High concentration of secondary hydrogen sulfide (sH₂S) in thermal recovery reservoirs of Liaohe Oilfield, NE China was concluded to originate from thermochemical sulfate reduction (TSR), and no biotic source of H₂S under abundant biomass has been reported in these presumed steam sterilized reservoirs ever before. In this study, we propose a new mechanism, biomass thermal decomposition for sulfur compounds (BTDS), to interpret the increasing of sH₂S. Sulfur of cells’ dry weight took 0.20%–1.92% of the active strains isolated from the in-situ thermal recovery reservoirs of Liaohe Oilfield. When microbial organic sulfur compounds (MOSC) in biomass were exposed to injected steam, it resulted in the BTDS process. The isolated Bacillus subtilis D3 (G+) and Pseudomonas aeruginosa XJ14 (G−) were chosen to simulate this process. About 36% of sulfur in MOSC emitted as H₂S in steam chamber by BTDS. The δ³⁴S of H₂S from produced gas ranged from 8.7‰ to 17.0‰, close to the δ³⁴S of H₂S 11.2‰ from BTDS simulation experiment. It provides new insight into the contribution and sulfur cycle made by subterranean microorganisms on H₂S formation.

DNA analysis is the core of biotechnology applied in petroleum resources and engineering. Traditionally accurate determination of DNA purity and concentration by spectrometer is the first and critical step for downstream molecular biology research. In this study, three different spectrophotometry methods, BPM, NDTT and NPMTTZ were compared for their performance in determining DNA concentration and purity in 32 oil samples, and molecule methods like quantitative real-time PCR (qPCR) and high-throughput sequence were also performed to help assess the accuracy of the three methods in determining DNA concentration and purity. For ordinary heavy oil (OHO), extra heavy oil (EHO) and super heavy oil (SHO), the characteristics of high viscosity (η), density (ρ) and resin plus asphaltene content will affect the DNA extraction and UV determination. The DNA concentration was decreased as density increased: OHO (11.46 ± 18.34 ng/μL), EHO (6.68 ± 9.67 ng/μL) and SHO (6.20 ± 7.83 ng/μL), and the DNA purity was on the reverse: OHO (1.31 ± 0.27), EHO (1.54 ± 0.20), and SHO (1.83 ± 0.32). The results suggest that spectrophotometry such as BPM and NPMTTZ are qualitatively favorite methods as the quick non-consumable methods in determining DNA concentration and purity of medium oil and heavy oil.