.
The three categories of reserves generally used are proven (reasonably certain to be producible), probable (50% chance of these being produced) and possible reserves (having 10% certainty of being produced).
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The government plans to sell 5% of its shareholding in ONGC in the follow-on public offer (FPO) next month to raise Rs 12,000-13,000 crore.
For the 12 foreign properties held by the firm's overseas subsidiary, ONGC Videsh, independent review put 1P, 2P and 3P reserves at 119.977, 288.299 and 639.706 million tonne of oil and oil equivalent gas.
GCA estimated 1P, 2P and 3P oil reserves at Mumbai High fields off the west coast at 134.20 million tonne, 239.70 million tonne and 269.50 million tonne of oil respectively.
These were higher than ONGC's estimate of 125.09 million tonne of 1P and 190.07 million tonne of 3P reserves, the report said.
For natural gas, GCA estimated 1P reserves of 42.54 million tonne oil equivalent at Mumbai High and 3P reserves of 80.40 million tonne.
These comapred to the ONGC's estimate of 48.68 million tonne of 1P and 73.97 million tons of oil equivalent 3P reserves.