By Chief Ugwokeh
Nnaemeka, B.Sc, P.Geol, CCNA (+2348067876251, nnngroup@yahoo.com)
ABSTRACT
Diapiric structures are the structures
formed by the deformation of the overlying strata by a volume of rock rising up
buoyantly because of its low density relative to its surroundings. These
deformations around and above the rising volume of rock (diapir) creates
potential petroleum traps. Diapiric materials are: salt, shale, clay, mud,
serpentinites etc.
Diapirs are
initiated by unequal loading of a layer of materials of relatively high
placidity (low equivalent viscosity). The common diapiric materials such as
salt and abnormally pressured clay may be less dense than normally compacted
sediments overlying them. Hence, once a diapir is initiated the forces of
buoyancy tend to elongate the deformation vertically.
The deformation
of the overlying sediments and the sediments through which the diapir has
penetrated may lead to hydrocarbon accumulations trapped either by the
anticline form or by truncation of a reservoir by the relatively impermeable
diapir. Such accumulations may also be affected by faulting that resulted from
the deformation.
CHAPTER ONE
INTRODUCTION
1.1 MEANING OF DIAPIRIC STRUCTURES
Diapiric
structures are the structures formed by the deformation of the underlying strata
by a volume of rock rising upward buoyantly because of its low density relative
to its surroundings.
According to
Braunstein and O’Brain (1968), the words “diapir”, “diapirism” and “diapiric”
are derived from a Greek word meaning “to pierce”. The words were first used to
describe anticlinal folds in the Carpathian Mountains
with salt core that pierced the overlying strata. The concept was originally
confined to the injection of sedimentary materials but was gradually expanded
to include all types of piercement including magmatic injection. But this
expansion of the term destroys its usefulness. Consequently, the term will have
to refer to the injection of any solid rock whether sedimentary, igneous or
metamorphic. In general, the body cuts across the adjacent rock, although
locally it may be concordant.
Billings (1982) noted
that injected rocks may range in physical properties from solid
- Solid rock which may have a small percentage of pore spaces and pore liquid.
- Solid rock that is thoroughly broken up and fractured with some liquids in the fractures.
- Solid rock that has become mobile due to partial melting, a feature that generally occurs only at a considerable depth in metamorphic terrains.
- A loose aggregate of particles buoyed up by gases or liquid that could be derived from either magmatic or sedimentary sources.
- Liquid.
The first two
categories are diapiric, the last two are not. A strong argument could be made
for classifying the third type as diapiric. The plastic or viscous flow of
solid rock is not the same as diapirism.
Chapman (1973)
stated that the processes of diapirism are dynamic and lead to structures that
range from minor displacements of plastic materials to major volume of
intrusions of large volume of materials through considerable thickness of
overlying rocks. For clarity, the stages of diapirism that precedes penetration
may be referred to as incipient diapirism (incipient diapir). In plan, diapir
tends to acquire a more or less circular outline; in section, the amplitude may
achieve dimension of thousands of meters. The scale of diapirism and incipient
diapiric structures range from centimeters (eg. Load cast) to kilometers. They
commonly occur in groups or in lines or in lines of groups. They may be intimately
associated with folding and faulting. They demonstrate that certain rock
materials under stress will flow as quasi-fluid or a viscous solid.
Confining our
attension to diapirs of sedimentary rocks materials, we find them only in
sedimentary basins, in rocks of most geological age from proterozoic to holocene
(recent) and in all contents except Antarctica.
They are common in the petroleum provinces of the Gulf coast of the united
state and offshore in the Gulf of Mexico, the middle east, the Caucasus and
adjoining regions to the north of the capsian sea, north
west Europe (Braunstein and
O’Brein, 1968). They may be equally common in non-petroleum pronvinces that
have not received the same intensity of geological and geophysical
investigation. Current researches indicates salt diapirism in sedimentary
sequences of the continental shelf of West Africa
with water depths up to 4,000m (Gussow, 1954).
Diapirs
commonly (but not invariably) occupy area of gravity minima. A local gravity
minimum over a diapir indicates a deficiency of mass despite the intrusion of
deeper materials to shallow depths (Chapman, 1973). Halbouty (1970) stated that
diapirs, of whatever sedimentary materials, are characteristically overlain by
a sequence of sediments that is, of course, younger than the materials of the
diapir. The accumulation of sediments must be taken as clear evidence that the
depositional surface over the diapir was subsiding during periods of
sedimentary accumulation. If the growth of a diapir is regarded as
accelerating, there may come a time when there is absolute upward movement at a
sufficient rate to inhibit sediment accumulation over it and stratigraphic
continuity will be broken. This is essentially the concept of down building.
