Could Mars have oil? What events could have to happen to Mars (which is thought to once look like Earth) that may have depleted the atmosphere and erase all proof of life ever existing at least on the surface? 

I know an asteroid could melt the planet but could there have been an event that could wipe the surface while leaving deposits under the ground intact?

There is no proof that Mars melted but there is proof that there at least was oceans.

http://en.yibada.com/articles/18182/20150309/red-planet-once-held-large-shallow-ocean-mean-positive-signs.htm

Where did Mars water go?
Planet solidifying.

No molten core iron -> no magnetosphere -> solar wind strips atmosphere.
Without atmosphere to cycle in exposed water also leaves. 

Why did Mars freeze solid &amp; Earth has not (yet).


is further from sun
smaller means less volume to hold heat
Earth may have the iron cores from two planets (moon forming impact)
Radioactive decay?


On oil, possibly.
If there was life there and it colonized the land and it found it needed to create a scaffolding structure to compete for photons such as ligand as happened here, and there were several hundred millions years until other life there figured out how to digest those ligands then there would be a layer there like our Carboniferous era in which a large amount of carbon is sequestered underground. 

But chances are vanishingly slim.
If there was life there it most likely   


recycled carbon just as we have here for all time except that little blip* after trees showed up and before bacteria figured out how to digest them.  
did not survive long enough to need to compete for photons by growing very tall


(*)  300,000,000 / 4,300,000,000  is about 1/14th of life on earth's history.