Hydrogeological characterization of the Mabou Group in the Picadilly region, New Brunswick

Abstract

The Mabou Group in the Picadilly region of New Brunswick consists predominantly of red siltstones with interbeds of sandstone and conglomerate. The Mabou is of importance in this region as it overlies evaporite deposits of economic value. Given that the Mabou is several hundred meters thick, yet possesses no significant marker beds, it has remained stratigraphically undifferentiated in the Picadilly region over the course of previous regional mapping efforts in the area. Given the lack of a stratigraphic framework, coupled with insufficient hydrogeological sampling and test data to delineate any laterally extensive flow zones, the hydro-stratigraphy of this sedimentary package has not been established.

Based on experience gained at other operations, Potash Corporation of Saskatchewan Inc. (PCS) recognized the need to take a proactive approach towards understanding the hydrogeology of the Picadilly mine development site. PCS has used a specialized technique in characterizing boreholes to measure depth-profiles of hydrophysical properties by logging the electrical conductivity of the borehole fluids over a period of time. Through the interpretation of these logging results, with the context provided by conventional geophysical logging data and nearby exploration drill core, this research seeks to identify zones of flow and assess whether or not any major units with lateral connectivity can be identified.

The results highlight four zones of interest within the Mabou: the Upper Mabou; vuggy porosity directly above gypsum-infilled fractures; vugs and fractures within the Medium Sandstone subdivision; and the lower Mabou Siltstone at the base of the Mabou Group (within ~15m of base). Three of these zones are of interest as they tend to have relatively high permeabilities, with permeabilities over 10 000 md measured within the Upper Mabou, as high as 1800 md in the vuggy porosity zone directly above gypsum-filled fractures and as high as 1900 md within the Medium Sandstone subdivision . The fourth zone, found at the base of the Mabou Group, is of interest because the modest to low permeability recorded (as high as 38 md) is higher than the surrounding rocks and is in close proximity to the caprock of future mining activity.

The permeabilities of the rocks studied in this research are believed to be dominantly controlled by secondary porosity. In the case of the permeable zone identified above the zone of gypsum-infilled fractures, relatively high porosities (9 to 15%) were interpreted from geophysical logs. These elevated porosities are due to the presence of vuggy porosity in this zone. However, no clear correlation between log-derived porosity and permeability was observed for the other permeable zones. This is believed to be due to the fact that fractures were the dominant control on porosity in these zones, and that the incremental contribution of fracture porosity to total porosity was too small to be readily identified.