Faults and Fractures in Carbonates
Figure 3: Field (A, B) and Microphotograph (C) Examples of Compactive Shear Bands – The Results of Mercury-injection Analysis are Shown in (D) A
B
D 100,000 10,000 1,000 100 10 1
100,000 10,000 1,000 100 10 1
C Capillary pressure Zone I, II 020 Zone III 40 60 Saturation Hg%
0.005 0.004 0.003
0.001 0.002
020 40 60 Saturation Hg% 80 100 0 100 10 1 0.1 Pore diameter (micrometres) 0.01 0.001 80 100
0.006 0.004 0.002 0
Pore size distribution
100
10
1
0.1 Pore diameter (micrometres)
0.01
0.001
1mm Host rock Zone III
Zone II
Zone I
Zone II
Zone III Host rock
1mm Grains Matrix (grain <0.05mm) Pores Stylolite Slip surface Fine-grained zone and residual material
Single bands and a zone of shear bands (A) in the Orfento Fm (upper-Cretaceous grainstones; Maiella Mountain) and (B) in Lower Pleistocene beach carbonate sediments (Favignana Island, Sicily). Microphotograph and relative line draw of a typical compactive shear band (C), where it is possible to recognise three different zones: a well-developed, continuous zone of grain size and porosity (φ) reduction, Zone I (grains-to-matrix ratio = 0.06–0.11; φ <1%); - a compacted grain zone where a high frequency of intergranular stylolites are recognisable, Zone II (grains-to-matrix ratio = 2.4–3.7; φ = 1.8–2.4%); a wide area where φ was reduced by calcite precipitations and no compaction occurred Zone III (grains-to-matrix ratio=1.35–1.58; φ=3–7%). Mercury-injection analysis (D) show that shear bands have sealing capacity with respect to the host rock: Zone I and II fall in class C seal (30–150m oil); Zone III fall in class E seal (<15m oil). Source: Sneider et al., 1997.31
•
Well-developed faults (which include discrete slip surfaces and cataclastic fault rocks) are more efficient than single bands and zones of bands. The D/T factor is about 3.5.
Microstructural and textural analyses are consistent with three major processes taking place in the nucleation and subsequent development of compactive shear bands into zones of bands and eventually, fault zones (see Figure 3C): (i) strain localisation into narrow bands; (ii) pressure solution; and (iii) shearing of pressure solution products, that is, shearing of stylolites. The products of the first process are compactive shear bands characterised by a particulate flow mechanism involving grain rotation, translation and pore collapse. Localised pressure solution at grain contacts formed stylolites, which overprinted the earlier compacted material present within the individual shear bands. The shearing of the intergranular stylolites produce secondary and tertiary sets of stylolites oblique to the orientation of compactive shear bands, determining grain size reduction within the bands. Larger amount of slip, in order of tens of centimetres to a few metres, are solved by faults that include cataclastic fault rock, which is therefore, consistent with cataclasis being one of the mechanisms also occurring during the fault growth in carbonates. Pressure solution within the bands also generates solved solids that precipitate in nearby pores, occluding thus the primary porosity of the host rock. Laboratory analyses of representative fault rock samples show that the tectonic structures described above have significant sealing capacities and may compartmentalise a subsurface fluid reservoir (see Figure 3D). In
32 fact, according to the Sneider Seal Classification,31 single compactive
shear bands fall between class E seal (<15 m oil) and class C seal (30–150 m oil).
Faulting of Low-porosity Carbonates
Carbonate rocks with low-porosity usually contain fault zones characterised by two fundamental components: damage zone and fault core (see Figure 4A).4,32
The two fault components often show
a very different hydraulic behaviour; in general, high permeability occurs in damage zones characterised by a pronounced dilation, whereas a low permeability characterises the fault core due to the grain size reduction, cataclasis, gouge formation, sealing and other physical-chemical processes which occur in fault rocks. The overall architecture of faults (damage zone versus fault core) and related fault rock fabrics control, therefore, fluid flow within faulted and fractured rock volumes. Caine et al.4
emphasise that the bulk
conduit/barrier behaviour of fault zones depends on the relative thickness of these two fault components; the overall fault permeability structure may be represented by four end-members (see Figure 4B). As shown in Figure 4C, the fault permeability structure can be easily inferred by means of simple numerical indices (Fa = fault zone width/damage zone width) which can be derived from quantitative structural analysis. Of course, the results of this type of analysis are strongly biased by the operator; due to lateral and vertical variability of both fault core and damage zone, the choice of the site for investigation is key in order to calculate the most appropriated fault indices.
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
Capillary pressure (psia)
Capillary pressure (psia)
Incremental intrusion (ml/g)
Incremental intrusion (ml/g)
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