Modeling fluid flow in sedimentary basins with sill intrusions: Implications for hydrothermal venting and climate change

Abstract

[1] Large volumes of magma emplaced within sedimentary basins have been linked to multiple climate change events due to release of greenhouse gases such as CH 4 . Basin-scale estimates of thermogenic methane generation show that this process alone could generate enough greenhouse gases to trigger global incidents. However, the rates at which these gases are transported and released into the atmosphere are quantitatively unknown. We use a 2D, hybrid FEM/FVM model that solves for fully compressible fluid flow to quantify the thermogenic release and transport of methane and to evaluate flow patterns within these systems. Our results show that the methane generation potential in systems with fluid flow does not significantly differ from that estimated in diffusive systems. The values diverge when vigorous convection occurs with a maximum variation of about 50%. The fluid migration pattern around a cooling, impermeable sill alone generates hydrothermal plumes without the need for other processes such as boiling and/or explosive degassing. These fluid pathways are rooted at the edges of the outer sills consistent with seismic imaging. Methane venting at the surface occurs in three distinct stages and can last for hundreds of thousands of years. Our simulations suggest that although the quantity of methane potentially generated within the contact aureole can cause catastrophic climate change, the rate at which this methane is released into the atmosphere is too slow to trigger, by itself, some of the negative d 13 C excursions observed in the fossil record over short time scales (<10,000 years).

Distinct Degassing Pulses During Magma Invasion in the Stratified Karoo Basin—New Insights From Hydrothermal Fluid Flow Modeling

“…Iyer et al (2013) showed that varying the density of methane-bearing fluids does not significantly affect the methane release rates at the surface, at least in single-phase flow models such as ours. We further assume that all released methane is dissolved in the pore fluid without affecting the fluid's physical and thermodynamic properties.…”

“…This table supports Figure 2.B starting with the stratigraphy on the left, followed by sill geometry and depth levels. Each sill in our simulations are considered to have a thickness of 100 m, the most common thickness considered for the Karoo sills (e.g., Aarnes et al, 2010;Iyer et al, 2013). Note that consistent with field observations, the Stormberg Group do not host any major sills (e.g., Chevallier & Woodford, 1999).…”

“…The goal is to precisely quantify the amount and rates at which thermogenic gases are mobilized and released by venting into the ocean and/or atmosphere. To provide a potential basin-scale degassing estimate, the surface area of sill intrusions in the sedimentary basin of interest, for example,~85,000 km 2 in the Vøring and Møre Basins (offshore Norway, Svensen et al, 2004) and~370,000 km 2 in the Karoo Basin (South Africa, Svensen et al, 2017), is used to upscale the quantitative results obtained from a single cooling sill model (e.g., Aarnes et al, 2010;Iyer et al, 2013). A calibrated kerogen cracking model considering the breaking of different kerogen bounds (Sweeney & Burnham, 1990) allows to calculate the amount of thermogenic gas that is converted from the total organic carbon (TOC) present in the host-rock.…”

“…A calibrated kerogen cracking model considering the breaking of different kerogen bounds (Sweeney & Burnham, 1990) allows to calculate the amount of thermogenic gas that is converted from the total organic carbon (TOC) present in the host-rock. Subsequent studies that additionally accounted for heat transport by hydrothermal fluid flow (Iyer et al, 2013(Iyer et al, , 2017 have estimated that the total amount of thermogenic gas mobilized by organic cracking and emitted at the top of these intruded sedimentary basins is sufficient to trigger an environmental crisis. To provide a potential basin-scale degassing estimate, the surface area of sill intrusions in the sedimentary basin of interest, for example,~85,000 km 2 in the Vøring and Møre Basins (offshore Norway, Svensen et al, 2004) and~370,000 km 2 in the Karoo Basin (South Africa, Svensen et al, 2017), is used to upscale the quantitative results obtained from a single cooling sill model (e.g., Aarnes et al, 2010;Iyer et al, 2013).…”

3D structure and formation of hydrothermal vent complexes at the Paleocene-Eocene transition, the Møre Basin, mid-Norwegian margin

“…The presence of long-lived seepage through vents from underlying strata Iyer et al, 2013) could also have contributed to the reduced compaction of the vent structure. If these fluids migrated and were trapped in the upper vent structure by the deposition of overlying sealing sediments, overpressure could have developed, which in turn may have inhibited further compaction.…”