At the foot of the giant Hebes Mensa salt dome on Mars, there is a feature called the "Oil Spill" (Adams et al., 2009). But, ccording to Adams et al., it consists of liquid brines that have been coloured black by dark dust particles. However, - this is not be true.
Figure 1. This is what the "Oil spill" feature looks like on Google Mars.
To me, this explanation is not probable. According to own earth-bound research, over the past 5 years, the most plausible and probable explanation is thatthe liquid substance consists of crude oil.