Seventh International Conference on Fault and Top Seals

A dedicated workflow, consisting of an integration of multidisciplinary and multi-scale approaches, has been developed and tested within Eni for caprock integrity assessment, by combining commercial software and AI-based solutions with proprietary tools and plugins, specifically devised for this goal.

This workflow provides geological and geophysical integrated deliverables, highlighting the correlations between faults distribution and areas more prone to possible gas escapes through the caprock, to be used as additional input for top seal evaluation, mitigating the risk associated to hydrocarbon exploration and storage activities.

The Flemish Pass Basin, spanning over 10,000 km², features sedimentary layers up to 10 km thick from the Mesozoic and Cenozoic eras. As part of the North Atlantic rift system, its tectonic history includes multiple rifting phases from the Triassic to Early Jurassic. Exploration efforts have focused on closures of rotated fault blocks, with varied success. The Late Tithonian and Lower Cretaceous synrift sandstone reservoirs show high porosity and permeability, while source rock intervals with type II kerogen offer good-to-excellent potential.

The timing of hydrocarbon charge relative to the paleo-seal capacity of Cretaceous rocks is crucial for assessing the petroleum potential of the basin. A flow-path migration modelling tool evaluates scenarios involving reservoir charge, migration efficiency, lithological facies, net-to-gross maps, and seal capacity of Cretaceous and Jurassic sediments. This study examines how the sealing properties of these rocks over time impact hydrocarbon accumulation.

Modelling results indicate a consistent pattern in the filling history of structures, with initial vertical hydrocarbon movement due to ineffective cap seals, followed by the establishment of effective seals. The timing of this transition varies across different structures in the basin. Discrepancies between modelled and observed accumulations highlight the need for refined input parameters and methodologies.

This study investigated the variability of seal rock characteristics in the Pleistocene Kazusa Group, a forearc basin succession in Japan. Regional- to local-scale variations in mudstone characteristics, including mineralogy, seal capacity, and permeability, were examined using closely spaced onshore outcrops with precise chronostratigraphic correlation based on volcanic ash beds. The results show remarkable downslope variations in the seal rock properties, especially in the shelf margin to the slope environments. In contrast, basin floor mudstones exhibit only minor downslope variations. These findings can be used as a geologic analogue for potential carbon storage sites in the offshore part of the Kazusa sedimentary basin, where subsurface data remains limited. Furthermore, it can be utilized as a geologic analogue for areas with similar geologic settings, such as young siliciclastic-dominated successions formed in active margin basins.

This study evaluates the sealing capacity of various lithologies of the Ionian zone in Greece, through the analysis of 11 outcrop and 6 well samples using X-ray Diffraction (XRD) and Mercury Injection Capillary Pressure (MICP) techniques. Samples were collected from Epirus and Lefkas (Greece) and wells in Crete. XRD analysis identified mineralogical compositions, particularly focusing on evaporitic lithotypes. MICP testing, conducted with a Poremaster60GT porosimeter, assessed pore size distribution and capillary entry pressures, providing estimates of the maximum hydrocarbon (HC) and CO2 column heights each lithology could retain. Results showed that shale samples exhibited homogeneous microporosity and high sealing efficiency, capable of retaining significant oil and CO2 columns, constrained primarily by geomechanical properties. Mudstones demonstrated slightly larger pore sizes but retained excellent sealing potential for oil (up to 1315 m) and CO2 (up to 269 m). In contrast, evaporitic samples, impacted by weathering and reduced plasticity, exhibited wider macropore distributions and significantly lower sealing capacities (oil: 5–66.61 m; CO2: ∼1–13.26 m). These findings highlight the strong capillary sealing potential of shales and mudstones, while suggesting that evaporitic rock performance in outcrop samples may underestimate true reservoir conditions due to surface degradation and MICP limitations in microporosity detection.

The Mercia Mudrock Group is a sequence of siltstones, claystones, and evporites that is an extensive regional caprock and a prospective host rock for radioactive waste disposal in the UK. However, numerous natural fractures pose a leakage risk to infrastructure projects relying on MMG’s low permeability. Precipitation of evaporitic minerals into the fracture space lowers the permeability, restoring the seal. However, this specific form of sealing may create mechanical contrast between the fracture fill and the host mudrock, which can be exploited by stress perturbations, brought on by reservoir pressurisation, tectonic forces, and tunnelling.

