REGIONAL-SCALE GEOTHERMAL AND HYDRODYNAMIC REGIMES IN THE ALBERTA BASIN: A SYNTHESIS

Stefan Bachu

Alberta Geological Survey, Edmonton, AB, Canada

ABSTRACT

The flow of water and heat in a sedimentary basin may be coupled through buoyancy effects, caused by temperature variations, and through heat advection by flowing formation waters. However, there are situations when the two processes can be partially or totally decoupled. Such situations are when variations in formation water salinity offset variations in water density caused by temperature differences, and when rock permeability is so low that the velocity offluid flow does not affect significantly the conduction ofterrestrial heat through the sedimentary succession. The role of heat advection versus heat conduction in a sedimentary basin can be established through either numerical or dimensional analysis, based on the geothermal and hydrodynamic characteristics of the basin or parts thereof.

The Alberta Basin in western Canada is complex in terms of hydrostratigraphy and flow off ormation waters. Several basin-scale aquitards and aquicludes separate various aquifers and aquifer systems. On a regional scale, the flow of formation waters in aquifers is driven in various systems and directions by past tectonic compression, erosional rebound in thick shales, and regional- and local-scale topography. The salinity increase with depth offsets the decrease in density resulting from temperature increase. Formation-scale rock permeability in aquifers is low, leading to formation water velocities of the order of 10.2 m/yr. Dimensional analysis shows that conduction dominates the terrestrial heat flow in the basin, except for the Middle Devonian Elk Point aquifer system at the northern edge of the basin.

The thermal conductivity of the sedimentary succession increases generally eastward, from 1.4 to 2.2 Wlm OK. The basement heat flow, calculated on the basis of rock lithology, surface and bottom-hole temperatures, and thermal conductivity measurements, increases generally northward, from 40 to 80 mW/m2• This corresponds on a basin scale to changes in basement structure from old Archean rocks in the south to younger magmatic arcs in the north. Accordingly, geothermal gradients range from less than 20 mKlm in the south to more than 45 mKlm in the north. Local-scale anomalies are superimposed on this general trend. These anomalies are the result of basement heterogeneity, variations in radiogenic heat production by basement rocks, and stratigraphic and lithologic variability caused mainly by erosion. Only

81

A. Förster et al. (eds.), Geothermics in Basin Analysis© Kluwer Academic/Plenum Publishers 1999

82 BACHU

at the northern edge of the basin, high permeability of the Elk Point aquifer system, caused by reefs, dolomitization, fracturing and karst processes, leads to focused flow along the reef barrier and to heat advection by formation waters stronger than heat conduction. The advection of heat by formation waters discharging from this aquifer at outcrop near Great Slave Lake explains the high geothermal gradients (> 70 mKlm) observed at shallow depths near the Precambrian Shield at the northeastern comer of the basin featheredge.

INTRODUCTION

Sedimentary basins are of great economic importance because of their energy and mineral resources. These are the result of a whole series of physical and geochemical processes among which the flow of formation waters and geothermal regime playa crucial role. The flow of formation waters may play an important role in the redistribution of terrestrial heat and minerals, in the migration and accumulation of hydrocarbons, and in the genesis of ore deposits. Depending on the flow strength, the geothermal regime in the basin, or parts thereof, can be significantly distorted, with cooling effects in recharge areas, and warming effects in discharge areas. On the other hand, understanding the geothermal regime in a sedimentary basin is important because of the link between temperature and the various physical and chemical processes leading to the generation of hydrocarbon resources and the genesis of Mississippi Valley and other types of ore deposits. In some situations, the temperature distribution in a basin may affect the flow of formation waters by inducing free convection in a cellular pattern. This also has an effect on redistribution of terrestrial heat and of energy and mineral resources. Thus, it is evident that the two main transport processes, fluid and heat flows, may be either intertwined or independent of each other, depending on the stage of basin evolution and its particular conditions and characteristics. Therefore, knowledge of both processes and of their interdependence, or lack of, is essential to understanding the formation of energy and mineral resources in a basin, and, therefore, it is an important exploration tool.

The geothermal regime in a sedimentary basin is influenced by the magnitude and distribution of heat sources and by the various mechanisms for transport of the terrestrial heat to the surface and their interaction. The main mechanisms for heat transfer in a sedimentary basin are conduction and convection by formation waters. If the flow of formation water is driven by external forces (forced convection or advection), then the relative importance of the two heat transfer mechanisms is indicated by the geothermal Peelet number (Bachu, 1988) defined as:

pwCwq D Gh Pe*= .-

Am Gv (1)

where A. is thermal conductivity, p is density, c is specific heat, q is the Darcy velocity ofthe water, D is the thickness of the respective aquifer, G is geothermal gradient, and the subscripts w, m, h, and v stand, respectively, for water, saturated porous medium, horizontal and vertical. The Darcy velocity is given in terms of hydraulic head (de Marsily, 1986) by:

kpwg ~p q = --- (VHo + -Vz) ~ po

(2)

where k is absolute permeability, J.l is water dynamic viscosity, g is the gravitational constant, WI" is the freshwater hydraulic gradient driving the flow of formation waters, ap is the density difference in the flow domain, and z is the vertical coordinate oriented upward.

