Friday, 7 August 2015

DIAPIRIC STRUCTURES: ORIGIN AND SIGNIFICANCE IN HYDROCARBON ENTRAPMENT.

DIAPIRIC STRUCTURES: ORIGIN AND SIGNIFICANCE IN HYDROCARBON ENTRAPMENT.
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  
  1. Solid rock which may have a small percentage of pore spaces and pore liquid.
  2. Solid rock that is thoroughly broken up and fractured with some liquids in the fractures.
  3. Solid rock that has become mobile due to partial melting, a feature that generally occurs only at a considerable depth in metamorphic terrains.
  4. A loose aggregate of particles buoyed up by gases or liquid that could be derived from either magmatic or sedimentary sources.
  5. 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.



                                               REFERENCES

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