Environmental Engineering Reference
In-Depth Information
Figure 13
Shale gas exploration geological parameter summary.
depth is also critical, with the shale at an economically drillable depth.
While this is variable between basins, in the USA shale gas production is
optimal at 3000-4000 m depths; less than 1000-1500 m increases the risk of
surface or ground-water-related stimulation issues while depths greater than
4000 m are generally uneconomic, except for prolific highly over-pressured
shales such as the Haynesville Shale or the Eagle Ford Shale. Other general
parameters are given in Figure 13.
Defining areas of potential shale resources is generally based on pre-
liminary screening of existing outcrop geochemical data (TOC, organic
richness, maturity, etc.), augmented by well log, drill cuttings or core-derived
parameters as detailed above, to high-grade areas of potential interest or
''sweet-spots''. These can be defined as areas where the shale has good or-
ganic content, quality and maturity, good ''frac'able'' characteristics (min-
eralogy), thickness and depth. Seismic data are increasingly being used in
''sweet-spot'' definition, based on seismic attribute evaluation. High organic
matter content within a shale affects its seismic character due its low density
and low elastic moduli. While there is a linear reduction in bulk density with
increasing organic content, seismic velocities show a non-linear reduction.
This is complicated by thermal maturity which, as discussed above, in-
creases kerogen density, particularly affecting shales within the gas window.
Seismic energy propagates through the sub-surface as both P-waves and
S-waves which have different properties with variable relationships. Key
seismic parameters are P-wave acoustic impedance (AI), which is a function
of rock density and P-wave velocity, and the ratio between P-wave and S-wave
velocities (V p : V s ratio). AI shows a very clear drop with increasing organic
content, which can indicate good potential reservoir, while the V p : V s ratio
(and Poisson's ratio) shows a weak decrease. Data from several resource
shales, such as the Marcellus and Eagle Ford, show that the cross-plot of AI
versus V p : V s is a good indicator for shale reservoir potential (see Figure 14),
 
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