--> The Influence of Fault-Controlled Dolomitization on Porosity Modification in the Western Canada Sedimentary Basin

AAPG ACE 2018

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The Influence of Fault-Controlled Dolomitization on Porosity Modification in the Western Canada Sedimentary Basin

Abstract

The differentially dolomitised Devonian carbonates of the Western Canada Sedimentary Basin (WCSB) have been extensively studied due to their importance as hydrocarbon reservoirs. Multiple phases of dolomitisation are known to have occurred, including so-called hydrothermal dolomitisation, along major faults within the WCSB. Middle Cambrian aged fault-controlled dolostone bodies exposed in the Rocky Mountains exhibit comparable textures to the Devonian; in particular they are non-stratabound and highly fractured and brecciated. In outcrop, stratabound dolostone bodies are texturally more homogenous, and are found distal to faults, suggesting that dolostone texture is at least in part controlled by distance from fault. To test this hypothesis in the subsurface, variably dolomitised cored intervals of Devonian Swan Hills and Leduc Formations and Wabamun Group were described, based on their proximity to mapped platform margin faults and/or regional Precambrian shear zones. Two contrasting types of dolomitisation were identified:

1. Fabric-retentive textures were found in the Swan Hills Fm. Partial dissolution of stromatoporoids created minor mouldic porosity, but most mouldic pores and fractures were occluded by saddle dolomite cements.

2. Fabric-destructive textures were found in cores from the Swan Hills and Leduc Fms., with a higher frequency of biomouldic pores than in fabric-retentive dolostone. Vuggy porosity is common, and lined with coarsely crystalline dolomite cement. Fractures are abundant and often cross-cut stylolites, indicating formation after burial.

Initial subsurface observations appear to support the outcrop based dolomitisation model proposed, with fabric-destructive dolomite forming proximal to faults, producing abundant secondary porosity and dense fracture networks. Fabric-retentive dolomite represents a more distal expression of fault-controlled dolomitisation, with minor dissolution and porosity generation. The presence of contrasting types of dolomitisation within individual formations indicates that lithological variation may not be the only controlling factor on dolostone texture. As such, this study will provide insights into how porosity may be distributed within other fault-controlled dolostone reservoirs within the WCSB and globally.