Boggs S. Principles of Sedimentology and Stratigraphy
Подождите немного. Документ загружается.
Precambrian
1. Green River
12.6 Correlation of Lithostratigraphic Units
427
2. Westwater
Figure 12.19
Correlation of strata between two localities on
the Colorado Plateau on the basis of similar
lithology of distinctive stratigraphic units.
[From Mintz, L W. , 1981, Historical geology,
The science of a dynamic Earth, 3rd ed., Fig.
1 0.1, p. 241 , reprinted by permission of
Charles H. Merrill Pub. Co., Columbus, Ohio.]
degree of condence with strata that are in a similar stratigraphic position with re
spect
to the control unit in other areas. If two or more marker beds are present in a
succession, they give even greater reliability to correlation of units that lie be
tween the marker beds. Obviously, correlation becomes more equivocal with in
creasing stratigraphic separation above or below e control units.
Coelation by Instmental We ll Logs
We ll logs are simply curves sketched on paper charts that are produced from data
obined from measurements in well bores. These traces record variations in such
rock
properties as electrical resistivity, transmissibility of sound waves, or adsorp
on and emission of nuclear radiation in the rocks surrounding a borehole. These
variations are a reection of changes in features such as gross lithology, mineralo
gy, fluid content, and porosity in the subsurface formations. Thus, correlation by
428
Chapter 12 I Lithostratigraphy
Figure 12.20
use of well logs is not based totally on lithology. Nonetheless, most of the rock
properties measured by well logs are closely related to lithology.
Well logs are obtained by the following procedure. After an exploratory well
is drilled by a petroleum company, the well is logged before being completed as an
oil or gas producer or abandoned as a dry hole. The logging procedure begins with
lowering an instrument called a sonde to the bottom of the well bore (Fig. 12.20).
The sonde may be designed to measure the electrical resistivity of a rock unit, nat
ural or induced gamma radiation emitted by the unit, the velocity of sound waves
passing through the rock, or other rock properties. As the sonde is slowly with
drawn from the bore hole through a succession of stratigraphic units, it continu
ously measures the particular property of the rock that it is designed to analyze,
and it electrically transmits this information to a digital tape and display unit lo
cated in a logging truck at the surface.
One common type of well log is the electric log, or resistivity log, which
records resistivity of rock units as the sonde passes up the bore hole in contact
with the wall of the hole. Resistivity is affected by the lithology of the rock units
and the amount and nature of pore fluids in the rock. For example, a marine shale
whose pore spaces are filled with saline formation water will have a much lower
electrical resistivity (higher conductivity) than a porous sandstone or limestone
Schematic diagram illustrating how instrumen
tal well logs are obtained in a well bore. [From
Desbrandes, R., 1985, Encyclopedia of well log
ging. Editions Te chnip, Paris, Fig. 2.1, p. 95.]
12.6 Correlation of thostratigraphic Units
429
filled with oil or gas. With experience in a given geological province, petroleum
geologists can recoize the particular signatures represented by the traces on the
log
and can relate these signatures
to particular types of lithostratigraphic
units or
to a specific formation. Lithology cannot be read directly from such logs, but the
characteristics of the log traces are a reflection of lithology (and fluid content).
Commonly, the lithology of cores and cuttings obtained from e well bore during
drilling are identified and a "lithologic log" is prepared from this information.
This lithologic log can then be matched against the instrumental log tracing to aid
in interpreting lithology from the instrumental log.
A variety of other types of logging tools are in common use by petroleum
companies. Gamma ray logs measure the natural gamma radiation in rock units.
Sonic logs measure the velocity with which a sound signal passes through rock
units. In addition to their usefulness in correlation, sonic logs can also be used to
determine the porosity of subsurface formations because of the fact that sound
waves are slowed in their passage through rocks by the presence of fluid-filled
pores. Foation density logs provide information about porosity and liology.
