UPDATE 1-BP's output cut by 100,000 boepd in Caspian-source

(Adds details, quotes)

LONDON, Aug 13 (Reuters) - BP Plc's (BP.L) share of oil and gas output cut at fields in the Caspian Sea is about 100,000 barrels of oil equivalent per day (boepd), a source close to the company said on Wednesday.

The amount affected is less than that implied by BP's equity stake in fields such as Azeri-Chirag-Gunashli and is because of the way the barrels are assigned under production-sharing contracts (PSCs). BP has lowered production at ACG oilfields in the Caspian Sea and has stopped pumping gas from the Shah Deniz field into a pipeline that runs to Turkey. It has not specified by how much output has been cut. "BP's net share is about 100,000 boe per day," said the source, who declined to be identified by name.

"Under the terms of the production-sharing contract, the net volumes lifted by the shareholder companies are less once cost oil has been recovered. It already has been recovered."

Under PSCs, oil companies can get a larger share of production from a field after it first starts production to be repaid their costs. This then declines over time.

The BTC oil pipeline was closed last week after a fire in Turkey. BP on Tuesday shut a pipeline from Azerbaijan to Supsa in Georgia and stopped pumping gas through Georgia because of the conflict with Russia.

The BTC pipeline closure forced the BP-led ACG fields, which feed the BTC line, to cut output from about 800,000 bpd to 250,000 bpd, according to a trade source.

BP is the largest shareholder in the ACG fields with a 34.1 percent stake. Its partners include Chevron Corp (CVX.N). (10.2 percent), StatoilHydro (8.6 percent) (STL.OL) and Exxon Mobil Corp. (8 percent) (XOM.N). (Reporting by Alex Lawler, editing by Barbara Lewis)