NUMERICAL ANALYSIS OF SEAWATER CIRCULATION IN CARBONATE
PLATFORMS: II. THE DYNAMIC INTERACTION BETWEEN
GEOTHERMAL AND BRINE REFLUX CIRCULATION
GARETH D. JONES*
†
, FIONA F. WHITAKER**, PETER L. SMART* and
WARD E. SANFORD***
ABSTRACT. Density-driven seawater circulation may occur in carbonate platforms
due to geothermal heating and / or reflux of water of elevated salinity. In geothermal
circulation lateral contrasts in temperature between seawater and platform groundwa-
ters warmed by the geothermal heat flux result in upward convective flow, with colder
seawater drawn into the platform at depth. With reflux circulation, platform-top waters
concentrated by evaporation flow downward, displacing less dense underlying ground-
waters. We have used a variable density groundwater flow model to examine the
pattern, magnitude and interaction of these two different circulation mechanisms, for
mesosaline platform-top waters (50 ‰) and brines concentrated up to saturation with
respect to gypsum (150 ‰) and halite (246 ‰). Geothermal circulation, most active
around the platform margin, becomes restricted and eventually shut-off by reflux of
brines from the platform interior towards the margin. The persistence of geothermal
circulation is dependent on the rate of brine reflux, which is proportional to the
concentration of platform-top brines and also critically dependent on the magnitude
and distribution of permeability. Low permeability evaporites can severely restrict
reflux whereas high permeability units in hydraulic continuity enhance brine transport.
Reduction in permeability with depth and anisotropy of permeability (kv << kh)
focuses flow laterally in the shallow subsurface (<1 km), resulting in a horizontally
elongated brine plume. Aquifer porosity and dispersivity are relatively minor controls
on reflux. Platform brines can entrain surficial seawater when brine generating
conditions cease but the platform-top remains submerged, a variant of reflux we term
“latent reflux”. Brines concentrated up to gypsum saturation have relatively long
residence times of at least 100 times the duration of the reflux event. They thus
represent a long-term control on post-reflux groundwater circulation, and conse-
quently on the rates and spatial patterns of shallow burial diagenesis, such as dolomiti-
zation.
introduction
Groundwater circulation in carbonate rocks is important for understanding topics
as diverse as management of groundwater resources (Oberdorfer and Buddemier,
1988), transport of contaminants (Muldoon and others, 1998) and hydrocarbon
migration (Tator and Hatfield, 1975). Groundwater circulation is also an important
control on carbonate diagenesis, which impacts subsequent flow via modifications to
the porosity and permeability network. Of particular significance is the role of
groundwater circulation in the formation of secondary replacement dolomites, which
remains enigmatic despite its fundamental role in controlling spatial variations in
hydrocarbon reservoir quality (Amthor and others, 1994; Saller and Henderson, 1998).
There has been increasing recognition over the past decade that early diagenesis
of carbonates does not cease beneath the zone of meteoric water, and that active
circulation of sea-water derived fluids can drive diagenesis within the shallow to
intermediate burial zone (Whitaker and others, 1994). In part I we examined geother-
mal circulation (fig. 1A), which arises from variations in fluid density controlled by
*School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom
**Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom
***United States Geological Survey, MS 430, Reston, Virginia 20192, USA
†
Present address: ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77027,
USA; email: gareth.d.jones@exxonmobil.com
[American Journal of Science, Vol. 304, March, 2004,P.250–284]
250

differences in temperature between platform groundwaters and the surrounding
ocean, in isolation from other potential drives for flow (Sanford and others, 1998). In
this companion paper we consider the interaction between geothermal circulation and
reflux, a flow system driven by differences in fluid density generated by spatial
variations in the salinity. Evaporation of isolated or restricted bodies of seawater can
result in the generation of brines, that subsequently flow downwards under the
influence of gravity displacing less dense pore fluids (fig. 1B). Conceptual and
analytical models suggest that refluxing brines will continue moving downwards until a
depth at which their density equals that of the surrounding sea-water, and then flow
laterally to discharge from the platform margins (Simms, 1984; Whitaker and Smart,
1993).
Reflux circulation was first proposed by Adams and Rhodes (1960) to explain
replacement dolomitization of Permian reefs in West Texas, which show a close
stratigraphic association with bedded evaporites. Reflux circulation has since been
widely advocated to explain dolomitization in many carbonate platforms, particularly
those proximal to hypersaline environments (see for example, Fisher and Rodda,
1969; Moore, and others, 1988; Shields and Brady, 1995; Pomar and Ward, 1999).
Adams and Rhodes (1960) developed the original model of reflux from associa-
tions in the rock record rather than observation of an active flow system. A number of
more recent hydrological studies confirm that reflux operates in nature at a range of
Fig. 1. Conceptual models of density-driven circulation in half a symmetrical isolated carbonate
platform, (A) geothermal circulation, (B) reflux circulation.
251G. D. Jones and others