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Clays and Clay Minerals

, Volume 22, Issue 4, pp 319–335 | Cite as

Diagenesis of Clay Minerals from Lower Cretaceous Shales of North Eastern British Columbia

  • A. E. Foscolos
  • H. Kodama
Article

Abstract

Clay minerals from shale outcrops of the Lower Cretaceous Buckinghorse Formation (4250 ft thick) were investigated in order to assess their degree of diagenesis and their oil-generating potential. Crystallinity index, sharpness ratio, per cent of illite which is the 2M polymorph and presence of discrete minerals have been studied in the whole clay fraction, while the very fine clay fraction has been subjected to X-ray diffraction, differential thermal, thermogravimetric, differential thermogravimetric, i.r. spectros-copy, surface area and chemical analyses. With information derived from these studies and from published data, a classification scheme was devised which relates variation of clay mineralogy to diagenetic stages and burial depth.

Data on the < 2 μm size fraction show that the crystallinity index decreases while the sharpness ratio and per cent of illite which is the 2M polymorph increase with burial depth. Results on the <0·08 μm fraction reveal that a three-component interstratified clay mineral exists. In addition, Fourier transform calculations and chemical and physicochemical analyses indicate that both the ratio of the amounts of non-hydrated clays (illite) to hydrated clays and the K2O content of clays increase with burial depth; cation exchange capacity and surface area decrease with burial depth.

Based upon a classification scheme, which was devised by combining criteria and data derived from the studies of Weaver (1961a), Kubier (1966), Burst (1969) and Dunoyer de Seconzac (1970), the upper and middle parts of the formation (upper 3250 ft) fall within the middle stage of diagenesis whereas the lower part (1000 ft) is allocated to the beginning of late diagenesis. In terms of Burst’s (1969) work, the upper 3250 ft are transitional between the stability and dehydration zones indicating that, prior to uplift, hydrocarbons may have been in the process of migration. The lower 10000 ft of the formation are in the restricted dehydration zone, indicating that hydrocarbon migration should have been completed.

Résumé

Des minéraux argileux provenant d’affleurements de schistes de la Formation Buckinghorse du Crétacé inférieur (4250 pieds d’épaisseur) ont été étudiés en vue de déterminer leur degré de diagénèse et leur potential de réservoir de pétrole. L’indice de cristallinité, le rapport de finesse des pics, le pourcentage d’illite qui est le polymorphe 2M et la présence de minéraux discrets ont été étudiés sur la fraction argileuse totale, tandis que la fraction argileuse très fine a été étudiée par la diffraction X, les analyses thermique différentielle, thermogravimétrique, thermogravimétrique dérivée, la spectroscopic infrarouge, la mesure de surface spécifique et les analyses chimiques. A l’aide des informations tirées de ces études et de résultats déjà publiés, un schéma de classification a été établi qui relie les variations de la minéralogie d l’argile aux étapes de la diagénèse et à la profondeur d’enfouissement.

Les résultats obtenus avec la fraction <2 μm montrent que l’indice de cristallinité diminue tandis que la rapport de finesse des pics et le pourcentage d’illite qui est le polymorphe 2M augmentent avec la profondeur d’enfouissement. Les résultats obtenus avec la fraction <0,08 μm rélèvent l’existence d’un minéral argileux interstratifié à trois composants. En plus, les calculs de transformées de Fourier et les analyses chimiques et physicochimiques indiquent que le rapport des teneurs en argiles non hydratées (illite) à celles des argiles hydratées, et que la teneur en K2O des argiles augmentent simultanément avec la profondeur; la capacité d’échange de cations et la surface spécifique diminuent avec la profondeur.

Si l’on se fonde sur un schéma de classification qui a été établi en combinant les critères et les résultats tirés des travaux de Weaver (1961a), Kubier (1966). Burst (1969) et Dunoyer de Segonzac (1970), les zones supérieures et moyennes de la formation (3250 pieds d’épaisseur) coincident avec l’étape moyenne de la diagénèse, tandis que la zone inférieure (1000 pieds) est attribuée au début de la dernière diagénèse. Selon les termes du travail de Burst (1969), les 3250 pieds supérieurs sont une transition entre les zones de stabilité et de déshydratation, indiquant que, avant la remontée, les hydrocarbures peuvent avoir participé au processus de migration. Les 1000 pieds inférieurs de la formation sont dans une zone de déshydratation restreinte, indiquant que la migration des hydrocarbures doit avoir été achevé.

