Hydrogen signatures in the Bourakebougou field, Mali: A petrophysical log analysis

The Bourakebougou hydrogen field in Mali, located in the southern part of the Taoudeni Mega-Basin, represents a unique natural hydrogen reservoir, offering valuable insights into hydrogen generation, migration, and accumulation processes. This study focuses on a detailed analysis of wireline log data from the Bougou-6 well, aiming to elucidate the petrophysical and fluid-rock interaction characteristics of hydrogen-bearing formations. Using gamma-ray (GR), bulk density (RHOB), neutron porosity (NPHI), and gas mixture (H₂ and CH₄) composition logs, the research investigates the relationship among lithology, porosity, density, and gas composition across different lithologies—Carbonate, Sandstone, Siltite, and Dolerite. The results reveal distinct patterns in NPHI-RHOB separation within zones shallower than 597m True Vertical Depth (TVD). Instead of expected gas separation, the NPHI-RHOB overlay exhibits shale-type separation with anomalously high NPHI values. This can be explained by hydrogen adsorption on pore walls, clay micropores, and fracture surfaces in shallow zones (above 597m TVD), while shallow zones (above 597 m TVD) contain negligible methane and exhibit adsorption-related anomalies, deeper sandstone intervals show a conventional ‘gas effect’ due to higher temperature, increased hydrogen solubility in water, and greater methane content, which collectively reduce adsorption. These findings highlight the importance of considering hydrogen adsorption responses in log interpretation for H₂-bearing reservoirs. Integrating petrophysical and gas composition data provides a preliminary framework for identifying hydrogen-rich intervals and optimizing exploration strategies. However, the analysis is limited to data from a single well, and future research should expand to include additional wells and integrate factors such as water salinity, temperature, and pressure differences for a more comprehensive understanding of the reservoir. This research contributes to the growing body of knowledge on natural hydrogen systems, supporting the development of consistent hydrogen exploration and production techniques in similar geological settings.