1.2
TYPES OF DIAPIRIC STRUCTURES
I.2.1 SALT DIAPIRIC
STRUCTURES
Salt
diapiric structure is a structure formed by the deformation of the underlying
strata by a volume of salt rising rising upward buoyantly because of its low
density relative to its surroundings. The salt here is the salt diapir.
Salt diapirs
occur under large areas of the gulf coast province
of North America,
Northwest Europe, Russia and
around the Arabian gulf. The expression of
these is varied: Some have reached the surface, or are at very shallow depth.
Salt diapirs may form the cores of diapiric anticlines, or takes the
self-explanatory shape of salt pillows. More pronounced forms are known as salt
domes, plugs or stocks (Braustein and O’Brein, 1968).
The mining of
salt domes near the surface has shown that the salt is intensively deformed
with complicated flow patterns (rather than folds) but very rare faults. The
salt domes, particularly those at shallow depths, which have developed a cap
rock are the less soluble residue from leaching of the salt by circulating groungwater.
Salt domes may be sheathed in a thin layer of anhydrite or clay “gouge”. The
cap rock sometimes contains sulphur in commercial quantities and the sulphur is
associated, probably biogenically, with oil-bearing diapiric structures
(Chapman, 1973).
1.2.2
SHALE DIAPIRIC STRUCTURES
Shale diapiric
structures are the diapiric structures formed by the deformation of the
underlying strata by a volume of shale rising upward buoyantly because of its
low density relative to its surroundings. The volume of shale rising up is
called shale diapir.
1.2.3
CLAY DIAPIRIC STRUCTURES
Clay
diapiric structures are the diapiric structures formed by the deformation of
the overlying strata by clay diapir. Clay does not seem to develop into such
clearly defined diapirs as salt. The expressions of clay diapirism are
typically two: diapiric cores to anticline and mud-volcanoes. Anticlinal clay
diapir occur very widely usually as incipient diapirs without penetration. Clay
diapir known at the surface are perhaps about as numerous as salt diapir at the
surface.
The younger
clay diapir in the subsurface characteristically contains interstitial fluid at
abnormal pressures. These are contained in a “sheath” or “gouge” of compacted
shale in a manner analogous to the abnormally pressured clay units. The
physical properties of diapiric clay are those associated within other
abnormally pressured clay ie high porosity and low bulk density relative to
normally compacted clay, low mechanical strength and low equivalent viscosity.
These properties suggest that the mother layer is an abnormally pressured clay
unit with retarded compaction that is a gravity loaded rather than tectonically
loaded clay.
1.2.4
SERPENTINITE DIAPIRIC STRUCTURES
These structures
are caused by the deformation of the overlying structures by serpentinite
diapirs.
Ultramafic
rocks are common in some parts of orogenic belts although some are unaltered
dunite (composed of olivine), peridotite (composed of olivine and pyroxene) and
other similar rock. They are commonly altered to serpentinite, a rock composed
of the mineral serpentine. The ultramafic rocks have been injected into the
enclosing rocks. Pure dunite would crystallize at about 17000oC and peridotite
at somewhat lower temperature. Nevertheless, the enclosing rocks commonly, but
not always, lack contact metamorphism. This implies that the ultramafic rock
was relatively cool at the time of injection. Intense fracturing, shear plane
and slickenside in the serpentinite are consistent with the conclusion that
they were injected in the solid state. The low density of the serpentinite is a
factor favouring solid emplacement (Billings,
1982).
CHAPTER TWO
ORIGIN OF DIAPIRIC
STRUCTURES
2.1
ORIGIN OF SALT DIAPIRIC STRUCTURES
Salt diapirs
result from the intrusion of solid halite into the surrounding sediments. This
salt is derived from the underlying source bed usually thousand of feet thick.
In the gulf coast, the sources are very probably the Lauann salt of jurasic or
Permian age and as much as 5000 feet thick. In Germany,
the source is the Permian zechstein which is as much as 3000 feet thick (Billings, 1982).
Bergun (1980)
esthablished that the original salt bed was deposited as a result of the
evaporation of seawater and subsequently buried by deposition of the overlying
sedimentary units. Billings
(1982) stated that the motivating force in the gulf coast results from the
difference in density between the salt and the overlying sediments. Rock salt
has a relatively uniform density regardless of depth. This is because salt does
not compact when buried unlike other sediments. The density of the rock salt is
2.2g/cm3. Between the surface and a depth of 2000 feet, the average density of
the sediments is 1.9 to 2.2g/cm3 but below a depth of 2000 feet the density of
the average sediment increases progressively to a value of 2.46 at a depth of 20,000
feet. Thus, below a depth of 2000 feet, an unstable gravitational situation
exist and the salt tends to move upward in the same way that a lighter fluid
rises through heavier overlying fluid.
