Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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GASES
IN
SEDIMENTS
principal
end
product within tlit: sLillatc-rcduction zone. Many
bacteria! groups operate
in
this zone
to
oxidize organic matter
bul llie
key
reaction
is
thai
of the
sulfate-reducing bacteria
(SRB,
notably Desutfoyitwio
sp.).
Taking carbohydrate
to
represent
the
bulk organic matter,
the net
reaction
can be
represented
by the
reaction:
2CH;O
+
SO4-
2HCO3 (Eq.
ir iron
is
freely available within this
/one. the HiS
will
be
buffered
to
form pyrite
(see
Sultides
in
Sediments). Where
insufficient iron
is
present, free HiS
may
build
up
although
the
ultimate concentration
is
limited
by
bacterial oxidation ofthe
sultidc
as it
diffuses upward into
the
suboxic
and
oxic zones.
Methane
The deepest bacterial zone
in
sediments
is
represented
by the
meihanogenie zone. Here,
two
competing bacterial reactions
generate free methane
gas:
acetate
(or
simitar) fermentation:
CH1COOH-CH4
+ CO:
carbon dioxide reduetion:
(Eq.
2)
(Eq.
3)
The acetate.
CO: and H2
substrates
in
both cases
are
provided
by
parallel, largely fermentation type, bacterial
reactions.
In
general,
CO;^
reduction
is
dominant
in
marine
sediments while fermentation reactions
are
dominant,
at
least
in tiie early stages,
in
freshwater environments although
in all
cases both reactions operate
to
some extent (Clayton. 1995).
The so-cailed "biogenic"
or
"bacterial""
gas
formed
in
this
way
is economically important
and may
account
for as
mueh
as
2O'yii
of the
world's natural
gas
resources (Rice
and
Claypool,
1981).
Since
no
external oxidizing agents
are
required
for
methane
production,
the
ultimate methane concentration
is
limited
largely
by the
availability
of
organie matter although some
methane
is
lost
by
anaerobic oxidation
at the
base
of the
sulfatc-reduction zone (Iversen
and
Jorgensen.
1985) and
methane generation ceases when
the
sediment
is
buried
lo
temperatures above approximately
60 65X due to
metabolic
limitations
o'i the
bacteria involved. Laboratory experiments
show that
the
optitnum temperature
for
methanogenesis using
natural assemblages
is
around
35 45 C
(Zeikus,
J.G. and
Winfre>,
M.R.,
1976). Under oplimum conditions,
up to
about
U)".,
of the
organic carbon tnay
be
converted
to
methane
(Clayton. 1992).
Carbon dioxide
Carbon dioxide
is the
ultimate fate
of
organic matter o.xidation
in
all
bacterial zones although
it is not
usually
as
abundant
as a
free
gas as is CH4.
There
aie a
number
of
reasons
for
this.
Firstly, during very shallow diagenesis,
the
majority
of
the CO:
is able
to
diffuse upward back into
the
overlying water column
(botloni waters
arc
Irequently enriched
in CO2
relative
to
surfaee
or
intermediate depth waters). Secondly,
the
eolleetive
bacterial reactions within each zone generally result
in CO^
released
in the
fortn
of
dissolved bicarbonate (HCO;i")
or
carbonate
(COr )
which
can be
precipitated
as
diagenetic
concretions (Irwin ciat.. 1977). FinaJly. CO:
is
itself consumed
by
the CO^
reduction pathway within
the
methanogenic zone
which generally results
in
lower CO: concentrations
in
marine
environments (where
CO:
reduction
is
dominant over acetate
fermentation) relative
to
freshwater environments (Clayton,
1995.
146).
Deeper within
the
sediment, thermal decarboxylation (i.e.,
abiogenic removal
of
carboxyl
and
similar oxygen bearing
functional groups from
the
organie matter) contributes further
CO2 which
may
reach appreciable concentrations. This process
occurs principally between
50 and 100'C
(Sweeney
and
Burnham,
1990) and to
some extent overlaps bacterial
methanogenesis
and
deeper thermogenic produetion
of
hydro-
carbons.
Nitrogen
Nitrogen
is
also released
by
bacterial processes during early
diagenesis
but is
generally
not as
abundant
as H:S and
CH4
as
a free gas.
It is
sourced
by
both dissolved nitrate (NO3
) and
organic bound nitrogen (pariicularly
in
proteins) although
the
latter
is
overwhelmingly dominant. Nitrate reduction
to
form
N2
is
generally considered
to be a
suboxic process
but in
practice there
is
considerable nitrate reduction within
the
aerobic zone also.
The
decomposition
of
proteins (deamina-
tion) occurs both within
the
aerobic zone
and
deeper
and
produces ammonia
(NHi) as an end
product, usually
in the
form
o\'
dissolved ammonium
(NH4 ). In the
presence
of
oxygen,
the
ammonia
may
then
be
oxidized
to
nitrate
(by
Nilro.somonas
sp.) or
nitrite
(NO: . by
.Mlmtnicler
sp.) and
can then
be
bacterially reduced
to
molecular nitrogen deeper
in
the sediment.
Migrated gases
Gases migrated from greater depth
are
locally abundant
in
surface sediments within petroliferous sedimentary basins (i.e.,
those containing eeonomic
oil and
natural
gas
accumulations).
These gases result from thermal breakdown
of the
residual
organic matter which
has
survived early diagenesis
(see
Kerogen). typically
at
temperatures above
lOO C
(Pepper
and Corvi, 1995). They
are
characteristically dominated
by
methane with varying eoncentrations
of
higher hydrocarbons
(ethane, propane, etc.),
CO2 and N2.
Where migration from
depth
is
highly active, sueh gases
may
escape
to the
water
column
as
active "seeps", often assoeiated with
a
number
of
assoeiated sedimentary features such
as
diapirism, pockmarks,
ehetnosynthetic communities, clathrates (q.v.)
and
earbonate
cementation
(see
Hovland
and
.ludd. 1988.
for a
full discussion
of these features).
Summary
Where present, free gases
in
sediments
are
dominated
by
methane, hydrogen sulfide, carbon dioxide
and
sometimes
traces
of
nitrogen
or
ammonia. Althotigh occasional migrated
deep-sourced thermogenic gases containing higher hydrocar-
bons
are
locally important,
the
gases
are
dominantly
the
result
of in si lit bacterial reactions during early burial diagenesis
of
organic rich sediments. Carbon dioxide
is
generated
at all
depths, N2
is
Ibrmed mainly
in the
suboxic zone, H2S
is
formed
306
GEODES
below this under sulfate reducing conditions and CH4 is
formed deeper still.
Chris J. Clayton
Bibliography
Clayton,
C.J.,
1992. Source voluinetrics
of
biogenic
gas
gcneralion.
In
Vially.
R.
(ed.). BacierialGas. Paris: Edition Technip, pp. 191-204.
Clayton.
C.S..
1995. Microbial
and
Organic Proeesses.
in
Parker.
A.
and Si;llwoo(J,
R.W.
(eds.). Quantiialive Diagenesis: Reeeni Devel-
opments
and
Appliealions
lo
Reservoir Geology.
