About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

CSPG Bulletin


Bulletin of Canadian Petroleum Geology
Vol. 36 (1988), No. 2. (June), Pages 158-167

Authigenic Illite in the Lower Silurian Cataract Group Sandstones of Southern Ontario

K.J. O'Shea, S.K. Frape


Approximately 50 percent of the natural gas produced annually in Ontario is developed from the Lower Silurian sandstones of the Cataract Group. The sandstones are cemented primarily with silica and are classified as quartz arenites. Minor amounts (<5%) of pore-filling authigenic illite (clay mica) occur in the sandstones.

Isolated patches of illite have formed via the transformation of smectite to illite. Most of the illite in the Cataract (Medina) sandstones is associated with corroded microcline feldspars, and measureable quantities of gas. Evidence suggests that this illite formed by direct precipitation from saline pore fluids, the component ions being leached from the potassium feldspars. Leaching of the feldspars, and transport of the Al3+ ions were possibly mediated by organic acids released during the early stages of hydrocarbon maturation.

The cores used in this study were air dried over a period of 4 to 12 years. The delicate, fragile nature of the authigenic illite as seen in SEM, is similar to illite observed in studies employing critical point drying and freeze drying techniques during sample preparation, but unlike the collapsed structure of the illite observed in cores air dried in ovens. This suggests that the rate of air drying may affect the observed clay fabric much more then the methods of core drying.



Approximativement 50 pourcent du gas naturel produit chaque annee en Ontario provient des gres Silurien inferieur du groupe Cataract. Les gres sont cimentes surtout avec de la silice et appartiennent a la classe des arenites a quartz. On retrouve des quantites mineures (<5%) d'illite authigene (mica argileux) remplissant les pores dans les gres.

Des plaques isolees d'illite se formerent grace a la transformation de smectite en illite. La plus grande partie de l'illite dans les gres Cataract (Medina) est associee a du microcline corrode et a des quantites mesurables de gaz. Selon toute evidence cette illite fut precipitee directement des solutions salines remplissant les pores, les ions composants ayant ete lessives des feldspaths potassiques. Le lessivage des feldspaths, ainsi que le transport des ions Al3+ furent peut-etre aides par des acides organiques liberees durant les premieres etapes de la maturation des hydrocarbures.

Les carottes utilisees dans cette etude ont ete sechees a l'air durant une periode allant de 4 a 12 ans. L'aspect delicat et fragile de l'illite authigene tel qu'observe au MEB, est semblable a celui de l'illite decrite dans des etudes utilisant les techniques de sechage au point critique et de sechage a froid durant la preparation des echantillons, mais dissemblable a la structure effondree de l'illite observee dans des carottes sechees a l'air dans des fours. Ceci suggere que le taux de sechage a l'air pourrait influer sur la fabrique des argiles beaucoup plus que les methodes de sechage des carottes.

Traduit par Marc Charest

Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Protected Document: $10
Internal PDF Document: $14
Open PDF Document: $24