ALBERT

Description: This study is an investigation of aquitard characteristics and hydrocarbon entrapment in the Upper Devonian strata of the Bashaw area of Alberta, Canada. Oil and gas are trapped at two stratigraphic levels, the Leduc-aged Bashaw Reef Complex (D-3) and the Camrose Member-Nisku Formation (D-2), which are separated by a low-permeability aquitard of the Ireton Formation, a marl with variable carbonate content. The Ireton aquitard provides the principal control to cross-formational fluid flow in the area. Over much of the Bashaw Reef Complex, the Ireton aquitard ranges from approximately 25 m (~82 ft) thick to less than 1 m ( 32.8 ft) of Ireton aquitard. Where the overlying Ireton aquitard drape is less than 10 m (

Description: The origin and residence time of brines in the Alberta Basin have been debated for more than 40 years, with conflicting conclusions reported by geochemical and hydrogeologic studies. Here, numerical models were used to determine hydrogeologically feasible scenarios for the origin of brines in the Alberta Basin, using salinity and Cl/Br ratios as geochemical constraints. The models simulated variable-density fluid flow, heat transport, solute transport, and sediment compaction and decompaction in the Alberta Basin over the last 100 m.y. Simulation results suggest that pore fluids in this basin represent a mixture of four geochemical end members: seawater, freshwater, brines formed by evaporation of seawater, and, contrary to prior interpretations of Cl/Br ratios, brines derived from halite dissolution. Sensitivity studies revealed that similar distributions of salinity and Cl/Br ratios could be obtained without dissolution of halite if extremely low permeabilities were used in the model, but this scenario conflicts with field-based permeability data and prior simulations of petroleum migration in the basin. The residence time of brines in the Alberta Basin has thus likely been overestimated. The presence of evaporites introduces significant uncertainty in the use of Cl/Br ratios for interpreting the origin of brines in sedimentary basins, but salinity and Cl/Br ratios provide valuable new constraints for regional-scale models.

Description: This study is an investigation of aquitard characteristics and hydrocarbon entrapment in the Upper Devonian strata of the Bashaw area of Alberta, Canada. Oil and gas are trapped at two stratigraphic levels, the Leduc-aged Bashaw Reef Complex (D-3) and the Camrose Member–Nisku Formation (D-2), which are separated by a low-permeability aquitard of the Ireton Formation, a marl with variable carbonate content. The Ireton aquitard provides the principal control to cross-formational fluid flow in the area. Over much of the Bashaw Reef Complex, the Ireton aquitard ranges from approximately 25 m (~82 ft) thick to less than 1 m (〈3.3 ft) over paleotopographic highs. Associated with thinning is a change in lithofacies and carbonate content from approximately 50% carbonate in the thicker, more basinal facies to more than 80% carbonate and shallower-water facies over the paleotopographic highs of the Leduc. Early replacement dolomitization, probably by density-driven reflux, generated intercrystal porosity on the order of 3 to 4% in the more carbonate-rich facies, which rendered these parts of the aquitard less effective to later hydrocarbon retention. Subsequent differential compaction of the underlying reef complex led to a raised rim morphology and, in places, to a thickness inversion of the Ireton aquitard, that is, a thicker Ireton aquitard draped over the Leduc raised rims. As a result, petroleum migration occurred primarily along the raised rims and was trapped within the present-day highs that are draped by more than 10 m (〉32.8 ft) of Ireton aquitard. Where the overlying Ireton aquitard drape is less than 10 m (〈32.8 ft) in thickness, breaching and remigration from Leduc traps into the overlying Nisku-Camrose traps may have occurred and/or can be suspected. Invariably, the Ireton aquitard was breached where its carbonate content exceeded approximately 80% and where it is less than 4 m (13.1 ft) thick. The critical controls to hydrocarbon breaching are pervasive dolomitization of the Ireton aquitard with high carbonate content, which enhanced porosity and permeability, and aquitard thickness. Many Nisku and Camrose pools are situated either directly above or updip from such breaches. Mark R. Hearn is a senior geologist at Talisman Energy, Inc., currently seconded to the Troung Son JOC in Ho Chi Minh City, Vietnam. He received his B.Sc. degree in geology from Southampton University, United Kingdom, in 1993 and his M.Sc. degree from the University of Alberta in 1996. After a short contract with the Geological Survey of Canada from 1996 to 1998, Mark has since worked as an exploration geologist for Shell Canada (1998–2004) and for Talisman Energy from 2004 onward. Hans G. Machel is a professor at the Department of Earth and Atmospheric Sciences, University of Alberta. His research involves carbonate/evaporate facies and diagenesis, low-temperature geochemistry, and petroleum geology, particularly dolomitization, diagenetic redox processes relevant to sour gas reservoirs, and karstification. He is a member of 10 professional organizations and associate editor of two international journals. Ben Rostron is a professor in the Department of Earth and Atmospheric Sciences at the University of Alberta. He obtained a B.A.Sc. in geological engineering from the University of Waterloo (1986), and an M.Sc. (1990) and Ph.D. (1995) in geology from the University of Alberta. His research interests are in the area of large-scale fluid flow; petroleum hydrogeology; regional groundwater flow; hydrochemistry; numerical modelling; and geological carbon sequestration.

Description: 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. 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.

Description: Banded iron formations are critical to track changes in Archaean to Palaeoproterozoic ocean chemistry, with deposition triggered by water column iron oxidation. Recently, however, it was suggested that reduced iron minerals were the primary precipitates, and these were subsequently oxidized by oxygen-bearing groundwater. If true, this would cast doubt on our understanding of how banded iron formations were deposited and their ability to record early ocean chemistry. Here we present a hydrogeological box model, based on the approximately 2.5 billion year old Hamersley Basin of Western Australia, developed to evaluate the plausibility of secondary iron oxidation. The box model calculates the time required for groundwater to flux enough oxygen through the basin to oxidize a given amount of ferrous iron. Less than 9% of nearly four million model iterations returned oxidation times less than the age of the basin. Successful simulations required simultaneously steep hydraulic gradients, high permeability and elevated oxygen concentrations. Our simulations show that the postdepositional oxidation of large banded iron formations is unlikely, except on a limited scale (that is, during secondary ore formation), and that oxidized iron phases were probably the precursor to large Palaeoproterozoic banded iron formations. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.