Apparatus and method for Raman-spectrography based measurement of the composition of gas mixture in a high-temperature borehole. The method includes any of determining molar densities of individual alkanes of the mixture, introducing refractive index corrections, utilization of reference species internally to the measurement apparatus, correction for the effect of self-absorption and cross-absorption, as well as minimizing fluorescence when a liquid fraction is present in the borehole. The apparatus is configured to detect vibrational bands of CH, OH, CC, HS, NN and CO functional groups as well as collective modes in the fingerprint spectral region.