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AAPG Bulletin, V.
Numerical modeling of reflux dolomitization in the Grosmont platform complex (Upper Devonian), Western Canada sedimentary basin
1School of Geographical Sciences, University of Bristol, United Kingdom; present address: ExxonMobil Upstream Research Company, P.O. Box 2189, Houston Texas 77027; [email protected]
2School of Geographical Sciences, University of Bristol, United Kingdom
3School of Earth Sciences, University of Bristol, United Kingdom
4Department of Earth and Atmospheric Sciences, University of Alberta, Canada
5Department of Earth and Atmospheric Sciences, University of Alberta, Canada
Gareth D. Jones is a geoscientist at ExxonMobil Upstream Research. Following assignments in exploration and production, he is currently researching and teaching aspects of carbonate reservoir characterization and predictive diagenesis. He has an M.Sc. degree in hydrogeology and received his Ph.D. from the University of Bristol, where he studied numerical modeling of fluid flow in carbonate platforms.
Peter L. Smart is a professor in the School of Geographical Sciences, University of Bristol. His research focuses on the geochemistry and hydrology of modern carbonate terrains in both ancient and Cenozoic limestones and development of predictive models for carbonate diagenesis and paleokarst development in the geological record.
Fiona F. Whitaker is a lecturer at the Department of Earth Sciences at Bristol University. Her research focuses on the role of ground water in geological processes, specifically, the hydrogeological, diagenetic, and sedimentary evolution of carbonate rocks using process-based field and experimental studies of modern carbonates and numerical modeling techniques.
Benjamin J. Rostron is an associate professor in the Department of Earth and Atmospheric Sciences, University of Alberta. His research area is petroleum hydrogeology, the application of hydrogeological and hydrochemical principles/techniques to petroleum exploration. Research topics include regional ground-water flow, numerical modeling, hydrocarbon migration/entrapment, paleohydrogeology, brine chemistry, and stable isotopes.
Hans G. Machel is a professor at the Department of Earth and Atmospheric Sciences, University of Alberta. His research involves carbonate/evaporite facies and diagenesis, low-temperature geochemistry, and petroleum geology of Alberta, particularly dolomitization, cathodoluminescence, and diagenetic redox processes relevant to sour gas, sulfur and MVT-sulfide deposits, and magnetic exploration for hydrocarbons.
This study forms part of the Ph.D. thesis of G.D. Jones funded by a University of Bristol scholarship and in part by the Natural Sciences and Engineering Research Council of Canada operating and equipment grants to B. Rostron. We thank Harald Huebscher for discussions and a core inspection and Laura Parnell at ExxonMobil for drafting the figures. ExxonMobil Upstream Research financed the reproduction of the color figures. Colin North and an anonymous reviewer provided insightful reviews.
The Upper Devonian Grosmont platform in the Western Canada sedimentary basin is a pervasively dolomitized giant heavy-oil reservoir with reserves of 317 billion bbl of bitumen. The principal type of Grosmont platform dolomite formed early and on the basis of stratigraphic and geochemical evidence is interpreted as early diagenetic reflux dolomite.
We use a numerical ground-water flow model to investigate the viability of reflux to dolomitize the Grosmont platform. We simulate reflux at four key stages of platform evolution, incorporating the transient effects of changes in platform architecture, rock properties, and the salinity of platform-top waters.
The pattern and magnitude of reflux is critically controlled by permeability and the distribution of platform-top brines, which are concentrated up to gypsum saturation. Reflux flow is focused in the relatively permeable carbonates of the Grosmont Formation and is from the platform interior toward the platform margin. The 120-m-thick shales of the Ireton Formation that separate the Grosmont and Cooking Lake formations restrict cross-formational flow and brine transport. During a 100-k.y. period of relative sea level rise and platform-top drowning, brines of reflux origin continue to sink and entrain platform-top waters (latent reflux). Where the intervening aquitards are thin or absent, reefs of the Leduc Formation capture reflux brines from the overlying Grosmont platform and focus cross-formational brine transport. Lateral contrasts in salinity are sufficient to drive a series of free convection cells in the relatively permeable reefs of the Leduc Formation.
Computed distributions of fluid flux in conjunction with magnesium mass-balance calculations that incorporate the range of uncertainty, particularly in permeability, support the suggestion that the reflux of gypsum-saturated brines could have formed much if not most of the dolomite in the Grosmont Formation in the 1.6 m.y. available.
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