GEOTHERMICS AND HYDRODYNAMICS OF ALBERTA BASIN 83

If the flow of formation water is driven by internal forces (buoyancy) in cellular convection, then the onset of free convection takes place when the Rayleigh number Ra* for flow in porous media, generally defined as:

(XW pw g k D ~ T (3) Ra* = cos e

JlKm I

is greater than a critical value, Rae" which depends on basin structure and boundary conditions (Nield, 1968; Ribando and Torrance, 1976; McKibbin and hvand, 1983). In relation to (3) a is the coefficient of water thermal expansion, Km is the thermal diffusivity of the saturated porous medium, II T is the temperature difference between the top and the bottom of the aquifer, and e is the angle of the aquifer with the horizontal.Where a saturated porous layer subjected to a constant temperature difference, Raer = 39.5, whereas for a porous layer subjected to a constant heat flux at the bottom, R~r= 27.1 (Nield, 1968; Ribando and Torrance, 1976).

Examination of relations (1) - (3) shows that rock permeability k is the most critical parameter in determining either the strength of forced convection relative to heat conduction, or the onset of free convection in the aquifers of a sedimentary basin. Many numerical studies of groundwater and heat flows in sedimentary basins implicitly take into account the interdependence between the two processes by simulating them, but seldom express explicitly this interdependence and the factors which affect it. On the other hand, most studies of the geothermal regime in various sedimentary basins based on observational data and their interpretation do not take into account the relation between the two processes. Yet in other situations, conclusions regarding the geothermal regime are drawn based on postulated assumptions, without ground-truthing them with data from the field of basin hydrodynamics. In such examples, wrong conclusions may be reached. However, without using sophisticated numerical simulations, just field data and dimensional analysis, it is easy to assess if indeed the two flow processes are interrelated or not.

The Alberta Basin in Canada is such an example where the geothermal regime was interpreted first in isolation, based only on temperature data and a postulated model of the flow of formation waters, as being controlled by forced convection. Subsequent geothermal and hydrodynamic studies, to be discussed in the following, have shown that the transport of the terrestrial heat to the surface is dominated by conduction. This paper showcases the Alberta Basin as an example of the importance of integrating basin hydrodynamics in the study of the geothermal regime in a sedimentary basin without having to use complex numerical models.

THE ALBERTA BASIN

The Alberta Basin, located in western Canada, is sitting on a stable Precambrian platform and is bounded by the disturbed Foothills thrust and fold belt and Rocky Mountains to the west and southwest, the Tathlina high to the north, by the Canadian Precambrian Shield to the northeast, and the Williston Basin to the east and southeast (Fig. lA). A brief basin history and geology is presented here based on Porter, Price, and McCrossan (1992) and Mossop and Shetsen (1994). The basin comprises a wedge of sedimentary rocks increasing in thickness from zero at the Canadian Shield in the northeast to close to 6000 m in the southwest (Figs. IB and 2A). The basin was initiated during the late Proterozoic by rifting of the North American craton. Subsequent thermal contraction led to the transgressive onlap of the cratonic platform from middle Cambrian to middle Jurassic time during the passive-margin phase of

84 BACHU

basin evolution. The corresponding sedimentary succession is dominated by shallow-water carbonates and evaporites, with some regional-scale intervening shales. Subsequent accretion to the western margin of North America of allochtonous terranes during the Columbian and Laramide orogenies led to isostatic flexure of the lithosphere which formed the foreland basin. Westward dipping of the passive-margin succession from middle Jurassic to early Cretaceous led to extensive erosion of eastwardly progressively older strata, which subcrop along the pre­Cretaceous unconformity (Fig. lB). During the Cretaceous and early Tertiary active-margin phase, the basin filled with synorogenic clastics, mainly shales, derived from the emerging Cordillera. Tertiary-to-Recent erosion has removed from up to 3800 m of sediments in the southwest to only 1000 m in the north. As a result ofthese depositional and erosional events, the present-day topography ranges in elevation from close to 1200 m in the southwest at the edge of the thrust and fold belt to slightly less than 200 m at Great Slave Lake in the northeastern comer of the basin near the Precambrian Shield (Fig. 2B).

B

w Cretaceous Sub·Cretaceous Unconformity

Figure 1. Major features of Alberta Basin: A, location; and B, hydrostratigraphic cross section.

E

GEOTHERMICS AND HYDRODYNAMICS OF ALBERTA BASIN 85

Figure 2. Characteristics of sedimentary succession in the Alberta basin: A, isopach; and B, ground surface (isolines in m).

