Quantification Analysis of Geometric Characteristics of Micro Crack Network On Fault Rock Surface

Fault is a common water conduit in coal mine, and the cracks of fault rock will greatly affect its permeability. In this study, three fault samples obtained in the mining area in Southwest Shandong of China was tested and observed by SEM, XRD and plane-polarized light microscope. The geometric characteristics, including crack density, fractal dimension and crack connectivity, of the crack network on the sample surface were calculated. Combined with the mineral content obtained by XRD, the nonuniformity coefficient of mineral composition in rock is defined. The results show that the crack geometric characteristics of the three samples are quite different and the above geometric parameters of crack network on three fault rock samples are correlated. The optical photomicrographs and SEM images show that the crack network is developed most in the fault rock samples with the least clay content. The study suggests that the nonuniformity coefficient of rock samples is positively correlated with the geometric characteristic of crack network. The difference in the crack network of fault rock samples is related to the coefficient of friction of clay.

Structural traps and seals for expanding CO2 storage in the northern Horda Platform, North Sea

The maturation of geological CCS along the Norwegian Continental Shelf is ongoing in the Norwegian North Sea, however, more storage sites are needed to reach climate mitigation goals by 2050. In order to augment the Aurora site and expand CO2 storage in the northern Horda Platform, regional traps and seals must be assessed to better understand the area’s potential. Here, we leverage wellbore and seismic data to map storage aquifers, identify structural traps, and assess possible top and fault seals associated with Lower and Upper Jurassic storage complexes in four major fault blocks. With respect to trap and seal, our results maintain that both prospective intervals represent viable CO2 storage options in various locations of each fault block. Mapping, modeling, and formation pressure analyses indicate that top seals are present across the entire study area, and are sufficiently thick over the majority of structural traps. Across-fault juxtaposition seals are abundant, but dominate the Upper Jurassic storage complexes. Lower Jurassic aquifers, however, are often upthrown against Upper Jurassic aquifers, but apparent across fault pressure differentials and moderate to high shale gouge ratio values correlate, suggesting fault rock membrane seal presence. Zones of aquifer self-juxtaposition, however, are likely areas of poor seal along faults. Overall, our results provide added support that the northern Horda Platform represents a promising location for expanding CO2 storage in the North Sea, carrying the potential to become a future injection hub for CCS in northern Europe.

Development, Evolution, and Episodic Charge History of Pop-up Structures in Southeast Abu Dhabi, UAE

The main goal of this paper is contributing to the understanding to the structural geology, development, and evolution of traps associated with strike-slip restraining bend and restraining step-over structures as a key petroleum system element in southeastern Abu Dhabi. We introduce a preliminary classification scheme for these relatively small, low-relief features defined here as pop-up structures. These structures represent different evolutionary stages of strike-slip restraining bends formed along prominent WNW-trending strike-slip fault systems in southeastern Abu Dhabi. The proposed classification scheme was summarized as a chart to illustrate the correlation between the degree of structural deformation and seal integrity, and estimates the likelihood of finding multiple, vertically stacked, productive reservoirs. It also leads to a more detailed discussion on others important characteristics of pop-up structures and provides a better understanding of sealing mechanisms such as fault juxtaposition, fault throw analysis, fault slip tendency, fault rock processes, and the role of the development of hybrid flower structures in the area. We will also show a simple case study based on two exploratory wells that targeted two pop-up structures with different degrees of deformation in southeast Abu Dhabi. This case study illustrates the complex relationship between pop-up evolution, timing of trap formation, seal integrity, trap preservation, and multiple petroleum generation and migration events. Pop-up structures are linked to multiple episodes of trap and seal evolution, where several episodes of hydrocarbon migration, charge, and leaking of hydrocarbons may occur.