Ve ry specialized logs include geochemical, formation microscanner, and maetic
susceptibility logs. All logs share the common characteristic that they consist of
electrically produced signatures or traces that represent some particular property
of subsurface lithostratigraphic unit that is related in some way to lithology, uid
content, bed thickness, or other properties.
One common type of log data display consists of two different types of
traces that are arranged on either side of a central column that represents the well
bore. This central colu is calibrated in feet (or meters) to show depth below the
surface. Figure 12.21 illustrates a section of a well log showing a sonic curve op
posite a gamma ray curve. e curve shapes generated by a particular liostrati
graphic unit are not unique, but a trained, experienced well-log analyst can lea
to recognize the signature of a particular formaon or succession of formations
and can match up e signatures in logs from one area to those from nearby wells.
e Asquith (1982), Desbrandes (1985), Rider (1986), and Whittaker (1998) for ad
ditional descriptions of well logs and log interpretations.
Characteristically, the well-log curves of adjacent wells a very similar, but
the degree of similarity decreases in more distant wells. By working with a series
of closely spaced wells, however, a geologist can carry a correlation across an en
ti sedimentary basin, even when pinch outs or facies changes occur. In fact, one
of the reasons why petroleum geologists find correlation of well logs so useful in
petroleum exploration is that correlation permits recoition of pinch-outs and fa
cies changes that may be potential traps for oil and gas. Figure 12.22 is an example
of correlation by gamma-ray and sonic logs across a portion of the East Irish Sea
Basin,
lying amid Ireland, Wales, Scotland, and England. These wells penetrate a
stratigraphic section consisng mainly of halite and mudstone with minor anhy
dte, dolomite, and sandstone. Geologis often add lithologic infoation ob
tained from drill cores or cuttings to the well logs; however, I stress again that
correlation by well logs is not necessarily correlation based entirely upon litholog
ic identity because e shapes of the curves can represent a variety of rock proper
ties, such as porosity and fluid content. Note, however, that there is generally
good agreement between the shapes of the well-log curves and the lithology. Cor
relation by well logs is actually based more upon the posion of each unit in a suc
cession of units represented on the logs rather than on the character of any
individual unit reflected in the curves. Correlation by well logs is thus the approx
imate subsurface equivalent of correlation of surface sections by position in the
succession.
430
Chapter 12 I Lithostratigraphy
l".nrn�rnl
"c'
I
�
�
WELL
FIELD
cou
:�
LOCATION
§���
GAMMA Y
API UNITS
_0
1120 _240
t
'
'
Figure 12.21
WATERS NO. 1
WILDCAT
NS
CSENE
����SeNic�
. . ·�-
I
I
�
INTERVAL TRANSIT TIME
MICROSECONDS PER FOOT
T 3��3R,
I
8
0
�
0
0
I
I
:
'
0
I
0
I
I
'
I
I
0
0
I
I
40
5
I
0
.
"
I
'
- �
�
'
,
I
0
�
�
'
I
I
A section of a sonic-gamma ray log. The gamma ray
curve is shown on the left. The sonic log (interval
transit time log) is on the right. Depths (in feet)
below the surface are shown in the central column.
Four distinct correlatable units are indicated.
t
0
0
0
I
�
Correlation by instrumental well logs can be a laborious process involvin
g
large numbers of logs; it is also subject to considerable subjectivity owing to the
similarity of the log curves or traces in different parts of a logged stratigraphic sec
tion. Differences between stratigraphic units may be manifested only by very sub
tle differences in the digital plots and can be difficult to disce visually. The
availability of computers and sophisticated statistical techniques now makes it
possible to apply automated approaches to stratigraphic correlation of well logs,
removing some of the subjectivity correlation. These approaches iv0lve using
digital tapes to segment the logs for use in computational systems. These systems
then provide a statistical match for correlation purposes. Details of automated
well-log correlation are given by Shaw and Cubitt (1978), Griffiths (1982), and
Olea (1988).
w
Sea
bed
�
�
�
�
�
o
�
Well No.
