Hydrothermal fluid flow within a tectonically active rift‐ridge transform junction: Tjörnes Fracture Zone, Iceland

Abstract

[1] We investigate the regional fluid flow dynamics in a highly faulted transform area, the Tjörnes Fracture Zone in northern Iceland which is characterized by steep geothermal gradients, hydrothermal activity, and strong seismicity. We simulate fluid flow within the Tjörnes Fracture Zone using a high-resolution model that was based on the available geological and geophysical data and has the aim to represent the complex geological structures and the thermodynamical processes that drive the regional fluid flow in a physically realistic way. Our results show that convective heat flow and mixing of cold and saline seawater with deep hydrothermal fluids controls the large-scale fluid flow. The distribution of faults has a strong influence on the local hydrodynamics by focusing flow around clusters of faults. This explains the nature of isolated upflow zones of hot hydrothermal fluids which are observed in the Tjörnes Fracture Zone. An important emergent characteristic of the regional fluid flow in the Tjörnes Fracture Zone are two separate flow systems: one in the sedimentary basins, comprising more vigorous convection, and one in the crystalline basement, which is dominated by conduction. These two flow systems yield fundamental insight into the connection between regional hydrothermal fluid flow and seismicity because they form the basis of a toggle switch mechanism that is thought to have caused the hydrogeochemical anomalies recorded at Húsavik before and after the 5.8 M earthquake in September 2002.

Improving the Estimation of Porosity and Permeability Distribution by Linking Basin Modelling to Diagenetic Evolution of Carbonate Reservoirs

“…In many sedimentary basins, convection in the fault zones drive large-scale fluid fluxes (e.g. Lewis and Couples, 1999;Matthäi et al, 2004;Davies and Smith, 2006;Wilson et al, 2007;Lupi et al, 2010;Saller and Dickson, 2011;Souche et al, 2013). Although PetroMod TM provides a complete set of equations to calculate the temporal thermal evolution of a sedimentary basin, it has some limitations for the systems considered here.…”

“…CSMP++ comprises algorithms for modelling the transient evolution of heat and brine and has been applied to a wide range of physical problems, including but not limited to fluid flow in sedimentary basins (e.g. Matthäi and Belayneh, 2004;Lupi et al, 2010Lupi et al, , 2011Crutchley et al, 2010;. CSMP++ solves the standard equations for single-phase heat and mass transfer by means of a combined finite elementfinite volume method :…”

“…This will provide us with better locations and more realistic geometries of the faults, which are potentially key controls on the basinscale fluid flow (e.g. Lewis and Couples, 1999;Mätthai et al, 2004;Davies and Smith, 2006;Wilson et al, 2007;Lupi et al, 2010;Saller and Dickson, 2011;Souche et al, 2013). The overall integration of the different software packages envisioned here is shown in Fig.…”

“…We base the latter part our new workflow on a large body of research that has investigated convective and diffusive heat and mass transfer in large-scale sedimentary basins: Bjørlykke et al (1988) pointed out, in their mathematical calculations of thermal convection, that convective flow in sedimentary basins is of minor importance as a mechanism for transport of solids in solution and for diagenetic reactions, except where there are high lateral temperature gradients (i.e. igneous intrusion or around salt dome or active faults); Fleming et al (1998) investigated the large thermal convective systems in steep fault zones within the North Sea; Lewis and Couples (1999) identified the relationship between Carboniferous Zn-Pb mineralisation in central Ireland that involves crustal convection-hydrothermal systems focused at faults that controlled the upwelling fluid plumes; Esteban and Taberner (2003) pointed out the relationship between secondary porosity as a result of interaction, during deep burial diagenesis, between brines and carbonate reservoirs; Machel (2004) pointed out that burial dolomitisation needs advection and convection; Whitaker et al (2004), Davies and Smith (2006) and Saller and Dickson (2011) investigated the conditions for hydrothermal dolomitisation and hydrothermal fluids; Wilson et al (2007) related the geothermal convection to dolomitisation in carbonate platforms; Lupi et al (2010) demonstrated that large-scale fluid flow in the Tjörnes Fracture Zone (offshore North Iceland), is probably controlled by convective heat flow and mixing of cold and saline seawater. To reiterate, our aim is to link basin fluid and heat movements, similar in scale to hydrocarbon migration and other structurally focussed fluxes to potential causes of burial diagenesis in carbonate reservoirs.…”

“…To achieve this aim, we use a basin modelling software (PetroMod TM ) to model the large-scale structural and sedimentary features of the evolving basin and use the resulting geometries to simulate heat and fluid mass transfer through them using an unstructured finite elementfinite volume simulator (CSMP++, Matthäi et al, 2007). CSMP++ is a specialised simulation software that allows us to simulate flow and transport processes in 2D and 3D high-resolution models of structurally complex geological reservoirs and basins (e.g., Matthäi et al, 2004;Lupi et al, 2010;Geiger and Matthäi, 2013;Crutchley et al, 2013). The paper is organised as follows: in the next section we describe the geologic setting (i.e.…”