EMRP1.18

The Alto do Sobrido (AS) and Ribeiro da Serra (RS) Mines are old Sb-Au explorations. These are located in Gondomar, Portugal, on the inverse limb of the well-known structure called Valongo Anticline. In the AS Mine, the mineralization occurs near the contact between the Schist-Greywacke Complex (CXG) (Precambrian and/or Cambrian(?)) and the breccia of the base of the Carboniferous. In the RS Mine, the mineralization occurs only on the CXG. In both mines, the Sb-Au mineralization occurs in quartz veins and some stockworks.

A spatial correlation between the Sb-Au mineralization and the post-orogenic granites occurs in the Dúrico-Beirã Region according to Gumiel & Arribas (1987). Couto et al. (2007) also acquired data that suggests a genetic connection between this mineralization and non-outcropping granites. These granites may have been the source of fluids and a heat source that improved hydrothermal circulation and they have been observed in one of the RS Mine’s galleries.

With this hypothesis in mind, we intend to compare the data from a radiometric survey, which is a method that is radiometrically sensitive to K, Th and U at the near-surface, to the data from a gravimetric survey, which is a method that is sensitive to density anomalies at greater depths, in order to show if these granites could have chemically influenced its embedding rocks.

To make this comparison, we used the residual anomaly map from our gravimetric survey and the four maps obtained in the radiometric survey (total concentrations, K, eTh and eU). Firstly, we normalized all the grid maps to obtain grids with values between -1 and 1. Once this was complete, we multiplied each of the four radiometry maps to the residual anomaly map, obtaining the comparison maps.

On the resulting maps, we can observe high values in 3 different areas. The first corresponds to a lower value of gravimetric anomaly and a lower value of concentrations of all the elements. This area is located where the hypothesized non-outcropping granites are situated. The second area corresponds to high values on both methods. This matches the location of the lithologies from the Middle Ordovician to the Carboniferous, which are rocks of higher densities and higher concentration values of K, eTh and eU. The third area consists of lower gravimetric anomalies and lower concentrations of K and eU, and coincides with the location of the Ordovician quartzites. This area isn’t as visible on the eTh map, which is consistent with what was observed on the field.

We consider this approach to be a practical method to correlate the results of these two methods and an attempt to understand how the granite located at depth could have influenced these lithologies that today outcrop.

References

Gumiel, P., Arribas, A., 1987. Antimony Deposits in the Iberian Peninsula. Economic Geology, Volume 82, pp 1453-1463.

Couto, H., Borges, F. S., Roger, G., 2007. Late Palaeozoic orogenic gold-antimony deposits from the Dúrico-Beirã area (North Portugal) and their relation with hidden granitic apexes. Ninth Biennial SGA Meeting, Dublin. pp 609-612.

How to cite: Carvalho, A., Ribeiro, R., Moura, R., and Lima, A.: Comparison Between Gravimetry and Radiometry Results: Alto do Sobrido-Ribeiro da Serra Case Study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-418, https://doi.org/10.5194/egusphere-egu22-418, 2022.

Carbon Capture Utilization and Storage (CCUS) is an essential technology to meet net-zero carbon emission targets. Due to CO₂ injection, original reservoir properties are altered. Early warning of potential CO₂ injection-induced reservoir instability depends on our correct interpretation of the geophysical remote sensing data, particularly seismic and electromagnetic datasets. Using joint elastic-electrical datasets is a proven effective approach to simultaneously characterize mineral skeleton properties and pore fluid distribution, and therefore a powerful reservoir monitoring tool for CCUS.

To interpret large-scale elastic-electrical datasets, original rock properties and fundamental CO₂-fluid-rock interactions are  preliminarily investigated by combining available well-logging data, lab-controlled experiments using rock samples, and rock physics theories; the latter two are inevitably dependent on one another. Despite we can mimic changing reservoir conditions in the lab and generate datasets that provide essential information to understand specific processes at the micro- and meso-scales (and serve as inputs for large-scale reservoir simulations), every experiment carries limitations inherent to the particular lab capabilities, together with the obvious time- and space-scale related uncertainties (i.e., core-scale experiments only partially describe the events occurring in the field). Then, we need theoretical rock physics to make experimental assumptions, and reciprocally we use the experimental data to validate models.

Physical and petrographic properties of reservoir rocks condition the degree of heterogeneity and anisotropy of the CO₂ storage unit that, in turn, influence the total storage capacity and fluid migration. Original clay content, grain size distribution, mineralogy, porosity and permeability are among the most influencing parameters, particularly for low reactive siliciclastic formations (i.e., desired CO₂ storage reservoirs). But these properties randomly change to some extent within any reservoir formation.

Here, we investigate how reservoir heterogeneity influences our geophysical interpretation of the potential CO₂ storage site Aurora, offshore Norway. Recent studies suggest high clay content and porosity variability within the Johansen Formation sandstone, Aurora’s primary reservoir. Due to lack of Johansen Fm. samples, we selected three sandstone samples from the Central Graben, Offsore UK (Forties Formation), formed in a similar depositional environment, with similar mineralogical composition, and porosity (20 to 28%), clay content (10 to 26%) and permeability (1 to 8 mD) ranges. The elastic (from P- and S-wave velocities) and transport (from permeability and resistivity) properties of the tested samples were used to assess the influence of their intrinsic properties on the pore fluid distribution during CO₂ injection and the permanent CO₂-induced changes in the Aurora reservoir complex. We apply well-known rock physics theories, including effective stress law, Archie’s relationship, and the Biot-Stoll and White and Dutta-Ode models, for both to impose the most similar reservoir conditions according to our lab limitations and to assess the experimental results. We observe (i) elastic and transport properties variations (up to 15% and 30%, respectively) between samples, mainly related to porosity differences, and (ii) more significant permanent alterations post-CO₂ injection in those with higher porosity and clay content. Our results show the importance of accounting for heterogeneity-related changes in sandstone reservoirs during/after subsurface CO₂ storage activities for enhanced geophysical interpretation.  