113/21
Gamma-
a
R
a
y
14a 14a
;
s
�
m
�
c
Sonic
40
�
;
1
Figure 12.22
Well No.
11a/2-5
Gamma-
a
R
a
y
140
140
Sonic
4a
ll No.
112-6
Gamma-
O
R
a
y
14a 140
Sonic
40
Well No.
11a/9-1
Gamma-
O
R
a
y
140 14a
Sonic
40
Well No.
110/7-1
Gamma-
a
R
a
y
140 140
Sonic
40
Sea
"b
�
i
"5
�
Vertical scale
[
F
No sample
Halite
Anh
y
drite
Dolomite
Mudstone and siltstone
Sendstone
Fautt
� Unconformit
y
--�·� Disconrmi
G
a
mma-ra
y
log scale in
API units
Sonic log sc
a
le in
rcroseconds/foot
An
illustration of correlation by use of well logs. The logs for each well contain a gamma ray and
a sonic log. Lithologic infor
mation has been added to the logs to aid in correlation; however, correlation is made primarily on the basis of the "signatures"
of the well-log cues. Selected boreholes from the Mercia Mudstone Group (Triassic), East Irish Sea Basin. [After jackson, D. 1.,
et
al., 1995, United Kingdom offs hore regional report: The geology of the Irish Sea, HMSO for the British Geological Suey,
London, Fig. 58, p. 75.]
432
Chapter 12 I Lithostratigraphy
FURTHER READING
Ager, D. , 1993, The new catastrophism: The importance of the
rare event in geologic history: Cambridge University Press,
Cambridge, 231 p.
Ager, D. , 1993, The nature of the stratigraphical record, 3rd ed.:
john Wiley & Sons, Chichester, 151 p.
Agterberg, P., 1990, Automated stratigraphic correlation: Else
vier, Amsterdam, 424 p.
Coe, A. L. (ed.), 2003, The sedimentary record of sea-level
change: The Open University, Cambridge University Press,
Cambridge, 287 p.
de Boer, P. L., and D. G. Smith (eds.), 1994, Orbital forcing and
cyclic sequences: International Assoc. Sediamentologists Spec.
Pub. 19, Blackwell Scientific Publications, Oxford, 559 p.
Doyle, P., and M. R. Bennett (eds.), 1998, Unlocking the strati
graphical record: Advances in modern stratigraphy: john
Wiley and Sons, Chichester, 532 p.
Doyle, P., M. R. Bennett, and A. N. Baxter, 1994, The key to Earth
history: An introduction to stratigraphy: john Wiley & Sons,
Chichester, 231 p.
Einsele, G., and A. Seilacher (eds.), 1982, Cyclic and event strati
fication: Springer-Verlag, Berlin, 536 p.
Einsele, G., Rieken, and A. Seilacher (eds.), 1991, Cvcles and
events in stratigraphy: Springer-Verlag, Berlin, 955 p.
Emery, D., and K. j. Meyers (eds.), 1996, Sequence stratigraphy:
Blackwell Science Ltd., Oxford, 297 p.
Hailwo, E. A., and R. B. Kidd (eds.), 1993, High resolution
stratigraphy: Geological Society London Spec. Pub. 70, 357 p.
Hallam, A., 1981, Facies interpretation and the stratigraphic
record: W. H. Freeman, San Francisco, 287 p.
Haq, B. U. (ed.), 1995, Sequence stratigraphy and depositional re
sponse to eustatic, tectonic and climatic forcing: Kluwer Aca
demic Publishers, Dordrecht, 381 p.
House, M. R., and A. S. Gale (eds.), 1995, Orbital forcing
timescales and cyclostratigraphy: Geological Society Special
Publication 85, 210 p.
Miall, A. D., 1997, The geology of stratigraphic sequences:
Springer-Verlag, Berlin, 433 p.
