Geology Reference
In-Depth Information
Modified HS upper bound
Shaley sands
Cemented sands
a)
b)
Constant
cement model
(2% cement)
0.5
6000
GR
0.4
5000
70
60
50
40
30
Cemented sands
0.3
4000
Cementing
trend
0.2
3000
Sandy
shales
Uncemented
sands
0.1
Uncemented
sands
2000
Sorting trend
Sandy shale
20
0
0
0.1
0.2
0.3
0.4
0.5
4000
6000
8000
12000
Porosity
AI (g/cm
3
.m/s)
c)
d)
Top cemented sands
Top uncemented sands
0.3
0.3
0.2
0.2
Shale/water sand
Shale/water sand
Shale/water sand
0.1
0.1
0
0
-0.1
-0.1
Shale/oil sand
Shale/oil sand
Shale/oil sand
-0.2
-0.3
-0.2
-0.3
0
0.1
0.2
0.3
0.4
0
0.1
0.2
0.3
0.4
sin
2
sin
2
θ
θ
Figure 5.19
Brine saturated data from a logged section of high-porosity sands - offshore Norway; (a) porosity vs V
p
, (b) AI vs PR, (c) AVO plot
showing top sand responses for brine (blue) and gas filled (red) in cemented sands, (d) AVO plot showing top sand responses for brine (blue)
and gas filled (red) uncemented sand.
which porosity reduction is through advanced
cementation (lower rate of velocity increase with
decreasing porosity). The porosity at which the
change occurs has been referred to as the consoli-
dation porosity (Vernik,
1998
). The consolidation
porosity is commonly between 22%
which sand and shale are mixed, with a gradation
from matrix supported shales and sandy shales to
grain supported sands and shaley sands and (b) rocks
composed of alternating laminae of sand and shale
(i.e. laminated sands). In shaley sands the clay may be
pore filling (often referred to as
30% and prin-
cipally depends on grain sorting (e.g. clay content)
and stress state (Vernik and Kachanov,
2010
). It
effectively defines the point at which sands change
from behaving as a granular material to sandstones
behaving as a solid with pores and cracks.
Shale content is an important element to consider
in the acoustic properties of sandstones. In general
two sand/shale situations are common: (a) rocks in
-
clay) or
structural (i.e. part of the load bearing framework) or
indeed both, dependent on the relative sizes of sand
and clay/shale
'
dispersed
'
. This simple picture is made
complicated by the fact that clays in sandstones may
also be authigenic in origin (i.e. derived in-situ from
the breakdown of other minerals such as feldspars).
While some generalised discussion of the effects of
clays in sandstones is given below each reservoir
'
grains
'
71
