Exxon Mobil's (XOM) discovery of several reservoirs in Keathley Canyon containing an estimated total resource of more than 700M barrels of oil equivalent is big by US Gulf of Mexico standards. 
It's larger than Chevron's (CVX) Tahiti field, which was discovered in 2002 and contains an estimated 400-500M boe. The Tahiti platform accounts for nearly 40% of Chevron's production in the area. XOM climbs nearly $1 premarket on the news, recently sitting up 0.6% at $80.50.