Page 146 - Fundamentals of Gas Shale Reservoirs

126 PETROPHYSICAL EVALUATION OF GAS SHALE RESERVOIRS

Unlike (the) uranium, thorium is extremely stable and DEPT 0. GR (api) LLD (ohmm)
310.
will rarely pass into solution; thus, its concentration can be (M) Fm. Tops 0.45 NPHI (dec) –0.15 2. 50.
directly attributed to the provenance (source area) of the
accumulated sediment. The relative immobility of thorium,
as a stable, conserved, trace element in the marine environment,
compared to the transient mobility of uranium due to fluctu­
ations in oxidation–reduction potential is a relationship that
can be used to delineate the possible sequence stratigraphy
in the target gas shale layer (Jacobi et al., 2008).

6.4.1.3 Neutron Log Neutron log is a porosity log 2350 Upper carynginia shale
(NPHI) that measures the amount of hydrogen in a formation.
Like the other conventional well log data, neutron log inter­
pretation in the gas shale layers is a complex task and needs
many parameters to be considered:
• Hydrogen in the organic matter
• Hydrogen in the structure of clay minerals (hydroxyl
groups)
• Hydrogen in water and hydrocarbons present in the 2400
formation Intra shale sand Mb.
Figure 6.9 shows the responses of GR, NPHI, and deep resis­
tivity in the Carynginia Shale which is a potential gas shale
layer in the Perth Basin, WA. As can be seen in this figure, due
to the clay effect, NPHI log response shows the higher value in
the lower and upper Carynginia Shale, while in the middle sec­
tion of the Carynginia, which is a sandy shale member, NPHI
values decrease. It is also expected that NPHI log response will 2450
be reduced in the gas shale layers due to the lower hydrogen
index (HI) of gas and organic matter compared to water,
although quantifying the effects of reducing porosity due to
lack of hydrogen in gas and organic matter is quite complex
(Glorioso and Rattia, 2012). To some extent, this effect can be Lower carynginia shale
observed on the lower Carynginia Shale in Figure 6.9. Between
2435 and 2475 m the resistivity is higher and NPHI is lower
compared to the lower part of this section (i.e., between 2475
and 2520 m), possibly due to the presence of gas. 2500
Apart from the aforementioned parameters based on the
studies by Zhao et al. (2007) and Labani and Rezaee (2012),
neutron porosity decreases with increasing thermal maturity
in the gas shale layers. The following explanations can jus­ FIGURE 6.9 Typical well log response from a well in the
tify this relationship: Carynginia Shale, Perth Basin, WA.

• HI of generated hydrocarbons in the final stages of
thermal maturity (i.e., gas window) is lower than oil 6.4.1.4 Density Log The density log measures the
window products; for example, HI of dry gas is less formation bulk density. Density log has been used for
than that of/in wet gas. source rock evaluation for a long time (Schmoker, 1979;
• By increasing thermal maturity, smectite converts to Schmoker and Hester, 1983; Passey et al., 1990). The
illite, and HI of transformed illite is lower than that of/ bulk density log data can be successfully employed in
in smectite. porosity modeling of the gas shale layers if the mineral
• Reduction of the water saturation at the high thermal composition (or matrix density) is properly determined
maturity levels causes a relatively lower HI values for using mineralogical tools (Vernik and Milovac, 2011;
the shale layers. Alexander et al., 2011).