Fluorescence spectroscopy and micro-thermometry of hydrocarbon inclusions from wells in Joetsu and Chuetsu areas, Niigata Prefecture, Japan

The characteristics of hydrocarbon inclusions in quartz from wells in the Joetsu and Chuetsu areas, Niigata Prefecture, Japan were examined by means of fluorescence spectroscopy and micro-thermometry. The fluorescence is emitted mainly from the liquid phase of the inclusions. The fluorescence spectra are similar to those of crude oils, having a narrow single peak around 440nm. In the fluorescence spectra of crude oils, both the wavelength of the peak (λ) and the relative intensity at 500nm (I500) increase with increasing specific gravity, indicating that heavier oils contain larger molecular-weight aromatic hydrocarbons and/or lower amount of saturated hydrocarbons. Crude oils and hydrocarbon inclusions are similar to each other in the λ-I500 relationship, suggesting that the compositional variation of hydrocarbon inclusions can be estimated by means of fluorescence spectroscopy. Liquid homogenizing temperature (ThL) and I500 of hydrocarbon inclusions in quartz from the Shinkumoide SK-1D well decrease with increasing depth, suggesting that, overall, molecules in the inclusions become lighter with depth. As ThL also decreases after pyrolysis experiment, the depth profiles of the two parameters in the Shinkumoide SK-1D well can be interpreted as the result of thermal cracking of the hydrocarbons. This means that the correlation between ThL and I500 for the well samples indicates a maturity trend of hydrocarbon inclusions. The hydrocarbon inclusions from various wells from Joetsu and Chuetsu areas would be divided, at least, into two groups with respect to the ThL-I500 relationship. One group is represented by the relationship observed in the Shinkumoide SK-1D well, while the other group is distinguished from that by the higher ThL.