Environmental Engineering Reference
In-Depth Information
Other workers report similar findings: Modica and Lapierre for the Mowry
Shale,
27
while Hover et al.
28
found no visible intercrystalline or interparticle
matrix porosity in low-maturity Antrim and New Albany shales, while in the
Woodford shale Curtis et al.
29
found no organic porosity development in
shales below VR of 0.9%, with the first appearance of pores occurring with
gas window maturity of 1.2% Ro: however, porosity development was vari-
able within samples, indicating that organic matter composition also con-
trols kerogen porosity development. This appears to be confirmed by a study
on the Marcellus Shale which found that variation in TOC was a stronger
control on organic porosity than thermal maturity,
30
noting that a rela-
tionship between TOC and porosity only held at lower TOC values, with
bitumen-like material predominating in organically richer shales.
Bitumen, generated as part of the maturation process in the oil window,
has been seen to contain nanopores in Marcellus shales with gas window
maturity of VR 1.1 and 3.1% Ro,
31
and in the Barnett Shale.
32
Bitumen
generation has been suggested as the cause of the reduction in organic
porosity in the late oil window, such as recorded in the Posidonia black
shales of the Hils Syncline located in the Lower Saxony Basin of Germany
where shale porosity, present at lower maturity levels in the oil window, was
found to decrease through the late oil window. Maximum porosity devel-
opment was recorded at gas-window maturities, suggesting that the thermal
cracking of these bitumens could be at least partially responsible for this
increase in organic porosity.
33
The thermal maturation process also affects
the density of the kerogen, which increases with maturity from 1.2 to 1.65
gc m
3
for immature Type I and Type II kerogens to greater than 1.65 g cm
3
at gas window maturities of VR greater than 2% Ro. This has an effect on
shale resource determination as kerogen density, derived from bulk density
logs, is a key parameter in porosity determination.
The importance of a kerogen ''network'' within organic-rich shales with
TOC 42.5 wt% is well documented as promoting the expulsion of hydro-
carbons into adjacent carrier beds.
16
As much of the porosity, particularly in
siliceous shales, is located within the kerogen as a result of thermal mat-
uration, the presence of a kerogen matrix is believed to be critical in the
permeability of the shale by providing an ecient route for gas to move from
pore to artificial fracture.
3.3 Permeability: Methods - Traditional Core Plug and
Crushed Core
Although resource shales are low permeability rocks by definition, for them
to be effective reservoirs they must have minimal matrix permeability to
allow significant hydrocarbon flow after stimulation, although clearly this
matrix permeability must not be large enough to allow leakage of the
hydrocarbon charge over geological time. This implies low permeability and
connectivity between pores in the natural state, allied to high capillarity,
particularly in shales where the pore system is water wet. Also, it should be
