Boggs S. Principles of Sedimentology and Stratigraphy

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16.7 Applications of Basin Analysis

577

used to document the regional structural trends and overall basin geometry as

well as to identify local structural features such as anticlines and faults that may

provide traps for hydrocarbons. These seismic techniques are described in

Chapter 13, which also discusses the application of seismic methods to stratigraph-

ic interpretation, including identification of seismic sequences and facies as well as

correlation of stratal reflectors. The most widespread application of paleomagnet-

ism in basin analysis is the study of magnetic polarity reversals as a tool for corre-

lation and other purposes, as described in Chapter 13. Airborne or ground-based

magnetic and gravity surveys are sometimes conducted also to provide informa-

tion about basin structure in addition to that provided by seismic data. Gravity

surveys determine the presence of gravity anomalies (abnormally high or low

gravity values), which can be related to the presence of dense (e.g., basalt) or less

dense (e.g., thick sediment) rock bodies in the subsurface. These magnetic and

gravity surveys furnish data that can be used to construct maps showing large-

scale characteristics of basins (e.g., major basin highs and lows); however, these

maps commonly do not have sufficient resolution to identify small-scale structur-

al features, that is, features such as small anticlines or faults that might constitute

petroleum traps.

16.7 APPLICATIONS OF BASIN ANALYS IS

Interpreting Geologic History

As pointed out, basin analysis is an integrated program of study that involves ap

plication of sedimentologic, stratigraphic, and tectonic principles to develop a full

understanding of the rocks that fill sedimentary basins and to use this information

to interpret the geologic history and evaluate the economic importance of these

rocks. Thus, one major goal of basin analysis is simply to develop a better under

standing of Earth history as recorded in particular depositional basins. Through

analysis of sedimentary textures, structures, particle and chemical composition,

fossils, and the stragraphic characteristics of sedimentary rocks (as revealed by

physical, biological, paleomagnetic, and seismic-reflection characteristics), geolo

gists are able to interpret the important tectonic and sedimentologic events that

transpired to generate and fill a particular sedimentary basin. Thus, these various

kinds of basin studies, commonly involving preparation of appropriate maps and

stratigraphic sections, allow geologists to interpret past tectonic, climatic, and sed

imentologic events and conditions (including source-area characterization and in

terpretation of depositional environments) to reconstruct the paleogeography and

paleogeology of Earth during specific times in the past.

A good example of the application of basin-analysis principles to interpreta

tion of the geologic history of a particular region is provided by Clifton, Hunter,

and Gardner (1988). These authors examined the influence of eustatic, tectonic,

and sedimentological processes on the generation of transgressive and regressive

depositional cycles in the Merced Formation near San Francisco, California. The

Merced Formation consists of approximately 2000 m of shallow-marine and

coastal nonmarine sediments of late Cenozoic age.

To work out the history of the Merced Formation, Clifton et al. first defined

the stratigraphic boundaries (base and top) of the formation through field study

and reference to previously published information. Ages of different parts of the

formation were established by a combination of paleontologic data and radiomet

ric dating of intercalated ash beds. Various depositional facies within the Merced

were then characterized on the basis of contained fossils, physical and biogenic

sedimentary structures, and textural characteristics (Table 16.3). Ten facies types

were identified, which (from the base of the formation upward) were determined

578

Chapter 16 I Basin Analysis, Te ctonics, and Sedimentation

Inferred

origi

Texture

Biota

Physical structures

Biogenic structures

Outer shelf

Sandy silt

Molluscs (in situ None

Intense bioturbation

and scattered shells)

