You should add "Soot Ground enters the battlefield tapped" before the offering-text instead.
Also I would recommend writing "[...] if you do, untap Soot Ground. Until end of turn, whenever you use an activated- or mana-ability of this card, copy it." instead of explicitly referring to mana;
If you keyword an ability, it is to give the player more of a connection to reoccurring abilities, to establish consistently present themes of a set (aka: FLAVOR).
To put it simply: If "Offering X" allows you to copy abilities used by a permanent, ANY kind of permanent, it can also be used to double the mana produced by "Soot Ground",
while the version you use at the moment is ONLY written for "Soot Ground", thus seeming less viable as a whole.
It is a great asset if you succeed in making easy to comprehend, elegant mechanics; It's like programming computer code, where you can use a simple function of three lines for nearly ANYTHING,
just by leaving some variables open to change.
Here are some things you can do, if you generalize your formula:
- A hellhound creature, that deals 2 Damage to target creature/player; Using Offering, it is able to deal double the amount in its first turn.
- A vampire with haste and "{r}{1}: +1/+1 counter on [name]. Use this ability only, if [name] dealt damage to a creature or player this turn"; Adding Offering to a creature like this would lead to some nice comboplay, by basically doubling the +1/+1-counter boost.
I could give more examples, but I think you alraedy know, what I mean.