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  The effects of Jurassic igneous intrusions on the generation and retention of gas shale in the Lower Permian source-reservoir shales of Karoo Basin, South Africa

Chukwuma, K., Tsikos, H., Horsfield, B., Schulz, H.-M., Harris, N. B., Frazenburg, M. (2023): The effects of Jurassic igneous intrusions on the generation and retention of gas shale in the Lower Permian source-reservoir shales of Karoo Basin, South Africa. - International Journal of Coal Geology, 269, 104219.

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Chukwuma, Kenneth1, Author
Tsikos, Harilaos1, Author
Horsfield, B.2, Author              
Schulz, H.-M.2, Author              
Harris, Nicholas B.1, Author
Frazenburg, Madelaine1, Author
1External Organizations, ou_persistent22              
23.2 Organic Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum, ou_146041              


Free keywords: Gas shale, Organic carbon, Igneous intrusions, Coke, Gas retention, Organic petrography Intrusion-sediment interactions
 Abstract: Shale gas potential of the Whitehill Formation (Permian), Karoo Basin, South Africa is strongly influenced by widespread Jurassic igneous intrusions. A high-resolution organic petrographic and geochemical study is reported here on two stratigraphic sections, the objective being to gain insights into the chemical structure and alteration of oil-prone sedimentary organic matter during contact metamorphism. One section unaffected by intrusion served as background, while the other section was intruded by two laterally continuous dolerite sills with an average thickness of 10.7 and 9.3 m, preferentially emplaced at the upper and lower contacts of the formation, respectively. Heat flow from the sills caused TOC and S2 values from programmed pyrolysis to decline over a distance of 8.7 m from 3.2 to 0.15% and 4.65 to 0.11 mg HC/g TOC, respectively, and cause the reflectance of vitrinite and solid bitumen to increase from 2.03 to 5.82% and 1.58 to 7.87%. Organic matter in the background samples is dominated by solid bitumen, a secondary thermal conversion product of oil-generative Type II kerogen. In the sill-hosting section, the thermal aureoles can be recognized by systematic changes to the optical texture and chemical structure and composition of solid bitumen. Solid bitumen particles develop anisotropy approaching the sills and become transformed into coke and pyrolytic carbon. δ13Corg values become less negative by up to 3.13‰ VPDB. These data suggest significant devolatilization of organic matter to methane and destruction of petroleum generation and retention potential in the contact aureole of the sills. The trend of these datasets, coupled with the presence of several sills and their laterally extensive nature throughout much of the study area suggests that the intrusions have been significant in increasing methane generation on a basinal scale. Together, this study indicates that the complementary application of combined light and electron microscopy (CLEM), programmed pyrolysis, and isotope ratio mass spectrometry (IRMS) is suitable for a comprehensive characterization of organic matter alteration through the maturation continuum. Results also show that contact metamorphic alteration of organic matter by intrusions produced structural and compositional changes that are distinctive from those expected from geological burial maturation where graphite is the end-product.


 Dates: 20232023
 Publication Status: Finally published
 Pages: -
 Publishing info: -
 Table of Contents: -
 Rev. Type: -
 Identifiers: DOI: 10.1016/j.coal.2023.104219
GFZPOF: p4 T8 Georesources
 Degree: -



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Title: International Journal of Coal Geology
Source Genre: Journal, SCI, Scopus
Publ. Info: -
Pages: - Volume / Issue: 269 Sequence Number: 104219 Start / End Page: - Identifier: CoNE: https://gfzpublic.gfz-potsdam.de/cone/journals/resource/journals219
Publisher: Elsevier