discount to brent was narrower in 2013 than in 2012 and 2011 .as a result of the significant increase in u.s .production of light sweet crude oil , the historical relationship between wti , brent and lls pricing may not be indicative of future periods .composition 2013 the proportion of our liquid hydrocarbon sales volumes that are ngls continues to increase due to our development of united states unconventional liquids-rich plays .ngls were 15 percent of our north america e&p liquid hydrocarbon sales volumes in 2013 compared to 10 percent in 2012 and 7 percent in 2011 .natural gas 2013 a significant portion of our natural gas production in the u.s .is sold at bid-week prices , or first-of-month indices relative to our specific producing areas .average henry hub settlement prices for natural gas were 31 percent higher for 2013 than for 2012 .international e&p liquid hydrocarbons 2013 our international e&p crude oil production is relatively sweet and has historically sold in relation to the brent crude benchmark , which on average was 3 percent lower for 2013 than 2012 .natural gas 2013 our major international e&p natural gas-producing regions are europe and e.g .natural gas prices in europe have been considerably higher than the u.s .in recent years .in the case of e.g. , our natural gas sales are subject to term contracts , making realized prices in these areas less volatile .the natural gas sales from e.g .are at fixed prices ; therefore , our reported average international e&p natural gas realized prices may not fully track market price movements .oil sands mining the oil sands mining segment produces and sells various qualities of synthetic crude oil .output mix can be impacted by operational problems or planned unit outages at the mines or upgrader .sales prices for roughly two-thirds of the normal output mix has historically tracked movements in wti and one-third has historically tracked movements in the canadian heavy crude oil marker , primarily wcs .the wcs discount to wti has been increasing on average in each year presented below .despite a wider wcs discount in 2013 , our average oil sands mining price realizations increased due to a greater proportion of higher value synthetic crude oil sales volumes compared to 2012 .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 aeco natural gas sales index and crude oil prices , respectively .the table below shows average benchmark prices that impact both our revenues and variable costs: .
[['benchmark', '2013', '2012', '2011'], ['wti crude oil ( dollars per bbl )', '$ 98.05', '$ 94.15', '$ 95.11'], ['wcs ( dollars per bbl ) ( a )', '$ 72.77', '$ 73.18', '$ 77.97'], ['aeco natural gas sales index ( dollars per mmbtu ) ( b )', '$ 3.08', '$ 2.39', '$ 3.68']]
wcs ( dollars per bbl ) ( a ) $ 72.77 $ 73.18 $ 77.97 aeco natural gas sales index ( dollars per mmbtu ) ( b ) $ 3.08 $ 2.39 $ 3.68 ( a ) monthly pricing based upon average wti adjusted for differentials unique to western canada .( b ) monthly average day ahead index. .

Question: by what percentage did the aeco natural gas sales index decline from 2011 to 2013?
Answer: -16.3%