PREVIOUS FLUID AND HEAT-FLOW MODEL

In the 1970's and early 1980's, the accepted model of flow offonnation waters in the Alberta Basin was that the basin-scale groundwater flow is at steady state with and driven by the present-day topography (Fig. 3). This model was based on the work of Hitch on (1969) and Toth (1963, 1978), who analyzed the distribution of hydraulic heads in various units in the basin using a limited database, and promoted the idea of gravity-driven flow of fonnation waters. In the general model of Hitch on (1969,1984), the fonnation waters in the southern and central parts of the Alberta Basin are driven in a single basin-scale flow system from the southwest to the northeast, with recharge in the Foothills and discharge in the Athabasca area near the basin edge at the exposed Precambrian Shield. In the northern part of the basin, the flow of fonnation waters, driven also by topography, is directed southwestward, toward the same discharge area in the Athabasca region. In the extreme south, topography is driving the flow of fonnation waters eastward from the Foothills to lake Winnipeg, across the Alberta Basin and the Canadian part of the Williston Basin.

Anglin and Beck (1965) were the first to note, based on a few carefully measured data, that heat generation in the Precambrian basement shows a trend of northerly increase, which they attributed to crustal structure. In the 1980s, some authors, whose most relevant papers are cited here, used only measured bottom-hole temperature (BHT) data (uncorrected first, and corrected for drilling disturbances in later papers) to analyze the geothennal pattern in the whole or various parts and stratigraphic intervals of the Alberta Basin (Majorowicz and Jessop, 1981,1993; Hitchon, 1984; Majorowicz and others, 1984, 1985; Jones, Majorowicz, and Lam, 1985). The limitations of this approach are that: (1) BHT data collected by industry are notorious for poor quality and inconsistencies (Chapman and others, 1984); (2) there are many uncertainties in correcting for thennal equilibrium, when these corrections are made; and (3) there is a great variability (scatter) in data distribution and resolution both areally and with depth because the measurements are made selectively in target areas of interest to the industry. Calculated values for geothennal gradients were distorted further by these authors by averaging gradients based on BHT data measured at various depths in stratigraphic intervals

86 BACHU

[

OCKY INTERIOR PLAINS

UNT"NS~~ ___ -------1 "",-. nallateral flow cischarge

lOCal k:Jca1 local d_ ...... oge __

-----./

A

~ Main flow directions

... B

Figure 3. Diagrammatic model of assumed steady-state topography-driven flow offonnation waters in Alberta Basin (after Hitchon, 1984): A, dip cross section; and B, plan view.

of different lithologies. In addition, some of the authors (Majorowicz and others, 1984, 1985; Jones, Majorowicz, and Lam, 1985) smoothed the results further by using an areally moving window of 3 x 3 townships (9.6 x 9.6 km2), and made the results more difficult to compare by using different data controls in different profiles, and different averaging techniques. Moreover, no documentation was provided with respect to the corrections applied, when this was done, and for the methodology used in the statistical processing of the respective data distributions. Nevertheless, regardless of the data distributions and methodology used, a general pattern of increase in geothermal gradients from the south-southwest to the north­northeast was observed by all of the mentioned authors. On the basis of the previously described hydrodynamic model, they explained this trend as the result of heat transport by vigorous flow of formation waters in a single regional-scale system across the entire sedimentary succession and basin, from recharge areas in the southwest to discharge areas in the northeast. However, neither pressure and water salinity data, nor permeability data were used in their analysis to establish the direction and quantify the strength of groundwater flow, and the relative importance of convective versus conductive heat transport. Their model of the geothermal regime in the Alberta Basin was based on a simple statistical trend analysis of uncorrected and corrected BHTs, and the postulated hydrodynamic model of Hitch on (1969, 1984). This model of convection-dominated heat transport implicitly assumes uniform or

GEOTHERMICS AND HYDRODYNAMICS OF ALBERTA BASIN 87

quasi-uniform basement heat flow. Based on available data at the time (Burwash and Cumming, 1976), Majorowicz and Jessop (1981) tried to determine a spatial correlation between the geothermal state of the sedimentary succession in the basin and basement heat production, but reached the conclusion that such a relation does not exist.

Based on corrected BHTs, and pressure and permeability data for the study of the hydrogeological and geothermal regimes in the Cold Lake and Swan Hills areas ofthe Alberta Basin, Bachu (1985, 1988) argued that the main mechanism for heat transport in the Alberta Basin is conduction and not advection by formation waters, because the regional-scale permeability values for aquifers in these areas are too low (of the order of 10.15 m2), leading to flow velocities of the order of 10.2 m/yr. At such low velocities, dimensional analysis shows that no significant heat transport by formation waters can take place (pe* < 0.1). Bachu (1985, 1988) linked regional- and local-scale variations in geothermal gradients, both vertically and areally, to the lithology of the sedimentary succession, and with assumed variations in basement heat flow. Using data of radiogenic heat production and U-Pb dates of basement rocks, combined with the concepts of heat-flow provinces and decline of heat flow with age, Jessop (1992) estimated the heat flow from the tectonic provinces of the Precambrian basement of the Alberta Basin. He noted that the derived patterns of basement heat flow show little agreement with the maps of heat flow in the Paleozoic and Mesozoic successions of the Phanerozoic cover produced previously by Majorowicz and others (1985), and pointed out to the effect of different data distributions and various methodologies used in processing and interpreting the geothermal data.

