Evaluation of the characteristics of biogenic gas production based on the CDOM spectral characteristics of shales

Objective The composition and degradability of dissolved organic matter (DOM) play a critical role in evaluating biogenic gas production during shale gas exploration. A simple and effective method needs to be developed to assess the composition, functional groups and microbial activity of DOM, which will provide crucial valuable information concerning the gas production mechanism in the field of biogenic shale gas. Methods Shale samples were collected from depths of 1 219 m to 1 309 m from the ST1 exploration well located at the edge of the Sebei No.1 gas field in the Qaidam Basin. According to the 3D fluorescence of the extraction and emission matrix (EEM) and the UV visible absorption spectrum characteristics of chromophoric dissolved organic matter (CDOM), the composition, functional group and degradation characteristics of CDOM were analysed to determine the distribution of methanol formation substrates. Results The DOM in the samples mainly consisted of the small molecular tryptophan-like fluorescent components C1 and C3, which are readily degradable by biotic processes and account for 70.54% of the total fluorescence intensity. The fluorescence components, fluorescence parameters (HIX, BIX), and absorbance ratios (E253/E203, SUVA254) confirmed that the core layers at depths of 1 219-1 222 m and 1 285-1 301 m were potential effective gas-generating formations. These layers contain low amounts of humified organic matter, weak aromaticity and high amounts of methanol functional groups. The active gas-generating formations are generally consistent with those in previous reports. Conclusion The results indicated that the CDOM spectrum is a valuable method for exploring the methanol formation substrate distribution and microbial activity. The CDOM-EEM method has the potential to serve as an effective indicator of the methanol formation characteristics of DOM in mud shale and provides an effective and essential way to assess geochemical-based gas production capacity in hydrocarbon source rocks.