Mid-shelf

Sand

y silt

Molluscs

Shell lags, parallel lamination,

Sharp-topped bio-

hummocky cross-stratification,

turbated intervals,

ripple lamination

between sets of

laminae

Inner shelf

Ve ry fine sand,

Molluscs, echinoids

Shell lags, parallel lamination, Locally intense

scattered small

hummocky cross-stratification

bioturbation

pebbles near top

Nearshore

Fine to coarse

Molluscs, echinoids

Lenticular gravel and sand

Ve rtical burrows,

sand, gravel

beds, high- and low-angle Macaronichnus

cross-bedding, parallel

(near top)

lamination

Foreshore

Fine to coarse

Molluscs, echinoids

Parallel lamination, heavy

Macaronichnus

sand, gravel

mineral layers near top

(near base)

fining-upward

trend

Embayment

Mud, sand,

Molluscs, ostracods

Fine sand-silt-day lamination,

Bioturbation,

gravel, some

ripples and ripple bedding

root-rhizome

fining-upward (sand), cross-bedding (sand

structures

cycles

and gravel), shell

Backshore

Fine sand None

Parallel lamination, low-angle

Root-rhizome

ross-bedding, climbing

structures,

adhesion ripple bedding,

vertebrate

ripple lamination

foorints, vertical

tubes

Eolian dune

Fine sd

None Medium- and large-scale Local mottling,

and medium- and high-angle

vertical tubes

cross-bedding

Alluvial

Gravel and

None

Indistinct stratification,

None

pebbly sand

lenticular bedding,

medium- and small-scale

trough cross-bedding

Pond/swamp/

Mud, peat, or

Freshwater diatoms,

Flat-bedded (mud)

Root structures,

marsh

lignite

insect wings,

burrows and

terrestrial vertebrate

intrastratal trails

bones

urce: Clifton, Hunter, and Gardner, 1988.

to represent ten different depositional environments: pond/swamp/marsh, allu

vial, eolian dune, backshore, embayment, foreshore, nearshore, inner shelf, mid

shelf, and outer shelf.

 vertical succession, these facies define alternating episodes of transgres

sion and regression that probably reflect Pleistocene eustatic sea-level fluctua

tions. The transgression-regression fluctuations were matched with reasonable

confidence to a Pleistocene sea-level curve, determined from oxygen-isotope data.

Finally, Clifton et al. were able to calculate that the Merced Formation was deposit

ed under shallow-marine/coastal conditions in a setting undergoing subsidence at

16.7 Applications of Basin Analysis

579

an average rate of 1 m/1000 years, a rapid rate that must have been influenced by

tectonic deformation.

This example illustrates how geologists use an armory of various sedimento

logical and stratigraphic approaches (i.e., basin analysis) to determine the physical

and biologic characteristics of a sedimentary formation, from which the sedimen

tological and tectonic history of the formaon can be interpreted. Several addion

al examples of this approach may be examined in Kleinspehn and Paola (1988).

Economic Applications

A second goal of basin analysis is to use the principles and techniques described

above to evaluate the economic importance of sedimentary rocks and identify eco

nomically exploitable deposits of minerals or fossil fuels. Basin analysis finds its

greatest economic application in the fields of petroleum geology and, to a lesser ex

tent, hydrogeology. In spite of the fact that petroleum geologists have been trying

for many years to locate petroleum (hydrocarbon) accumulations by geochemical

analysis of surficial rocks and soil overlying such deposits, no successful method

has

yet been developed for direct detection of hydrocarbon deposits. To fd an oil

or gas deposit, geologists must (1) explore basins that have the right conditions for

the formation and migration of hydrocarbons and (2) locate a suitable trap, such as

a structural anticline,  which the hydrocarbons may have accumulated.

What, exactly, are the right conditions for formation and migraon of hydro

carbons? Most petroleum geologists believe that petroleum originates from ne

size plant or animal organic matter through complex biochemical processes that

take place during sediment burial. Because organic matter is preserved preferen

tially in fine-grained sediments, organic-rich shales are believed to be the princi

pal source rocks for petroleum. Thus, the first condition for successful hydrocarbon

exploration is to find a bas that contains suitable source rocks. Furthermore, the

source rocks should have been buried to depths where the temperature is ade

quate to "crack" oil and gas from the organic matter ( �90°-l25°C) but not so

high that e hydrocarbons are destroyed. After oil and gas are generated within

the shale source rocks, they migrate, in some (poorly understood) manner, from

the source rock into associated porous and permeable rocks called reservoir rocks.

