Elements having the same number of protons in their nuclei but differ in the number of neutrons. Isotopes have the same atomic number, different atomic weight, and almost but not quite the same chemical properties. Important isotopes in petroleum geochemistry include carbon (13C), hydrogen deuterium (2H), nitrogen (15N), oxygen (18O), and sulfur (34S). Isotopes are used for oil-oil correlations, oil-source rock correlations, and identification of the genetic origins of gas and their thermal maturity during generation.