The
Netherlands:
Kiuwer AcademiL- Publishers,
pp.
125-160.
Hovland.
M., and
Jiidd, A.G.. 1988. Seabed Poekniarks and
Seepages.
Impact on
Geology.
Biotogy
and
the Marine Fnvironnwnl. London:
Graham
&
Trotman.
Irwin,
H..
Cnrtis.
C, and
Coleman, M.L., 1977. Isotopic evidence
for
the source
of
diiigenetic carbonates formed during burial
of
organic-rich sediments, Nature. 269:
209 213.
Iversen.
N,. and
Jorgensen, B.B,. 1985. Anaerobic methane oxidaiion
rates
at the
sulfiite-methane transition
in
marine sedmients from
the Kattegat
and
Skagerrak (Denmark). Limnology
and
Oeeano-
graphy.
30: 944 955.
Pepper.
A.S.. and
Corvi. P..I..
1995.
Simple kinetic models
of
petroleum formation. Part
I: oil and gas
generation from kerogen.
Marine
d
Pelroteum Geotogy.
U:
291
319.
Rice,
D.[),,
and
Claypool, G.E.. 1981, Generation, accumulation
and
resource polerlial
of
biogenic
gas.
Ameriean Association
oj
Petro-
teum
Geotogi.sts
Bullelin.
65: 5 25.
Sweeney.
J.J., and
Buinham.
A.K.. 1990.
Evalnation
of a
simple
kinetic model
of
vitrinite retleclance based
on
chemical kinetics,
Ameriean Assoeiation
of
Peiroleum Geotogists Bulletin.
74: 1559
1570.
Zeikus,
J.G.. and
Winfrey.
M.R.. 1976,
Temperatnie limitation
of
methanoiienesis
in
aquatic sedimenis. Applied Fnvironmenlat
Mi-
erohiotogy. 31:
99 107.
Cross-references
Bacteria
in
Sediments
Black Shales
Clalh rales
Hydrocarbons
in
Sediments
Kerogen
Maturation, Ortianic
Peat
Sulfide Minerals
in
Sediments
GEODES
Figure
G2 lA)
Geode trom
a
volcanic host rock.
A
thakeduny
rim
lollowed
by
tOiirse quartz crystals
is
A
widely observed sequence
in
bolh volcanic and sedimentary geodtfs.
A
late precipitate
of
caicite
is
observed
In
llie f^eode interior. Diameter
15 tm.
Barron Collection,
Texas Memorial Museum, University
of
Texas
at
Austin.
(B)
CalcJte
gende
in
<i clfilomitic limestone. Many geodes
ot
this type form
by
replacement
of
earlier-formed nodular anhydrite. Diameter
approximately 12 cm. Collection of the author. Photos by
Joe
jaworski.
Introduction
Geodes are nodules (i.e.. mineral aggregates having a
composition that contrasts strongly with that of the surround-
ing rock) containing an interior cavity lined with macroscopic
crystals (i.e., crystals sufficiently large to be distinguishable
without magnification). Geodes occur primarily within lime-
stones, dolomites, and mudrocks, but some are found in
volcanic rocks. The term geode is largely synonymous with
"vug" (a crystal-lined eavity), but has the additional connota-
tion that the object has, or cotild, weather free by virtue of its
compositional and textural distinctiveness from the host rock.
The sizes and shapes of geodes vary widely, but most are in the
range of 5 to 10 cm across and roughly spherieal or oblate.
Geodes are recognized as a group partly on the basis of their
appearance and distinctiveness, in particular their attractive-
ness to mineral collectors. (Figure G2).
Geode formation
All geodes share certain genetic aspects. Foremost, crystals in
geodes are emplaced by precipitation from aqueous solution
(authigenesis) within ca\ities that are anomalously large in
eomparison to typical pore spaces in the surrounding rock.
These essential characteristics and the attendant necessity for
substantial volumes of mineral-bearing fluids to flow through
low- penneability roeks were reeognized in some ofthe earliest
reports on geode formation (e.g.. Dana and Shepard, 1845;
Shaler, 1899). Abnormally large pore spaces form in a variety
CFODFS 307
ot" vviiys. Cavities crcitted by dissolution ot^ earlier-formed
nodules, ga.s vesicles in volcanic rocks, cavities within fossils,
desiccation cracks within early-formed concretions (see Sep-
tarian Nodules), and pore spaces associated with teetonie
IVactures can all become the locus of geode formation.
Categories of geodes
Most yeodes belong to one of two major eategories. Quartz
replacement of former gypsum and anhydrite is responsible for
the caulifloweruodulcs found in Carboniferous shallow-marine
limestones and dolomites of the upper Mississippi Valley
region ofthe central United States. Theclassie locality for sueh
geodes is at Keokuk. Iowa (e.g.. Van Tuyl, 1916; Hayes, 1964;
Sinoite. 1969). Similar silicified evaporites are known world-
wide in shallow marine roeks ranging in age from Preeanibrian
\o Ccnozolc (e.g.. Chowns and KIkins. 1974: Siedleeka. 1976;
Kisher, 1977; Milliken, 1979; Radke and Nieoll. 19SI; Matzko
and Naqvi. 1983: Elor/a and Rodriguez-Lazaro. 1984; Maliva,
19S7:
Ulmci-Scholle. Scholle
('/(//.
"l99.'*). Many calcite geodes
in sedimentary rocks also have features consistent with
formation by evaporite replacement.
An intriguing aspeet of silicitied evaporite nodules is
expansive growth during their early development. Formation
of evaporite nodules is a syn-sedimentary or very early
diagenetie proeess. occurring prior to lithiftcation of the
enelosing sediment. The e\aet nature of mieroen\ironments
responsible for evaporite loealization is uncertain, but it is
elear that in some cases preeipitation was localized within the
skeletal remains
oi^
marine invertebrates leading to dramatic
expansion or '"explosion*" ofthe fossils (Figure 03) (Maliva.
1987;
Foerstcr, 1991). Fossils expanded in this manner
attraeted great interest from early students of geodes (e.g..
Shaler. 1899; Bassler. 1908), but a convincing mechanism for
expansion eluded these workers. Growth within uncoiisoli-
dated sediment, either through direct displacivc precipitation
o\'
anhydrite, or by expansion (approximately 3O'!''n) during
hydration of anhydrite to gypsum provides a plausible
inechanism tor the development of exploded fossils. Replaee-
ment of the early-formed nodules during later diagenesis is
accomplished by dissolution ofthe evaporite minerals followed
by precipitation of authigenic quartz into the resulting pore
space. In kUippcrsicins some of the later-formed quart/
crystals, perhaps onee encased in the now-vanished evaporite
mineral, are loose within the geode interior and rattle audibly
against the inner uatls when the geode is shaken.
Figure G3 Ciuliflower chert nodules lexpioded fossilst lormed by
siliciiiLalton ot anhydriie. Specimen on the lefl (icm talll is a silicitied
blastoid of the genus Fvntrcmilcs. The three specimens on the right
show progressive disruption of similar blastoids by displacive growth
of the anhydrite, now completely replaced by quartz. Specimen on the
t.ir right is a klapperstein (see text). Collection of the author. Photo by
)i)e l.iworski.
