The oil is contained in the four main horizons of Cretaceous age: Wara (sandstone), Mauddud (limestone), Burgan Third Sand (3S) and Burgan Fourth Sand (4S). Burgan Third Sand is in turn subdivided into Third Sand Upper (3SU), Third Sand Middle (3SM) and Third Sand Lower (3SL). Historically the production has come mainly from the 3SM unit. By 1992 the second and third sand units had been swept by water.
