Page 156 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

chamosite (2SiO0,A1,0,3FeOnH,O) in the Liassic shales by weight). The alteration products in this example are
of the North Sea is sufficiently rich to be seen on the biotite, sericite, kaolinite, montmorillonite and chlorite,
neutron log (Figure 10.22). The zone is characterized by a]l of which themselves have varying amounts of water
an increase in the neutron log value associated with a {Table 10.8) (Nelson and Glenn, 1975).
corresponding increase in the bulk density (see also A series of basalt flows from the Middle Jurassic of the
Section 10.7, Neutron-density combination). North Sea also show high neutron values, here diagnosti-
cally associated with Jow gamma ray readings and high
Volcanic and intrusive rocks
densities (Figure 10.24). A characteristic profile of
The neutron log is especially useful in the recognition of
increasing values upwards is developed in the neutron log
intrusive and volcanic rocks: most of these show high
in each individual lava flow (Figure 10.24). The response
neutron log values and high densities (as in many shales)
appears to be typical of subaerial flows and is also seen in
but associated with low gamma ray values (unlike shales).
the Deccan traps of India (Buckley and Oliver, 1990).
The high neutron-log values in igneous rocks are due to
Two proposals exist for this neutron response. The first
their high content of chemica!ly-bound water. The bound:
suggests that it is related to alteration products, such as
water may be original or associated with alteration prod-
chlorite, which occur progressively towards the tops of
ucts, mainly clay minerals, which result from the reaction
the flows and formed during soil development and weath-
of hydrothermal fluids with the original intruded rock.
ering. The second suggests that the increase in vesicles
An example is shown of an altered diabase with no
upwards is the cause, either enclosing water or water rich
porosity (Figure 10.23). The neutron log is seen to be
chemicals such as zeolites.
responding entirely to the bound water (between 14%

35 NEUTRON LOG counts/second
9 2500 2000 1500 moe ave |

I
!
3 |
2 *BOUND WATER wt % 2! ~ NEUTRON POROSITY %
@
=
= ° limestone matrix
5 0 1 2 3 4
t
!
!
3504. 7: dG G oOo 485 _ _ 15_ _0 _-16
A
GAMMA RAY API |$ 7 1 ( 3

5 K DENSIT
° 50 709 J fg ULK DENSITY gio og
po
dt
po tt
DIABASE --
7

400-47 dound water

density

ra

450-4- >

(m) 25m

depth

50m

75m

Figure 10.23 Neutron response to volcanic rocks with bound
water: the example is of a diabase with 1-2% of sulphide Figure 10.24 Neutron log response to basait flows. The
mineralization. It has no porosity. *Bound water values from Jurassic age flows from the North Sea show low gamma ray
analyses of 10ft (3m) composites on pulp. (Re-drawn from values and high neutron log values. The flows are interbedded
Nelson and Glean, 1975). with iron-rich, clay soil.