A seeond important category of geodes eorresponds to
silicified vesieles in tuffaeeous voleanie rocks ranging in
composition from rhyolite to basalt. Weathering of unstable
minerals and voleanie glass provides high concentrations oi
dissolved silica to pore fluids in such rocks. In the northern
Mexieo state of Chihuahua, quartz-lined vesieles weathered
from rhyolitic and andesitic tuffs supply an abundance o^
geodes to mineral collectors (Finkleman. 1974; Keller. 1979;
Cross.
1996).
Precipitation of cavity-filling minerals
Although geode development must postdate the (orniation of
the enelosing rock, the timing of mineral precipilation can vary
from near-syndepositional to mueh later in the roek's history.
Temperatures in the range of
25'
to 100 C have been estimated
for quartz in silicified evaporite nodules, based on two-phase
lluid inclusions and stable isotopic evidence (Milliken. 1979;
Roedder. 1979): similar temperatures are estimated for quartz
in volcanic geodes (Keller. 1979). The necessity for a large,
open pore space through whieh significant volumes of mineral-
bearing fluids can flow most likely restricts get)de formation to
relatively shallow depths in the crust (<6 km?).
Quarts in a wide variety of textural and color subtypes is the
most common geode-filling mineral. In both sedimentary and
voleanie rocks, there is a eommon sequenee of mineral
formation from the outer rim to the eenter of the geode.
Microcrystaliine tjiiartz. either eqiiant or fibrous (chalcedony)
mieroqiiartz, forms the initial precipitate on the interior
surface of the cavity, followed by megaquarlz whieh may
develope inward-projecting euhedra around the central eavity.
Other common geode-forming minerals are ealcile, dolo-
mite,
and, in voleanie roeks, zeolites. Most minerals that occur
in geodes postdate the early-formed quartz and belong to a
mineral assemblage that also occurs in associated veins and
fracture fills. Aragonite. ankerite. celestite. magnetite, hema-
tite,
marcasite, pyrite. millerite. chalcopyrite, galena, sphaler-
ite,
fluorite. and kaoiinite are widely reported in silicified-
evaporite-type geodes (e.g.. Goldstein and MeKenzie. 1997).
Many of the same minerals are reported to oeeur in volcanic-
associated geodes along with a wide variety of zeolites and
authigenic feldspars (e.g., Finkleman. 1974; Frnst and Hall.
1975).
Particularly intriguing to collectors are geodes that retain
fiuids within the interior cavity. Water-bearing geodes. cnhy-
(Iros. are obtained from volcanic deposits in Brazil (e.g.,
Boutakoffand Whitehead, 1952; Prashnowsky. 1990). Petro-
leum-filled geodes are reported in limestones near Niota.
Illinois (e.g.. Robertson. 1942; Spitznas. 1949).
Research on geodes
Much infortnation on geode occurrence is found within the
mineral collecting literature. In the academic arena, geodes
have received surprisingly little attention despite the fact that
the large pore spaces within geodes permit the growth of
relatively large erystals that represent aeeessible satnples of
authigenie minerals that may otherwise be pervasive through
the host rock but also rare or cryptie. The sequenee of mineral
precipitation observable within geodes and the elemental and
isotopie zoning within the geodc's large crystals can record an
eminently readable history o\' fluid/rock interaction across a
308
GEOPHYSICAL PROPERTIES
OF
SEDIMENTS (ACOUSTICAL, ELECTRICAL, RADIOACTIVE)
substantial range of diagenetic conditions (e.g., Milliken,
1979).
Summary
Authigenic mineral precipitation on the walls of large cavities
in sedimentary and volcanic rocks produces the nodules
known as geodes. The large crystals of authigenic minerals
found within geodes have interest from the dual standpoints of
their esthetic qualities and the intriguing record of rock
chemical history that they represent.
Kitty L. Milliken
Bibliography
Bassler,
R.S.,
1908.
The
formation
of
geodes, with remarks
on the
silicification
of
fossils.
Proceedings
of the
United
States National Mu-
seum,
133-154.
Boutakoff,
N., and
Whitehead,
S. 1952.
Enhydros
or
water-stones.
Mining atid
Geological
Journal, 4(5): 14-18.
Chowns, T.M.,
and
Elkins, J.E. 1974. The origin
of
quartz geodes
and
cauliflower cherts through
the
silicification
of
anhydrite nodules.
Journalof Sedimentary
Petrology,
44:
885-903.
Cross,
B.L., 1996.
The Agates of Northern Mexieo. Edina, Minnesota,
Burgess International Publishing Group.
Dana,
J.D., and
Shepard,
C.U.,
1845. Origin ofthe constituent
and
adventitious minerals
of
trap
and
allied rocks. Ameriean Journalof
Science, 49: 49-64.
Elorza,
J.J., and
Rodriguez-Lazaro,
J., 1984,
Late Cretaceous quartz
geodes after anhydrite from Burgos, Spain. Geological Magazine,
121:
107-113.
Ernst, W.G.,
and
Hall,
C.A. Jr.,
1975. Feldspathic geodes near Black
Mountain, western
San
Luis Obispo County, California. Ameriean
Mineralogist,
60:
1105-1112.
Finkleman,
R., 1974. A
guide
to the
identification
of
minerals
in
geodes from Chihuahua, Mexico. Lapidary Journal,
27: 1742-
1744.
Fisher,
I.S., 1977.
Distribution
of
Mississippian geodes
and
geodal
minerals
in
Kentucky. Econotnic
Geology,
72:
864-869.
Foerster,
R.,
1991. Fossil geodes. Lapidary Journal, 45(3): 102-105.
Goldstein,
A., and
McKenzie,
B. 1997.
Halls
Gap,
Lincoln County,
Kentucky. The Mineraiogieal
Reeord,
28: 369-384.
Hayes,
J.B., 1964.
Geodes
and
concretions from
the
Mississippian
Warsaw Formation, Keokuk region, Iowa, Illinois, Missouri.
Journal of Sedimentary
Petrology,
34:
123-133.
Keller,
P.C,
1979. Quartz geodes from near
the
Sierra Gallego area.
Chihuahua, Mexico. The Mineraiogieal
Reeord,
10(4): 207-214.
Maliva, R.G., 1987. Quartz geodes: early diagenetic silicified anhydrite
nodules related
to
dolomitization. Journalof Sedimentary Petrology,
57:
1054-1059.
Matzko,
J.J., and
Naqvi,
J.M.
1983. Geodes from Saudi Arabia.
La-
pidary Journal, 37(8): 1140-1156.
Milliken, K.L., 1979.
The
silicified evaporite syndrome:
two
aspects
of
silicification history
of
former evaporite nodules from southern
Kentucky
and
northern Tennessee. Journal of Sedimentary
Petrol-
ogy,
49:
245-256.
Prashnowsky, A.A., 1990. Biogeoehemical study
of
enhydros (Brazil).
Mitteilungen aus dem
Geologisch-Palaeontotogisehen
Institut der Uni-
versitaet Hamburg, 69:
35-43.
