** Oil and gas producer down as much as 26.6 pct to22 pence, lowest in 2-1/2 years, after announcing a fall inproven and probable reserves
** Stock is biggest drag on FTSE AIM Oil and Gas Index
** Company says after accounting for production, provengross field reserves at end 2014 fell 6 pct to 16.2 millionbarrels of oil; proven and probable gross field reserves fell19.5 pct to 24.55 million barrels of oil. (venkateshasoumithri.mamidipudi.thomsonreuters.com@reuters.net)