Low-flux, tectonically-induced squeegee fluid flow (“hot flash”) into the Rocky Mountain Foreland Basin

Abstract

This study investigates paleofluid flow through the Devonian carbonate aquifers in the deep part of the western Alberta Basin, using the distribution and geochemical composition of deep burial, late diagenetic, sparry calcite and dolomite cements, as well as the Precambrian metasediments that make up the underlying basement and much of the adjacent Rocky Mountain Main Ranges. The objective of this study is to characterize regional, tectonically-induced (squeegee-type) fluid flow in these Devonian aquifers, the fluxes and thermal anomalies, their timing and duration, as well as their possible influence on hydrocarbon migration and trapping. New data from the Southesk-Cairn complex and the surrounding shales show intriguing patterns. Firstly, the data define the regional (background) value of the MAximum Sr Isotope Ratio of BAsinal Shale (MASIRBAS) at 0.7120. Comparison with literature data shows that MASIRBAS is valid for the entire stratigraphic section from the Pleistocene glacial till down to at least the Middle Devonian, probably also to the base of the Cambrian. Consequently, any carbonate (or sulphate) cement with a 87 Sr/ 86 Sr-ratio of >0.7120 is likely to have formed from formation waters with a basin-external (extra-basinal), metamorphic component that is derived from Precambrian rocks. Many carbonates close to the deformed belt host sour gas pools that are hydrologically relatively isolated at present. However, highly elevated 87 Sr/ 86 Sr-ratios (up to 0.7370) indicate sublateral injection of fluids from a basin-external, probably metamorphic source during deep burial diagenesis in some pools close to the deformed belt, such as at Obed. At other locations, such as Simonette or Kaybob-South, basin-external fluids appear to have been injected via subvertical faults from below, with highly radiogenic Sr derived from the metasedimentary basement or sublaterally via the intervening Cambrian clastics. The fluxes and possible geothermal anomalies during injection of these external fluids appear to have been low, as estimated from the volumes of late calcites with elevated 87 Sr/ 86 Sr-ratios and from their fluid inclusion homogenization temperatures. Reasons for the low fluxes include low volumes of expelled fluids and a relatively poor hydraulic connection between the source of these fluids and the Devonian aquifers, reminiscent of an ill-fitting garden hose connection. Tectonically-induced fluid flow appears to have been laterally rather limited, extending perhaps only 100 to 200 km into the foreland basins, where the radiogenic Sr signal disappears. Another important finding is that late diagenetic carbonate cements from outcrops in thrust sheets of the Rocky Mountain Main Ranges do not exhibit the highly elevated 87 Sr/ 86 Sr-ratios found in the subsurface. These findings have several important implications. Firstly, large-scale hydrothermal dolomitization, which requires large fluxes and has been advocated recently for the deep part of the Alberta Basin and similar geologic settings elsewhere, is not possible by squeegee-type flow, unless the fluids are focused into and funnelled through relatively narrow confined aquifers. Secondly, the pervasive Laramide-aged remagnetization of much of the sedimentary sequence close to the limit of the disturbed belt in Alberta is not possible by this type of flow. Lastly, the recognition that metamorphic Sr, albeit present in the deep basin, is absent from at least some of the thrust sheets in the Main Ranges, indicates a different type of hydrologic regime during their formation.