North American Commission on Stratigraphic Nomenclature,
1983, North American Stratigraphic Code: Am. Assoc. Petro
leum Geologists Bull., v. 67, p. 841-875.
Posamentier, H. W., C. P. Summerhayaes, B. U. Haq, and G.
Allen (eds.), 1993, Sequence stratigraphy and facies associa
tions: Blackwell Scientific Publications, Oxford, 644 p.
Salvador, A. (ed.), 1994, International stratigraphic guide: A
guide to stratigraphic classification, terminology, and proce
dure: International Union of Geological Sciences and Geolog
ical Society of America, Inc., Trondheim, Norway, 214 p.
Schwarzacher, W., 1993, Cyclostratigraphy and the Milankovitch
theory: Elsevier, Amsterdam, 225 p.
Weimer, P., and H. W. Posamentier (eds.), 1993, Siliciclastic se
quence stratigraphy: Recent developments and applications:
AAPG Memoir 58, American Association Petroleum Geolo
gists, Tu lsa, Okla., 492 p.
Wilgus, C. K., B. S. Hastings, C. G. St. C. Kendall, H. Posamen
tier, C. A. Ross, and j. C. Van Wa goner (eds.), 1988, a-level
changes: An integrated approach: Soc. Econ. Paleontologists
and Mineralogists Spec. Pub. 42, 407 p.
Seismic, Sequence, and
Magnetic Stratigraphy
13.1 INTRODUCTION
S
eismology is the study of earthquakes and the structure of Earth on the basis
of the characteriscs of seismic waves. Although the broad subject of seis
mology lies outside e scope of this book, some aspects of seismology have
a very important application to stratigraphy. The emphasis of this chapter on
what is commonly referred to as exploration seismology and, more specifically, on
application of the techniques of exploration seismology to stragraphic study. Ex
ploration seismology deals with the use of articially generated seismic waves to
obtain information about the geologic structure, stratigraphic characteristics, and
disibutions of rock types. The techniques of exploration seismology were devel
oped initially to locate structural traps for petroleum deposits, and they are still
used extensively for that purpose; however, seismic methods can also be applied
to stratigraphic problems.
Seismic stratigraphy is thus the study of seismic data for the purpose of
extracting stratigraphic information. Seismic stratigraphy is a relatively new sci
ence, born in the 1960s. Because of its wide applicability to subsurface study
both on land and at sea, where other types of stratigraphic data are few, it has al
ady achieved an important position alongside the more traditional branches of
stratigraphy.
Sequence stratigraphy is an outgrowth of seismic stratigraphy, although the
practice of sequence stratigraphy is not limited to study of seismic records. Se
quence stratigraphic concepts can be applied also to outcrop and well data. A sed
imentary sequence is a stratigraphic unit composed of a relatively conformable
succession of genetically related strata that is bounded at its top and base by un
conformities or their correlative conformities (Mitchum, Va il, and Sangree, 1977).
A sequence represents one cycle of deposition bounded by nonmane erosion, de
posited during one significant cycle of rise and fall of base level. Because base
level in marine basins is controlled by sea level, a sequence is thus the product of
a
cycle of rise and fall of sea level. The sequence concept has assumed such an im
portant role in stratigraphic thinking that we now refer to application of sequence
concepts as sequence stratigraphy.
Magnetostratigraphy is also a relatively new branch of stratigraphy. It is
based on the principle that iron-bearing minerals such as magnetite become mag
netized at the time they crystallize in a magma or lava ow. The magnetized
433
434
Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy
minerals acquire magnetic polarity (north and south pole alignments) in keepg
with Earth's magnetic polarity, a property called remanent magnetism. Geologists
discovered in the 1960s that the remanent magnetism of some volcanic rocks dis
plays reversed polarity, indicating that Earth's magnetic field has reversed at
times in the geologic past. Sedimentary rocks can also become magnetically polar
ized because small iron-bearing minerals are mechanically aligned with earth's
magnetic field as they settle in water. Pattes of magnetic reversals in ancient
sedimentary rocks constitute a powerful tool for stratigraphic subdivision of these
rocks, which can be applied to a variety of geologic problems such as correlation,
geochronology (study of time in relation to Earth history), and paleoclimatology.
