BUZZ-Amerisur Resources: Fall in reserves hurt
** Oil and gas producer down as much as 26.6 pct to 22 pence, lowest in 2-1/2 years, after announcing a fall in proven and probable reserves
** Stock is biggest drag on FTSE AIM Oil and Gas Index
** Company says after accounting for production, proven gross field reserves at end 2014 fell 6 pct to 16.2 million barrels of oil; proven and probable gross field reserves fell 19.5 pct to 24.55 million barrels of oil. (venkateshasoumithri.mamidipudi.thomsonreuters.com@reuters.net)