S.K. Hamilton

Stratigraphic and depositional framework of the Walloon Subgroup, eastern Surat Basin, Queensland

The Middle Jurassic Walloon Subgroup is a prolific coal seam gas (CSG) resource in the Surat Basin, Queensland. Sedimentary framework models constrain stochastic reservoir models of the geological heterogeneity, but there is limited basin analysis information in the public domain. Here we present a regionally consistent stratigraphic framework model for the Walloon CSG play in the eastern Surat Basin. Lithostratigraphic correlation of open-file industry and government wireline logs supports the interpretation of six subunits in the eastern Surat Basin (oldest–youngest: Durabilla Formation; Taroom Coal Measures; Tangalooma Sandstone; and Juandah Coal Measures, informally divided into three members named the lower Juandah Coal Measures, Juandah sandstone and upper Juandah Coal Measures). Important findings are that subunits within the Walloon Subgroup do not correlate along the entire CSG play area; in many places, the overlying Springbok Sandstone (Upper Jurassic) has incised to the lower Juandah Coal Measures level, removing the upper coal seam groups. The Walloon Subgroup thins to the south through a combination of depositional thinning and truncation. Lithofacies analysis and isopach maps support deposition in a southerly prograding fluvial system or clastic wedge. This stratigraphic and depositional interpretation informs models for hydrogeological studies of the Walloon Subgroup and underpins a regional assessment of controls on microbial methane distribution.

Characterisation of readily bioavailable compounds in Surat basin walloon coals for biomethane production using exogenous culture

This experimental study aims to characterize the bioavailability of six Surat Basin Walloon coals to exogenous methanogenic consortia, and the possible compositional and environmental factors that control bioavailability. Finely crushed coal cores samples were inoculated with digested sludge culture sourced from domestic wastewater treatment plants in biomethane potential bottles (BMP bottles) maintained at mesophilic temperature. Degradation of coal compounds was demonstrated via GC-MS characterization of methanol and dichloromethane (DCM) extracts of coals, as well as analysis of volatile fatty acids and alcohols and total dissolved organic carbon (TOC) in water eluents of coals conducted before and after biodegradation. The resulting methane yields ranged from 14 to 33 μmol/g, with an average of 21 μmol/g (0.515 m/t) achieved within 30 days. Organic solvent-extractable materials accounted for 3.8 to 12% of coal weight. Aliphatic compounds, primarily medium-long-chain n-alkanes, n-alcohols and esters dominated the solvent extracts. Aromatics were detected up to three fused rings, and are rich in dibenzofuran, alkyl benzene, alkyl polyaromatic hydrocarbons, and acetyl diphenyl. The abundance of solvent-extractable matter was found to rely on liptinite content, particularly suberinite. Preservation of these compounds was thought to be facilitated by vitrinite, such as telinite and collotelinite that are rich in micropores, serving as storage for the hydrocarbons. On the other hand, environmental factors, such as microbes-carrying groundwater might compromise coal extractability by converting coal hydrocarbons to biogas. The study has revealed three levels of dependence regarding coal bioavailability: 1) Water solubility - An average 98% of aqueous compounds (based on TOC) was eliminated via biodegradation. These were mainly volatile fatty acids and alcohols, and to a lesser degree, medium-chain n-alcohols, esters and aliphatic amine; 2) Solvent extractability – approximately 35% of solvent-extractable compounds were biodegraded on average, with aliphatics being more bioavailable than aromatics; 3) Heterogeneous moieties, particularly aliphatic hydroxyl, ester bond, ether bond and C-N bond in aliphatic amine - These functional groups are characteristics of compounds that were heavily degraded. The study is to our knowledge, the first coal bioavailability research that demonstrated a detailed linkage between biomethane generation and bio-elimination of coal extractable compounds with connections to petrographic composition and possible environmental factors.