Kurzreferat

Tonminerale aus Schichtköpfen von Schiefern der Buckinghorse Formation der Unterkreide (4250 Fuß mächtig) wurden untersucht, um das Ausmaß der Diagenese und ihre Fähigkeit zur Ölbildung abzuschätzen. Der Kristallinitätsindex, das Schärfeverhältnis, der Prozentgehalt des 2M Polymorphs am Illitanteil und das Vorkommen besonderer Minerale wurden in der gesamten Tonfraktion bestimmt, während die feinste Tonfraktion durch Röntgenbeugung, differentialthermoanalytisch, thermogravimetrisch, differentialthermogravimetrisch, infrarotspektroskopisch und durch Bestimmung der spezifischen Oberfläche und der chemischen Zusammensetzung untersucht wurden. Mit Hilfe der aus diesen Untersuchungen erhaltenen Informationen und veröffentlichter Ergebnisse wurde ein Klassifikationsschema entwickelt, das tonmineralogische Veränderungen zum diagenetischen Umwandlungsgrad und der Lagerungstiefe in Beziehung setzt.

Die Werte der Korngrößenfraktion <2 μm zeigen, daß der Kristallinitätsindex mit zunehmender Lagerungstiefe abnimmt, während das Schärfeverhältnis und der als 2M Polymorph vorliegende Illitanteil ansteigen. Die mit der Fraktion <0·08 μm erhaltenen Ergebnisse lassen erkennen, daß ein aus 3 Komponenten bestehendes Wechsellagerungstonmineral vorliegt. Darüberhinaus ergeben Fourier-Analysen sowie chemische und physikochemische Analysen, daß sowohl das Verhältnis des Anteils nichthydratisierter Tone (Illite) zu dem hydratisierter Tone als auch der K2O-Gehalt der Tone mit zunehmender Lagerungstiefe ansteigen. Kationenaustauschkapazität und spezifische Oberfläche nehmen mit der Lagerungstiefe ab.

Auf der Grundlage eines Klassifikationsschemas, das durch Kombination von Kriterien und Werten aus den Untersuchungen von Weaver (1961a), Kubler (1966), Burst (1969) und Dunoyer de Seconzac (1970) aufgestellt wurde, fallen die oberen und mittleren Teile der Formation (die oberen 3250 Fuß) in den mittleren Bereich der Diagenese, während der untere Teil (1000 Fuß) dem Beginn einer späten Phase der Diagenese zuzuordnen ist. Nach den Begriffen der Arbeit von Burst (1969) stellen die oberen 3250 Fuß einen Übergang zwischen den Stabilitäts- und Dehydratationszonen dar. Dies deutet an, daß vor Eintreten der Hebung die Kohlenwasserstoffe im Wanderungsprozeß begriffen waren. Die unteren 1000 Fuß der Formation sind der Zone beschränkter Dehydratation zuzuordnen, was zeigt, daß die Kohlenwasserstoffwanderung abgeschlossen sein sollte.

Резюме

Исследовали глинистые минералы обнаженных пород глинистого сланца нижнего мелового периода Букингхорса (толщиной 4250 фут), чтобы определить степень их диагенеза и их потенциал генерации нефти. На целой фракции глины изучали показатель степени кристал-лизации, остроугольность, процентное содержание иллита, являющимся полиморфом 2М и присутствие разрозненных частиц минералов, а очень малую фракцию глины подвергали рентгенографическому структурному, дифференциально-термическому, термогравиметри-ческому, дифференциально-термогравиметрическому, ИК-пектроскопическому анализам и также анализу поверхностной площади. Посредством информации полученной этими исследо-ваниями и из опубликованных данных, разработали схему классификации, относящую разно-образия минералогии глины к диагенетическим периодам и к глубине залегания.

По данным о фракции 2 μм видно, что в зависимости от глубины залегания степень крис-таллизации уменьшается, в то время как остроугольность и процентное содержание иллита, являющимся полиморфом 2М, повышаются. Результаты исследования фракции <0,08 μм показали, что существует переслаивающийся трехсоставной глинистый минерал. Кроме того, преобразование Фурье и химический и физико-химический анализы указали, что как соотно-шение негидратировавшейся глины (иллит) к гидратировавшейся глине, так и содержание К2O в глине повышаются чем глубже залегает минерал, а катионообменная способность и площадь поверхности при этом понижаются.

На основании схемы классификации, выработанной комбинацией критерий и данных взятых из исследований Вивера (1961а); Кублера (1966); Бурста (1969) и Дуноейра де Секонзака (1970), верхняя и средняя части формации (верхняя 3250 фут) приходятся на среднюю стадию преобразования осадков в горные породы, в то время как нижняя часть (1000 фут) относится к началу позднего диагенеза. По работе Бурста (1969), верхние 3250 фут находятся в переходной стадии между зонами устойчивости и дегидратации указывающими, что до взброса, угле-водороды, вероятно, подвергались перемещению.

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Copyright information

© Clay Minerals Society 1982

Authors and Affiliations

  • A. E. Foscolos
    • 1
  • H. Kodama
    • 2
  1. 1.Institute of Sedimentary and Petroleum GeologyCalgaryCanada
  2. 2.Soil Research InstituteDepartment of AgricultureOttawaCanada

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