HYDRODYNAMICS OF THE ALBERTA BASIN

The stratigraphy and lithology ofthe sedimentary succession in the Alberta Basin plays an important role in determining the interplay between conductive and advective heat transfer in the basin and in establishing its geothermal regime. Extensive, basin-scale aquitards, such as the Ireton, Exshaw-Banff, and Cretaceous shales, and aquicludes, such as the Lotsberg, Cold Lake, Muskeg, and Prairie evaporites, do not allow communication between various aquifers in the sedimentary succession, except where reefs breach through these aquitards or aquicludes, or along erosional unconformities near the eastern edge of the basin. Thus, full­scale basin circulation across the entire sedimentary succession, of the type postulated initially by Hitchon (1969, 1984), is precluded. Convective heat transport can take place only in aquifers, whose thickness D ranges between several meters and up to 300 m (Mossop and Shetsen, 1994).

Until the 1990's, what was missing from the debate regarding the influence or lack thereof of advective heat transport in the Alberta Basin was a good knowledge and understanding of the directions and mechanisms driving the flow of formation waters in the basin, of the chemistry of formation waters, and of permeability distributions both areally and with depth in the hydro stratigraphic succession. As a result of several hydrogeological regional-scale studies (Hitchon, Bachu, and Underschultz, 1990; Bachu and Underschultz, 1993, 1995; Parks and Toth, 1993; and Bachu, 1995, 1997), a clear image of the basin-scale flow in the Alberta Basin emerged, totally different from the topography-driven model prevalent in the 1980's. The flow of formation waters is more complex, both areally and stratigraphically (Fig. 4). South of the Peace River Arch, several regional flow systems are active, driven in different directions by different mechanisms (Bachu, 1995). A long-range flow system is active in the Upper Devonian-to-Carboniferous succession, driven north-northeastward by basin-scale topography,

88 BACHU

from a recharge area at outcrop in Montana in the south to a discharge area in northeastern Alberta (Athabasca region) near the Precambrian Shield (Bachu and Underschultz, 1993, 1995). An underlying flow system in Cambrian and Middle Devonian aquifers, driven east­northeastward from the Rocky Mountains in the southwest to the Precambrian Shield in the east, most probably is driven by past tectonic compression during the Laramide Orogeny (Bachu, 1995). Finally, inward flow systems are present in the overlying Cretaceous and Tertiary aquifers, driven toward the thrust and fold belt by erosional rebound in thick Cretaceous and Tertiary shales (Bachu and Underschultz, 1995). Hydraulic heads in these aquifers reach values near the thrust and fold belt lower than the lowest elevation in the basin at Great Slave Lake, situated at more than 1,500 km to the northeast. The erosional shale rebound, leading to these flow systems, is the result of Tertiary-to-Recent erosion in southern Alberta of up to 3,800 m of sediments (Bustin, 1992). Local-scale flow systems are present at the top of the sedimentary succession, driven by local topography. The complex pattern of formation-water flow south of Peace River Arch shows that the flow systems did not reach yet steady state and are evolving toward reaching equilibrium with the present-day boundary conditions.

w

Local-scale lopography dn'o'(l:n Row

RegIOnal-scale rlow doveo by erOSlooal rebound

BaSin-scale lopography--drlvon flow

Basin-scale !low of leclonlc origin with sirong buoyancy cnecls

Am.basca and Cold Lake 0<1 SandS

== = MIXing zone

Basln·scale lopoglaphy dnven

A

!low normal 10 the plane of cross section

E

B

Figure 4. Diagrammatic representation of main fluid-flow systems in Alberta Basin: A, in plan view; and B, in cross section (see Fig. I for location of line of cross section W-E).

GEOTHERMICS AND HYDRODYNAMICS OF ALBERTA BASIN 89

North of the Peace River Arch, the flow pattern is simpler (Bachu, 1997), but different from the one postulated by Hitchon (1984). Namely, the flow of formation waters in the entire Phanerozoic succession is driven northeastward by regional-scale topography, from a recharge area near the fold belt and Bovie Lake fault, to discharge at aquifers outcrop at Great Slave Lake (Bachu, 1997). Because only up to I km of sediments were removed by Tertiary-to­Recent erosion (Kalkreuth and McMechan, 1988), the process of erosional rebound in the thick Cretaceous shales ended in this area, and the flow systems are at steady state with the present­day topography.

