Depositional environment, thermal maturation, burial history and porosity of Late Mississippian black shales and black carbonates (“Hangender Alaunschiefer” and “Kohlenkalk”) in western Germany

Depositional environment, thermal maturation, burial history and porosity of Late Mississippian black shales and black carbonates (“Hangender Alaunschiefer” and “Kohlenkalk”) in western Germany | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Depositional environment, thermal maturation, burial history and porosity of Late Mississippian black shales and black carbonates (“Hangender Alaunschiefer” and “Kohlenkalk”) in western Germany Ralf Littke, Zhongrui Wu, Thorsten Bauersachs, Mila Kannemacher, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9377858/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract In 2025, an about 1000 m deep geothermal exploration well, drilled in western Germany, recovered cores from a thick succession of Mississippian black shales and dark, partly dolomitized limestones. Two intervals were investigated in detail using petrographic and geochemical methods: the Upper Alum Shale from the uppermost Mississippian (Namurian A, Seltersberg Formation) and the Viséan Kohlenkalk (Heiligenhaus Formation), drilled about 500 m deeper in the well. The Upper Alum Shale consists predominantly of black shales with moderate to high organic carbon contents. Deposition occurred under marine to brackish conditions, with predominantly anoxic bottom waters that gradually became more oxygenated toward the end of the depositional period. Paleoproductivity proxies show no significant trends, suggesting that changes in hydrodynamic conditions, particularly water shallowing, rather than productivity, controlled the observed decline in organic carbon preservation and accumulation. Climate proxies, such as CIA, indicate a shift from relatively cool and arid conditions to warmer and more humid conditions. This transition likely enhanced chemical weathering on adjacent landmasses, including the Mid-German Crystalline Rise, the rising Variscan Mountains, and the London–Brabant High. The Viséan Kohlenkalk interval comprises both partly dolomitized limestones and interbedded marlstones and shales with significantly lower carbonate contents. The marlstone–shale intervals are enriched in organic matter and were deposited under oxygen-depleted conditions, similar to those of the Upper Alum Shale. In contrast to the shales and marlstones, which represent former petroleum source rocks, the carbonate units represent a former petroleum reservoir. In both successions, the organic matter is dominated by solid bitumen with very high reflectance (pyrobitumen), accompanied by only minor amounts of vitrinite and inertinite. In the black shales, original Type II kerogen has largely been transformed into residual pyrobitumen. In the carbonates, pore spaces were initially filled with oil that was subsequently cracked into natural gas and pyrobitumen. High vitrinite and solid bitumen reflectance values combined with numerical basin and petroleum system modeling indicate a burial depth of approximately 5,000 m and maximum temperatures of at least 230 °C. Despite this deep burial, significant porosity is preserved—approximately 5% in the Upper Alum Shale and more than 10% in the Kohlenkalk interval—making these units promising targets for shale gas and geothermal exploration. This comparatively high porosity is likely due to elevated pore pressures during rapid burial, which inhibited complete porosity loss. In addition, the carbonates developed secondary porosity through processes such as dolomitization, fracturing, and/or karstification. Mississippian Visean Namurian Black Shale Kohlenkalk Upper Alum Shale Major and Trace Elements Carbon Isotopes Organic Petrography Krefeld High Pyrobitumen Porosity Full Text Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 21 Apr, 2026 Reviewers invited by journal 20 Apr, 2026 Editor assigned by journal 16 Apr, 2026 First submitted to journal 16 Apr, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9377858","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":626501247,"identity":"87d07820-a7be-477c-9a8c-e649d10161e5","order_by":0,"name":"Ralf 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Two intervals were investigated in detail using petrographic and geochemical methods: the Upper Alum Shale from the uppermost Mississippian (Namurian A, Seltersberg Formation) and the Viséan Kohlenkalk (Heiligenhaus Formation), drilled about 500 m deeper in the well.\u003c/p\u003e\n\u003cp\u003eThe Upper Alum Shale consists predominantly of black shales with moderate to high organic carbon contents. Deposition occurred under marine to brackish conditions, with predominantly anoxic bottom waters that gradually became more oxygenated toward the end of the depositional period. Paleoproductivity proxies show no significant trends, suggesting that changes in hydrodynamic conditions, particularly water shallowing, rather than productivity, controlled the observed decline in organic carbon preservation and accumulation. Climate proxies, such as CIA, indicate a shift from relatively cool and arid conditions to warmer and more humid conditions. This transition likely enhanced chemical weathering on adjacent landmasses, including the Mid-German Crystalline Rise, the rising Variscan Mountains, and the London–Brabant High.\u003c/p\u003e\n\u003cp\u003eThe Viséan Kohlenkalk interval comprises both partly dolomitized limestones and interbedded marlstones and shales with significantly lower carbonate contents. The marlstone–shale intervals are enriched in organic matter and were deposited under oxygen-depleted conditions, similar to those of the Upper Alum Shale. In contrast to the shales and marlstones, which represent former petroleum source rocks, the carbonate units represent a former petroleum reservoir.\u003c/p\u003e\n\u003cp\u003eIn both successions, the organic matter is dominated by solid bitumen with very high reflectance (pyrobitumen), accompanied by only minor amounts of vitrinite and inertinite. In the black shales, original Type II kerogen has largely been transformed into residual pyrobitumen. In the carbonates, pore spaces were initially filled with oil that was subsequently cracked into natural gas and pyrobitumen. High vitrinite and solid bitumen reflectance values combined with numerical basin and petroleum system modeling indicate a burial depth of approximately 5,000 m and maximum temperatures of at least 230 °C.\u003c/p\u003e\n\u003cp\u003eDespite this deep burial, significant porosity is preserved—approximately 5% in the Upper Alum Shale and more than 10% in the Kohlenkalk interval—making these units promising targets for shale gas and geothermal exploration. This comparatively high porosity is likely due to elevated pore pressures during rapid burial, which inhibited complete porosity loss. 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