This study uses optical microscopy, SEM, and EDX, to analyse the interface between fracture fill and host rock at a small scale, as to assess possible reactivation.

The main finding is that the interface is not always a discrete surface. Coarse grained mudrocks may share a cementing mineral with the fracture, which should limit the mechanical contrast, in contrast to clay rich mudrocks.

This work presents a method for establishing weak reflectivities that indicate the hydraulic behaviors of potential fluids within and around these seals, as well as in damage zones. By utilizing concepts of pristine amplitudes and further inversion, we analyze gentle signal responses and texture patterns to understand fluid behavior in these structures. This approach not only helps identify potential hydrocarbon accumulations but also enhances our understanding of fluid interactions with various types of seals.

This work illustrates the importance of capitalizing on hydrocarbon column heights trapped against active faults for evaluating the hydraulic and mechanical properties of faults. This is crucial for understanding fault behavior, evaluating CO2 containment and risks associated with fault reactivation and leakage.

A method allowing to determine the strength of active faults, or faults close to critical stress is presented, based on case studies evaluating hydrocarbon column heights in fault-bounded traps from the East African Rift and the Niger Delta. This method consists in comparing the in-situ hydrocarbon fluid pressure data with a theoretical fault slip threshold calculated using Mohr-Coulomb theory to determine the static friction coefficient best fitting the maximum pressure trend. The values determined match those obtained in laboratory shearing experiments on fault gouge material with a range of compositions, supporting this method for determining in-situ mechanical fault properties.

This work is directly applicable to CO2 storage screening studies for potential site selection where the trapping mechanism is a fault-bounded structure. It also helps in better understanding the hydraulic and mechanical processes involved in faulting by providing access to more realistic static fault properties to be used as input parameters in geomechanical models for CO2 storage containment.

Environmental Impact Assessment Statements (EIS) for Mining and Coal Seam Gas (CSG) projects are generally required to consider the potential hydrological impacts of faults on surface and near-surface groundwater assets. Three distinct end-member geological scenarios and outlines methods for characterising fault-related groundwater flow within a risk assessment context have been proposed ( Murray& Power, 2021 ).

This presentation considers the environmental hydrological aspects of risk assessment for mining and CSG developments. Following water resources and environmental risk assessment, we refer to features such as lakes, rivers, groundwater-dependent ecosystems, and valuable aquifers as groundwater assets or simply assets. Damage to a groundwater asset may include (for example) groundwater depletion or changes to the flow regime caused by mining and CSG development-induced pressure changes. Pressure changes may cause increased or decreased flow along pre-existing flow pathways, or cause the activation of new or previously unimportant flow paths. Should this flow be significant enough to degrade the asset, then the flow pathway is called a causal pathway.

Fault zone permeability remains a challenging parameter to address both in outcrop and subsurface. Limited experimental data providing permeability measurements on natural fault rock samples from sealing parts of the fault are available. More data is needed to improve our understanding of fluid migration in faults and dynamic changes related to subsurface injection and the risk of fault reactivation and alteration of the seal capacity.

In this work we measure permeability for a natural fault gouge sample from Little Grand Wash Fault zone in Utah using a novel direct shear device. The low permeability measured is in line with a sealing fault core and supports field observations of the damage zone as the main pathway for CO2 migration. The shear loading phase of the experiment suggest a reduction in permeability during shearing and provides key data for improving our understanding of fault reactivation and effects on fluid migration.

The Middle-Upper Triassic Mercia Mudstone Group (MMG) of England and Wales, and equivalent units on the UK offshore continental shelf, is a successful top seal to hydrocarbon reservoirs and is now a potential host rock for radioactive waste disposal and top seal to Carbon Capture and Storage (CCS) reservoirs in the UK. A significant challenge for these projects is that the MMG is both vertically and laterally heterogeneous. The distribution and causes of heterogeneity are poorly understood. We will here show that this variability has arisen from spatial-temporal changes in depositional environments combined with complex differences in the temperature-time history of major UK Triassic basins. This research therefore takes an integrated approach by combining published compositional, sedimentological and burial diagenetic data to synthesise the depositional and diagenetic controls on the spatio-temporal variability of this heterogeneous rock. As well as improving general understanding of the variability of the MMG, the findings of this research provide crucial insights into the representativity of natural analogues for CCS and radioactive waste disposal projects in the UK.