Page 249 - The Geological Interpretation of Well Logs

15

SEQUENCE STRATIGRAPHY

AND STRATIGRAPHY

15.1 Introduction 15.2 Well logs and high resolution
siliciclastic sequence stratigraphy
As a means of correlation, the use of wireline logs is
obvious. As an aid to stratigraphic analysis, logs are
Principles and generalities
invaluable. But as a tool in sequence stratigraphy, their Sequence stratigraphy is ‘the study of genetically related
use so far, is seriously underdeveloped. This chapter prin- facies within a framework of chronostratigraphically
cipally describes the use of logs in sequence stratigraphy, significant surfaces’ (Van Wagoner et al, 1990). It was
by building on the techniques Jaid down in Chapter 14, developed to a very high degree of sophistication by
and developing them in terms of sequence stratigraphy. [t Exxon and is illustrated by the ‘type’ exposures of Book
also describes a more modern and thoughtful, computer- Cliffs in Utah and Colorado, USA (Van Wagoner er ai.,
based methodology for such analyses. The chapter ends 1990). It is a concept that explains the vertical and later-
with a description of logs used in classical correlation and al variations of sedimentary successions in terms of
stratigraphy. These come Jast as the ideas of sequence relative sea leve] changes. The sequence (Figure 15.1) is
stratigraphy have caused such a revolution that earlier the fundamental stratal unit of sequence stratigraphy and
ideas can never again be seen in the same light. records between 0.5~-3 Ma of sedimentation (Vail ef ai.,

A, The sequence-lithological schema and systems tracts

apparent
truncation

CO,

MFS
incised valley $B
MFS
: fg I in
ae coastal plain (silts) a] phate Marine [ oa - SB

&
Sn) fluvial estuarine submarine fan submarine fan

L_——] sands sands
B. Log (GR) schema with key surfaces
\
mes — |
SBTSE == 2 LS =

3S ————— Sa
——-—
MFS —
SN
is

a ll, = Se aés

— SS Se

Figure }5.1 The depositional mode} of sequence stratigraphy defined for the DS, depositional sequence (after Exxon) and the GSS,
genetic stratigraphic sequence (after Galloway). A. The sequence as a lithological schema with sequence tracts (forced regression is
not included, the upper, incised valley was cui at lowstand, filled during later ransgression). B. The same model with gamma ray tog
traces and key surfaces (from various sources including, Van Wagoner ef a/., 1990, Mitchum & Van Wagoner 1991, Bhattacharya
1993). MEFS = maximum flooding surface: TSE = transgressive surface of erosion: SB = sequence boundary: fs = flooding surface.
LST, TST, HST, = Lowstand, Transgressive, Highstand Systems Tracts.

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