The chemistry of formation waters in the Alberta Basin (Hitchon, Bachu, and Underschultz, 1990; Bachu, 1995, 1997) shows that the connate waters in Paleozoic aquifers (corresponding to the platform-margin in basin development) are generally saline, except for small regions in recharge and discharge areas where water of meteoric origin enters the system. This indicates incomplete flushing of the original basinal waters, consistent with low flow velocities which would not distort the conductive heat regime in a sedimentary basin (Deming and Nunn, 1991). In contrast, the formation waters of meteoric origin in Mesozoic aquifers (which correspond to the foreland stage in basin evolution), are less saline. A zone of mixing (Fig. 4B) occurs along the sub-Jurassic and sub-Cretaceous unconformities (Bachu, 1995). The high salinity of Paleozoic formation waters overcomes the decrease in density caused by increased temperature with depth. Thus, the Paleozoic formation waters in the Alberta Basin are heavier than the Mesozoic waters, particularly in areas where Lower and Middle Devonian evaporite beds are present. This difference in salinity adds another component to the flow of formation waters in that buoyancy plays an important role, in this case impeding full-scale water circulation from the surface to the basement.

GEOTHERMAL REGIME IN THE ALBERTA BASIN

As described previously, the Alberta Basin is sitting on a stable crystalline Precambrian basement. No volcanic centers were active in the past nor are present in the basin. Tectonic activity in the basin also is limited (Burwash, McMechan, and Potter, 1994). According to Ross, Broome, and Miles, (1994), the basement is comprised of Archean rocks in the south, younger accretted terranes, Proterozoic plutonic and metaplutonic rocks, and magmatic arcs in the north. After the rifting in late Proterozoic which initiated the basin, and compression in Jurassic-Cretaceous time, the most recent thermal activity is related to an Eocene tectono­extensional event at 50 Ma localized at the southern edge of the basin. The effects ofthermal events older than 100 Ma have already dissipated, whereas the effects of the Eocene event, if present yet, are local and limited in nature. Based on the maximum thickness of the sedimentary cover, the characteristic time for propagation to the surface of basement thermal events is less than 3 Ma. Thus, the heat flow from the basement is at steady or quasi-steady state, depending on location. Based on basin history and current conditions, it follows that the two main sources of heat are the mantle heat flow and the heat generated internally in the crust by the decay of radioactive elements. All other heat sources are either absent or minor (Rybach, 1981; Hitchon, 1984; Bachu, 1993). Having established the main sources of heat flow in the basin, the next step in the analysis of the geothermal regime is to determine the relative importance of the two heat-transport mechanisms: conduction through the entire sedimentary succession, and convection (forced and free) in aquifers. Further evaluation of the relative importance of convective versus conductive heat transport needs estimating regional-

90 BACHU

scale representative values for the aquifer hydraulic and geothennal parameters in the expressions of the Peelet and Rayleigh numbers (relations 1-3).

Rock Permeability and Porosity

On a regional scale, the rock penneability k of aquifers in the Alberta Basin is low, of the order of 10-15_10-14 m2 (Bachu, 1985, 1988, 1997; Hitchon, Bachu, and Underschultz, 1990; Bachu and Cao, 1992; Bachu and Underschultz, 1992, 1993), notwithstanding the even lower permeability of the intervening shaly aquitards and evaporitic aquieludes. This low penneability, in conjunction with regional-scale hydraulic gradients V'H of the order of 1-10 mIkm, leads to low fluid-flow velocities, of the order of 1O-QO-1 rn/yr. Only along the Presqu'ile barrier reef in Middle Devonian carbonates along the northern edge of the basin, permeability values in the reef complexes are high (up to 10-12 m2; Bachu, 1997), leading to relatively high flow velocities (of the order of 1 m1yr), capable of focusing the flow of formation waters along the barrier reef. The porosity cp of aquifer rocks ranges between 2 and 35 % (Bachu and Underschultz, 1992,1993; Bachu, 1997).

Effective Thermal Conductivity and Heat Capacity

Based on complete logs of lithological analysis, the local thennal conductivity A,,20 of sedimentary rocks comprising fractions f; ofN lithological types An was calculated from rock thennal conductivity measurements performed at laboratory conditions for various lithologies (with values ranging between 1.2 W/m K for shales and 5.8 W/m K for anhydrite; Bachu, 1993), using the geometric average (Chapman and others., 1984; Bachu, 1991, 1993):

N

A..20 = II Anfi (4)

These values were corrected for in-situ conditions at the temperature T eC) corresponding to the burial depth according to the relation (Chapman and others., 1984):

A. = A.,.20 [293/(T +273)] (5)

Finally, the thennal conductivity A,., of the water-saturated porous medium was calculated also as the geometric average of the two components of the binary system (Chapman and others., 1984; Bachu, 1991):

(6)

after correcting the water thennal conductivity Aw for variations with temperature (depth) based on the relation (Deming and Chapman, 1988):

Aw = 0.5648 + 1.878 x 10-3 T -7.231 x 10-6 T2 for T ~ 137°C (7)

which corresponds to the temperature range encountered in the Alberta Basin. Using this methodology, surface-to-basement logs ofthennal conductivity variations with

depth were constructed for 1453 selected wells (one per township, or 9.6 x 9.6 km2) which reach the Precambrian crystalline basement and for which complete lithological logs were available (Bachu, 1993).

