2022 lower 2008 storage margins related to storage risk management positions and the impact of changes in natural gas prices on these positions ; and 2022 fewer opportunities to optimize storage capacity due to the significant decline in natural gas prices in the second half of 2008 ; o a decrease of $ 9.7 million in physical storage margins due to a lower of cost or market write-down on natural gas inventory ; and o a decrease of $ 2.1 million due to colder than anticipated weather and market conditions that increased the supply cost of managing our peaking and load-following services and provided fewer opportunities to increase margins through optimization activities , primarily in the first quarter of 2008 ; partially offset by o an increase of $ 15.8 million from changes in the unrealized fair value of derivative instruments associated with storage and marketing activities and improved marketing margins , which benefited from price movements and optimization activities .
operating costs decreased primarily due to lower employee-related costs and depreciation expense .
2007 vs .
2006 - net margin decreased primarily due to : 2022 a decrease of $ 22.0 million in transportation margins , net of hedging activities , associated with changes in the unrealized fair value of derivative instruments and the impact of a force majeure event on the cheyenne plains gas pipeline , as more fully described below ; 2022 a decrease of $ 5.0 million in retail activities from lower physical margins due to market conditions and increased competition ; 2022 a decrease of $ 4.3 million in financial trading margins that was partially offset by 2022 an increase of $ 4.9 million in storage and marketing margins , net of hedging activities , related to : o an increase in physical storage margins , net of hedging activity , due to higher realized seasonal storage spreads and optimization activities ; partially offset by o a decrease in marketing margins ; and o a net increase in the cost associated with managing our peaking and load following services , slightly offset by higher demand fees collected for these services .
in september 2007 , a portion of the volume contracted under our firm transportation agreement with cheyenne plains gas pipeline company was curtailed due to a fire at a cheyenne plains pipeline compressor station .
the fire damaged a significant amount of instrumentation and electrical wiring , causing cheyenne plains gas pipeline company to declare a force majeure event on the pipeline .
this firm commitment was hedged in accordance with statement 133 .
the discontinuance of fair value hedge accounting on the portion of the firm commitment that was impacted by the force majeure event resulted in a loss of approximately $ 5.5 million that was recognized in the third quarter of 2007 , of which $ 2.4 million of insurance proceeds were recovered and recognized in the first quarter of 2008 .
cheyenne plains gas pipeline company resumed full operations in november 2007 .
operating costs decreased primarily due to decreased legal and employee-related costs , and reduced ad-valorem tax expense .
selected operating information - the following table sets forth certain selected operating information for our energy services segment for the periods indicated. .

Years Ended December 31,
Operating Information
2008
2007
2006
Natural gas marketed(Bcf)
1,160
1,191
1,132
Natural gas gross margin($/Mcf)
$0.07
$0.19
$0.22
Physically settled volumes(Bcf)
2,359
2,370
2,288

our natural gas in storage at december 31 , 2008 , was 81.9 bcf , compared with 66.7 bcf at december 31 , 2007 .
at december 31 , 2008 , our total natural gas storage capacity under lease was 91 bcf , compared with 96 bcf at december 31 , natural gas volumes marketed decreased slightly during 2008 , compared with 2007 , due to increased injections in the third quarter of 2008 .
in addition , demand for natural gas was impacted by weather-related events in the third quarter of 2008 , including a 15 percent decrease in cooling degree-days and demand disruption caused by hurricane ike. .

Question: Analyse this data from a financial earnings document. what was the percentage change in natural gas gross margin ( $ /mcf ) between 2007 and 2008?
Choices:
-0.78947
-0.00545
-0.12
-0.0001
-0.63158
Correct choice index: 4