If a small
anticinal flexure exists on top of the original salt bed, upward movement
starts here, and salt is drained away from the surrounding region. Eventually,
the salt bed in the adjacent area may become so thin and constricted that
further addition of salt is impossible. Salt diapiric structures can also be
formed by differential pressures on the buried salt due to unequal loading of
sediments (Chapman, 1973).
2.2
ORIGIN OF CLAY/MUD/SHALE DIAPIRIC STRUCTURES
Clay, mud
and shale diapiric structures have the same mode of origin. They are formed by
the abnormal pressure or overpressure on clay, mud or shale at depth by the
overlying sediments. Rapidly deposited and buried clay, which do not have time
to loose their fluid before being covered by younger sediments. This is typical
of tertiary sedimentary area eg Trinidad gulf of Mexico, Burma,Indonesia and Niger Delta of
Nigeria. In case of serial field, Brunei, the sand being higher in
specific gravity than clay makes clay to rise and penetrate the overlying
strata (Weber and Daukoru). The overpressure on the clay makes it to have lower
specific gravity than the overlying strata.
Chapman
(1973) stated that the physical properties of diapiric clay are those
associated with other abnormally pressured clay. They have high porosity and
low bulk density relative to normally compacted clay with low mechanical
strength and low equivalent viscosity. These properties suggest that the mother
layer is abnormally pressured clay with retarded compaction. That is a gravity
loaded rather than tectonically loaded clay. Mud diapirs are clay diapirs that
expressed themselves at the surface.
CHAPTER THREE
HYDROCARBON ENTRAPMENT
3.1
TRAP
A trap is
any arrangement of strata that permits the accumulation of hydrocarbon in
commercial quantities (North, 1984). Traps occur in fundamentally different
forms and can enclose very different volumes of pore spaces and hence
petroleum. The maximum total holding capacity of enclosed column of a trap is
the volume between the highest point and the “spilling plane” or outflow level
at the bottom (Tissot and Welte, 1984).
A trap has
two functions: it receives the hydrocarbons and prevents them from migrating
further. All traps have a single feature in common, a porous rock (eg.
Sandstone) that is at least partially enclosed in rocks that are relatively impervious
(ie the permeability of the enclosing rock must be too low for pressure and
temperature condition of the oil or gas in the trap to take advantage of it).
In addition to receiving oil and gas, a trap must be able to expel water at
depth and later re-admit.
The roof rock
(above the trap) and the wall rocks along it may be impermeable not only to oil
and gas but to water under the reservoir’s pressure conditions. If this is the
case, the accumulating oil and gas will displace downward the water originally
in the trap and the oil pool will contain bottom water. If the wall rock is not
impermeable to water, it must be water saturated and the pool will be bounded
laterally by edge water.
3.2
CLASSIFICATION OF TRAPS
3.2.1
STRATIGRAPHIC TRAPS
Stratigraphic
traps are the traps formed by depositional features such as a sand body
embedded in and sealed by shale in transgressive sequence or a porous reef rock
buried dense limestone and shale (Gussaw, 1954). According to North (1984),
stratigraphic traps are created by any variation in the stratigraphy that it is
essentially independent of structural deformation other than erision or uncomplicated
tilting.
3.2.2
STRUCTURAL TRAPS
Structural
traps are the traps that are formed by tectonic events ie when the strata are
involved in any kind of secondary post depositional structure eg folding,
drapping, faulting, piercing, unless the involvement is in erosion or tilting
only. Examples of structural traps are: anticinal traps, fault traps and
diapiric traps.
3.2.2.1
ANTICLINAL TRAPS
Anticlinal
traps are the traps that are formed when strata containing an impervious
stratum overlying the permeable one is folded convexing upward. The impervious
stratum forms the roof rock and the wall rock to prevent the hydrocarbon from
migrating further. The permeable rock forms the reservoir rock. The oil and gas
accumulate at the highest part of the fold. The gas highest, and the oil next,
both floating on the water that saturates all permeable formations (Louerson,
1967).
3.2.2.2
FAULT TRAPS
The displacement
at a fault may place a dipping permeable sandstone bed opposite impermeable
shale creating a trap for oil.
