Determination of Light and Condensate Oil Categories in a Complex Petroleum System by Fluorescence Parameters: A Case Study on the Northern Tazhong Uplift, Tarim Basin, China

Light and condensate oils are high-quality fossil energy sources. Because light and condensate oils have complex origins and are generally dominated by light hydrocarbons with few diagnostic biomarkers, conventional geochemical methods have difficulty identifying their categories, especially in complex petroleum systems. In this study, the fluorescence lifetime (τ_oil) and fluorescence spectral parameters (λ_max, Q_510/430, and Q_650/500) of light and condensate oils in the northern Tazhong Uplift of the Tarim Basin were systematically analyzed. The results indicate that the light and condensate oils in this area can be divided into three categories according to their fluorescence characteristics. For the TZ-I, TZ-II, and TZ-III oils, τ_oil progressively increases, and the fluorescence spectra gradually shift blue with decreases in Q_510/430, Q_650/500, and λ_max, which results from the successive decreases in gas invasion extent for the three types of Tazhong oils. Light hydrocarbons mainly consisting of saturated hydrocarbon fractions were carried by highly mature gaseous hydrocarbons from deep sources to relatively shallow reservoirs and mixed with early accumulated crude oils. The charged saturates reduced the fluorophore (polycyclic aromatic hydrocarbons, PHA) concentration in crude oil, weakened fluorescence quenching, and promoted fluorescence emission, which changed the fluorescence characteristics of crude oils. Correlation diagrams based on different fluorescence parameters as well as other parameters, including physical, geochemical, and associated gas parameters, provide a favorable method for determining light and condensate categories. Moreover, the fluorescence method exhibits great application potential for direct correlations between reservoir oils and inclusion oils in complex petroleum systems.