The heat capacity (pc)m of the saturated porous medium was calculated as the porosity­weighted average of water and rock heat capacities (Cheng, 1978):

GEOTHERMICS AND HYDRODYNAMICS OF ALBERTA BASIN 91

(pc)m = ql (pc)w + (l-ql ) (pc), (8)

where c is specific heat. Literature values were used for rock specific heat (de Marsily, 1986) and density (Daly, Manger, and Clark, 1966), resulting in values for (pc)m of the order of2.3 x 106 J/m3 K.

Dimensional Analysis

The thermal diffusivity of the saturated porous medium, K,n, was calculated according to its definition as the ratio of thermal conductivity to heat capacity. The coefficient of water thermal expansion, IXw, ranges from 0.1- 1.0 x 10-4 K-I for temperatures encountered in the Alberta Basin (de Marsily, 1986). Finally, the water dynamic viscosity /l and density Pw at in situ temperature and salinity conditions were calculated according to the values published by Kestin, Khalifa, and Correia, (1981) and Rowe and Chou (1970), respectively. They range between 300 and 1300 /lPaos for viscosity, and 990 and 1350 kglm3 for density, the former being influenced mainly by temperature and the second by salinity, particularly in the vicinity of evaporitic beds. Finally, regional-scale geothermal characteristics such as gradients Gv and Gh across and along aquifers are on the order of30 mKlm and 0.2 mKlm, respectively (Bachu, 1985, 1988; Bachu and Cao, 1992).

Using the given characteristic values for the parameters needed in dimensional analysis, the following regional-scale values were obtained for the geothermal Peelet and Rayleigh numbers representative for aquifers in the Alberta Basin:

Pe* < 0.01 and Ra* < 0.1

which indicate that heat transport by forced convection (flow of formation waters in aquifers) is negligible with respect to heat conduction through the sedimentary succession, and that aquifer permeability is too low for free convection to develop in a cellular pattern (Ra* < Ra.,). With respect to the high-permeability Presqu'ile reef barrier at the northern edge of the basin, where permeability reaches formation-scale values of 10-12 m2, the corresponding Peelet and Rayleigh numbers are of the order of:

Pe* '" 4 andRa*:;;; 1

This indicates that, although cellular free-convection flow would not develop in this aquifer, the lateral transport of terrestrial heat by the flow off ormation water is significant, at least of the same order as heat conduction, thus locally distorting the conductive geothermal pattern. It must be emphasized here that all the parameter values and dimensionless numbers Pe* and Ra* are representative for the Alberta Basin at a regional- to basin-scale. Locally, areas of high permeability, high hydraulic gradients or even high geothermal gradients may lead to local­scale disturbances ofthe conductive heat flow by convection of formation waters.

Relation between Basin Hydrodynamics and Heat Transport

The analysis of the hydrodynamic regime of formation waters in the Alberta Basin, and the dimensional analysis of convective versus conductive heat transport in the basin show that the previous basin-scale model of topography-driven flow of formation waters and convective

92 BACHU

heat transport from recharge in the southwest to discharge in the northeast is not correct for the following reasons:

(1) The velocity of formation waters is too low, because of low regional-scale permeability, such that advective heat transport is negligible in comparison with conductive heat transfer, except only for the Presqu'ile reef barrier at the northern edge of the basin. The low permeability of rocks in aquifers precludes also the onset of free convection in a cellular pattern.

(2) The rebounding Cretaceous and Tertiary strata in southwestern Alberta act as sinks for any meteoric waters recharging the basin in this region, impeding their reaching of deeper aquifers. In addition, the flow in the aquifers in this succession is oriented downdip, southwestward. Thus, these waters do not flow and carry heat on a basin scale from the southwest to northeast, regardless of their velocity (although too low).

(3) Extensive, basin-scale aquitards and aquicludes also impede deep vertical flow and penetration, hence cooling and subsequent heating, of any meteoric water recharging the basin.

(4) The salinity difference between Paleozoic and Mesozoic formation waters leads to "negative" buoyancy which also impedes the deep penetration of cool, meteoric water where it would be heated and carry out the terrestrial heat toward discharge areas in the northeast.

(5) The various basin- and regional-scale flow systems carry the formation waters in different directions, such that there is no single flow system carrying heat from the southwest to the northeast, as previously postulated.

With respect to the relation between basin-scale hydrodynamics and geothermal regime in the Alberta Basin, the main conclusion of this analysis is that the two are independent, and that the main mechanism for the transport of terrestrial heat from the basement to the surface is vertical conduction through the sedimentary succession. As a result, geothermal gradients and basement heat flow can be calculated using a simple one-dimensional conductive model, given the layered structure of the basin and the absence of significant short-scale topographic variations (like in mountain regions) which would introduce heat-refraction effects.

