STRUCTURALLY CONTROLLED DIAGENESIS OF A CARBONATE RAMP (BANFF FORMATION, ALBERTA, CANADA)

The Lower Carboniferous Banff Formation represents a homoclinal carbonate ramp. Its layer-cake geometry and homogeneous lithology over a large area make it ideal to study palaeostructures and palaeofault systems. The study of 607 wells in the area east of Jasper Park outlines major diagenetic features that are probably structurally related. In each well, the average lithology has been broken down into limestone, dolomite and shale, with the dolomite content and the porosity being mapped. Because of the varying density of wells, an "anomaly mapping technique" has been used that is based on flagging anomalies (wells outside the norm) after searching in six out of eight octants. The contour maps indicate that anomalous zones of porosity and oil stain (migration pathways) are related to dolomitisation trends. Their linear geometry and their location indicate a structural control. Three major trends (NW-SE, NE-SW and NNE-SSW) are recognised; they have varying characteristics. Comparison of the anomaly maps for dolomite and porosity shows, locally, a typical pattern of overdolomitisation in the NE-SW trends. Assessment of the relative timing of the various dolomitisation and porosity phases has been tentatively made. In one case a lateral throw of a previously non-recognised fault has been measured (65 km). A comparison with known Devonian sedimentary and diagenetic features is tentatively made in order to validate the outlined trends and their interpretations.