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Black Shale—Its Deposition and Diagenesis

Published online by Cambridge University Press:  01 July 2024

Harry A. Tourtelot
Harry A. Tourtelot*
Affiliation:
U.S. Geological Survey, Federal Center, Denver, Colorado 80225
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Abstract

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Black shale is a dark-colored mudrock containing organic matter that may have generated hydrocarbons in the subsurface or that may yield hydrocarbons by pyrolysis. Many black shale units are enriched in metals severalfold above expected amounts in ordinary shale. Some black shale units have served as host rocks for syngenetic metal deposits.

Black shales have formed throughout the Earth's history and in all parts of the world. This suggests that geologic processes and not geologic settings are the controlling factors in the accumulation of black shale. Geologic processes are those of deposition by which the raw materials of black shale are accumulated and those of diagenesis in response to increasing depth of burial.

Depositional processes involve a range of relationships among such factors as organic productivity, clastic sedimentation rate, and the intensity of oxidation by which organic matter is destroyed. If enough organic material is present to exhaust the oxygen in the environment, black shale results.

Diagenetic processes involve chemical reactions controlled by the nature of the components and by the pressure and temperature regimens that continuing burial imposes. For a thickness of a few meters beneath the surface, sulfate is reduced and sulfide minerals may be deposited. Fermentation reactions in the next several hundred meters result in biogenic methane, followed successively at greater depths by decarboxylation reactions and thermal maturation that form additional hydrocarbons. Suites of newly formed minerals are characteristic for each of the zones of diagenesis.

Резюме

Резюме

Чёрные сланец является темно окрашенной глинистой породой, содержащей органический материал, который мог явиться источником углеводородов в недрах или из которого можно получить углеводороды пиролизом. Во многих видах чёрного сланца содержание металлов в несколько раз превышает их содержание в обыкновенных сланцах. Некоторые виды чёрного сланца служат вмещающими породами для сингенетических отложений металлов.

Чёрные сланцы формировались в течение всей истории земли и во всех частях мира. Это указывает на то, что геологические процессы и не геологические условия являются определяющими факторами при формировании чёрных сланцев. Геологические процессы включают отложение, в результате которого накапливаются первичные осадки чёрного сланца, и диагенез, присходящий в результате увеличивающейся глубины захоронения осадков.

Процессы отложения включают целый диапазон отношений между такими факторами как органическая продуктивность, скорость седиментации, интенсивность окисления, разрушающего органическое вещество. Если присутствует достаточное количество органического вещества, чтобы истощить кислород в среде, образуется чёрный сланец.

Диагнетические процессы включают химические реакции, определяемые природой составных частей и режимами давлений и температур, вызванных продолжающимся захоронением. На глубине в несколько метров под поверхностю земли восстанавливается сульфат и могут отложиться сульфидные минералы. На глубине в несколько сотен метров в результате реакций брожения образуется биогенетический метан, а на больших глубинах в результате реакций декарбоксилирования и температурного метаморфизма образуются дополнительные углеводороды. Комплексы вновь образованных минералов характеризуют каждую зону диагенеза.

Resümee

Resümee

Schwarzschiefer ist ein dunkler Schieferton, der organische Substanz enthält, die durch Überlagerung oder durch Pyrolyse Kohlenwasserstoffe bilden kann. Viele Schwarzschieferschichten zeigen eine Anreicherung an Metallen, sodaß ihre Metallgehalte um einiges höher liegen als die der üblichen Schiefer. Einige Schwarzschieferschichten dienten als Muttergestein für syngenetische Metallablagerungen.

Schwarzschiefer wurden während der ganzen Erdgeschichte und überall auf der Erde gebildet. Daraus folgt, daß geologische Prozesse, nicht geologische Gegebenheiten die ausschlaggebenden Faktoren für die Ablagerung von Schwarzschiefer sind. Diese geologischen Prozesse sind die Ablagerung, durch die das Ausgangsmaterial für den Schwarzschiefer sedimentiert wurde und die Diagenese infolge der zunehmenden Überlagerung.

Ablagerungsprozesse werden durch die Einwirkung mehrerer Faktoren beeinflußt, wie z.B. organische tätigkeit, Absatzgeschwindigkeit klastischer Sedimente, und Intensität der Oxidation, durch die organisches Material zerstört wird. Schwarzschiefer entsteht, wenn genügend organisches Material vorhanden ist, um den Sauerstoff der Umgebung zu verbrauchen.

Diagenetische Prozesse beinhalten chemische Reaktionen, die durch die Art der Komponenten sowie durch die herrschenden Druck- und Temperaturverhältnisse kontrolliert werden, die durch zunehmende Überlagerung entstehen. Denn unter einer Überlagerung von einigen Metern wird Sulfat reduziert, und Sulfidminerale können abgelagert werden. Fermentationsreaktionen in den nächsten hundert Metern führen zur Bildung von biogenem Methan. Ihnen folgen mit zunehmender Tiefe nach und nach Decarboxyli-erungsreaktionen und thermische Alterung, die weitere Kohlenwasserstoffe bilden. Abfolgen von neu gebildeten Mineralen sind charakteristisch für jede der Diagenesezonen.

Résumé

Résumé

L'argile shisteuse noire est une roche argileuse foncée contenant de la matière organique qui peut avoir généré des hydrocarbones dans le sous-sol ou qui peut donner des hydrocarbones par pyrolyse. Beaucoup d'unités d'argile shisteuse noire sont enrichies de quantités de métaux plusieurs fois plus importantes que celles aux quelles on s'attendrait dans l'argile shisteuse ordinaire. Certaines unités d'argile shisteuse noire ont servi de roches hôtes pour des dépôts de métal syngénétique.

Les argiles shisteuses noires ont été formées tout au long de l'histoire terrestre et dans toutes les parties du monde. Ceci suggère que ce sont des procèdes géologiques et non des lieux géologiques qui sont les facteurs controllants dans l'accumulation de l'argile shisteuse noire. Les procédés géologiques sont: la déposition par laquelle les matières premières d'argile shisteuse sont accumulées et la diagénèse répondant à une profondeur d'enterrement croissante.

Les procédés de déposition comprennent une étendue de relations entre des facteurs tels la productivité organique, la vitesse de sédimentation élastique et l'intensité d'oxidation par laquelle la matière organique est détruite. S'il ya assez de matière organique pour épuiser l'oxygène de l'environnement, il en résulte une argile shisteuse noire.

Les procédés diagénétiques comprennent des réactions chimiques contrôlés par la nature des composants et par les régimes de pression et de température imposés par l'enterrement continuel. A une épaisseur de quelques mètres sous la surface, la sulphate est réduite et des minéraux sulphides peuvent être déposés. Les réactions de fermentation dans les prochaines centaines de mètres résultent en de la méthane biogènique, suivie successivement à de plus grandes profondeurs de réactions -de decarboxylation et de maturation thermale qui forment d'avantage d'hydrocarbones. Des suites de minéraux nouvellement formés sont caractéristiques de chacune des zones de diagénèse.

Keywords

Black shaleDepositionDiagenesisOrganic matterSyngenetic ores
Type
Research Article
Information
Clays and Clay Minerals , Volume 27 , Issue 5 , October 1979 , pp. 313 - 321
Copyright
Copyright © 1979, The Clay Minerals Society

Footnotes

1

Invited paper at 1978 Clay Minerals Conference, Bloomington, Indiana.

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