Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Gas Generation Reactions in Highly Mature Gas Shales
(1) Organic Geochemistry, GFZ German Research Centre for Geosciences, Potsdam, Germany.
(2) Chemistry and Physics of Earth Materials, GFZ German Research Centre for Geosciences, Potsdam, Germany.
As a new and abundant energy resource, gas shales have already impacted worldwide energy supply. Currently, worldwide exploration activities are drastically increasing. This unconventional gas resource constitutes complex self-contained source-reservoir systems, in which three distinct processes together result in the formation of thermogenic gas: (1) the decomposition of kerogen to gas and bitumen; (2) the decomposition of bitumen to oil and gas; and (3) the secondary cracking of oil to gas and a carbon-rich coke or pyrobitumen residue. While the basic stoichiometries and source-sink relationships are understood, the chemical and structural variability of gas shales at the submicrometer scale is still poorly documented. Consequently, hydrocarbon generation and retention processes occurring within such unconventional systems are still poorly constrained.
As part of the European GASH project (GAs SHales) coordinated by the GFZ-Potsdam, organic-rich calcareous mudstone samples from Northern Germany at varying stages of thermal maturation have been characterized using an original combination of compositional organic geochemistry and spectromicroscopy techniques, including synchrotron-based STXM (scanning transmission X-ray microscopy) allowing in situ imaging of organic-rich samples with a chemical-based contrast at a 25-nm spatial resolution while providing spatially-resolved information of organic constituent speciation [1].
Within the selected gas shale samples, we document multi-scale chemical and mineralogical heterogeneities, from the bulk scale down to the nanometer scale. Different types of bitumen, very likely genetically derived from thermally degraded organic precursors, have been detected in close association with authigenic minerals. Macroscopic fracture-filling bitumen with the same organic chemical signatures as one family has been documented. Chemical rock-fluid interactions could be inferred in the case of another family. The porosity evolution with thermal maturation has also been documented using TEM at the nanometer scale. Our observations provide key constraints on the thermal history of this gas shale formation and shed new light on the influence of the organic precursor chemistry on the thermal generation and retention of the various organic moieties which can be encountered in gas shales.