(Porosity refers to that percentage of the volume of a rock occupied by pore space;

permeability refers to the ability of a rock to transmit a fluid.) Drilling experience

has shown that the best reservoir rocks for hydrocarbons are sandstones and lime

stones. Roughly 55 percent of e world's oil and 75 percent of its natural gas

occur in sandstone reservoirs; most of the remaining world reserves are in carbon

ate reservoirs.

Most of the pore spaces in porous sandstones and carbonate rocks in the sub

surface are filled with water. en hydrocarbons migrate from source beds into

these overlying or underlying (?) reservoir rocks, some of the pore water must be

displaced to make room for the hydrocarbons. Because the hydrocarbons have a

lower specific gravity than water, they tend to move updip to get above the water

(oil floats on water). Thus, by progressive displacement of pore water, hydrocar

bons will gradually move up the regional dip of a sedimentary basin. This migra

tion will continue until the oil reaches a trap or, if no trap is encountered, until it is

expelled at the surface as an oil seep.

A hydrocarbon ap is some kind of structural or stratigraphic feature of a

rock into which oil or gas can easily migrate but from which it is diicult to es

cape. The three most common kinds of petroleum traps are anticlines, faults, and

stratigraphic pinch-outs (see Fig. 7.25). Oil or gas will rise to the structurally high

est position in an anticline, fault trap, or stratigraphic pinch-out, where it is pre

vented from further movement by an impermeable rock layer (e.g., shale or

evaporite) that lies above e reservoir bed and seals the trap. Thus, the search for

580

Chapter 16 / Basin Analysis, Te ctonics, and Sedimentation

a petroleum deposit requires that a trap be located and drilled in a basin that con

tains suitable source and reservoir rocks. The trap, when drilled, may or may not

contain petroleum. Only about 10 to 15 percent of the traps drilled in new explo

ration areas commonly yield commercial quantities of petroleum. Structural traps

(e.g., anticlines and faults) are located primarily by reflection seismic techniques

(Chapter 13). Stratigraphic traps are located through detailed analysis of strati

graphic information obtained by study of stratigraphic cross sections (based on

subsurface lithologic logs and well logs) and seismic cross sections

Basin analysis, to the petroleum geologist, thus means locating within depo

sitional basins suitable source rocks, reservoir rocks, and traps. To do this success

fully calls into play most of the principles of sedimentology and stratigraphy, as

well as the various techniques for analyzing sedimentary successions and present

ing data, discussed in this book. Petroleum geologists must have a thorough un

derstanding of the physical, chemical, and biological characteristics of sedimentary

rocks; must understand depositional environments and depositional systems;

must be well grounded in all aspects of stratigraphy; must have a working knowl

edge of the principles of geophysics and structural geology; and must have a basic

understanding of the principles involved in the flow of fluids through porous,

subsurface rocks. No single geologist is normally capable of carrying out the

many complex studies required to develop a major, successful petroleum "play."

Basin assessment is commonly carried out by teams of workers, which may in

clude stratigraphers, sedimentologists, sedimentary petrologists, paleontologists,

geophysicists, and hydrologists. These teams of scientists work together to devel

op an understanding of the sedimentary, stratigraphic, and structural factors that

control the accumulation of oil and gas in drilled basins where commercial hydro

carbon deposits have already been found. They seek then to extend the knowl

edge gained from such studies to exploration for new hydrocarbon deposits in

untested basins.

The application of basin-assessment teciques to developing a petroleum

play is described by Allen and Allen (1990). Before new exploratory wells ( e.g.,

Fig. 16.23) are drilled in an untested basin, petroleum geologists must first work

out the stratigraphic characteristics of the basin to see the kinds of potential source

rocks and reservoir rocks that are present and the thickness and characteristics of

these rocks. They then collect and analyze samples of potential source rocks (or

ganic-bearing shales) to see if enough organic matter (kerogen; Chapter 7), and the

right kind of kerogen, is present to generate economically significant quantities of

petroleum or natural gas. Some kinds of kerogen generate liquid petroleum; oth

ers generate gas. Through outcrop study, and possibly drilling of small-diameter

holes for information purposes, they determine which potential reservoir ros

have adequate porosity and permeability to make them targets for drilling.