Radke, B.,
and
Nicoll, R.S. 1981. Evidence
for
former evaporites
in the
Carboniferous Moogooree Limestone, Carnarvon Basin, Western
Australia. BMR Journal of Australian
Geotogy
and
Geophysies,
6(1):
106-108.
Robertson,
P., 1942.
Bituminous matter
in
Warsaw geodes [Illinois].
Transaetions
ofthe Itlittois State Academy of Seienee, 35: 138-140.
Roedder,
E., 1979.
Fluid inclusion evidenee
on the
environments
of
sedimentary diagenesis,
a
review.
In
Scholle,
P.A. and
Schluger,
P.R. (eds.), Aspeetsof Diagenesis. Tulsa, Oklahoma, SEPM, 26:
pp.
89-107.
Shaler, N.S., 1899. Formation
of
dikes
and
veins. Geologieal Soeietyof
Ameriea Bulletin, 10: 253-262.
Siedlecka,
A., 1976.
Silicified Precambrian evaporite nodules from
northern Norway;
a
preliminary report. Sedimentary Geotogy,
16:
161-175.
Sinotte,
S.R., 1969.
TheFabutousKeokukGeodes-M
Volume
I.
their ori-
gin,
formation,
and
development
in the
Mississippian lower Warsaw
beds
of
southeast Iowa atid adjaeent states.
Des
Moines, Iowa,
Wallace-Homestead
Co.
Spitznas, R.L., 1949. Petroliferous geodes, their occurrence
and
origin
[Tyson Creek area, Illinois]. Earth Seienee Digest, 3(11): 15-18.
Ulmer-Scholle,
D.S.,
Scholle,
P.A. et al., 1993.
Silicification
of
evaporites
in
Permian (Guadalupian) back-reef carbonates
of the
Delaware Basin, West Texas
and New
Mexico. Journal
of
Sedi-
mentary Petrology, 63: 955-965.
Van Tuyl,
F.M., 1916. The
geodes
of the
Keokuk beds. Ameriean
Journal of Seienee, 42(192): 34-42.
Cross-references
Authigenesis
Cements
and
Cementation
Diagenesis
Diagenetic Structures
Septarian Concretions
GEOPHYSICAL PROPERTIES
OF
SEDIMENTS
(ACOUSTICAL, ELECTRICAL, RADIOACTIVE)
Introduction
Geophysics is an important tool for investigating sediments in
the Earth's subsurface. The conversion of geophysical data
into geological information requires an understanding of the
connection between the nature of the sediments and the
physical properties that govern their geophysical response, a
discipline called petrophysics or rock physics.
Sediments are heterogeneous systems; their effective physi-
cal properties are a function of the properties and volume
fraction of the individual components (i.e., the minerals that
compose the grains and the fluid species filling the pores), as
well as the geometric configuration of these components. This
heterogeneity occurs on a wide range of size scales, and these
scaling effects on the macroscopic properties of rocks are an
active field of investigation. This article will assume for
simplicity that size scale of the heterogeneities are on the
order of the individual rock grains and pores, forming a single
lithologic unit for the larger size scale at which the effective
properties are measured.
Even with this assumption, sediments are very complex
systems from the petrophysical point of view. A specific
physical property can have a broad range of values for a given
lithology, making the analysis and interpretation of the
geophysical properties of sediments in terms of geological
factors a difficult task. With this in mind, the aim of this article
to introduce the reader to petrophysical principles involved in
the acoustical, electrical and radioactive properties of sedi-
ments; in-depth information on this subject can be found in a
number of recent texts (e.g., Gueguen and Palciauskas, 1994;
Mavko etal., 1998; Schon, 1996).
Acoustical properties
The propagation of elastic waves through geological materials
is the basis for seismic methods and acoustic (or sonic) logging.
GEOPHYSICAL PROPERTIES OF SEDIMENTS (ACOUSTICAL, ELECTRICAL, RADIOACTIVE) 309
Table Gl A list of compressional (Vp) and shear (Vs) wave velocities for a variety of sedimentary rocks
Lithology
Vp (in m/s)
;
(in m/s)
Chalks (water saturated)'
Dolomites (water saturated)'
Limestones (water saturated)'
Sandstones (water saturated)'
High porosity sandstones (water saturated)'
Poorly consolidated sandstones (water saturated)'
Tight-gas sandstones (dry)'
Shales^
Anhydrite^
Gypsum^
Halite^
1530-4300
3410-7020
3390-5790
3130-5520
3460-4790
2430-3140
3810-5570
1830-5180
6090
5800
4590
1590-2510
2010-3640
1670-3040
1730-3600
1950-2660
1210-1660
2590-3500
3470
2660
Sources: 'Mavko etat., 1998; ^Schon, 1996; ^Gueguen and Palciauskas, 1994.
With minor exceptions, these methods use body waves (i.e.,
waves that travel through a body's interior) to investigate the
Earth's subsurface. There are two types of body waves that
differ in terms of their particle motion relative to the
propagation direction of the wave. P waves (or compressional
waves) have particle displacement in the direction of wave
propagation; S waves (or shear waves) have particle displace-
ment perpendicular to the propagation direction. The com-
monly measured and analyzed acoustic properties of
geological materials are the velocities of these two body waves.
For a homogeneous and isotropie medium, the P and S
wave velocities are given by
and
Vs
=
\fWp-,
(Eq- I)
respectively, where k and ^l are the effective bulk and shear
moduli of the medium and p is its density. The elastic moduli
quantify the deformation a material undergoes when subjected
to different types of applied stress. The bulk modulus k relates
volumetric changes to variations in applied pressure. The shear
modulus [I describes the shape distortion due to a pure shear
stress;
there is no volumetric change accompanying the pure
shear stress. The dependence on density is due to inertial
effects, such as those described by Newton's laws of motion.
The overwhelming majority of seismic and acoustic well
logging data involves the propagation of P waves; however,
information about S wave propagation is becoming more
readily available. A list of P and S wave velocities for a variety
of sedimentary rocks is given in Table Gl.
The velocity ratio
VplV^
is also used in the analysis of
velocity data; Table G2 give the ranges of theis ratio for
different sedimentary lithologies. It can be seen from Equation
(1) that this ratio only depends on the elastic moduli. The
composition and microstructural elements of sediments affect
k and \i differently; these differences are refiected in the
ratio.
Acoustic velocities of carbonates and sandstones
Due to their importance as potential hydrocarbon reservoirs,
the acoustic velocities of carbonates and sandstones have been
more extensively studied than other sedimentary lithologies.
An obvious factor affecting these velocities is the composition
of the solid grains. Typically, carbonates have higher Vp than
sandstones having the same porosity (Table Gl). This is
related to the differences in Vp for the rock forming minerals
calcite (6650 m/s), dolomite (7370 m/s) and quartz (6060 m/s).
Table G2 Typical range of the velocity ratio
lithologies (Domenico, 1984)
for common
Lithology
Dolomites'
Limestones'
Sandstones'
Shales^
1.78-1.84
1.84-1.99
1.59-1.76
1.70-3.00
Sources: 'Domerico, 1984; ^Schon, 1996.
