Heavy oil compositional changes through bioupgrading process in the presence of Geobacillus stearothermophilus

The exploitation and processing of unconventional heavy oil resources are constrained by high viscosity, abundant heavy fractions, and associated environmental concerns. Biological upgrading has therefore attracted interest as a potentially lower-impact approach for modifying heavy-oil composition. This study investigated compositional changes in heavy crude oil following incubation with Geobacillus stearothermophilus at 60 °C for different treatment periods. Biotic experiments and abiotic controls were conducted in duplicate. Among the tested conditions, the most pronounced changes occurred after 14 days. Quantitative FTIR analysis showed that the aromaticity index decreased from 0.21305 to 0.08230, whereas the aliphatic index increased from 0.71945 to 0.85934. GC–MS analysis further showed that cyclohexane, 1,1,3-trimethyl-, and 1R,2c,3t,4t-tetramethylcyclohexane were no longer detected under the applied analytical conditions, while the estimated contents of octacosane and dodecane, 2,6,10-trimethyl-, increased by approximately 21% and 86.9%, respectively. Abiotic controls showed only minor changes in the corresponding FTIR indices and viscosity. Overall, the findings provide preliminary evidence that G. stearothermophilus can modify heavy crude oil composition and viscosity under the tested laboratory conditions. Further validation under pressurized, saline, anaerobic, and reservoir-relevant conditions is required before practical application can be assessed.