Adds comment from BP's Gordon Birrell

June 1 (Reuters) - British energy major BP BP.L said on Monday commercial production of non-associated gas has begun at the Azeri-Chirag-Gunashli (ACG) field off the coast of Azerbaijan.

ACG, which has been producing oil for nearly three decades, holds estimated non-associated gas resources of 4 trillion cubic feet of recoverable reserves, with potential upside to 6 trillion cubic feet, BP said.

Non-associated gas refers to natural gas extracted from wells without significant crude oil, and is produced on its own rather than as a byproduct of oil extraction.

The development comes as BP has been pivoting back to fossil fuels from a renewables push. Last week, Reuters reported that BP will become the operator of the Babek gas field in Azerbaijan, citing industry sources.

BP, which operates ACG with a 30.37% stake, said the first well was drilled from the existing West Chirag platform in the Azerbaijani sector of the Caspian Sea, establishing it as a critical integrated oil and gas asset.

BP's operations chief Gordon Birrell said it had also discovered a thin oil rim between the gas and water zones in the deeper reservoir, adding further potential to the project.

ACG co-venturers include Azerbaijan's state energy company SOCAR, MOL, INPEX, ExxonMobil XOM.N, TPAO, and ONGC Videsh.