Connected sub-seismic features compromise cap rock integrity: the influence of faults and permeable sandstone lenses
Abstract
Mudstones and shales can form caprocks creating a barrier retarding vertical migration of fluids from Geological Disposal Facilities (GDF) and Carbon Capture and Storage (CCS) schemes. During site characterisation for a GDF or CCS it is essential to consider the evolution of the geosphere over the lifetime of the scheme (≈103 years for CCS and 106 years for a GDF). Predicting the location of connected flow pathways through caprocks is important for making a safety case. The sealing ability of a caprock can be compromised by the presence of connected pathways along fault zones. Faults can be detected using seismic surveys, however their limited resolution makes it unlikely that small faults (<10m throw) can be located. These subseismic faults may still have a significant influence on overall leakage through a caprock. This investigation considers the influence that small faults can have on the hydraulic properties of caprocks and how these properties will change with time. We investigate the structural detail of subseismic normal faults cutting through mudstone from the Myoch Formation, Girvan, UK. The Myoch Formation is a 20m thick sequence of silty mudstone with ≈1cm thick coarse grained sandstone lenses. Three main faults have been identified in the study area, two of which intersect creating an area of high fracture density. Fracture lengths are from 0.3m to 3m and are orientated +/- 30° perpendicular to the bedding. Fracture density is lowest in the hangingwall and highest in the fault intersection. Two generations of flow are evidenced by carbonate cement infill and green alteration halos. Carbonate was cemented from over saturated fluids flowing in the fracture network and the green halos were caused by a subsequent phase of reducing fluids. Carbonate fill varies in width from <1mm to several cm, the green halos are generally <6mm wide but can be up to 3cm. Crosscutting relationships show carbonate cementation precedes the reducing fluid migration. Some fractures contain only carbonate fill, some have only a green halo and a subset show evidence of both flow events. This latter subset have original orientations close to vertical and are consistent with fractures being held open for flow in a normal stress regime. The sandstone lenses were utilised as part of the flow network during both flow events. Three closely spaced areas of distinct hydraulic behaviour have been identified (several metres apart), which are separated by the subseismic faults. These areas show differing fracture properties (e.g. length and density) and differing interactions with the sandstone lenses that run through the mudstone (e.g. termination, step over or cross-cutting). Interaction between subseismic faults can create a connected fracture network compromising the integrity of a caprock. Our results show that outside of the fault interaction area the fracture network alone is not enough to create a connected pathway through the mudstone. It is only when the thin permeable lenses are considered that we get an accurate representation of the flow network through the mudstone. We conclude that caprock permeability may be governed by small subseismic features that are commonly ignored during logging and site investigation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H13A1186M
- Keywords:
-
- 1832 HYDROLOGY / Groundwater transport;
- 8010 STRUCTURAL GEOLOGY / Fractures and faults