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| Titel |
Stratigraphic controls on fracturing in black and grey shale-dominated sequences |
| VerfasserIn |
Jonathan Imber, Howard Armstrong, Liam Herringshaw, Ken McCaffrey, João Trabucho-Alexandre, Jon Trevelyan |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250081299
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| Zusammenfassung |
| Regularly-spaced arrays of opening-mode fractures develop in brittle layers (sandstone,
limestone) that are embedded within a weaker matrix (shale) and subjected to layer-parallel
extension. Where cut by tectonic faults, such brittle layers display a range of features
including drag folds, shear fractures and rotated blocks, in addition to opening-mode
fractures. In both cases, the weaker matrix deforms in a macroscopically ductile manner.
Knowledge of the “mechanical stratigraphy” can therefore aid fracture prediction in
subsurface reservoirs. A key question for unconventional hydrocarbon exploration is to what
extent does the concept of mechanical stratigraphy apply to thick, shale-dominated
sequences? The Toarcian (Lower Jurassic) Whitby Mudstone Formation (WMF) is a ca. 105
m thick, shale-dominated sequence that crops out within the Cleveland Basin, NE England
and was deposited in < 7.4 Myr. The WMF contains both black and grey shale intervals. The
lowermost Mulgrave Shale Member (“Jet Rock”; Harpoceras falciferum Zone, Cleviceras
exaratum Subzone) of the WMF is characterised by high total organic carbon (TOC < 18
%), but low SiO2, Al2O3 and K2O. TOC decreases, but SiO2, Al2O3 and K2O all
increase above the exaratum Subzone (“Bituminous Shales”). These geochemical
variations are consistent with an upward increase in quartz and clay content within the
Mulgrave Shale Member. The upper part of the Alum Shale Member (“Cement Shales”;
Hildoceras bifrons Zone, Zugodactylites braunianus Subzone) is characterised by low
TOC, Al2O3 and K2O, but high SiO2 consistent with a high quartz and low clay
content. The Jet Rock and Bituminous Shales are cut by regularly-spaced arrays of
sub-vertical, calcite-filled opening-mode fractures that abut against sub-horizontal,
bedding-parallel fractures with ca. 1 m vertical spacing. By contrast, the Cement Shales are
characterised by arrays of dipping (dip < 60Ë ) shear fractures with consistent
extensional offsets. We hypothesise that large fluid overpressures (λ - 1) generated
during clay diagenesis and/or kerogen maturation within the highly stratified (i.e.
mechanically isotropic) Mulgrave Shale Member contributed to the development of
bedding-parallel veins within the Jet Rock and Bituminous Shale. More speculatively, the
spacing of the bedding-parallel veins may be controlled by metre- and sub-metre scale
variations in organic and/or clay content throughout the Mulgrave Shale Member, which
in turn may reflect primary sedimentary discontinuities. The regular spacing of
compositional variations and discontinuity surfaces is likely to be a consequence of
allogenic forcing mechanisms. By contrast, the SiO2-rich, TOC- and clay-poor
Cement Shales appear to be compositionally more homogeneous. The Cement Shales
deformed under conditions of higher effective normal stress (λ < 1), giving rise to
shear fractures with classic Andersonian geometries. Our observations suggest that
stratigraphy exerts a strong control on fracturing within thick, shale-dominated sequences.
However, the relationship between “mechanical stratigraphy” and fracture spacing and
orientation appears to be more complex than in conventional clastic reservoir sequences. |
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