Title : Structural and geological controls on hydrogen generation and migration in neoproterozoic carbonates of the Taoudeni basin: A comparative study between Mali and Mauritania
Abstract:
The Taoudenni Basin one of Africa’s largest intracratonic basins remains underexplored but hosts a distinct Paleozoic petroleum system. Drilling of key wells (Ta-7, Ta-8, Ouasaa-1, Abolag-1) confirmed thermally mature Ordovician–Devonian source rocks, widespread siliciclastic reservoirs, and oil shows. Natural hydrogen seeping out as a gas from hydrothermal systems in mid-oceanic ridges has been detected in the 1970’s (Whelan and Craig, 1979; Charlou et al., 2002; Marcaillou et al., 2011). Hydrogen sourcing out has also been observed few years later in ophiolites (Neal and Stanger, 1983; Abrajano et al., 1988; 1990). In Mali, natural hydrogen is trapped within flower structures by dolerite sills along the Taoudeni Basin margin, hosted in Neoproterozoic-age rocks. Similar dolerite sills have been observed in seismic data and drilled wells within the Taoudeni Basin in Mauritania. These structures and associated rocks were identified in the Abolag-1 and Ouasaa-1 wells following the interpretation of multiple datasets, including dolerite sills, dykes, and feeder stocks; shallow and deep faults; shallow and deep basement structures; iron-rich zones; deep magmatic centers; and surface circular features Fig-2. Exploration of natural hydrogen (H?) in Mauritania integrates seismic, well, gravity, magnetic, migration pathways, and traps. Seismic and well logs image shallow and deep faults, basin margins, shear zones, and structures like flower faults and dolerite-filled zones that act as H? conduits. Iron-rich terrains, potential H? sources, are detectable via gravity and magnetic signatures, which also help identify crustal thickness, deep faults, and dolerite sills (e.g., Mali H? analogue). Integrating seismic, well, gravity, and magnetic data enhances interpretation, velocity models, and mapping of igneous intrusions. Well logs provide density data for modelling, and wells can be tested for H?. Satellite and surface data aid in detecting hydrogen seeps, enabling a multi-scale, comprehensive exploration approach. The presence of substantial hydrogen gas in all the wells located in the vicinity of the Bourakebougou structure implies the existence of a large natural hydrogen system, A production of a gas predominantly composed at 98% of hydrogen, 1% of nitrogen and 1% of methane was reported at that time (Alain Prinzhofer, 2018). The thermal history of Abolag-1, calibrated to the well, is similar to Ouasa-1, with burial shallower by 1100 m, leading to lower source rock maturity. Optical measurements indicate 1.3% VR equivalent at TD, requiring a Caledonian thermal event to reach this maturity. Abolag-1 (1974 – TD 2941 m, Lower to Middle Infracambrian, Mauritania) tested gas at 13,600 m³/d, condensate unknown. Fluids: light hydrocarbons, 70% oil / 30% gas (Include Hydrogene, H?S: CO?: possible). Reservoir >300 m thick, lithology: stromatolitic limestones and quartzitic sandstones, possible bitumen stains. This study shows that natural hydrogen systems observed in Mali’s Taoudeni Basin, trapped within flower structures and dolerite sills, have analogous features in Mauritania. Integration of
seismic, well, gravity, magnetic, satellite, and surface data identified shallow and deep faults, basin margins, shear zones, dolerite sills, feeder dykes, iron-rich zones, deep magmatic centers, and circular surface features in the Abolag-1 and Ouasaa-1 wells. These findings enabled the construction of a geological, structural, and geochemical model demonstrating the existence of Structural and Geological Controls on Hydrogen Generation and Migration in Neoproterozoic Carbonates of the Taoudeni Basin: A Comparative Study between Mali and Mauritania H? systems in Mauritania similar to those in Mali, highlighting the Taoudeni Basin as a prospective area for natural hydrogen exploration.
