Catuneanu Principles Seq Stratigraphy book[1]
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Catuneanu Principles Seq Stratigraphy book[1]


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envi-
ronment (Willapa Bay, Washington; photo courtesy of M.K. Gingras).
The borings are sand-filled, which provides their typical mode of
preservation, and are made by the terenid bivalve Bankia. The wood-
ground has the significance of a transgressive tidal-ravinement
surface. The association between Teredolites and transgressive coast-
lines is generally valid for both in situ and allochthonous wood-
grounds.
sedimentological details of an actual core. Such \u2018virtual\u2019
cores allow visualization of details at a millimeter
scale, including sediment lamination, cross-stratifica-
tion, bioturbation, etc., in three dimensions (Fig. 2.30).
Well logs have both advantages and shortcomings
relative to what outcrops have to offer in terms of
facies data. One major advantage of geophysical logs
over outcrops is that they provide continuous informa-
tion from relatively thick successions, often in a range
of kilometers. This type of profile (log curves) allows
one to see trends at various scales, from the size of
individual depositional elements within a deposi-
tional system, to entire basin fills. For this reason, data
provided by well logs may be considered more
complete relative to the discontinuous information that
may be extracted from the study of outcrops.
Therefore, the subsurface investigations of facies rela-
tionships and stratigraphic correlations can usually be
accomplished at scales much larger than the ones
possible from the study of outcrops. On the other hand,
nothing can replace the study of the actual rocks,
hence the wealth of details that can be obtained from
outcrop facies analysis cannot be matched by well-log
analysis, no matter how closely spaced the boreholes
may be (Cant, 1992).
Log
Spontaneous
potential
Conventional
resistivity
Micro resistivity
Gamma ray
Sonic
Neutron
Density
Dipmeter
Caliper
Property measured
Natural electric potential
(relative to drilling mud)
Resistance to electric current flow
(1D)
Resistance to electric current flow
(3D)
Natural radioactivity
(e.g., relater to K, Th, U)
Velocity of compressional
sound wave
Hydrogen concentration in pores
(water, hydrocarbons)
Bulk density (electron density)
(includes pore fluid in measurement)
Orientation of dipping surfaces
by resistivity changes
Borehole diameter
Units
Millivolts
Ohm-metres
Ohm-metres
and degrees
API units
Microseconds/metre
Per cent porosity
Kilograms per cubic
metre (g/cm3)
Degrees (azimuth
and inclination)
Centimetres
Geological interpretation
Lithology, correlation, curve
shape analysis, porosity
Identification of coal, bentonites,
fluid types
Borehole imaging, virtual core
Lithology (including bentonites,
coal), correlation, shape analysis
Identification of porous zones,
tightly cemented zones, coal
Porous zones, cross plots with
sonic and density for lithology
Lithologies such as evaporites
and compact carbonates
Paleoflow (in oriented core),
stratigraphic, structural analyses
Borehole state, reliability of logs
FIGURE 2.29 Types of well logs, properties they measure, and their use for geological interpretations (modified from Cant, 1992).
FIGURE 2.30 New micro resistivity logs combine resistivity with
dipmeter data to produce \u2018virtual cores\u2019 in three dimensions. Such
detailed borehole imaging, with a vertical resolution of less than
8 mm, allows the observation of sedimentary structures in the absence
of mechanical core (modified from data provided by Baker Atlas).
WELL LOGS 41
42 2. METHODS OF SEQUENCE STRATIGRAPHIC ANALYSIS
Well Logs: Geological Uncertainties
Well logs provide information on physical rock
properties, but not a direct indication of lithology.
Spontaneous potential and gamma ray logs are
commonly used for the interpretation of siliciclastic
successions in lithological terms, but one must always
be aware of the potential pitfalls that may occur 
\u2018in translation\u2019. Changes in rock porosity and pore-water
chemistry (fresh vs. sea water) may induce different
responses on spontaneous potential logs, including
deflections in opposite directions, even if the lithology
is the same. Similarly, gamma ray logs are often inter-
preted in grading terms (fining- vs. coarsening-upward),
or worse, as it adds another degree of unconstrained
interpretation, in bathymetric terms (deepening- vs.
shallowing-upward trends). In reality, gamma ray logs
simply indicate the degree of strata radioactivity,
which is generally proportional to the shaliness of the
rocks and/or the amount of organic matter.
Zones of high gamma ray response may correspond
to a variety of depositional settings, from shelf and
deeper-marine to coastal plains, backshore marshes and
lacustrine environments. In fully subaqueous settings
(marine or lacustrine), high gamma ray responses
correlate to periods of restricted bottom-water circula-
tion and/or with times of reduced sediment supply.
Such periods favor the formation of condensed sections,
which most commonly are associated with stages of
shoreline transgression, and hence with maximum
flooding surfaces (Galloway, 1989). However, due to
the wide variety of environments which may result in
the accumulation and preservation of organic matter
and/or fine-grained sediment, the mere identification
of high gamma ray zones is not sufficient to unequivo-
cally identify condensed sections (Posamentier and
Allen, 1999). At the same time, condensed sections
may also be marked by a variety of chemical and
biochemical precipitates formed during times of sedi-
ment starvation (e.g., siderite, glauconite, carbonate
hardgrounds, etc.), thus exhibiting a wide range of log
motifs which may not necessarily fit the classic high
peaks on gamma ray logs (Posamentier and Allen, 1999).
The equivocal character of well logs, when it comes
to geological interpretations, is also exemplified by the
fact that fundamentally different depositional systems
may produce similar log motifs. Figure 2.31 illustrates
such an example, where comparable blocky log
patterns formed in fluvial, estuarine, beach, shallow-
marine, and deep-marine environments. Similarly,
jagged log patterns are not diagnostic of any particular
depositional system, and may be found all the way
from fluvial to delta plain, inner shelf, and deep-water
(slope to basin-floor) settings (Fig. 2.32). Such jagged
log motifs simply indicate fluctuating energy condi-
tions leading to the deposition of alternating coarser
and finer sediments (heterolithic facies), conditions
which can be met in many nonmarine, marginal-
marine, shallow-marine, and deep-marine environments.
Monotonous successions dominated by fine-grained
sediments may also be common among different depo-
sitional systems, including deep-water \u2018overbanks\u2019
(areas of seafloor situated outside of channel-levee
complexes or splay elements) and outer shelf settings
(Fig. 2.33).
At the same time, one and the same depositional
system may display different well-log signatures as a
function of variations in depositional energy, sediment
10
 m
10
 m 1
0 
m
10
 m
10
0 
m
1. 2. 3. 4. 5.
Gamma ray (API) Gamma ray (API) Gamma ray (API) Gamma ray (API) SP (mV)
\u2212120 20
FIGURE 2.31 Well logs from
five different siliciclastic deposi-
tional systems, each including a
\u2018blocky\u2019 sandstone unit. 1\u2014fluvial
channel fill; 2\u2014estuarine channel
fill; 3\u2014sharp-based shoreface
deposits; 4\u2014deep-water channel
filled with turbidites; and 5\u2014
beach deposits (modified from
Posamentier and Allen, 1999, and
Catuneanu et al., 2003a). Note the
potentially equivocal signature of
depositional systems on well logs.
For this reason, the correct inter-
pretation of paleodepositional
environments requires integration
of multiple data sets, including
core, rock cuttings, biostratigra-
phy, and seismics. Abbreviation: