Quantitative fluorescence techniques and their applications in shale oil reservoir research: a case study of Permian Fengcheng Formation in Mahu Sag, Junggar Basin

The shale oil reservoirs of the Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin are characterized by a source and reservoir integration. Their formation process is affected by various factors, including sedimentation, diagenesis, and organic matter evolution, resulting in highly complex source rock properties and reservoir characteristics. Currently, the effects of inorganic mineral development and organic matter evolution on reservoir and shale oil properties remain unclear. To address these issues, this study extends the application of quantitative fluorescence (QF) techniques, which are widely used in conventional reservoir research, to continental shale oil reservoirs. Techniques such as quantitative grain fluorescence on extract (QGF-E) and total scanning fluorescence (TSF) were utilized. By combining QGF-E analysis and rock pyrolysis, it was found that free hydrocarbon (S1) was positively correlated with QGF-E intensity, and their variation range was significant. This indicated that oil saturation was mainly controlled by S1. Under the same testing conditions, the normalized TSF spectra's maximum intensity and two-dimensional nuclear magnetic resonance (2D NMR) experimental results showed that the differences in shale oil density (API gravity) were related to the adsorption of hydroxyl-rich heavy organic matter by clay minerals. Higher clay mineral content was found to adsorb more organic matter. The crude oil maturity index (R1) further indicated that the differences in bio-precursors under the original depositional environment and the variations in pore types and structures after diagenetic alteration were key factors affecting the properties of shale oil in the reservoir. These analytical techniques and methods serve as a bridge connecting different parameters, facilitating a deeper understanding of the characteristics of the shale oil reservoir and providing valuable references for the exploration and development of unconventional oil and gas resources.