suitable source and reservoir rocks are present in the basin, the next step is

locate one or more traps that are large enough to hold a commercial quantity of

oil or gas. As mentioned above, traps are located mainly by seismic prospecting

and detailed stratigraphic analysis. An essential component of a trap is that reser

voir rocks in the trap must be covered or capped by some kind of nonpermeable

rock such as shale or evaporites to prevent oil in the reservoir rock from escaping

upward. Finally, geologists study the fluid-flow patterns in the subsurface rocks of

the

basin to see if fluids are flowing with adequate pressure to move petroleum

into a well bore that penetrates the reservoir rock. If all the requisite conditions of

source rock, reservoir rocks, and trap are present, the well is drilled. For additional

insight into application of the principles of basin analysis to petroleum explo

ration, see Cross et a!. (1995) and We lte, Horsfield, and Baker (1997).

Further Reading

581

Figure 16.23

An exploratory gas well

(Chevron Cm Sup Watrton)

dnlling in the northwest cor

flPr of lht western Canada

S�imenta Basin, Alberta,

Can.ld. The well tested Mi

si��ipptan and Devoman car

bonate formations and

encountered gas betow "

depth ot 4000 m. [Photo

graph courte�y of Sidney

Smith.j

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Appendix A

Form and Roundness of Sedimentary Particles

582

Definition and Measurement of Sphedty (Form)

Numerous mathematical measures for expressing form have been proposed, but

spherici is probably the most widely used. The concept of sphericity was intro

duced by Wadell (1932), who defined sphericity mathematically as the ratio of the

diameter of a sphere with the same volume as a particle to the diameter of the

smallest circle that would just enclose or circscbe the outline of the parcle.

Wadell determined the volumes of large particles by immersing the particle in water

and measuring the volume change of the water. Krumbein (1941) modified

Wadell's sphericity concept slightly to express sphericity  by the relationship

 =

volume of the particle



volume of the circumscribing sphere

(A-A.1)



The volume of a sphere is given by 6

, where D is the diameter of the

sphere. The approximate volume of natural particles can be calculated by assum

ing that the particles are triaxial ellipsoids havg three diameters DL, D1, and D5,

where L, I, and S refer to the lengths of the long, intermediate, and short axes of

the ellipsoid. By substituting appropriate values for volume into Equation A-A.1,

we can express sphericity by

(A-A.2)

The sphericity of a particle determined by this relationship is called tercept

sphericity and can be calculated by measuring the long, inteediate, and short

axes of a particle and substituting these values into Equation A-A.2.

Sneed and Folk (1958) suggest that Krumbein's intercept sphericity does not

correctly express the behavior of particles as they settle in a fluid or are acted on

by uid flow. A rod-shaped particle, for example, settles faster than a disc, al

though the intercept sphericity formula suggests the opposite. Particles fallg

through water tend to settle with maximum projection areas (the plane of the

Definition and Measurement of Sphericity (Form)

583

long and intermediate axes) perpendicular

to the direction of motion, and small

particles resting on the bottom orient themselves perpendicular to current flow di-

rection. Sneed and Folk thus proposed a different sphericity measure called

maximum projection sphericity ( p ), which they claim better expresses particle be-

havior. Maximum projection sphericity is defined mathematically as the ratio be-

tween the maximum projecon area of a sphere with the same volume as the

particle and the maximum projection area of the particle:

(A-A.3)

Maximum projection sphericity has gained favor for expressing the shape of par

ticles deposited by water. Conceptually, it is not necessarily more valid than inter

cept sphericity when applied to other modes of particle transport and deposition,

e.g., transport by ice and sediment gravity flows.

Regardless of the sphericity measure used, experience has shown that parti

cles having the same mathematical sphericity can differ considerably in their over

all shape. Some additional measure or index is needed to more specifically define

the form of particles. Two additional form indices that permit a more graphic rep

resentation of form are in wide use. Zingg (1935) proposed the use of two shape

indices DJIDL and Ds/ Dr to define four shape fields on a bivariate plot: oblate

(disc), equant (spheres), bladed, and prolate (rollers) (see Fig. 3.9A in e text).