13.2 SEISMIC STRATIGRAPHY
Early Development of Seismic Methods
The use of seismic methods for obtaining information about subsurface rocks and
structures involves the natural or artificial propagation of seismic (elastic) waves.
These waves pass downward into Earth until they encounter a discontinuity and
are reflected back to the surface, where they can be picked up by detectors. Seis
mic waves travel at velocities ranging from less than 2 km/s (some sediments) to
more than 8 km/s (some ultramafic rocks), depending upon the kinds of rocks
through which they pass and their depth below Earth's surface. If we know the
velocity with which seismic waves travel through a particular kind of rock and if
we can time their passage downward to a reflector and back to the surface, then
we can calculate the depth to the reflecting horizon. This principle forms the basis
for application of seismic methods to geologic study.
Much of the theory of elasticity and propagation of seismic waves through
rock materials that constitutes the theoretical basis for seismology was developed
in the early part of the 19th century. An English seismologist, Robert Mallet, was
the first scientist to measure the velocity of seismic waves in subsurface materi
als. He initiated experimental seismology in 1848 by measuring the speed at
which seismic waves passed through comparatively near-surface materials. He
used black powder as an energy source to create a disturbance in the rocks and
the surface of a bowl of mercury as the detector for the arriving seismic waves.
The possibility of using seismic techniques to define the characteristics of subsur
face rocks was apparently first put forward by a scientist named Milne in 1898.
Two other interesting applications of the principles of seismology were experi
mented with in the early part of the 20th century. These were a method for de
tecting icebergs (after the sinking of the Titanic by an iceberg in 1912) and the use
of mechanical seismographs to detect the position of large enemy guns during
Wo rld War I.
Principles of Reflection Seismic Methods
On-Land Sueying
Practical application of seismology to the detection of rock structures began im
mediately after the end of World War I in both the United States and Europe, es
pecially in Germany and England. The first applications were in petroleum
exploration in Germany and in the Gulf Coast region of the United States, particu
larly in exploration for petroleum traps associated with salt domes. These early
exploration efforts used the refraction seismic method for determining the struc
ture of subsurface formations. This method is based on the principle that arti
cially generated seismic waves (produced in the early years of exploration by
explosives) are refracted or bent at discontinuity surfaces as they travel downward
13.2 Seismic Stratigraphy
435
below the surface. The waves en travel along these discontinuities before being
fracted back to the surface, where their arrival is picked up by detectors placed
at various distances away from the explosion (shot) point. The time that elapses
during passage of the seismic waves downward to the disconnuity and back to
the surface is used to compute the depth to the disconnuity.
Although several shallow petroleum deposits in salt domes were discovered
during the early years of seismic exploration by the refraction method, this
method did not work well for deeper structures because of the excessive distances
required between shot points and detectors. erefore, it was soon largely sup
planted in petroleum exploration by the reflection seismic method. In the reflec
on method, waves created
by an explosion are reflected back to
the surface
directly from subsurface rock interfaces without beg refracted and traveling lat
erally along discontinuity surfaces. Therefore, detectors can be located at relative
ly
short distances om the shot points, and reflection seismic techniques can be
used for delineating very deep structures. After introduction of the reflection
method about 1930, it quickly became the primary tool in the petroleum industry
for locating buried anticlines and other structural traps for oil.
A very brief summary of basic principles of reflection seismology is given
h
ere. Additional details of the physical principles upon which reflection methods
are based can be found in standard seismology textbooks, such as Lay and Wallace
(1995) and Stein and Wysession (23). As mentioned, the reflection seismic meod
for delineating the structure of subsurface rock units is based on the principle that
elastic or seismic waves travel at own velocities through rock materials. These
velocities vary with the type of rock (typical average velocities: shale = 3.6 km/s;
sandstone = 4.2 km/s; lestone = 5.0 km/s; Christie-Blick, Mountaiun, and
Miller, 1990). Where the subsurface lithology is own relatively well from drill
hole
information, it is possible to make accurate calculations of the time required
for a seismic signal to travel from the surface to a given depth and then reflected
back to e surface.
