What do I fill this hole with after removing old gas fireplace? Fillers, sealants and adhesives for all types of DIY work. Help, advice and information on all aspects of this subject

[caned_monkey]

Hi,

I have removed the old gas fireplace that was in my living room and I'm left with a rectangular recess in the floor that is a uniform 30mm deep.

My question is, what to fill it with to bring it up to level?

The floor is made of concrete and I'm going to be laying oak parquet floor over it so the end results needs to be flat.

Is floor leveling compund enough or do i need to fill it with concrete to match the rest of the floor?

Thanks