Fluid-driven Cyclic Reorganisation in Shallow Basaltic Fault Zones

Faults represent a critical heterogeneity in basaltic sequences, yet their architectural and hydromechanical evolution is poorly constrained. We present a detailed multi-scale characterisation of passively exhumed fault zones from the layered basalts of the Faroe Islands, which reveals cyclic stages of fault evolution. Outcrop-scale structures and fault rock distribution within the fault zones were mapped in the field and in 3D virtual outcrop models, with detailed characterisation of fault rock microstructure obtained from optical and SE-microscopy. The fault zones record localisation from decametre-wide Riedel shear zones into metre-wide fault cores, containing multiple cataclastic shear bands and low strain lenses organised around a central principal slip zone (PSZ). Shear bands and the PSZ consist of (ultra-) cataclasites with a zeolite-smectite assemblage replacing the original plagioclase-pyroxene host rock composition. Low-strain lenses are hydrothermal breccias of weakly altered host rock, or reworked fault rocks. PSZ-proximal zones show significant late-stage dilatation in the form of hydrothermal breccias or tabular veins with up to decimetre apertures. We interpret these structures as evolving from alternating shear-compaction and dilation through hydrofracture. The fault core preserves PSZ reworking, evidencing repeated shear zone locking and migration. The alternating deformation styles of shear compaction and dilatation suggest episodic changes in deformation mechanisms driven by transient overpressure and release. The fault zone mechanical properties are thus governed by the combined effects of permanent chemical weakening and transient fluid-mediated mechanical weakening, alternating with cementation and healing.

Quantitative determination of the lowest density domain in major fault zones via medical X-ray computed tomography

Determination of the youngest active domains in fault zones that are not overlain by Quaternary sedimentary cover is critical for evaluating recent fault activity, determining the current local stress field, and mitigating the impacts of future earthquakes. Considering the exhumation of a fault zone, the youngest active domain in a fault zone is supposed to correspond to the activity at the minimum fault depth of a buried fault, such that the most vulnerable area, which possesses the lowest rock/protolith density ratio, is assumed to be indicative of this recent fault activity. However, it is difficult to measure the density of fault rocks and map the rock/protolith density ratio across a given fault zone. Here, we utilize medical X-ray computed tomography (CT), a non-destructive technique for observing and analyzing materials, to investigate the fault characteristics of several fault zones and their surrounding regions in Japan, and attempt to determine the lowest density domain of a given fault zone based on its CT numbers, which are a function of the density and effective atomic number of the fault rock and protolith. We first investigate the density, void ratio, and effective atomic number of active and inactive fault rocks, and their respective protoliths. We then calculate the CT numbers after reducing the beam-hardening effects on the rock samples and study the relationships among the CT number, density, and effective atomic number. We demonstrate that the density, effective atomic number, and CT number of the fault rock decrease as the youngest active zone, identified by outcrop observation, are approached, such that the region with the lowest CT number and rock/protolith density ratio defines the lowest density domain of a given fault zone. We also discuss the relationship between the lowest density domain and the youngest active domain in major fault zones and investigate the points to be considered when the youngest active domain is identified from the lowest density domain determined by the CT number.

A novel flow-based geometrical upscaling method for representing fault zones with two-phase fault rock properties into a dynamic reservoir model

Faults are generally represented in conventional upscaled models as 2D planar surfaces with transmissibility multipliers used to represent single-phase fault properties. However, faults are structurally complex 3D zones in which both single-phase and two-phase fault rock properties can be significant. Ignoring this structural and petrophysical complexity within faults may impart considerable inaccuracy on the predictive performance of upscaled models. This study has developed a two-phase flow-based geometrical upscaling method capable of representing simultaneously the complex geometry and saturation-dependent two-phase flow properties of realistic fault zones. In this approach, high-resolution sector models are built of small portions of the fault zones and assigned appropriate single-phase and two-phase fault rock properties. Steady state two-phase flow simulations at different fractional flows of oil and water are used to determine the saturation dependent upscaled pseudo relative permeability functions which are incorporated into upscaled models. The method is applied to an example model containing two 3D fault zone components and tested by comparing the flow results of upscaled model with those of a high-resolution truth model. Results show that two-phase flow-based geometrical upscaling is a promising method for representing the effects of two-phase fault rock properties and complex 3D fault zone structure simultaneously.

