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 (τoil) and fluorescence spectral parameters
(λmax, Q510/430, and Q650/500) 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, τoil progressively
increases, and the fluorescence spectra gradually shift blue with
decreases in Q510/430, Q650/500, and λmax, 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.