Well first define why you're doing the blanketing.
Fuel quality or explosion risk?
For me there isn't that much in it, but your fixed roof won't like very much in the way of extra pressure ( inches water column). Your supply needs to be pretty good otherwise you can collapse the tank when you draw the Jet A1 out. what is more available?
0707 - that link doesn't work for me. why not post the image into the post direct using the Image button above.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Typical refinery fuel gas is high dewpoint fuel gas, which will contain some high mol wt components that will gradually dissolve in the jet fuel and mess up your jet fuel IBP. So you would be better off with N2. And you have the added benefit of a smaller Zone 2 area around the vents at tank top with N2 blanketing (I would guess, check with hazardous area classification engineer)
I would echo the concern with fuel gas negatively impacting product quality. Fuel gas is generally high in H2S and other less than desirable components.
While it's great because it's free, it can be inconsistent in availability and pressure.