Aging Reposado tequila is normally used oak barrel. Aging tequila will allow tequila to soften taste such as tannin. But tequila will develop more richer and complex flavor. White oak is usually used for aging tequila. Popular oak barrel is from US, France, and Canada. In some companies, they might burn the wood barrel before aging to give some smoky flavor to tequila. Moreover, aging tequila sometimes use used barrel that was used from aging wine or whisky as well.
Aging Añejo tequila uses barrel that used for Reposado tequila. In some companies, they will normally use barrels that were already used for aging whisky. Used barrels from Jack Daniels, US, and Canada are very popular to be used for aging Añejo tequila. This is because these used barrels will add some darker color and complex flavor to Añejo tequila.