Fault Rock Property Prediction On Jurassic Clastics Of The Barents Sea/Norway

We analysed the fault rocks of a compartmentalized field in the Barents Sea, in an area with several tectonic elements, which formed at different tectonic events. Standard Fault Seal Analysis (FSA) was conducted to predict the shale content of the fault rock (SGR). A static cellular model based on well data, seismic data and geological concepts served as input. The fault rock calibration workflow required various data acquired by different methods. We analysed the Mid-Triassic to Upper Jurassic clastic deposits to reconstruct the tectonic history. Apatite fission track and (U-Th)/He thermochronology were used to determine the maximum burial depths and exhumation history. The results of high-resolution shale ductility analysis (BIB-SEM), a compaction trend study, kinematic analysis and structural modelling (section balancing) served as additional input constraints for fault rock calibration. The evaluation of the results helped to reconstruct the following tectonic evolution: The orthogonal faults of the analysed area developed at an early stage, during Late Triassic to Early Jurassic times at relatively shallow depth, below 1000 m. Ongoing subsidence created accommodation space for Upper Jurassic to Cenozoic deposits with a maximum burial depth of 2000 m for the analysed Mid-Jurassic rocks. Exhumation of the area started around 10 Ma and continued through to Quaternary times. The calculated values for fault rock permeability show a wide range when using poorly constrained input for fault rock calibration: 10 to 1000 mD for SGR values around 0.08 at reservoir/reservoir juxtaposition. Fault rock calibration using above described results concluded in reliable values for fault rock permeability and ultimately, for transmissibility multipliers. The reason for the sensitivity of the fault rock calibration is a combination of multiple factors: highly permeable reservoir sandstone, shallow depth of initial faulting, maximum burial depth and low shale content at the primary reservoir level. The study shows that an accurate reconstruction of the geohistory provides essential parameters for fault rock calibration and fault rock permeability calculation. The range of values can widely scatter if preconditions are not acknowledged. Well-constrained fault rock calibration reduces the uncertainty on possible flow scenarios, increases the reliability on production forecasts and helps to determine the most efficient drainage strategy.

Fault rock heterogeneity produces fault weakness and promotes unstable slip

Geological heterogeneity is abundant in crustal fault zones; however, its role in controlling the mechanical behaviour of faults is poorly constrained. Here, we present laboratory friction experiments on laterally heterogeneous faults, with patches of strong, rate-weakening quartz gouge and weak, rate-strengthening clay gouge. The experiments show that the heterogeneity leads to a significant strength reduction and decrease in frictional stability in comparison to compositionally identical faults with homogeneously mixed gouges typically used in the lab. We identify a combination of weakening effects, including smearing of the weak clay; differential compaction of the two gouges redistributing normal stress; and shear localization producing stress concentrations in the strong quartz patches. The results demonstrate that small-scale geological heterogeneity has pronounced effects on fault strength and stability, and by extension on the occurrence of slow-slip transients versus earthquake ruptures and the characteristics of the resulting events, and should be incorporated in lab experiments, fault friction laws, and earthquake source modelling.

The Horseshoe Abyssal plain Thrust could be the source of the 1755 Lisbon earthquake and tsunami

The southwest Iberia margin is widely believed to have hosted the 1755 Great Lisbon earthquake and ensuing tsunami, one of the most destructive natural events in European history. Here we combine geophysical data and numerical tsunami modelling to investigate the source and mechanism responsible for this event. We find that an intra-plate, lithospheric¬-scale thrust fault located at the Horseshoe Abyssal Plain coincides with the location and focal mechanisms of the largest regional earthquakes and is likely to have suitable dimensions and fault-rock properties to account for the magnitude of the 1755 event. We present tsunami simulations with this fault as the source, and find that they reproduce reported tsunami energy propagation patterns, arrival-times and run up heights more successfully than other modelled sources. We propose that a reverse dip-slip mechanism on the northwest verging Horseshoe Abyssal plain Thrust, combined with the two-state mechanical behaviour of serpentinite, is the most likely candidate for the source of the 1755 Great Lisbon earthquake and for other recent large regional earthquakes.