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Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 The role of structural in characterization

J. W. COSGROVE

Department of Geology, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK

Abstract: In this brief review of the role of in reservoir characterization a comparison is made between the original role of structural geology, which focused on the three-dimensional geometry and spatial organization of structures and which involved a statistical treatment of the spatial arrangement of structures such as faults and folds and on static analyses, and the more recent trend in structural geology which is concerned with the dynamics of structure formation and the associated interplay of and fluid migration.

Traditionally the role of structural geology in the an individual amplifies, it may cause the location and definition of hydrocarbon reser- initiation of other folds on either side and so voirs has been to define the geometry and spatial begin the formation of a wave-train. The work organization of structures such as folds and frac- of Dubey & Cobbold (1977) has shown that as tures. observations, theoretical analyses adjacent wave-trains propagate, they too may and analogue modelling of these various struc- interact with each other by the process of 'link- tures has led to a sound understanding of their ing' or 'blocking' depending on the relative wave- likely three-dimensional geometry and their lengths and positions of the two trains. Some of spatial relationships. For example field observa- the different ways in which they can interact are tions and analogue modelling of buckle folds shown in Fig. 3. It can be seen that linking of has shown that these structures have a periclinal folds from two fold-trains can also give rise to geometry, i.e. have the form of an elongate , structures which, in plan view, have deflections basin or , and that this geometry is charac- in their hinges (Fig. 3c(ii)). Alternatively folds teristic of buckle folds on all scales. Figure 1 may bifurcate (Figs 3c(iii) & d(ii)). shows three examples which illustrate this As as having a limited extent along their point. The folds range in scale between folds hinges, folds often die out relatively rapidly in with a wavelength of less than a centimetre to profile section. A typical profile section through folds with wavelengths in excess of 10 km. a fold in a multilayer is shown in Fig. 4a. The It is clear from Fig. la that in plan view the random initiation of folds within a multilayer folds are arranged statistically in an en echelon would give rise to the type of fold distribution manner. Analogue modelling (e.g. Dubey & shown in Fig. 4e. It can be seen therefore that Cobbold 1977; Blay et al. 1977) has shown how the folds are arranged in an en echelon manner this pattern of distribution emerges as the folds both in plan view and in plan. For an extended are initiated and amplify into finite structures. discussion of this the reader is referred to Price Initially isolated folds form and as & Cosgrove (1990). continues these amplify and extend along their Exactly the same spatial arrangement can be hinges. As they do so they may approach other found for faults. For example normal faults are folds. The way in which they interact is found often arranged in an en echelon manner in both to depend on the separation between the hinges profile and plan view and considerable work is of the two folds (Fig. 2). If the hinges of two inter- at being carried out in an attempt to fering folds are off-set but the amount of off-set is understand the stress distribution and only a small fraction of the wavelength, the folds patterns likely to develop in the relay zones link to form a larger fold with a deflection in the between these overlapping . hinge line (Fig. 2a). Experimental observations It can be seen from the above discussion that also show that if the hinges of two folds are sepa- the shortening along any line normal to the rated by more than about half their maximum fold hinges or the extension along any line wavelength but are still close enough to interact, normal to the strike of the faults will be constant they lock up, each preventing further propaga- regardless of where the line is drawn, but the tion of the other, so that they become arranged exact location of the individual folds or faults in an en echelon fashion (Fig. 2b). along any particular line cannot be determined. In the above discussion the linking and block- Although an understanding of the three- ing of individual folds is considered. However, as dimensional geometry and spatial organization

COSGROVE, J. W. 1998. The rode of structural geology in reservoir characterization. In: COWARD,M. P., DALTABAN, T. S. & JOHNSON, H. (eds) Structural Geology in Reservoir Characterization. Geological Society, London, Special Publications, 127, 1 13. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

2 J. W. COSGROVE

(a)

(b) (c)

Fig. 1. Three examples of folds displaying periclinal geometry and an en echelon spatial organization. (a) A crenulated schist from the Lukmanier Pass, Switzerland. Coin 1 cm. (b) En echelon periclines in a folded metasediment near Luarca, North Spain. Coin 2 cm. (c) Large periclinal folds in the Zargros . The pericline in the centre of the image is 20 km long. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

STRUCTURAL GEOLOGY AND RESERVOIR CHARACTERIZATION 3

( L )