The reflection technique involves first generating elastic waves at the surface
at a point source, originally called a shot point because explosives were rst used
to create the seismic waves. Nonexplosive energy sources located on the surface
now also in common use. These nonexplosive energy sources include vibrato
ry devices that produce continuous vibrations at the surface or devices that drop
heavy weights onto a metal plate placed on the ground surface. Seismic detectors,
called geophones, are laid out in arrays extending outward from the shot point.
Seismic
waves reflected back from subsurface discontinuities are picked up by
these detectors and fed electronically to a recording device. The principal discon
nuities that reflect seismic waves are bedding planes and unconformities. By
multiplying the travel velocity by one-half of the travel time elapsed from initia
tion of the elastic waves at the point source to their arrival at the detector, geo
physicists can accurately calculate depths to the discontinuities. This procedure
thu s allows the subsurface position of the discontinuities to be determined. The
data obtained in this manner can then be displayed as seismic sections or profiles
that depict the structure of the major rock uni as they appear in cross section.
Alternatively, the data may be used to prepare structure contour maps (see
Chapter 16) on the tops of particular reflecng horizons.
The general prciples of on-land reflecon seismic "shoong" are illustrat
ed in the interesting old diagram (Fig. 13.1) from Nettleton's 1940 Geophysical
Prospecting fo r Oil. The equipment and techniques for surveying locations, shoot
ing, and recording and processing seismic data have changed and significantly
improved since e 1940s; however, the basic principles illustrated in this figure
sl apply. As seismic waves pass downward and outward from the point energy
source through the subsurface formations, they are reflected from successively
deeper formations back to e surface where ey are picked up by e electronic
436
Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy
Figure 13.1
Diagram illustrating the
equipment and procedures
used in seismic exploration in
the 1940s. [From Nettleton,
L. L., 1940, Geophysical
prospecting for oil, Fig. 155,
following p. 332,
McGraw-Hill Book Co.]
detectors. The signal from the detectors is then amplified, filtered to remove ex
cess "noise," digitized, and fed to a recording truck to be recorded on magnetic
tape or disk.
The data recorded on the magnetic tape or disk must then presented in vi
sual form for monitoring and interpretation. Prior to the use of magnetic tapes or
disks for recordg, the visual seismic records, or seismograms, were mechanically
produced, wiggly-trace cords such as that shown in the sample reflection seismo
gram in Figure 13.1. Photographic or dry-paper recording methods are now used
for visual display, and several modes of displaying the amplitude of arriving seis
mic waves against arrival time are in use. A common type of display, called a
vaable-densi mode display, is generated by a technique by which light intensity
is varied to display differences in wave amplitude (one-half the height of a wave
above the adjacent trough) by producing alteting light and dark areas on film or
paper, ereby accentuating the amplitude of waves from a parcular reflecting sur
face. For example, all wave traces having an amplitude greater an a given value
are shaded black; traces with lower amplitudes a unshaded. Thus, a strong reflec
tion event will show up as a black line on the record, as illustrated in Figure 13.2.
Marine Seismic Sueying
Early seismic surveys were carried out on land; however, reflection seismic meth
ods can be used also in the ocean or in lakes. Some marine operations in very
shallow water began in the late 1920s and 1930s, but extensive marine seismic sur
veys did not get underway until about the middle 1940s. Marine seismic opera
tions employ the same principles as those used on land, but they differ in the speed
at which they take place and in the specific details of the shooting and detection
processes. Sound sources and detectors are towed behind the survey ship, which
can
operate at a speed of 6 knots or more on a continuous 24-h/ day basis. In the