How to cite: Falcon-Suarez, I. H., Dale, M., and Mondol, N. H.: Implications of reservoir heterogeneity for CO2 storage monitoring: A combined experimental and theoretical rock physics study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2416, https://doi.org/10.5194/egusphere-egu22-2416, 2022.

In this study we analyze the role of model weighting functions for resistivity and potential field data in both separate and joint inversion. We show that the model weighting function built with depth weighting and compacting factor, formerly formulated for the gravity and magnetics inversion, can be useful also for DC resistivity data modelling. The comparison was made using the depth weighting with different exponents and the roughness matrix under L1- and L2-norm Constrained Optimization. We then analyze the 2-D joint inversion of DC resistivity and potential field data, based on the above model weighting function and the cross-gradients constraint. We provide a number of synthetic cases to discuss the pro and cons of each model-weighting function and to examine the feasibility of the joint inversion algorithm. We then provide results from two real case datasets for mining and archeological exploration. The results show that the value of the β exponent is decisive for potential field problems, but it also leads to a faster convergence for the resistivity data inversion. Similarly, the role of compactness is important for modelling compact source from gravity and magnetic, and to warrant an even faster and compact solution for DC resistivity. On the other hand, the results of the joint inversion reveal that the cross gradient constraints allow a successful joint inversion even when resistivity and magnetic data are often not easily comparable.

How to cite: Milano, M., Varfinezhad, R., and Fedi, M.: Joint inversion of DC resistivity and potential field data under different model weighting functions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4190, https://doi.org/10.5194/egusphere-egu22-4190, 2022.

It is known that geological reservoir characterisation can be improved by the joint modelling and inversion of both electrical and elastic data, however the relationship between a rock’s electrical and elastic properties, which is intrinsic to these methods, is relatively uncertain. On top of this, estimating reservoir pressure from geophysical measurements is an essential part of the 3D and 4D monitoring of CO₂ injection and hydrocarbon production, and while electrical and elastic properties are affected by pressure, the effect of pressure on electrical-elastic relations is less obvious.

Here we use the Cross-Property Differential Effective Medium approximation to model public-domain electrical-elastic laboratory measurements made on brine-saturated clean and mixed sandstones cores at 6 effective pressures ranging from 8 MPa to 60 MPa. Although the approximation is able to realise a large proportion of the electrical-elastic space bounded by the Hashin-Shtrikman bounds using a range of permissible parameter values, we find the model parameter, equivalent pore aspect ratio, varies very little as a function of pressure when modelling the measured data. Interestingly, we see equivalent pore aspect ratio changes exponentially as a function of pressure with an R² value of over 0.99 when modelling clean sandstones, a trend which has been observed previously in single-property inclusion modelling. This small variance in the model parameter as a function of pressure corresponds to an observably small change in the samples’ electrical-elastic measurements with pressure.

We conclude the electrical-elastic properties of the examined clean and mixed brine-saturated sandstones are only weakly dependent on pressure and we demonstrate how a single, pressure-independent model parameter is able to model the electrical-elastic measurements of both the clean and mixed sandstones with reasonable accuracy over the full range of experimental pressures.

How to cite: Cilli, P. and Chapman, M.: Assessing the pressure (in)dependence of cross-property relations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9251, https://doi.org/10.5194/egusphere-egu22-9251, 2022.

Postseismic relaxation after large earthquakes induces transient deformation of the solid Earth, particularly in the deeper part of the crust. The deformation of the upper and lower crust are mainly controlled by the rheological behavior of quartz and feldspar, respectively. The mechanical properties of quartz and feldspar at steady-state creep conditions are well constrained and flow law parameters are known from experimental calibrations. However, the physical mechanism underlying transient creep is poorly understood and the corresponding flow law parameters are unknown so far. Here, we constrain a constitutive framework that captures transient creep and steady state creep consistently using the mechanical data from laboratory experiments. The constitutive framework represents a Burgers assembly with a thermally activated nonlinear stress versus strain-rate relationship for the dashpots. Using the Markov chain Monte Carlo (MCMC) method, we uniquely determine the flow law parameters for both quartz and feldspar. We find an activation energy of 70±20 kJ/mol and a stress exponent of 2.0±0.1 for transient creep of quartz. For feldspar, the best-fit activation energies are 280±30 and 220±20 kJ/mol with stress exponents of 1.0±0.2 and 0.9±0.1 under mid- and high-temperature conditions, respectively. The stress exponents and activation energies of transient creep are consistently smaller than those of steady-state creep for both quartz and feldspar. The flow law parameters determined in this study could be used to quantify the contribution of transient creep in the postseismic deformation following a large continental earthquake. 

How to cite: Masuti, S., Muto, J., and Rybacki, E.: Transient rheology of the continental crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4536, https://doi.org/10.5194/egusphere-egu22-4536, 2022.