Page 267 - The Geological Interpretation of Well Logs

- SEQUENCE STRATIGRAPHY AND STRATIGRAPHY -

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32
Rédby Fin.
LE .
\Y
; F Zz
<3
z¢
Figure 15.21 Palaeontologically-controlled correlation. The
mid-Cretaceous across the Buchan Horst, North Sea.
Outside wells 6km apart. GR = gamma ray API units. 160
GAPI
Sonic = At microseconds per foot. MFZ = microfaunal zone. GAPI__ 180, 160m

PS = Praegiobotruncana stephani biozone. RC = Rotalipora GaP 160m >
one . . * 0
cushmani biozone. HB = Hedbergella brittonensis biozone. oF :
GB = Globigerinelloides benionensis biozone. (From Burnhill
and Ramsay, 1981). — fF 20m

20m
sp

20m 4 — s
Correlation and depositional or erosional topography
The topography of the present day surfaces of sediment
deposition must be similar to those that existed in the - 40m

past. The environments of shelf, slope and basin existed 40m _
in past geological time, as now. Long distance correla-
Lo
PN
tions should reflect these topographic elements. “om
|
The Pennsylvanian to Permian deposits of West Texas Me
were apparently laid down close to the shelf edge (Van
Siclen, 1958). Correlations over 10 km in these beds
A. ‘SUBSURFACE’ corretation
across the palaeo-shelf edge show the depositional topog-
raphy as it then was (Figure 15.23). The topography is
associated with changes in lithology and, in this case,
limestone and reef development. Limestones develop on
the platform, reefs on the platform edge and shales in the
basin off the shelf slope.
Equally important as depositional topography, is
erosional topography. Much importance is put in
sequence stratigraphy on identifying ‘valley fills’
(Section 15.2). These are valleys cut during a relative fall
in sea level and filled subsequently when sea level rises
again. The modern example is the erosional valley
beneath the present day Mississippi (Fisk and McFarlan,
1955), filled with the present day river deposits. In
ancient sediments these deposits can be made evident in
well correlations (Figure 15.24) if the correct datum is
B. REAL {outcrop} correlation
chosen, normally a key surface (Section 15.2).
To show depositional and erosional topography the
Figure 15.22 The iimitations of subsurface correlation
choice of datum is obviously critical. In the example
itlustrated by gamma ray profiles measured in sands and
(Figure 15.23), the datum used is near structural and
shales along a quarry wail. A. expected correlations from
still allows the palaeotopography to be seen. However,
using gamma ray curves alone. B. The real correlations
this may not be the case and the chosen datum must based on actually following beds along the outcrop (from
correspond to a facies recognised as most likely to have Slatt ef al., 1992). .
257