Method and apparatus for providing real-time data indicative of the isotopic
composition of formation fluids during drilling. The method includes the steps
of: (a) providing a reference fluid having a known isotopic composition in a reference
cell; (b) capturing a sample of formation; (c) providing at least one laser beam;
(e) passing a beam through the reference fluid, measuring the reference-measurement
beam before and after it passes through the reference fluid; (f) and passing a
beam through the sample, measuring the beam before and after it passes through
the sample, and calculating a first isotope concentration from those measurements.
The measurements can provide information relating to the carbon isotopic composition
of individual compounds in hydrocarbon gas mixtures, with the individual compounds
including methane, ethane, propane, iso- or normal butane, or iso- or normal pentane.
