I have two grand soul gems at a petty level. When I try to trap the soul of a summoned skeleton I get the message that I don't have a stone large enough to capture the soul.

A grand soul gem has a capacity of 1600. If it is at the petty level does that mean it needs another 1450 (1600 - 150) to fill it? A skeleton has a petty soul which is 150.

You can′t do that. You need a completely empty gem in order to capture a soul. Filling up a Grand Gem which only has a petty charge to a full 1600 charge is not possible I′m afraid.

So to fill a grand soul gem I need to fill an empty one with a grand soul? Somehow I got two filled grand soul gems but I don't know how I did that.

Any soul will go into a Grand one (except a human soul of course). Maybe you used the trap spell or enchantment on a small creature

Edit: Did you mean they′re filled to 1600?

Yes, I should have said "Somehow I got two fully filled grand soul gems".

Ok, and they were empty at first? Do you use a weapon that′s enchanted with Soul Trap perhaps?

Perhaps you found or bought them? Particularly likely if your character is below level 20.

They probably were not empty. I have a custom spell with soul trap but I'm pretty sure I haven't killed any grand souls.

My character is well below level 20. I may have found them. I thought you could only buy empty soul gems.

You can buy empty or filled soul gems. I doubt you can buy a grand soul gem filled with a grand soul much below level 20 though.

Heh. Since you are well below level 20, if you filled a grand soul gem with a grand soul yourself, you would remember it. The Uderfryke Matron or a master level summon is about the only source to fill one yourself at that level.

I agree with you that it sounds most likely that you found it.

They'll be available alot sooner than that, levels 10 - 15 IIRC. Somewhere in that range.

@ 0sync0

If you're playing on a PC, there's a mod made by The Nice One that allows you to fill up under-utilized soul gems. It's called Soul Gem Magic.

Maybe you got the filled grand soul gems from the locked display case in the Archmage's lobby?

My alteration skill isn't high enough yet to open that case.

After doing some research, I think I got the gems from the Chorrol Mages Guild (filled to greater level) and I was mistaken about them being filled to the grand level. I already used them for enchanting.