Lines of equal intercept sphericity can be drawn on the Zingg shape fields (Fig.

3.98), illustrating that particles of quite different form can have the same mathe

matical sphericity.

Sneed and Folk (1958) used two somewhat different shape indices to con

struct a triangular form diagram (Fig. A-A.1), in which Ds/ DL is plotted against

DL Dr/ DL - Ds to create ten form fields (e.g., compact, C; platy, P; bladed, B;

elongate, E). Lines of maximum projection sphericity drawn across the field

(Fig. A-A.1) again illustrate the disparity between mathematical sphericity and

actual form.

Platy

0.1

Compact

Bladed

L = Long diameter

I = Intermediate diameter

S Short diameter

Elongated

Figure A-A.l

Classification of pebble shapes, after Sneed

and Folk. The symbol V refers to the adjec

tive very (e.g., very platy, very bladed, very

elongated). [After Sneed, E. D., and R. L.

Folk, 1958, Pebbles in the lower Colorado

River, Te xas, a study in particle morphogene

sis: Jour. Geology, v. 66, Fig. 2, p. 119.

Reprinted by permission of University of

Chicago Press.]

584

Appendix A I Form and Roundness of Sedimentary Particles

Figure A-A.2

Definition and Measurement of Roundness

Wa dell (1932) defined mathematical roundness as the arithmetic mean of the

roundness of the individual corners of a grain in the plane of measurement.

Roundness of individual corners is given by the ratio of the radius of curvature of

the corners to the radius of the maximum-size circle that can be inscribed within

the outline of the grain in the plane of measurement. The degree of Wadell round

ness ( Rw) is thus expressed as

(r/R) (r)



(A-A.4)

where r is the radius of curvature of individual corners, R is the radius of the max

imum inscribed circle, and N is the number of corners. The relationship of r to R is

illustrated in Figure A-A.2.

Owing to the numerous radius measurements that must be made, it is very

time-consuming to determine the Wadell roundness of large numbers of grains.

Simpler roundness measures have been proposed that require only that the radius

of the sharpest corner be divided by the radius of the inscribed circle (Wentworth,

1919; Dobkins and Folk, 1970); however, Wadell's roundness measure is still used

by most workers. Even if the simpler roundness formula is used, measuring the

radii of large numbers of small grains is a very laborious process, requiring use of

either a circular protractor or an electronic particle size analyzer to measure en

larged images of grains (Boggs, 1967a). Consequently, visual comparison scales or

charts consisting of sets of grain images of known roundness are often used to

make rapid visual estimates of grain roundness. The visual charts of Krumbein

(1941) and Powers (1953) are the most widely used of these comparison scales.

Maximum

scribed

- Grain

"

Diagram of enlarged grain image illustrating the method of measuring the

roundness of a sediment grain. (R) refers to the radius of the largest circle

that can be inscribed inside the grain; (r) is the radius of curvature of the

corners of the grain. [From Boggs, S., )r., 1967, Measurement of roundness

and sphericity parameters using an electronic particle size analyzer: )our.

Sed. Petrology, v. 37, Fig. 3, p. 91 2. Reprinted by permission of Society of

Economic Paleontologists and Mineralogists, Tulsa, Okla.]

Measuring mark i

proper position for

measuring radii of

urvature of

orners

Appendix B

Paleothermometry

Estimating Diagenetic Paleotemperatures

Because temperature has a particularly significant effect on diagenetic processes,

geologists are greatly interested in estimating the temperatures at which particu

lar diagenetic reactions take place. Considerable research has been carried out to

develop reliable techniques for paleotemperature analysis (e.g., Staplin et al.,

1982). Tools used for determining paleotemperatures are called geothermometers.

The principal techniques now in use for determining diagenetic paleotempera

tures include the following.