Geothermal Pattern and Basement Heat Flow

Average geothermal gradients across the entire sedimentary succession were calculated based on multiannual ground-surface temperatures (Fig. 5A) and temperatures at the top of the Precambrian basement (Fig. 5B). Although geothermal gradients based on temperature measurements at both ends of a stratigraphic succession do not explicitly take into account the lithological variations within that interval, they represent a true weighted-average of the geothermal gradients in each individual lithologic unit in the respective succession (Bachu, 1985). Ground-surface temperature values were calculated based on multi annual air­temperature averages recorded at climate stations across the basin, after applying a correction to account for the ground snow-cover during winter (Bachu and Burwash, 1991). The temperature distribution at the bottom of the sedimentary succession was constructed based on BHTs measured immediately below the crystalline Precambrian surface in the 1453 control wells distributed across the basin which reach the basement (Bachu and Burwash, 1991). For 1086 wells with multiple BHT readings, the formation temperature was individually estimated in each well by applying the Homer method to the raw data (Chapman and others, 1984; Bachu and Burwash, 1991). For the 367 wells with a single temperature measurement, a statistical correction with depth was applied based on regression analysis of the multiple-readings temperature measurements (BHT correction versus depth; Bachu and Burwash, 1991). Different statistical corrections apply in the southern, north-central, and northern parts of the

GEOTHERMICS AND HYDRODYNAMICS OF ALBERTA BASIN 93

basin (Bachu and Burwash, 1991). The ground-surface temperature distribution (Fig. 5A) exhibits an expected pattern consistent with latitude and altitude variations (Fig. 2B), whereas the temperature distribution at the top of the Precambrian basement (Fig. 5B) presents generally a pattern consistent with the westward dip of the basin (Fig. 2A) and local topographic variations (Fig. 2B). A few local-scale anomalies, such as the one in the southeast near the Bow Island Arch (Fig. 5B) are related to local high heat-generation by radioactive decay in the crystalline basement (Bachu and Burwash, 1991). A significant geothermal anomaly near Great Slave Lake in the northeast, where temperatures in the 40°C range were measured at shallow depths « 600 m) is the result of convective effects of heat transport by formation water in the highly permeable Presqu'ile reef barrier aquifer which discharges near the lake under a thin veneer of unconsolidated Quaternary sediments.

A B

--~'- - ------USA ..oN1',,"A USA

MONT"'~

Figure 5. Main characteristics of geothennal regime in Alberta basin: A, multiannual ground-surface temperature (OC); B, temperature distribution at top of Precambrian basement eC); C, geothermal gradients (mKlm); and D, effective thennal conductivity of sedimentary succession (W/m K)

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The distribution of geothermal gradients (Fig. 5C) shows a general trend of increasing values from less than 20 mKlm in the south to more than 40 mKlm in the north, consistent with the general trend observed by previous authors. The differences in detail between this map and previously published maps stem from using different data distributions, and different methods for BHT correction and statistical processing. Local-scale anomalies are better evidenced here than by the temperature distribution (Fig. 5B) where the temperature increase with depth plays a masking effect, and are all the result of variability in radioactive heat generation at the top of the basement (Bachu and Burwash, 1991). The geothermal anomaly near Great Slave Lake is caused by local convective heat transport in the Presqu'ile reef barrier and reaches values in the 70 mKlm range (Fig. 5C). Although all the studies to date of the geothermal regime in the Alberta Basin indicate the north-northeastward trend of increasing geothermal gradients, they differ in: (1) data distributions and methods used for processing; (2) resolution and detail; and (3) interpretation of the observed patterns.

In a steady- or quasi-steady-state conductive heat transfer problem, the temperature distribution, hence geothermal gradients, inside any domain of interest are controlled by the boundary conditions, and heat generation and variations in thermal conductivity inside the domain. For sedimentary basins, the respective boundary conditions are the temperature at the top ofthe succession and the heat flow at the bottom of the succession. In the inverse problem, the heat flow at the base of the succession can be calculated from knowledge of the top boundary condition (ground-surface temperature), distributions of geothermal gradients and thermal conductivity, and location and strength of internal heat sources caused by radiogenic decay mainly in shales. Another way to calculate the heat-flow distribution at the base of the sedimentary succession would be to calculate local geothermal gradients in the same well using BHTs from different depths in the basement, close to its top, and thermal conductivity values for the respective rocks. Unfortunately, such data are not available because of the depths involved and lack of industry interest in collecting these data.

In order to estimate the basement heat flow, the well-scale thermal conductivity of the sedimentary succession was calculated using the Bullard method (harmonic average) for the selected 1453 wells which reach the Precambrian basement, based on the thermal conductivity distribution across the sedimentary succession in each well. The resulting effective thermal conductivity of the entire sedimentary succession (Fig. 5D) ranges between 1.4 W/m K in the northwest, where shales are predominant, to 2.2 W/m K in the east, where the post-Jurassic siliciclastic cover is almost absent because of Tertiary-to-Recent erosion, and higher conductivity carbonates and evaporites dominate the sedimentary succession (Bachu, 1993). Local-scale variations in effective thermal conductivity are the result of local depositional and erosional events which affected the lithology of the sedimentary succession.