However, variations in porosity and microstructure produce a
range of acoustic velocities for each lithology. These factors
produce significant overlapping of the velocity ranges that
makes it difficult to differentiate between carbonates and
sandstones using only Vp.
Ambiguity is reduced in the lithology-velocity relationship
by analyzing the velocity ratio VplVs. It can be seen from
Table G2 that this ratio is commonly larger for carbonates
than for sandstones. Further, the velocity ratio VplVs has been
used to infer composition in mixed siliceous carbonate
systems. Lithological discrimination using the velocity ratio
is based on the large difference in VplVs between quartz {Vpl
Ks=1.47) and carbonate minerals (VplVs = 1.93 for calcite
and Kp/Fs= 1.85 for dolomite).
Another lithological element affecting acoustic velocities is
clay content in silicielastic rocks. Small amounts of clay in
sandstones can reduce its elastic moduli, producing lower
velocities. The effects of clay content are greater for Vs due to
the lessened rigidity or stiffness (i.e., lower /i) associated with
clay; this produces higher VplVs ratio with increasing clay
content. Further, it has been found that clay effects are
determined by its position among the sand grains. Clay in
load-bearing positions, such as contact clay cements, affects
sandstone stiffness; non-load-bearing clay that occupies pores
has little impact of sandstone stiffness.
A significant factor affecting acoustic velocities of carbo-
nates and sandstones is the porosity and pore structure.
Increasing porosity lowers Vp and Vs', the magnitude of this
effect depends on the shapes of the added pore space.
Experimental and modeling results show that the cracks and
fractures lowers the acoustic velocities of rocks much more
than the same amount of more spherical pores such as vugs
and main intergranular voids. This effect is due to the
increasing ease with which flatter pores can be deformed.
310 GEOPHYSICAL PROPERTIES OE SEDIMENTS (ACOUSTICAL, ELECTRICAL, RADIOACTIVE)
Further, pore shape affects fi more than k; hence, Vs is more
sensitive to the presence of crack-like pores than Vp.
Increasing VplVs ratio is used as indicator of fracturing.
The grain composition and pore structure are dependent on
many geological factors such as the source material, deposi-
tional environment and diagenesis. Attempts have been made
to connect these geological factors with elastic wave velocities,
starting with Faust (1951) relating Vp of silicielastic rocks to
age of burial. More recently, a number of systematic studies
have been published that analyze the relationship between
acoustic velocities and geological parameters for carbonate
rocks (e.g., Anselmetti and Eberli, 1997) and silicielastic rocks
(e.g., Vernik and Nur, 1992).
Acoustic properties of shales
While shales constitute a majority of the sedimentary rock
mass,
their acoustic properties have not been well documented
in the past due to their lack of importance as reservoir rocks
and the exceptional difficulty of performing laboratory
measurements on them. However, their importance as source
rocks and their effects on seismic imaging is leading to greater
interest in their properties. From Tables Gl and G2, it can be
seen that shales have a wide range of acoustic properties. Their
acoustic velocities rise with an increasing degree of induration
due to growing stiffness; hence, the VplVs decreases with
induration.
The preferred alignment of clay platelets in shales produces
significant velocity anisotropy (i.e., directionally dependent
acoustic velocities). Both Vp and Vs are consistently lower in
the direction perpendicular to bedding than parallel to
bedding. Velocity differences up to 30% for Vp and 35% for
Vs have been measured.
Both the presence of kerogen and its maturity are factors
that infiuence the acoustic properties of shales. Acoustic
velocities decrease with rising organic content due the lower
elastic moduli ofthe kerogen. Acoustic velocities also decrease
with kerogen maturity; several factors have been suggested for
this behavior, such as the formation of free phase hydro-
carbons and fracturing associated with the development of
overpressuring. The orientation of these fractures will affect
the velocity anisotropy of the shale.
Unconsolidated sediments acoustic velocities
Uneonsolidated sediments have distinctly lower velocities than
their equivalent consolidated lithologies. For instance, a
typical range of Vp is 200-500 m/s for dry sediments and
1600-2000 m/s when water-saturated. These lower velocities
are due to the low rigidity possessed by unconsolidated
sediment, especially near the transition to a fiuid supported
suspension. This condition produces the extreme low Vs valves
observed in situ for some unconsolidated sediments (e.g.,
~ 100 m/s in water saturated marine sediments). Hence, very
large values of VplVs (i.e., 3 and greater) are possible for
unconsolidated media.
These sediments can be divided into two groups: granular
media, such as sands and gravels, and muds principally
composed of clay and silt. Experimental and theoretical
investigations have been chiefiy directed toward the behavior
of granular sediments. In granular materials, acoustic wave
propagation through the solid framework is dependent on the
number of grain contacts per particle and the contact
micromechanics.
The number of contacts per grain determines how elastic
energy is transmitted throughout a granular medium. The
number of contacts per grain is determined by the nature of
grain packaging, particle shape and grain-size distribution. For
example, the compact hexagonal packing of identical spheres
gives 12 contacts per sphere, whereas the simple cubic packing
has 6 contacts. A dense random packing is a more realistic
representation of unconsolidated sediments, these systems
have an average of 6.8 contacts per sphere with 2.1 near
contacts that can close to form additional contacts as pressure
conditions change.
The contact micromechanics describe how elastic energy is
transmitted between adjacent grains at an individual contact.
Different types of contact micromechanics have been used to
explain the velocity data for granular sediments. These contact
phenomena are dependent on a number of parameters such as
bulk grain properties, grain shapes, surface energy, surface
roughness and the macroscopic sense of the applied stresses. In
unlithified sediments, these interactions occur at the relatively
small contact areas. As lithification occurs, pressure solution
and cementation modify the nature of the contact micro-
mechanics by strengthening the bond between spheres and
increasing the contact area. The lithification process enhances
the rigidity of the sediments and is refiected in increased
acoustic velocities, especially Vs. Hence, the lithification
process is accompanied by decreasing
Acoustic velocities of evaporates and coal
Evaporates (e.g., halite, gypsum) are essentially zero porosity
rocks;
the velocities of these lithologies strongly reflect their
mineral content (Table Gl). However, these velocities are not
significantly different from porous sedimentary rocks. Coal is
distinguished from other sedimentary rocks by its very low Vp.
P-wave velocities in coals are a function of grade, ranging from
a low of 1790 m/s for lignite to 3050 m/s for anthracite.
Dependence on pore fluids
Pore fiuid properties determine the elastic behavior of the pore
space; they are strongly dependent on pore fiuid composition,
temperature and fiuid pressure. At seismic frequencies (i.e., less
than 2 kHz), Vp is affected by variations in pore fiuid
properties. Vs is relatively insensitive to these changes,
responding only to fiuid density variations. This condition
results from lack of rigidity in fiuids (i.e.,
fif\mds
=
O)
sfd
complete pressure equilibration throughout the pore structure.
