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| Titel |
Characterizing flow pathways in a sandstone aquifer at multiple depths |
| VerfasserIn |
Giacomo Medici, Jared West, Nigel Mountney |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250139744
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| Publikation (Nr.) |
 EGU/EGU2017-3043.pdf |
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| Zusammenfassung |
| Sandstone aquifers are commonly assumed to represent porous media characterized by a
permeable matrix. However, such aquifers may be heavily fractured where rock properties
and timing of deformation favour brittle failure and crack opening. In many aquifer types,
fractures associated with faults, bedding planes and stratabound joints represent preferential
pathways for fluids and contaminants. This presentation reports well-test results and
outcrop-scale studies that reveal how strongly lithified siliciclastic rocks may be entirely
dominated by fracture flow at shallow depths (≤ 150 m), similar to limestone and crystalline
aquifers. The Triassic St Bees Sandstone Formation of the UK East Irish Sea Basin represents
an optimum succession for study of the influence of both sedimentary and tectonic aquifer
heterogeneities in a strongly lithified sandstone aquifer-type. This sedimentary succession
of fluvial origin accumulated in rapidly subsiding basins, which typically favour
preservation of complete depositional cycles, including fine-grained mudstone and silty
sandstone layers of floodplain origin interbedded with sandstone-dominated fluvial
channel deposits. Vertical joints in the St Bees Sandstone Formation form a pervasive
stratabound system whereby joints terminate at bedding-parallel discontinuities.
Additionally, normal faults are present through the succession and record development of
open-fractures in their damage zones. Here, the shallow aquifer (depth ≤150 m BGL) was
characterized in outcrop and well tests. Fluid temperature, conductivity and flow-velocity
logs record inflows and outflows from normal faults, as well as from pervasive
bed-parallel fractures. Quantitative flow logging analyses in boreholes that cut fault planes
indicate that zones of fault-related open fractures typically represent ∼ 50% of well
transmissivity. The remaining flow component is dominated by bed-parallel fractures.
However, such sub-horizontal fractures become the principal flow conduits in wells that
penetrate the exterior parts of fault damage zones, as well as in non-faulted areas.
Optical televiewer logs show development of karst-like conduits in correspondence of
bedding fractures and faults up to 150 m below the ground surface, where recharge
water containing dissolved carbonic acid enlarges fractures; these features may be
responsible for the relatively high field-scale permeability (K∼0.1−1 m/day) of the
phreatic zone at these depths. Below this ‘karstifed’ zone, field-scale permeability
progressively decreases from K∼10−2 to 10−4 m/day from 150 m to 1100 m depth.
Notably, differences between plug and field-scale permeability, and frequency of well
in-flows seen in temperature and conductivity logs, also decrease between intermediate
(150 to 450 m) and elevated (450 to 1100 m) depths. This confirms how fracture
closure leads to a progressively more important matrix contribution to flow with
increasing lithostatic stress, leading to intergranular flow dominance at ∼ 1 km depth. |
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