APPLYING TOTAL SCANNING FLUORESCENCE TO ESTIMATE THERMAL MATURITY AND GEOCHEMICAL CHARACTERISTICS OF SHALES IN THE BOHAI BAY AND SICHUAN BASINS, CHINA

The Total Scanning Fluorescence (TSF) technique has been widely employed to characterize the fluorescence of conventional reservoir oil or inclusion oil, due to its high sensitivity, fast-turnaround time, cost-effectiveness and minute amount of sample requirement compared to the microscopic and GC-MS methods. Considering the extremely low permeability of shales, hydrocarbons in shales can be preserved as “inclusions”, recording pristine history of the thermally evolving shales. A preliminary experiment was conducted to evaluate the potential of using the TSF method to estimate thermal maturity of shales, predicting aromatic composition and concentration, discriminate organic matter types and determine oil properties, using samples from three oil fields in the Bohai Bay Basin and the Sichuan Basin, China using in situ micro sampling in conjunction a pre-cleaning and analytical procedure. Artificial pyrolysis experiments of shales were employed to obtain a set of known thermal maturity samples to calibrate the TSF method. It has been found that fluorescence can still be detected in over-mature shales with Ro values in excess of 2.5 and the fluorescence signature in shale extracts can be used as a thermal maturity indicator for shales: (1) TSF intensity is proportionate to the amount of aromatics, a proxy for the organic matter abundance and quality of hydrocarbons; (2) TSF R₁, defined as the radio of the three-ring aromatic over single-ring aromatic hydrocarbons, is negatively correlated with the vitrinite reflectance (Ro) values in the pyrolysis rock samples. The TSF R₁ values also increase with decreasing Ts/(Ts+Tm), a well-established maturity biomarker ratio and burial depth, thus can be used to effectively reflect the thermal maturity; (3) With increasing thermal maturity the R₁ parameter and TSF intensity decrease, while the fluorescence excitation-emission spectral peak becomes narrow and moves towards the short wavelength region on the TSF plot; (4) Shales of different thermal maturities and the organic matter types can be readily distinguished by using the R₁ parameter and TSF intensity. It is concluded that the TSF technique can be used to approximate the thermal maturity of shales, providing an alternative to the conventional vitrinite reflection method. This will have a particular application for evaluating the early Paleozoic shale formations, in which vitrinite is absent.