A barrel is a volumetric measure so you can't compare it with area. This said according to the link at the bottom, there are 6.29 barrels per cubic metre. this equates to 9.1mill/6.29 cubic metres per day = 1.447 million cubic metres, or nearly 1 and a half mill cubic metres. can't think of anything that is 1.5mill cubic metres big to compare it with. sorree! (but it ain't as big as London for sure, there are 1 billion cubic metres to a cubic kilometre, so our barrels equates to roughly 3/2000ths of a cubic kilometre)