Pressure equilibration between the pore spaces means its
contents act as a single effective pore filling. The effective bulk
modulus ^eff for a pore space containing water (k^) and gas
(/Cg) phases is given by
{\ -
(Eq. 2)
where 5,^ is water saturation (i.e., volume fraction of the
porosity filled with water). Since the bulk moduli of liquids is
much greater than those of gases. Equation (2) predicts that
the presence of a gas phase, even in small amounts, will
significantly lowers the bulk modulus of the pore filling. For
example, consider a pore space containing 98% water
(k^j
=
2.2 X
10*^
Pa (or N/m^) and 2% air {kg= 1.55 x lO' Pa); it acts
as if it is filled with an effective fiuid with k^ff=l.l x 10* Pa.
GEOPHYSICAL PROPERTIES OF SEDIMENTS (ACOUSTICAL, ELECTRICAL, RADIOACTIVE)
311
Hence, the introduction of gas can produce large decreases in
Vp relative to its value when the medium is liquid saturated.
Given the insensitivity of Vs to changing pore fluid properties,
a decreased VplVs ratio is used an indicator for natural gas.
Further, the presence of a dissolved gas phase, such as occurs
in live oils, can reduce the pore fluid bulk modulus and
lower Vp.
Effects of pressure and temperature conditions on
velocities
Pressure effects are primarily dependent on effective (or
differential) pressure Ap whicli is the difference between the
external confining (or overburden) pressure p^ and the internal
fluid pressure pc. Increasing Ap causes pore closures in
consolidated reservoir rocks, resulting in velocity increases.
The velocity changes are more rapid at lower Ap values and
slacken as Ap grows. This behavior is due to the swift closure
of the more compliant intergranular cracks and fractures,
leaving the more spherical pores that are much harder to
collapse. In unconsolidated materials, increasing Ap narrows
the distance between grains in the medium. This phenomenon
produces stronger bonding at existing grain contact and
creates new contact as smaller intergranular gaps close.
Conversely, the existence of geopressure will reverse this
closure process and produce lower velocities. Variations in p^
can also affect pore fluid properties, particularly the gas phase
compressibility and density, as well as gas solubility in liquids.
Temperature change generally has a relatively small affect
on the elastic wave velocities of reservoir rocks with increasing
temperature causing minor velocity decreases. Major varia-
tions in velocities that are observed are primarily due to
changes in pore fluid properties. Significant temperature
induced change can results if it associated with fluid phase
transformations (e.g., freezing) or large bulk modulus and
viscosity changes in heavy oils.
Velocity dispersion
Dispersion is the variation of a physical property with
measurement frequency. Most laboratory measurements of
elastic wave velocity are performed at ultrasonic frequencies
(i.e.,
above 100 kHz) while acoustic well logging and seismic
exploration are done at lower frequency ranges (i.e., 10-20 kHz
and below 1 kHz, respectively). Since there is a significant
difference in measurement frequencies, an understanding of
dispersion is necessary for relating laboratory results to the
interpretation of geophysical field measurements.
Both Vp and Ks increase with increasing measurement
frequency for porous rocks containing pore liquids. While
there are very few direct measurements of velocity dispersion;
it can be estimated using ultrasonic measurements on dry
samples to predict low frequency velocities. This method
predicts that the magnitude of velocity dispersion can range up
to 20% or more; however, it is frequently 10% or less for water-
saturated sedmentary rocks. Dispersion is enhanced by the
presence of intergranular cracks; hence, dispersion decreases as
cracks close with increasing effective pressure. In addition,
dispersion can be significantly large for sedimentary rocks
containing gas-liquid mixtures and viscous oils.
Several mechanisms have been proposed as the cause of
velocity dispersion in porous rocks. Current evidence indicates
that the local flow mechanism (Murphy etal., 1986) is the
dominant mechanism in most porous rocks. At low frequen-
cies,
fluid pressure equilibration is maintained by fluid flow
between pores in response to varying degrees of deformation
within sections of the pore space. Dispersion due to this
mechanism results from the breakdown
oi"
interpore fluid flow
as frequency increases due to viscous forces. In the absence of
this flow, individual pores act as isolated units within the
porous rock at high frequencies. Under these conditions, pore
spaces, in particular the intergranular cracks, become harder
to deform and result in larger k and
pi
for the porous rock.
Another dispersion mechanism that is proposed for
sedimentary materials is the Biot (1956) dynamic poroelasti-
city. Dispersion due to this mechanism is due to the
progressive decoupling of the average motion of the solid
component from tliat of the pore fluid as frequency increases.
At low frequencies, the motions of the solid and pore fluid are
perfectly coupled. At high frequencies, the decoupled motions
iead to inertial effects that change the effective elastic moduli
and density of the rock. Laboratory results and theoretical
modelling indicate that this mechanism is important in
materials with open pore structures where local flow effects
are small.
Petrophysieal relationships for acoustic velocities
A wide variety of relationships are available for analyzing and
predicting the elastic wave velocities of reservoir rocks. Some
are empirical relationships between velocity and parameters
(e.g., lithology, porosity, clay content, pressure) obtained from
the statistical analysis of laboratory and field measurements.
However, an empirical relationship could have limited
accuracy when applied to sediments different from those used
in its development.
One of the most commonly used relationships is the time-
average equation of Wyllie etal. (1956); this relationship has
been extensively used in acoustic well logging analysis. It
assumes that the traveltime for a P-wave through a unit
thickness of porous medium is the sum of the traveltimes
through the volumetric fractional thicknesses of the solid
matrix and pore fluid. The resulting relationship between the
velocities of the porous medium, solid matrix and fluids {V,
and Km, Kf, respectively) is
4>
V
(Eq. 3)
where
cp
is porosity. Table G3 gives a list of solid matrix and
pore fluid velocities for use in this relationship. While this
mixing formula can give reasonable estimates for Vp, its
neglects the effects due to the geometrical configuration of the
constituents, in particular the pore structure. This element of
the time-averaging approach is partially reflected in the range
of Km values in Table G3 used for each lithology.
Relationships given by Gassmanri (1951) are frequently used
to analyze the effects of pore fluids on acoustic yelocities.
These relatioh'ships are'valid for any porous rhedium'"wheh
complete fluid pressure communication occurs throughout the
pore space (i.e., perfect interpore fluid flow); this condition is
approximated during seismic wave propagation. However, the
Gassmann relationships use effective moduli to implicitly
incorporate pore structure effects; this feature limits its
predictive capacity.
312
GEOPHYSICAL PROPERTIES
OF
SEDIMENTS
(ACOUSTICAL,
ELECTRICAL,
RADIOACTIVE)
Table
G3
Range
of
solid matrix
and
pore fluid velocities used
in
Wyllie's time average relationship (Matrix values from
Schon,
1996)
Material
or Vf (in
m/s)
Sandstone (general)
Sandstone (consolidated)
Sandstone (semi-consolidated)
Sandstone (unconsolidated)
Limestone
Dolomite
Fresh water
Brine
(200
kppm NaCI)
Oil
Air
3049-5977
5797-5882
5291-5482
2500-5184
6402-7008
6969-7008
1524
1737
1311
332
Another approach used to analyze the elastic wave velocities
of sedimentary rocks is the inclusion-based formulation. This
formulation treats a porous rock as a solid matrix and
embedded inclusions representing individual pore spaces.