Conodont Color Alteration

Conodonts are marine crofossils composed of the neral apatite, but also con

taining trace amounts of organic material. They range in age from Cambrian to Trias

sic and occur principally in carbonate rocks and shales. Color changes in conodon

om pale yellow ( <�S0°-80°C) to brown (60°-140°C) to black (300°, owing

to carbon fixation within trace amounts of organic material in the conodonts, as a

result of increase in temperature. Extremely high temperatures may change the

color to white (extreme thermal alteration) because of loss of carbon and water.

Vitrinite Reflectance

Vitrinite is structured or unstructured woody tissue plus tissue impregnations

that occur as disseminated grains  sediment and as a major constituent in coals

(Chapter 7). Light reflection from vitrinite (thermally altered organic grains) is

measured quantitatively with

reflectance microscope. Reflection increased with

increasing degree of thermal alteration (metamorphism). Up to about 240°C, per

cent vitrinite reflectance is related to minimum temperature by a scale that ranges

from <0.48% at <100°C to 2.14% at 240°C.

Graphitization Levels in Kerogen

Kerogen, which occurs principally in shales, is e disseminated organic matter of

sedimentary rocks (insoluble in nonoxidizing acids, bases, and organic solvents).

585

586

Appendix B I Paleothermomet

Organic matter initially deposited in sediments is converted to kerogen during di

agenesis by thermocatalytic processes. The carbon atoms in kerogen become in

creasingly well ordered (structured) with increasing levels of thermal diagenesis,

a process called graphitization. Increase in the degree of ordering, or level of

graphitization, as deteined by X-ray diffraction methods, can thus be related to

increase in diagenetic temperature. The method can be used to estimate tempera

tures up to about 600°C

Analysis of Clay-Mineral Assemblages

Clay minerals (Chapter 5) are most abundant in shales but occur also as a matrix

in sandstones and minor amounts in limestones. With increasing temperature and

metamorphic grade, smectite clay minerals convert to illite through a mixed-layer

illite/ smectite series; chlorite appears, and kaolinite (as well asK-feldspar) disap

pears. Thus, the relative abundance of these clay minerals in rocks can be related

to paleotemperatures (e.g., smectite occurs at temperatures below about 100°C;

mixed-layer clays are stable up to about 200°C; illite forms at temperatures above

200°C). The useful temperature range of the method is up to about 300°C.

Analysis of Zeolite Facies Mineral Assemblages

Zeolite facies minerals form authigenically in volcaniclastic sediments through al

teration of chemically reactive volcanic materials. ey form in the overlapping

temperature range of diagenesis and metamorphism and show a progression of

mineral facies that is a reflection of temperature (and pressure) of buriaL Temper

ature theoretically exerts a strong control on the types of zeolite minerals that occur

together. For example, heuldite and analcite tend to form at temperatures below

about 100°-l25°C, whereas laumontite and pumpellyite form at temperatures be

tween 100°-l25°C and 175°-200°C. Thus, temperatures, up to about 200°-250°C,

can be established roughly on e basis of the assemblages of zeolite minerals p

sent in a rock.

Analysis of Auid Inclusions

Fluid inclusions occur in a variety of sedimentary materials: geodes, vugs, and

veins in sediments; sedimentary ore deposits; carbonate and quartz cements in

sedimentary rocks; salt and sulfur deposits; petroleum reservoir rocks; and spha

lerite

(zinc-bearing ore mineral) in bituminous coal beds. Recrystalzation of min

erals or formation of overgrowths on minerals during diagenesis may trap uids

as minute inclusions in the crystals. Fluid inclusions commonly consist of a liquid

plus a bubble of gas. Presumably, the inclusion consisted of a single uid phase at

the time of formation, which separated into two phases upon cooling. By reheat

ing the mineral until the phase boundary between e liquid and gas can be seen

to just disappear, the approximate temperature at which the inclusion formed can

be established-taking into account an estimate of the pressure of formation. The

meod is useful at temperature ranges from about 25°C to 150°-200°C.

Oxygen Isotope Ratios

The fractionation factor

between two coexisting minerals is defined as

esa;t

A-B = eso;t

60)