3.2.2.3
DIAPIRIC TRAPS
Diapiric
traps are the traps formed by the deformation of overlying strata by a volume
of rock rising up buoyantly because of its low density relative to the
surroundings. Unlike fault and anticline, many traps can be formed as a result
of one diapirism since diapir can pierce through and deform an alternation of
impervious and porous strata forming different traps at a time.
CHAPTER FOUR
THE SIGNIFICANCE OF DIAPIRIC STRUCTURES IN HYDROCARBON
ENTRAPMENT AND CASE STUDY.
4.1 THE
SIGNIFICANCE OF DIAPIRIC STRUCTURES IN HYDROCARBON ENTRAPMENT
The deformation of the overlying
sediments and the sediments through which the dome has penetrated may lead to
petroleum accumulation trapped either by the anticlinal form or by truncation
of a reservoir by the relatively impermeable salt. Such accumulation may also
be affected by faulting that resulted from the deformation (Chapman, 1973).
According to Braunstein and O’brein (1968), hydrocarbon is trapped in the sediments
that flank the core of rock salt and some instances it has been found in the
cap rock.
Chapman (1973) noted that diapirs
may rise as sediments are expelling water and oil and so traps form as
sediments are yielding oil. Mud diapir that form diapiric structures contain
much organic matter and may act as a source of methane which may accumulate in
diapir and associated traps. Loverson
(1967) stated that the relationship between the oil accumulation and the diapir
is only structural and there are many ways to provide suitable traps in
association with salt intrusion. The traps may be formed along folded or
faulted flanks of the salt plug or on top of the plug where arching and /or
faulting are produced in the overlying sediments.
4.2 CASE STUDY
The case study is the Niger Delta
in Nigeria which has three
formations: Akata , Agbada and Benin
formations. Akata being the oldest while Benin being the youngest. Weber and
Daukoru (1975) noted that the shale upheaval ridge occurring in Nigeria is of three
different kinds:
1. There
are zones behind major growth faults.
2. Shale
bulges in front of growth faults are often observed and these bulges can
sometimes act as positive elements causing collapsed crest structures and
unconformities.
3. Along
the continental slop, shale bodies were extruded in a seaward direction as a
result of differential loading on the plastic marine shale.
With continued sedimentation,
these offshore clay upheaval ridges are buried like salt domes, their growth
can continue. Finally, the clay ridges may develop into true diapiric
structures. Result of marine survey have discovered diapiric structure beneath
the continental slope and rise about 100km SE of the delta. These diapirs appear to root deeply in the
sedimentary section and the source layer may be of aptian-albian age. The
absence of any notable magnetic anomaly indicates that the structures are not
of volcanic origin. The seismic reflection profiles show several pillar-shaped
piercement structures that look very much like salt diapirs. Thus the evaporite
basins known from offshore Angola
and Gabon
may extend to the Nigerian offshore area.
SUMMARY
AND CONCLUSION
Salt diapirism
under the gravitational load of overlying sediments deforms those sediments. In
general, this occurs during compaction of these sediments and contemporaneous
with fluid expulsion from them. The deformation creates traps for any petroleum
generated and expelled from a source rock and ultimately such accumulations may
be displaced to the flanks of a salt diapir when penetration of the overburden
takes place. Since the salt itself has no known causal association with
petroleum accumulation due to salt diapirism must be regarded as a coincidence
in which the deformation of potential reservoir rock happens to take place near
source bed during their compaction.
Clay diapirism
both from an observational and theoretical point of view is a process that can
begin soon after loading by burial under a permeable sequence of sediments. Mud
volcanism in general is clay diapirism that expresses itself at the surface.
The apparent rarity of penetrative clay diapir at depth and their occurrence at
shallow depths is consistent with relative density considerations for the
density inversion between clays and sands is greatest above equilibrium depth
of about 500m. The stability of clay under load may remain until the clay is
buried to depth greater than those at which petroleum is commonly found. Such
instability and the structures resulting from it are necessary contemporaneous
with the expulsion of the bulk of the interstitial fluid of a clay. In the
geological context, such loading occurs typically during regressive phase of
the development of a sedimentary basin where the clays or marls are loaded by a
prograding permeable sequence of sediments. In petroleum context, this
instability is necessarily contemporaneous with the diagenesis of organic
matter in clay source rock. Hence, if the clay is wholly or partly a source
rock, diapiric structures are formed contemporaneously with petroleum
generation and its primary migration from the source, and petroleum may
accumulate at a point lower on the fluid potential gradient between the clay
and the surface.
The association
between growth structures, clay diapirism, and petroleum accumulation is
therefore inferred to be a closed one, of major significance in the geology of
petroleum.
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