Hence, the pore structure of the rock is defined in terms of
the inclusion shapes and concentration. These formulations
commonly assume that no fiuid pressure communication
between pores (i.e., complete interpore fluid flow breakdown);
this condition replicates the high frequency condition for the
local fiow dispersion mechanism. Therefore, inclusion-based
formulations have been found to be more accurate than the
Gassmann's relationships when predicting ultrasonic velocity
measurements. The connection between Gassmann's relation-
ships and inclusion-based models has now been established,
permitting a framework to analyze the effects of pore geometry
and pore fluid properties on velocity dispersion.
Electrical properties
Electrical measurements of various types are made along the
Earth's surface or within boreholes. These geophysical
methods involve either the fiow of an electrical current
between electrodes or the propagation of an electromagnetic
(EM) field from a transmitter to a receiver. The behavior ofthe
current or field in a medium is dependent its electrical
resistivity, or its reciprocal electrical conductivity, which
quantifies the transport of electric current. It is one of the
most variable physical properties for geological materials,
extending over a range of values greater than 10^ for typical
sediments.
Resistivity of porous sediments
The extensive use of electrical and induction logging in
reservoir evaluation has lead to wide-ranging research on the
electrical resistivity of sandstones and carbonates. Since the
sediment grains are essentially insulators, the electrical
resistivity of clean (i.e., clay-free) sediments is controlled by
electrolytic (or ionic) conduction through the pore water. The
magnitude of the electrolytic conduction is a function of the
pore water resistivity p^ and the pore structure.
The value of p^ is directly related to the concentration ofthe
dissolved ions, and both can vary widely. For instance, at 75°C
water resistivity will decrease from 10 to 0.04 ohm-meters
(fi-m) as the dissolved NaCI concentration increases from 500
to 250000 ppm (i.e., from freshwater to brine). Further, ionic
species differ in terms of valence and mobility; hence, the ionic
composition of the pore water is also important. While the
porosity establishes the number of charge carriers available in
the porous medium, it is the pore space connectivity that
determines the paths followed by the ionic charge carriers.
Materials with open pore structure, such as those occurring in
well-sorted sands and gravels, permit more efficient current
fiow than in media with narrow, tortuous pore spaces.
Eleetrieal resistivity is also affected by surface conduction
along silica-fluid interfaces; this conduction is the combined
result of different surface phenomena. The magnitude of the
surface conduction for a material is proportional to its specific
surface area (i.e., surface area per unit pore volume); hence, it
is important conduction mechanism in clayey materials such as
shaly sandstones. It has been found that the style of clay
distribution infiuences this mechanism. Dispersed clay that
occurs between grains and along grain surfaces has more
surface area than clay laminations or framework grains. Clay
mineralogy is also a factor as surface area varies widely with
clay type. The strength of the surface conduction has been
observed to be nearly independent of
p^,',
it becomes the
dominant conduction mechanism in clayey materials when
fresh or low salinity water (i.e., large p^) is present.
Archie's equation
for
resistivity
The petrophysical relationship most commonly used for the
electrical resistivity of clean sediments was given by Archie
(1942).
This relationship states that the electrical resistivity ofa
water saturated porous medium p is given by
p
=
a4>-"'p^
(Eq. 4)
where a and m are empirical parameters. The combined terms
acj)'"'
are also referred to as the formation factor F
{
=
plp^).
The proportionality coefficient a takes on values between 0.5
and 3.5.
The parameter m implicitly incorporates the effects of pore
structure on resistivity. The observed value of
w
systematically
varies with the degree of consolidation in sandstones (Table
G4a);
hence, it is often called the cementation exponent. A
correlation between the value of m and porosity type also has
been noted for carbonates (Table G4b). For unconsolidated
samples, the exponent m has been related to partiele shape,
varying from 1.3 for spherical grains to 1.9 for thin disc-like
grains.
The presence of immiscible pore fluid species (e.g., air,
hydrocarbons, chlorinated solvents) is another factor affecting
electrolytic conduction porous sediments. Since immiscible
pore fluids are commonly electrical insulators, their presence
impedes electrolytic conduction through the pore space,
leading to higher resistivity values. Immiscible fluid effects
on the resistivity of porous media are described by the second
form of Archie (1942) equation:
p = «r"'VPw (Eq. 5)
where S^, is the water saturation and n is an empirical constant
called the saturation exponent. The observed values of n for
sandstones and limestones range between 1.1 and 2.6; it is
commonly assumed to be 2 when laboratory measurements are
not available. Further, it has been found that n for a given
sample is dependent on wetting conditions, immiscible species
present and saturation history. These factors influence the pore
scale fiuid distribution, affecting the geometry of the electro-
lytic conduction path through the water phase.
GEOPHYSICAL PROPERTIES
OF
SEDIMENTS
(ACOUSTICAL,
ELECTRICAL,
RADIOACTIVE)
313
Table
G4 The
typical
values
of the
cementation
exponent
m for
(a)
sands
with
varying
degrees
of
consolidation
and (b)
carbonates
with
different
porosity types
(Doveton, 1986)
Table
C5 The
abundance
of potassium,
uranium
and
thorium content
in
sediments
and
rock
forming
minerals.
Brackett
indicates average
value
(Bateman, 1985)
(a)
Unconsolidated
sands
Very
slightly cemented
sands
Slightly
cemented
sands
Moderately
cemented
sands
Highly
cemented
sands
(b)
Fractured
carbonates
Chalky
limestones
Crystalline
and
granular
carbonates
Carbonates
with
vugs
Petrophysieal relationship for shaley
The combined effects of electrolytic and :
m=l.3
m
1
4 1 S
/// —
1
.*+—
1 .J
m=
1.5-1.7
m=
1.8-1.9
m =
2.0-2.2
07=1.4
m=
1.7-1.8
»?=
1.8-2.0
/n =
2.1-2.6
sediments
surface conduction in
clay bearing sediments are generally treated as an equivalent
circuit consisting of parallel conductors;. A commonly used
petrophysieal relationship based on this model for water-
saturated shaly sands given by Waxman
and Smits (1968) is
Lithologies
Carbonates
Sandstones
Shales
Colorado
Oil
Shale
Minerals
Quartz,
Calcite,
Dolomite
Plagioclase
Orthoclase
Glauconite
Montmorillonite
Kaolinite
Illite
Micas
Phosphates
Zircon
K (%)
0.0-2.0
[0.3]
0.7-3.8
[1.1]
1.6-4.2
[2.7]
<4.0
0
0.54
11.8-14.0
5.08-5.30
0.16
0.42
4.5
6.7-9.8
-
—
U (ppm)
0.1-9.0
[2.2]
0.2-0.6
[0.5]
1.5-5.5
[3.7]
up to 500
0
-
2-5
1.5-3
1.5
-
100-350
300-3000
Th (ppm)
0.1-7.0(1.7]
0.7-2.0(1.7]
8-18 [12]
1-30
0
<0.01
<0.01
14-24
6-19
-
<O.OI
1-5
100-2500
•
+
BQ,
(Eq. 6)
The electrolytic conduction term
\la4>
"'pw is obtained from
the first Archie equation (Equation 4). In the surface
conduction term, Qv is the total charge in the electrical double
layer per unit pore volume and is related to the cation
exchange capacity of the material; hence, it can vary widely
with clay mineralogy. The empirical parameter B is the
counterion mobility. A similar relationship is available for
partially saturated shaly sands that accounts for the pore space
occupied by the insulating nonaqueous fluid phases.
