Alban Souche

Thermal structure of supra-detachment basins: a case study of the Devonian basins of western Norway

We investigate the peak temperatures of the Devonian Hornelen, Kvamshesten and Solund basins in SW Norway to constrain their thermal history. These basins are the three largest Devonian units exposed in Norway and were formed as supra-detachment basins in the hangingwall of the Nordfjord Sogn Detachment Zone. The peak temperatures of the basins were obtained using a geothermometer based on Raman spectroscopy of carbonaceous material on detrital carbonaceous plant fossils. The data confirm an anchizone to low greenschist-facies metamorphism with temperatures (±30 °C) of 284–301 °C in the Hornelen and Solund basins and a significantly higher temperature, 345 °C in the Kvamshesten basin. The temperature increases toward the detachment fault and cannot be explained by ordinary burial alone. In the Kvamshesten basin this temperature increase is close to 100 °C. The new data demonstrate that exhumation of high-grade rocks in the footwall in the Nordfjord Sogn Detachment Zone played an important role in controlling temperatures in the hangingwall. We conclude that the dynamic evolution along large-scale detachments may introduce heat at the base of the hangingwall and thereby control the thermal state of supra-detachment basins formed during extension.

Storage and Transport of Magma in the Layered Crust—Formation of Sills and Related Flat-Lying Intrusions

Abstract Even though dykes are the main upward magma pathways through the Earth’s crust, the last two decades of research showed that significant parts of volcano plumbing systems consist of flat-lying igneous intrusions, namely sills. Sills form mainly in the layered parts of the crust, principally in volcanic deposits and sedimentary basins. Sills exhibit various shapes, strata-concordant, transgressive sheets, and saucer-shaped. Lateral magma flow through sill complexes and networks can reach several hundred kilometres. Sills represent intermediate feeder structures for volcanic eruptions, and therefore better understanding of sill emplacement and evolution is essential for assessing volcanic hazards. Sills emplaced in sedimentary basins also deeply affect petroleum systems and are essential components in exploring hydrocarbons. Finally, the massive and fast emplacement of sills resulting from LIPs in sedimentary basins triggered catastrophic climate changes and mass extinctions during Earths history.

Shear Versus Tensile Failure Mechanisms Induced by Sill Intrusions: Implications for Emplacement of Conical and Saucer‐Shaped Intrusions

Sills, saucer-shaped sills, and cone sheets are fundamental magma conduits in many sedimentary basins worldwide. Models of their emplacement usually approximate the host rock properties as purely elastic and consider the plastic deformation to be negligible. However, many field observations suggest that inelastic damage and shear fracturing play a significant role during sill emplacement. Here we use a rigid plasticity approach, through limit analysis modeling, to study the conditions required for inelastic deformation of sill overburdens. Our models produce distinct shear failure structures that resemble intrusive bodies, such as cone sheets and saucer-shaped sills. This suggests that shear damage greatly controls the transition from flat sill to inclined sheets. We derive an empirical scaling law of the critical overpressure required for shear failure of the sill’s overburden. This scaling law allows to predict the critical sill diameter at which shear failure of the overburden occurs, which matches the diameters of natural saucer-shaped intrusions’ inner sills. A quantitative comparison between our shear failure model and the established sill’s tensile propagation mechanism suggests that sills initially propagate as tensile fractures, until reaching a critical diameter at which shear failure of the overburden controls the subsequent emplacement of the magma. This comparison also allows us to predict, for the first time, the conditions of emplacement of both conical intrusions, saucer-shaped intrusions, and large concordant sills. Beyond the application to sills, our study suggests that shear failure significantly controls the emplacement of igneous sheet intrusions in the Earth’s brittle crust.

