pre_text: crude oil , and political unrest in the middle east and elsewhere . later in 2008 , crude oil prices dropped more rapidly than they had climbed as the u.s . dollar rebounded and other countries entered recessions which decreased demand . during 2008 , the average spot price per barrel for wti was $ 99.75 , up from an average of $ 72.41 in 2007 , but ended the year at $ 44.60 . the average spot price per barrel for brent was $ 97.26 in 2008 , up from an average of $ 72.39 in 2007 , but ended the year at $ 36.55 . the differential between wti and brent average prices widened to $ 2.49 in 2008 from $ 0.02 in 2007 . our domestic crude oil production is on average heavier and higher in sulfur content than light sweet wti . heavier and higher sulfur crude oil ( commonly referred to as heavy sour crude oil ) sells at a discount to light sweet crude oil . our international crude oil production is relatively sweet and is generally sold in relation to the brent crude oil benchmark . natural gas prices on average were higher in 2008 than in 2007 . a significant portion of our u.s . lower 48 states natural gas production is sold at bid-week prices or first-of-month indices relative to our specific producing areas . the average henry hub first-of-month price index was $ 2.18 per thousand cubic feet ( 201cmcf 201d ) higher in 2008 than the 2007 average . natural gas sales in alaska are subject to term contracts . our other major natural gas-producing regions are europe and equatorial guinea , where large portions of our natural gas sales are subject to term contracts , making realized prices in these areas less volatile . as we sell larger quantities of natural gas from these regions , to the extent that these fixed prices are lower than prevailing prices , our reported average natural gas prices realizations may decrease . e&p segment income during 2008 was up 57 percent from 2007 , with revenue increases tied to these increases in average commodity prices accounting for almost half of the income improvement . liquid hydrocarbon and natural gas sales volumes were also higher in 2008 than 2007 . oil sands mining oil sands mining segment revenues correlate with prevailing market prices for the various qualities of synthetic crude oil and vacuum gas oil we produce . roughly two-thirds of the normal output mix will track movements in wti and one-third will track movements in the canadian heavy sour crude oil marker , primarily western canadian select . output mix can be impacted by operational problems or planned unit outages at the mine or upgrader . during 2008 , our average realized price for synthetic crude oil and vacuum gas oil was $ 91.90 per barrel , up from 2007 , but ended the year at $ 24.97 per barrel impacted by a heavier yield in december and a seasonal decrease in the value of our heavy output . the operating cost structure of the oil sands mining operations is predominantly fixed , and therefore many of the costs incurred in times of full operation continue during production downtime . per unit costs are sensitive to production rates . key variable costs are natural gas and diesel fuel , which track commodity markets such as the canadian aeco natural gas sales index and crude prices respectively . the table below shows benchmark prices that impact both our revenues and variable costs , listing high and low spot prices during the year. .
post_text: wti crude oil ( dollars per barrel ) $ 145.29 july 3 $ 33.87 december 19 western canadian select ( dollars per barrel ) ( a ) $ 114.95 july $ 23.18 december aeco natural gas sales index ( canadian dollars per gigajoule ) ( b ) $ 11.34 july 1 $ 5.42 september 19 ( a ) monthly pricing based upon average wti adjusted for differentials unique to western canada . ( b ) alberta energy company day ahead index . our osm segment reported income of $ 258 million for 2008 , reflecting synthetic crude oil and vacuum gas oil sales averaging 32 mboepd . derivative instruments intended to hedge price risk on future sales have impacted revenues in the periods presented , with net gains of $ 48 million in 2008 and net losses of $ 53 million in 2007 . in the first quarter of 2009 , we entered into derivative instruments which effectively offset certain of our open derivative positions . refining , marketing and transportation rm&t segment income depends largely on our refining and wholesale marketing gross margin , refinery throughputs , retail marketing gross margins for gasoline , distillates and merchandise , and the profitability of our pipeline transportation operations. .
table: Benchmark High Date Low Date
table: WTI crude oil<i>(Dollars per barrel)</i> $145.29 July 3 $33.87 December 19
table: Western Canadian Select<i>(Dollars per barrel)</i><sup>(a)</sup> $114.95 July $23.18 December
table: AECO natural gas sales index<i>(Canadian dollars per gigajoule)</i><sup>(b)</sup> $11.34 July 1 $5.42 September 19
question: Analyse this data from a financial earnings document. what is the difference between the high wti crude oil and western canadian select prices?