Regional-scale Hydrostratigraphic Analysis of the Paskapoo Aquifer Using Normalized Gamma-ray Logs

The Paskapoo Formation is the major groundwater aquifer system in western Alberta, Canada. The aquifer system covers approximately 65,000 km2, has over 69,000 groundwater wells completed in it, has a maximum thickness of 800 m and consists of a fluvial mudstone and sandstone complex. The formation is divided into the upper Dalehurst Member, the mud dominated Lacombe Member and the basal Haynes Member. In order to develop hydrostratigraphic models of the Paskapoo Fm., it is necessary to understand the distribution and connectivity of the sandstone remnants. Mapping these features is a particularly difficult problem given the size of the region and the complexity of fluvial systems. In 2006 the Government of Alberta mandated that all wire line log readings, including natural gamma-ray logs, must be collected to surface. Tens of thousands of new exploration and production wells have been drilled and logged since 2006. Since wire line logs are in the public domain, it has created a large source of new data for mapping shallow aquifer systems. However, much of the shallow data of interest is collected through casing. A gamma-ray normalization algorithm has been developed to approximately correct for the dampening effects of the casing so that the cased and uncased API log values are equivalent. The algorithm is based on estimating the second and ninety-fifth percentiles of the API values in uncased zones in wells with similar geology to the cased zones that are to be corrected. The API values of cased zones are then adjusted so that their second and ninety-fifth API percentiles are the same as the uncased values. Mudstone and sandstone zones in the logs can be discriminated and the continuity of sandstone units can be investigated. The Haynes member has been previously described as made up of set of stacked sandstone channels. Logs show that the Haynes member is a thick sandstone unit up to 100 m thick and extending over 100 km perpendicular to the Rocky Mountain's thrust front. Logs also show that the Haynes Member is absent in the northern Paskapoo Fm. The overlying Lacombe member is clearly distinguishable from the Haynes member and it consists of smaller channel sandstone remnants which cannot be correlated over large distances, consistent with its description of a mud dominated system. Areas of higher and lower sand fraction are evident in the records. Mapping sand fractions is extremely important as it exerts first order control on regional hydraulic conductivity, vertical conductivity and connectivity within the aquifer. The normalized gamma logs offer a powerful new tool for regional aquifer characterization in Alberta.