** European oil and gas stocks hit 2015 high as thesector is biggest outperformer following Shell's $70bln deal to buy BG Group
** Stoxx 600 Oil & Gas up 6 pct and poised for bestday in more than 6 years
** Sector dominates proceedings across region as oil-relatedstocks top the Stoxx 600 and regional mid and small-capindices on price and volume
** BG Group up 38 pct, well below the 50 pct premium toTuesday's close offered by Shell, though volumes in the stockhit $200 mln within first 15 minutes of trading
** BG was top FTSE performer on Tuesday
** Shell A shares down 1.3 pct (RM: vikram.subhedar.thomsonreuters.com@reuters.net; RM:tricia.wright1.thomsonreuters.com@reuters.net)