Basement – Indoor fuel tank removed – How to eliminate lingering smell

Background: I recently replaced my home heating system (an oil burning furnace) for a modern heat pump (air-water). There is a room in our basement (2.9m x 3.5m, or roughly 10ft x 13ft) where the 10'000 liter (~2500 gallons) tank used to be. The room was completely sealed off from the basement, save for a small window-sized opening through which you could enter to service the tank, should that be necessary.

I had a contractor cut out a door opening, and then a specialized firm came to clean the tank, cut it into smaller pieces, and recycle the steel chunks. I have now and empty room that reeks of oil, which I want to turn into a home gym. It's been some 4 weeks since we cut a door opening, and opened up the grilled vent to the outside, and although I've let enough air circulate, it still reeks as much as it did on the first day.

I assumed some oil may have leaked through the years onto the floor, which had a sort of epoxy coating, stained brown in some places. So I carefully removed it all. It took a lot of effort with a putty knife and paint scrapers. But even with a completely clean floor, it still smells. The lower half of the walls, which have the same paint/epoxy coating, have brown stains all over, which I imagine may have significant deposits of oil, perpetuating the lingering smell in the room. I read that spraying concentrated hydrogen peroxide helps eliminate all smells, so I carefully sprayed 12% hp on the walls (wearing a full body suit, goggles and a breating mask), which didn't have any significant effect overnight.

The question: is there any reasonable way (that doesn't involve overly expensive or extremely time-consuming efforts) to get rid of the smell, before I apply the decorative textured plaster on the walls? Would a controlled burn (e.g. with a small gas torch, with proper ventilation) help at all? Do you think applying plaster over the existing walls wouldn't do enough to cover the oil smell?

These pictures show the level of stains on the walls. Here you can see what I believe was pretty much the original color of the coating (first pic) vs the back wall.

Edit / Update: on Friday I primed and then used self-levelling compound (about 750lb / 350kg) to raise and level the floor, which had over 1 ¼ / 30mm height difference. The smell is completely gone. So as several of you pointed out, the oil had somehow leaked and penetrated the floor, which is now completely covered by a non porous concrete layer. Thank you all for your kind participation !!!