Are petroleum residues in your yard harmful to plants or animals? This is a difficult question. So I will start my answer with a quote: “All things are poisons, for there is nothing without poisonous properties. It is only the dose which makes a thing a poison.” Paracelsus, 1493-1541. This quote is the answer to the question. Crude oil, as it comes out of the well, is a natural product. Research has shown that there are microbes that will live in and feed on oil. This knowledge has led to the use of microbes in helping to clean up oil spills. If you spill oil in your yard, there are microbes already present, probably at very low populations, that will digest that oil over time and make it disappear. Other studies have shown that crude oil can be used as a plant nutrient. Up to one percent crude oil in a soil mix will act as a fertilizer. Some plants can tolerate higher amounts, but usually symptoms of toxicity will begin to show when oil concentrations are above one percent. If plants in your yard are not growing properly, there can be many factors besides oil that are causing the problem. Soil compaction is a very common problem associated with old oilfield subdivision soils. Poor soil drainage is also a problem. Chronic poor drainage can result from the compact soil or from sandstone lens in the soil. Poor growth may also be due to lack of soil nutrients. Many of the upland sand dune soil sites are seriously lacking in essential plant nutrients. Adverse soil pH and/or high soil salts could also be causing a problem.
Some homeowners are concerned about the presence of methane gas in old oil field sites. Plants are a very sensitive indicator of the presence of methane. If there is an odor or the plants are showing odd symptoms or are dying in areas of the yard, you may wish to have your gas company check the air and soil for methane.