Page 269 - The Geological Interpretation of Well Logs

- SEQUENCE STRATIGRAPHY AND STRATIGRAPHY -

A. SAND-BAR
Path diagram
aR GR Well a Well b
b, 6, b,b, ,
so 4

a, b, |*-b, a,

a,
40-5 SN GR b, 3,
Pby]a,-%
a,
:
4 GR
6,
8,
30-7 —- MaRINE‘SHALES a,
3 4 S
~ ~
g 24
a, | a
E 4

2 Pl b, N .
€ 105
= — “N
& =
1
— — DATUM
GR all GA
—» gapina } gap in b
4 GR
30-] —— “a N match (angle change with
relative thickness}
20-4
vw Figure 15.25 Computer correlation. Schematic illustration of
g the sequence matching technique {re-drawn from Fang et al.,
2 +04 1992).
3 4
2
= o4——_. —— DATUM
2
ae An entirely different approach is to use the computer,
104 erosion surface not for the actual correlauion procedure, but to create a
B, VALLEY FILL new format which is more easily correlated. Modern
software allows standard single curve logs to be plotted as
a colour image or rather, a colour “bar code’ (Figure 13.1).
Figure 15.24 The importance of choice of datum. If the base
With such a presentation, the eye has a much greater
of the sand is taken as a datum, A.. a sand bar is interpreted
facility for comparison. The example shows gamma ray
(unlikely). 1f the flooding surface at the top of the sand is
taken, B., valley-fill is interpreted and the covering sequence logs from eight closely spaced wells in a producing
shows a constant thickness. field, formatted as colour images rather than typical
curves (Figure [5.26). The eye is able to follow a greater
amount of detail than is possible with only the curves.
“90
10
20

30

50 metres

60

Figure 15.26 Gamma ray logs plotted as colowr images allowing a better visual correlation. The gamma ray scale has been
normalised and the 8 wells are closely spaced (imaging routine from GeoScene software).