Method and apparatus for detecting formation hydrocarbons in mud returns, and the like

A method and logger apparatus for analyzing mud returns to determine the presence of formation hydrocarbons. The logger takes a gas sample from a gas trap formed in the mud return line, purges it of all non-helium gases either by condensing them or chemically reacting them out of the sample. The sample is then fed into a special helium mass spectrometer that identifies how much of each helium isotope (3He and 4He) is present. This data is then fed to a correlator/computer and an isotope ratio calculated and monitored. A significant increase in this ratio is indicative of the presence of formation hydrocarbons. The computer uses additional input to track the sample vs its original downhole location to enable proper identification. The method and apparatus are particularly useful with oil-based drilling fluids.