Biogenic Shale Gas Formation in the Lower Jurassic Posidonia Shale (Northern Germany): Modelling Studies and Laboratory Experiments
Schulz, Hans-Martin¹; Krüger, Martin²; van Berk, Wolfgang³; Arning, Esther T.¹
¹Dep. 4.3 Organic Geochemistry, GeoForschungsZemtrum Potsdam, Potsdam, Germany.
²Microbiology, Federal Institute for Geosciences and Natural, Hannover, Germany.
³Hydrogeology, Technical University of Clausthall, Clausthal-Zellerfeld, Germany.
Biogenic methane can be formed in black shales during different evolutionary stages of a sedimentary basin: i) after early diagenetic formation in the first tens of metres, ii) at around 80 degrees Celsius due to the ideal temperature for the growth of methanogens, and iii) in sediments with oil window maturity when suppressed basinal brine salinity enhances methanogenesis such as in the Antrim Shale in the Michigan Basin. Similar processes may have developed in the Liassic Posidonia Shale of our test region, the Hils half-graben system in northern Germany.
Early diagenetic formation of biogenic methane shortly after deposition was retraced using PeaCH4 (Arning et al., 2011). It was an additional aim to analyse the fate and behaviour of such early formed methane (whether dissolved, gaseous or solid as hydrate). Besides methane prediction, the approach was also applied to model other gas phases together with early diagenetic cementation and dissolution patterns of solids.
The Antrim Shale geological setting serves as an analogue to hydrogeochemically model similar scenarios for the Posidonia Shale in northern Germany. At oil window maturity free bitumens in the rock matrix may serve as convertible compounds similar to processes at oil-water contacts. We transferred a similar scenario to northern Germany which also experienced coverage by Pleistocene glaciers. Reduced brine salinities may have created subsurface environments where water soluble hydrocarbons can be converted. The model calculates mass balances based on chemical thermodynamics. The approach considers how degradation products CH4, CO2, H2 and acetate react with the Posidonia Shale matrix. The results of this approach demonstrate that the hydrostatic pressure controls both modelled gas composition and carbonate species behaviour (dissolution or precipitation). This approach offers an efficient control of carbon dioxide contents in the production gas. Moreover, carbonate solubility can be predicted, which controls the occurrence of scaling.
To test todays sensitivity of Posidonia Shale to be a current source of microbially generated methane, samples of different thermal maturities were treated in culture media with different microbial communities (incubations). Biogenic methane forms in all investigated samples, but methane production is highest for samples with oil window maturity. Moreover, the produced methane has lowest δ13C values for samples with oil window maturity.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012