Page 165 - Fundamentals of Gas Shale Reservoirs

OVERPRESSuRE-GEnERATInG mEchAnISmS 145
by each end point of the three components. The distance from This can be better viewed on a neutron–density cross‐plot
the water–quartz line indicates the clay contents, and clay (Fig. 7.9, left). The difference between neutron porosity and
content is proportional to the difference between neutron density porosity is a measure of clay content (Katahara,
porosity and density porosity. however, the difference between 2008). As can be seen in the figure, there is a break in the
neutron porosity and density porosity will remain commen- slope occurring at the threshold between sands and shales,
surate with the clay content variation. and this can be used for shale discrimination. On a log plot
(Fig. 7.9, right), the difference between the neutron and
1000 density porosities, which defines shale and sand intervals, is
Grain-supported Clay-supported illustrated in the green and yellow windows, respectively.
sand shale

100 Water

Resitivity ( m) 10 Density porosity Clay volume

Smectite
1
Quartz
Illite

40 80 120 160 200 240 Neutron porosity
Gamma ray (API)
FIGURE 7.8 Graphic illustration of where the three components
FIGURE 7.7 Shale discrimination based on the gamma ray log. of shale stand on density porosity versus neutron density cross‐plot.

1 2 3
GR (API)
3
2.95 300 GR (GAPI) Depth RHOB (g/cm )
0. 250. (M) 1.95 2.95
Clay 0.45 NPOR (%) –0.15
250 Cutoffs 0.45 PHND (%) –0.15
2.75
Sand
Shale
200
RHOB (g/cm 3 ) 150 2800
2.55

2.35
100
2900
2.15
50
Sand Shale 3000

1.95 0
–0.15 –0.05 0.05 0.15 0.25 0.35 0.45 3100
NPOR-PHND (%)

FIGURE 7.9 Shale identification based on the difference between neutron porosity and density porosity showing on cross‐plot (left)
and log‐plot (right).