Liquid gets destroyed

This is an example of liquid destruction. There is a puddle of polluted water, covered by water. Crude Oil is dripping into this puddle. 

Expected behavior: Crude Oil would collect at the bottom of the puddle, pressing polluted water and water upwards. No mass lost.

Observed behavior: Crude Oil does not appear at the bottom, disappears. The volume of polluted water is reduced with each drop.