* Regulator rules company measured gas output incorrectly
* Ruling applies to two platforms in Albacora Field
* ANP may question royalty payments in other fields
RIO DE JANEIRO, April 3 (Reuters) - Brazil's state-controlled oil company Petrobras may be forced to pay additional natural gas royalties after Brazil's petroleum regulator ruled that the company erred in the way it measures output, a source with direct knowledge of the subject said Wednesday.
The regulator, known as the ANP, ruled in March that Petrobras, formally known as Petroleo Brasileiro SA, improperly measured natural-gas production from two platforms, the source said.
The ANP decision could result in an additional royalty levy of 200 million reais ($100 million) on gas output from the P-25 and P-31 production platforms in the offshore Albacora field in the Campos Basin northeast of Rio de Janeiro.
Company officials were not immediately available for comment.
The regulator has also found additional underpayments on other production units, according to the source, who said Petrobras on Tuesday appealed the ruling.
A decision on the company's appeal on the two platforms could be announced by the middle of April, the source said.
The source said the ANP is looking into measurement practices on other platforms, but said there was no way to immediately know how much Petrobras may have underpaid.
Petrobras apparently discounted some gas output from royalty calculations to compensate for so-called wet gas, or natural gas saturated with hydrocarbon liquids, the source said. Such discounts are allowed, but Petrobras did not have the equipment needed to properly measure this discount in accordance with regulations, the source said.
The presence of liquids in natural gas complicates the measurement of gas volumes.