Resistivity of shales, coal and evaporites
Shales generally have lower resistivity than sandstones and
carbonates due to their water content and the surface
conduction associated with clay minerals. An important factor
that can affect shale resistivity is the degree of thermal
maturity of their organic matter. Hydrocarbons are released
and displace pore water as kerogen matures, increasing shale
resistivity. Coals are often considered to have high resistivity.
However, both anthracite and lignite can have low resistivity.
In the case of lignite, resistivity can be used as a measure of
water content. Due to their lack of porosity, evaporites have
very high resistivity; this is used as a diagnostic property in
well log analysis.
Temperature and pressure effects on resistivity
Temperature changes affect aqueous solutions resistivity with
declining p^ as temperature rises. This response is due to
lowering of fluid viscosity with warmer temperatures that leads
to greater ion mobility. The dependence of p^ on temperature
is significant (e.g., a 2% change in p^ for a 1°C variation at
20°C);
hence, temperature effects need to be considered in the
analysis of resistivity data.
Fluid pressure changes have a minimal influence on p^, and
its effect is generally neglected. However, applied pressure
variations alter the pore structure and affect pore connectivity.
Pore closure with higher effective pressure increases tortuosity
that results in higher p for porous rocks.
Radioactive properties
Radiometric methods, such as natural y-ray well logging, are
used to measure the radioactivity of sediments due to the
spontaneous decay of radionuclides present in these materials.
There are three series of natural radionuclides that are
common in the Earth's crust: the uranium
(^•'^U
and ^^*U)
series,
thorium (^^•^Th) series and potassium
(''"K).
Both the
thorium and uranium series involve a decay chains with a
number of daughter products. The isotope
''"K.
decays to '"'Ca
and emits a y-ray. The relative abundances of these radio-
nuclides are variable in sedimentary rocks (Table G5). While
total y-ray tools count all y-ray without differentiation,
spectral y-ray tools determine the contribution due to each
series.
Shales and clays produce a higher y-ray count due to clay
minerals that contain K and Th; hence, total y-ray count is
used as measure of shale content. Since clay minerals differ
widely in terms of their Th/K ratios (Table G5), it has been
suggested that spectral y-ray data be used for clay mineral
identification. Organic matter in black shales preferentially
adsorb U ions; the U concentration is a function of U ion
species, amount and degree of organic matter maturity, and
pH/Eh conditions. Therefore, the Th/U ratio is useful in source
rock characterization.
Pure quartz sandstones contain very small concentrations of
radioactive isotopes and exhibit low y-ray count. The presence
of K-rich minerals (i.e., feldspars and micas) produces
significantly higher y-ray count and can be used as a sediment
maturity indicator. Some heavy minerals (e.g., zircon) are Th
and U bearing, and sandstones with these minerals also
produce significantly higher y-ray counts. These two cases can
be identified using the Th/K ratio. Both calcite and dolomite
have very low K, Th and U contents that explain the very low
y-ray activity commonly measured in carbonates. Elevated y-
ray count in carbonates can be attributed to the presence of
clay mineral or U associated with organic material and
phosphate minerals.
Coals commonly have little, if any, K or Th. Their y-ray
activity is very low; coals may have readings even lower than
clean sandstones on y-ray logs. Natural y-ray logging is an
314 GEOTHERMIC CHARACTERISTICS OF SEDIMENTS AND SEDIMENTARY ROCKS
excellent method for distinguishing coal from interbedded
shales. Potassium bearing evaporites (e.g., sylvite) are very
radioactive which combined with their very high resistivity can
be used for lithologic identification. Other evaporites are non-
radioactive, and their low y-ray readings separate them from
most shales.
Summary
The physical properties of sediments are dependent on the
properties and abundance of the individual components as well
as their geometrical configuration. Since sediments are
complex heterogeneous systems, their physical properties can
vary widely for a given lithology. However, there are general
petrophysical principles that govern the physical properties of
sediments.
The acoustic wave velocities for sediments are a function of
the elastic moduli and densities of the rock forming minerals
composing the solid grains and the fluid species that fill the
pore space. For seismic waves, the pore fluids act as a single
effective fluid that is very sensitive to the presence of
gases.
The
acoustic velocities decrease with increasing porosity; this effect
is greater for crack and fractures.
Since solid grains are insulators, the electrical resistivity of
sediments is primarily controlled by electrolytic conduction
through the pore water. The transport of electrical current by
this mechanism is dependent on the water resistivity and the
pore structure. In addition, electrical current can flow along
solid-fluid interfaces due to surface conduction. This mechan-
ism can significantly contribute to transport of electrical
current in materials with large surface area such as clayey
lithologies.
The natural radioactivity of sediments is a function of their
potassium, uranium and thorium content. Total y-ray count is
used as measure of shale content due to clay minerals
containing potassium and thorium. Source rocks can have
elevated levels of uranium due to preferential absorption by
organic matter. The presence of potassium bearing minerals
can significantly increase the radioactivity of sandstones and
evaporites that otherwise have low y-ray count.
Anthony L. Endres
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Cross-references
Classification of Sediments and Sedimentary Rocks
Compaction (Consolidation) of Sediments
Geothermal Properties of Sediments
Heavy Minerals
Kerogen
Magnetic Properties of Sediments
Maturation, Organic
Mudrocks
Porewaters in Sediments
GEOTHERMIC CHARACTERISTICS OF
SEDIMENTS AND SEDIMENTARY ROCKS
It has been known for centuries that temperature in the Earth
increases with depth. This increase towards the interior is the
natural process of general cooling of the lithosphere and where
a portion of the heat flow is generated by the decay of
radioactive constituents contained in the crust. The escape of
the heat (the heat flow), is measured by determining differences
in temperature with depth (the gradient), at various places
usually in mines, tunnels, or well bores and the type of rock
through which the heat is transmitted (rock conductivity).
Heat flow is expressed as:
Q = -K {dT/dx) (Eq. 1)
where Q is the heat flow, K is the thermal conductivity, Tis the
temperature, and x is the distance (depth).
Heat flow is computed either by the: (I) interval method; or
(2) BuUard method (thermal depth method) (Jessop, 1990).
Both methods utilize the temperature gradient (measured in a
well bore, mine, or tunnel) and rock conductivity value(s) for
each geological unit through which the gradient is determined.
Where the crystalline basement (crust) is covered by younger
sediments or sedimentary rocks, the heat flow, may be
influenced by the blanket effect of the cover. Physical
properties of the sedimentary cover, including the rock type
and contained radioactive elements, thermal conductivity, and
their contained fluids or gas, affect the thermal regime. In
addition, topography and past climatic conditions may be
important factors.