PhD of Dr. Thomas Thielemann (2000)
Balance of methane exchange across coal-bearing sedimentary basins:The Ruhr Basin and the Lower Rhine Embayment, Germany
Aim of this thesis was a comprehensive balance of methane exchange across coal-bearing sedimentary basins, carried out for the Ruhr Basin and the Lower Rhine Embayment, Germany. The study comprises a mass balance of organic matter in the gas source rocks, the reconstruction of methane generation and migration, and measurements of the recent gas exchange between litho- and atmosphere. Carbon isotope measurements of methane emitted in very different settings allowed to determine the origin of methane. The result was a reconstruction of the fossil and recent coal bed methane history, from which the climatic impact of coal-related methane in the past and present can be deduced.
Thermogene methane generation affected 1,800 billion metric tons (Gt) of organic matter in the Carboniferous rocks of the Ruhr Basin. Inside the Lower Rhine Embayment, 5.3 Gt of organic material were subject to methane formation in the Erkelenz mining district, 25.5 Gt in the Wurm district and 3.5 Gt in the Inde district. Open system non-isothermal pyrolysis experiments with coal samples delivered reaction kinetic data of coal bed methane generation. Activation energies and frequency factors rise with maturity. By combining these data with the temperature history of the coal-bearing Carboniferous in the Ruhr Basin and Lower Rhine Embayment (after BÜKER et al. 1994, LITTKE et al. 1994a, BÜKER et al. 1995, BÜKER 1996 and KARG 1998) the timing of thermogenic methane generation was reconstructed. Methane was formed in uppermost Carboniferous and lowermost Permian times, apart from the Erkelenz district, where a younger, second phase of methane generation might have occurred. Considerations by LOMMERZHEIM (1988, 1991a, 1994) of an extensive methane generation in post-Carboniferous times are not supported. Methane generation in the Ruhr Basin could be quantified to have reached about 16,000 m³ of methane per m² unit area. Taking facies variations into account, 100,000 to 110,000 km³ of methane have been generated in the Ruhr Basin.
Carbon isotope measurements revealed an enrichment of the isotope 13C in the course of degradation and maturation of organic matter. The isotope composition of methane released outside bog areas at the grounds surface varied between -32 ‰ and -43 ‰ (vs. PDB). Hence, all these gas emissions were coal-related. Depth trends in methane isotope compositions are related to adsorption and desorption processes. It could be shown that not only in bog areas, but also in the lignite opencast mine Hambach in the Lower Rhine Embayment bacterial methane is produced.
To reconstruct the history of coal bed methane migration in gas phase in coal-bearing sedimentary basins, the balance procedure fossil migration was developed in this project. It allowed concluding that methane migration in the Ruhr Basin was mainly focussed on three emission events, 304-298 Ma, 252-248 Ma and 100-90 Ma ago. Between these events, coal bed methane was adoptively bound to coaly organic matter. Differences in the recent gas contents of coal seams point to minor differences in subsidence history between the eastern and western part of the Ruhr Basin. Methane transport dissolved in water can be neglected in the Lower Rhine Embayment as groundwaters there are methane-undersaturated. This is also true for groundwaters in the Ruhr Basin. Oversaturation and methane degassing can occur where thermal brines locally ascend or salt-rich mine waters are produced.
The methane exchange between litho- and atmosphere could be subdivided into consumption and emission processes. In aerobe soils bacterial methane consumption was widespread and turned out to depend on methane diffusion, soil temperature, and soil utilization. The non-sealed areas of the Ruhr Basin and the Lower Rhine Embayment were subdivided into methane consumption classes. For each class the consumption was balanced. The results were a bacterial methane consumption of around 2727 t/a in the Ruhr Basin and of 2049 t/a in the Lower Rhine Embayment.
Thermogenic methane emissions across the grounds surface lack in the Lower Rhine Embayment. In the Ruhr Basin, however, natural and mining-induced emissions of thermogenic methane have been recorded. Recent natural emissions only occur alongside faults with an accumulation of coal bed methane below the top Carboniferous. Mining-induced emissions of thermogenic methane are restricted to areas of abandoned and active coal mining in the eastern Ruhr Basin. They are of no climatic relevance and contribute 0.0007 % to the national coal-bound methane emissions. Methane is mainly released via upcast mining shafts. The German contribution to international coal-related methane emissions is as small as 1.4 to 2.5 % and has been halved between 1987 and 1998. This was the main reason for the retreat of national annual methane emissions. This national trend contrasts the expected global future trend. Mainly due to a fall of methane emissions by coal mining the German contribution to the global methane release of 0.9 % will become even smaller in the future.