Effect of Paleo-Fluid-Flow-Induced Mineralization of the Ardley Coals on Development of a Major Regional Seal in the Alberta Basin, Canada

Kaush Rakhit¹ and Basim Faraj^2
1 Rakhit Petroleum Consulting Ltd, Calgary, AB
² Faraj Consultants Pty Ltd. and EOG Resources Canada Inc, Calgary, AB

The Scollard and Coalspur Formations in west-central Alberta contain up to 20 metres of aggregate coal in four major seams referred to as the Ardley Coal Zone. Covering over 25,000 km2 the coals dip westward from outcrop to a depth of over 1,000 metres, forming one of the most important top seals in the Alberta Basin.

Regionally, the dominant northeast-southwest oriented face cleats are preferentially mineralized in comparison to the less frequently developed butt cleats, reflecting the control of Laramide compressional stress on fluid flow. Calcite is the dominant mineral in cleats and non-coal fractures within the coal seams. Stable isotope and fluid inclusions data indicate that calcite precipitated from highly 18O depleted meteoric water (calcite del 18O between -22.6 and -24.5 per mil). We believe calcite mineralization occurred in conjunction with a basin-wide ‘paleo-fluid flow' event during the cycle of burial and uplift associated with the Laramide Orogeny.

Interestingly, a majority of the calcite mineralization is restricted to the uppermost (Val D'Or coal) versus the lower seams of the Arbour, Silkstone and Mynheer. Mineralization of the Val D'Or coal zone has its most pronounced effect on the hydrogeology of the Scollard and overlying Paskapoo Formations. Above the Val D'Or coal zone, the sands of the Paskapoo Formation form a regionally continuous aquifer adjusted to present day topography. A 2,000 to 3,000 kPa pressure discontinuity across the coal seam defines a major top seal that separates the overlying water wet system from a regionally pervasive, underpressured gas-saturated system. The vertical flow gradient set up across the Val D'Or coal zone has caused continual interaction between the “surface wet” system and underlying gas saturated system. This interaction is likely responsible for the variability in measured gas content of the Ardley Coals.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005