The conductive surface heat-flow density in the Alberta Basin was calculated everywhere except in the vicinity of Great Slave Lake (because of the convective heat-transport effects) based on the distribution of geothermal gradients and effective thermal conductivity. Because measurements of radiogenic heat production in the sedimentary rocks in the basin are missing, literature values (Rybach, 1988) were used to estimate the heat production in the sedimentary column and its areal distribution. The basement heat flow at the top of the Precambrian was estimated by subtracting the heat generated in the sediments from the surface heat flow. Its distribution (Fig. 6A) shows a regional-scale northward trend of increasing values from less than 40 mW/m2 in the south to values in the 50-60 mW/m2 range in the north, with local values reaching 70-80 mW/m2• This trend is consistent in a broad sense with the basement age and structure (Fig. 6B), which is comprised of old Archean rocks in the south and younger plutonic rocks and magmatic arcs in the north (Ross, Broome, and Miles, 1994). This broad

GEOTHERMICS AND HYDRODYNAMICS OF ALBERTA BASIN 95

correlation trend was noted also by Jessop (1992). A detailed statistical analysis of the degree of correlation between basement heat flow and structure is not possible because ofthe different data distributions and resolution of determining the two, that is aeromagnetic surveys and broad delineation for structure (Ross, Broome, and Miles, 1994), and point (well) data for heat flow. Local-scale thermal anomalies superimposed over this general trend are the result of basement heterogeneity (Bachu, 1993) and variable basement heat generation caused by the radiogenic decay ofU and Th, and K metasomatism (Bachu and Burwash, 1991). It is worth noting here the total dissimilitude between the regional-scale patterns of formation-water flow (Fig. 4) and geothermal regime (Figs. 5C and 6A) in the basin.

B

--""'"---,-----USA MONTANA U.SA MONTANA

0 Arcnean [Z2;l Magmallc arcs

D Accretod GSLSZ Greal Stave Lake lerranes Sheaf Zona

Figure 6. Characteristics of Precambrian basement underneath Alberta Basin: A, heat flow (mW/m2); and B, structure (after Ross, Broome, and Miles, 1994).

CONCLUSIONS

In assessing the geothermal regime in a sedimentary basin, it is important to consider all possible sources of terrestrial heat and mechanisms for heat transport to the surface. Particularly, the interplay between conductive and convective heat transport can be evaluated using either numerical simulations or dimensional analysis. The latter is particularly well suited for situations of complex hydrodynamic systems, extremely variable rock properties and a multitude of factors influencing both the hydrodynamic and geothermal regimes in a basin, situations which are too difficult to be treated numerically unless significant simplifying assumptions are made. A proper dimensional analysis is based on a good understanding of flow processes in a sedimentary basin, and on a good data set which allows a proper estimation of the numerical values ofthe parameters involved in the analysis.

Based on the wealth of data available for the Alberta Basin and on recent studies of basin hydrodynamics and geothermics, application of dimensional analysis to heat transport processes in the basin shows that:

96 BACHU

(1) Because of low rock permeability, the main, basin-scale mechanism for heat transport in the basin is conduction through the entire sedimentary succession. Only at the northern edge of the basin, high permeability of the rocks in the Presqu'ile reef barrier leads to convective heat transport in this aquifer, with the result of a significant thermal anomaly (high geothermal gradients) at aquifer discharge near Great Slave Lake.

(2) At the basin-scale, the south-northward increase in terrestrial heat flow and associated geothermal gradients most probably corresponds to changes in basement structure from older and "cooler" Archean rocks in the south, to younger and "hotter" rocks in the north.

(3) The stratigraphy and lithology of the sedimentary cover plays an important role in establishing the geothermal pattern in the basin because of variability in thermal conductivity and thickness of various strata.

(4) Local-scale anomalies in the basin-scale geothermal pattern are caused by heterogeneity in basement structure and heat generation, to local changes in the stratigraphy and lithology of the Phanerozoic cover, and to local topographic variations. Beside the thermal anomaly at Great Slave Lake, the flow of formation water does not affect the terrestrial heat flow. Local influences are possible in zones of high permeability, but these are undetectable with the existing data resolution and are insignificant on a regional scale.

This example of application of hydrodynamics, geothermics and dimensional analysis can be applied to any sedimentary basin for assessing the importance of conductive versus convective heat transport in the basin, and, consequently, of basin thermal history and maturation. Furthermore, it shows that a multidisciplinary approach, use of various data categories, and data quality and distributions play an important role in the study of flow and transport processes in sedimentary basins, and in developing sound and consistent conceptual models oftheir hydrodynamic and geothermal regimes.

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