Page 253 - The Geological Interpretation of Well Logs

- SEQUENCE STRATIGRAPHY AND STRATIGRAPHY -

of x

15 DT
GR as| 5 [6 NPHI
30 100 ~ 1.7 1 RHOB l -.15/130 69
= =>

-¢- minor &S
— condansed o

es horizon ‘
bloturbated —S
=
- 7 transgressive on SUEERCE 4070m-
Le ——__ | sand <<

roots 7
L 4 4080m 4

Figure 15.6 Coal followed by a flooding surface. There appears to be no erosion associated with the flooding event (marked by a
very thin, bioturbated, heterogeneous sand, too thin to be seen on the logs). Open marine, deeper water, organic rich shales of a
minor condensed sequence follow the flooding.
The second example (Figure 15.6) shows another Vail, 1988). Such surfaces mark the passage from
expression of a marine flooding surface, this time non-marine to marine sedimentation. The sequence strati-
immediately following coal deposition. Coal represents graphic interpretation of this erosion just before or during
slow accumulation at sea leve} with little detrital sediment flooding is that it represents a transgression and sea level
input which, in this example, the core shows to have a deepening (Nummedat! and Swift, 1987). In Exxon terms
seat earth and so be in situ. It is commonly associated this will be the case with the first marine flooding surface
with retrogradational episodes, that is coastal retreat following maximum regression and in shallower areas of
(Milton et ai., 1990). Over the coal is 30 cm of dark, the shetf, where it is associated with erosion, it is vari-
organic-rich, laminated shale with pyrite: a significant ously called the ravinement surface or the transgressive
deepening of the environment of deposition is evident. surface of erosion (Nummedal and Swift, 1987; Baum
On the logs this succession is seen as an obvious coal and Vail, 1988).
(low density, high neutron cf. Figure 10.28), followed On the logs, a transgressive surface will show similarities
abruptly by a shale with high gamma ray and very high to a flooding surface but the log responses will tend to be
neutron responses, (indicating the high organic content, more abrupt. The example (Figure 15.7) shows a medium
ef. Figure [0.20) interpreted as a minor condensed grained sandstone deposited in cross-beds, with thin car-
sequence. This grades upwards to shales with a normal bonate cemented zones and no bioturbation. On the core,
log response which the core shows to be bioturbated shale the topmost surface appears to truncate a bedform and is
and silty shale. very abruptly covered by a 20 cm, intensely bioturbated
The detail of the core shows that, in fact, immediately bed: there is also bioturbation at the junction. On the logs,
over the coal is a 20 cm burrowed, transgressive sand, not the abrupt upper surface of the sandstone is evident
resolved by the logs (masked by the coal), which has a and the bioturbated bed shows in the high density and
few scattered, very coarse sand grains and coal frag- low interval transit time (high velocity) responses, as
ments: at tts base is the flooding surface. As far as the logs described above (Figure 15.7). Clean, laminated and
are concerned and in the absence of core, it is the high slightly organic-rich shale rapidly follows the bioturbated
gamma ray and high neutron responses (condensed bed. At the base, where organic richness is at a maximum,
sequence) immediately following the coal, which are there is a gamma ray high, a neutron high, density low and
very typical and suggestive of the marine flooding event. sonic high (velocity low). The sonic response is as much
This example demonstrates the need for fine detail when caused by the fine shale laminations as by the organic
examining key surfaces, satisfied in this case, by core. matter content (Chapter 8). As detrital input increases
Image logs can also supply fine detail and, in the absence so the logs trend towards a normal shale response
of core, can give invaluable information (Figure 13.19). (Figure 15.7). These log trends are diagnostic (Creaney
This is true not just for a flooding surface, but any of the and Passey, 1993) and generally, where organic content
key surfaces described in this chapter. The use of the is highest, assumed to represent an anoxic, condensed
image logs in this area is developing rapidly (Chapter 13). section (see surfaces and intervals of slow deposition
below). The log responses in this example are typical.
— transgressive surface (ravinement surface) The identification of this as a transgressive surface
In the two previous examples there is no evidence for rather than a flooding surface depends on two things: the
significant erosion and certainly no truncation at the abrupt log responses and the position in the vertical
level of the flooding surface. This is normally the case. sequence. The abruptness, clearly, is suggestive of an
There are, however, flooding surfaces across which there erosional break. However, it is the position which gives
is evidence of considerable erosion, such as the presence the most significant clues. Most flooding surfaces occur
of a lag, mineral, especially galuconite concentrations at the top of prograding, coarsening-up successions.
and cementation of the underlying surface (Baum and Transgressive surfaces may not. They will cut into valley
243