A leak alarm system for a petroleum pipeline includes one or more detectors installed in the ground adjacent to a pipeline, each detector having a tubular enclosure with a diode light source and photosensor located at the opposed end of the tube. The tubular enclosure is fabricated of a microporous material that has microfiltration properties: it is hydrophobic, so that soil water will not enter the enclosure, but also oleophilic to enable petroleum and related substances migrate through the material of the tubular enclosure and accumulate therein. The petroleum-like materials scatter the beam and diminish the sensor signal, causing a leak alarm signal to be generated.