Three‐Dimensional Failure Patterns Around an Inflating Magmatic Chamber

Bedrock failure around an inflating magma chamber is an important factor that controls the occurrence of volcanic eruptions. Here, we employ 3D numerical models of elastoplastic shear failure around an inflating crustal reservoir, to study how the induced failure patterns depend on the geometry of the chamber, on the host rock strength and on the gravitational field. Our simulations show that either localized of diffuse plastic failure domains develop in 3 stages. Failure initiates (stage 1) after a critical overpressure is reached, the value of which depends on effective host rock strength. Next, and with increasing applied overpressure, either distributed (for zero friction angle) or localized plastic failure zones (for 30 ◦ friction angle) form (stage 2), until they finally connect to the surface (stage 3). Cylindrical chambers develop prismatic shear zones that simultaneously merge from the surface and chamber walls. For spherical and prolate chambers, diffuse conical zones of failure develop from the chamber’s crest, whereas for oblate symmetrical chambers, shear bands initiate at the horizontal tips but bend back above the center of the chamber to reach the surface. In contrast for asymmetrical oblate chambers, shear bands initiate in their cylindrical section and vanish along the elongated direction. Here, magmatic fluids may migrate both through diffuse elastic dilation zones at the tips, and through localized shear zones from the crest. Our results thus suggest that natural observations may be used to constrain the mode of failure occurring underneath a volcano. We discuss several natural examples in this context.

Interrelation between surface and basement heat flow in sedimentary basins

Petroleum system models (PSM) result critically depend on the computed evolution of the temperature field. As PSM typically only resolve the sedimentary basin and not the entire lithosphere, it is necessary to apply a basement heat flow boundary condition inferred from well data, surface heat flow measurements, and an assumed tectonic scenario. The purpose of this paper is to assess the use of surface heat flow measurements to calibrate basin models. We show that a simple relationship between surface and basement heat flow only exists in thermal steady-state and that transient processes such as rifting and sediment deposition will lead to a decoupling. We study this relationship in extensional sedimentary basins with a 1D lithosphere-scale finite element model. The numerical model was built to capture the large-scale dynamic evolution of the lithosphere and simultaneously solve for transient thermal processes in basin evolution, such as sedimentation, compaction-driven fluid flow, and seafloor temperature variations. Our analysis shows that several corrections need to be applied when using surface heat flow information for the calibration of basement heat flow in PSM. Not doing so can lead to significant errors of up to 30-50 °C at typical petroleum reservoir and source rock depths. We further show that resolving sediment blanketing effects in basin modeling is crucial, with the thermal impact of sediment deposition being at least as important as rifting-induced basement heat flow variations.

Models for Calcium Carbonate Precipitation in the Near-Well Zone by Degassing of CO2

Abstract: Calcium carbonate scale formation is a well known problem for water producing wells. Although there are several types of scale forming processes, we investigate the case of calcium carbonate precipitation when the degassing of CO2 causes the calcium equilibrium concentration to decrease towards a production well. We study a simplified system of carbonate chemistry, which allows for analytical expressions for the porosity loss as a function of time. The precipitation process normally goes from flow-limited away from the well to precipitation-limited close to the well. We derive an expression that estimates the transition zone between these two regimes. Furthermore, we present analytical estimates for the porosity reduction at a given radius as a function of time, including an estimate for each of these precipitation regimes. These analytical results are tested against numerical solutions for the porosity loss, which account for the full set of equations of the model. The analytical models give an accurate estimate of the linear porosity reduction with time, until at least half the porosity is lost. Examples of scale formation are given for the two regimes. Reasonable values for the precipitation kinetics indicate that most production operations have a kinetics-limited regime close to the well. The models also show that this type of scale formation takes place very close to the wells, typically within a few well radii from the walls of the well.

Interpretation of Microseismicity from Hydraulic Fracture Modelling

We have presented a finite-element based model for simulation of hydraulic fracturing. Each element is assigned strength in terms of a strain limit, and each element may have its individual strength. The element fractures when the strength limit is exceeded. One or more elements may break in one event. We show a 2D example of hydraulic fracturing of a low-permeable rock, where the fracture geometry is obtained. The bottom hole pressure is computed, which shows the pressure drops that follows each event. Finally, the magnitude of the fracture events is plotted in both space and time, which can be compared with the data from micro-seismic monitoring. A calibration of the model may provide effective parameter values for the rock. The presented formalism can also be coupled to reservoir simulators and calibrated against rupture propagation theories.