Oil forms one to three miles deep (on earth, deeper on mars due to lower gravity leading to lower pressures at a given depth). Sedimentation can reach this order of magnitude on earth[0], so it's reasonable that lipids could have gotten buried that deeply on mars. The trick would be to have a sufficient amount life early enough in Martian history that it would overlap with the planet's period of geological activity. Like xrange said below, this would be interesting to colonists as a source of hydrogen, carbon and water for industry and agriculture. (It used to be life, so we'd dig it up to make it back in to more life.)

Now, for anyone who knows more than me about this, I have a question: can methane ice comets get buried during accretion, eventually climb up the molecular mass ladder into heavy hydrocarbons?

[0]https://www.ngdc.noaa.gov/mgg/image/sedthick9.jpg