..... @&- --~--

(b) x' -y-@ ..... -- -U---...... -o- ...... 7---

Fig. 2. (a) The amplification and coalescence of two periclines (shown in plan) separated by a distance X which is somewhat less than half the wavelength of the structure. (b) Two periclines separated by X' which is greater than half the wavelength. The structures overlap and lock up, each preventing further propagation of the other. of folds and faults has proved extremely useful in criteria of extensional failure. Figure 5b and c the location of potential , it does not show extensional fractures and fractures provide any insight into the possible fluid migra- respectively, and the principal stresses with tion paths that may have operated during the which they are associated. These stress states initiation and amplification of these structures. are represented as Mohr circles on Fig. 5a More recent studies (e.g. Sibson 1990; Sibson where it is clear that in order for shear failure et al. 1975, 1988; Cosgrove 1993) have been to occur the Mohr circle must be sufficiently concerned with dynamics of deformation and large to touch the shear failure envelope. It the interplay of deformation, stress and fluid follows from the geometry of the failure envelope flow. that this can only occur if the diameter of the In the following section, brittle failure and Mohr circle (i.e. the differential stress G1 -a3) hydraulic fracturing are briefly considered is greater than four the tensile strength of together with the stress fields that generate the (T). It also follows from the geometry them in order to illustrate this approach and of the failure envelope that the angle 20 between show its possible relevance to the migration and the normal stress axis and the line joining the concentration of hydrocarbons. centre of the Mohr circle to the point A where The discussion is then extended to include the it touches the failure envelope, is the angle links between brittle structures and fluid flow and between the two conjugate shear fractures (Fig. ductile structures and fluid flow. 5c). For extensional failure to occur the Mohr circle must touch the failure envelope at point B. This can only occur if the differential stress Brittle failure and hydraulic fracturing is less than four times the tensile strength of the rock. In this section the geological expression of Thus the type of brittle failure indicates hydraulic fracturing is considered with particular whether the differential stress during fracturing reference to fracturing of a sedimentary succes- was greater or less than 4T. Occasionally it is sion during burial and . The clear that both extensional and shear failure of brittle failure and hydraulic fracturing is dis- occurred together during a single deformation cussed in most structural texts (e.g. Price 1966; . An example where this has occurred is Phillips 1972; Price & Cosgrove 1990), to which illustrated in Fig. 6, which shows a line drawing the reader is referred, and only a brief summary of incipient in a relatively thick of these concepts relevant to the ideas discussed sandstone layer in the Carboniferous in this paper are presented here. at Millook, north Cornwall. The individual Figure 5a is a summary diagram showing the boudins are separated from each other by failure envelope for brittle failure. This is deter- veins; some of these are single extensional mined in part by the Navier-Coulomb criteria veins which completely across the layer and for shear failure and in part by the Griffith are an expression of extensional failure; others Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

4 J. W. COSGROVE

general, in a rock undergoing extensional failure, 'W*l the individual tensile fractures, although aligned, f;i ] I ! | rl are randomly distributed whereas during shear I failure they are organized in such a way as to (a) (i) (ii) define either one or a conjugate pair of en eche- lon extension fractures (see Kidan & Cosgrove 1996). [i I ,ii ' x , [ t i !; The orientation of extensional fractures ! ; 1 I : I (ii) ! 1 The stress states represented by the Mohr circles (b) (i) shown in Fig. 7a all have a differential stress less than 4T and because they all touch the failure envelope they will all generate extensional

I I failure. The differential stresses vary between ; , ,i, , i , I I I I just less than 4T (circle i, Fig. 7a) and zero i , : : : ,, i I, I i ]1' , I,'l'i i%l il I as t (circle iv, Fig. 7a). When o.1- o.3 is zero the stress state is hydrostatic and the Mohr circle ,,, , l)ili/[i,, collapses to a point. (c) (i) I The relationship between extensional fractures (iii) (i and the principal stresses generating them is shown in Fig. 7b (e.g. Anderson 1951; Price 1966). These fractures form parallel to the maxi- mum principal compression o.1 and open against i ! i I the least principal compression o-3 . In the stress state represented by Mohr circle i (Fig. 7a), | which has a relatively large differential stress, :lli I'i1: i'li : :1:: there is a definite direction of easy opening for the tensile fractures, i.e. parallel to o-3 . Thus the (d) (i) (ii) fractures that form will have a marked alignment normal to this direction (Fig. 7b(i)). However, Fig. 3. Simplified plan view illustration of the interference between two fold complexes of different the differential stress for the stress states phase and wavelength. The continuous and broken represented by the Mohr circles ii-iv becomes lines represent anticlinal and synclinal axes progressively smaller until, for the hydrostatic respectively. (a) (i) Wave-trains in phase approaching stress represented by the Mohr circle iv, the along z resulting in direct linking along z (ii). (b) (i) differential stress is zero. In a hydrostatic stress Wave-trains in phase approaching along y resulting in field the normal stress acting across all planes is direct linking along y (ii). (c) (i) Wave-train out of the same and therefore it is equally easy to phase approaching along y resulting in oblique linking open fractures in all directions. Thus the frac- (ii) and fold bifurcation (iii). (d) (i) Wave-trains of tures will show no preferred orientation and if different wavelengths approaching along y resulting in fold bifurcation, B (ii), and blocking, producing steeply they form sufficiently close to each other they plunging terminations, T (after Dubey & Cobbold will generate a breccia texture (Fig. 7b(iv)). It 1977). can be argued therefore that as the differential stress becomes progressively lower the tendency for the extension fractures to be aligned will become less and less (Cosgrove 1995). form part of an en echelon array and are a manifestation of shear failure. It can be con- cluded therefore that the differential stress Hydraulic fracturing during boudin formation was around four times the tensile strength of the sandstone at The state of stress in the 's tends to be that . compressional and true tensile stresses are It is interesting to note that the boudin necks thought to be uncommon. This will be particu- defined by shear failure and extensional failure larly true for the stress states in a sedimentary zones are both made up of tensile fractures pile undergoing burial and diagenesis in a and that the only difference between the two is tectonically relaxed basin. Nevertheless exten- the spatial organization of these fractures. In sional fractures occur commonly and this Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

STRUCTURAL GEOLOGY AND RESERVOIR CHARACTERIZATION 5

""", .., "'%.

~ ('0

",,, "\ "-

J (d)

/ ,

,"~~~•,.._~ ...... t ~ "v. I

,4 - v_",q. ~ I

Fig. 4. Typical three-dimensional geometry and spatial organization of multilayer folds. apparent contradiction has been satisfactorily occur. It can be seen that stress state i will explained by arguing that failure occurs by cause shear failure, stress state ii will cause hydraulic fracturing (e.g. Phillips 1972). It is aligned extensional failure and stress state iii argued that the internal fluid in the will cause brecciation of the rock by the for- or rock acts so as to oppose the applied mation of an almost random array of extension stresses and that the rocks respond to the effec- fractures. tive stresses 0-1-P, 0-2-P, 0-3-P, where p is It is a common misconception that the result of the fluid pressure. Thus a state of lithostatic hydraulic fracturing in and rocks is the stress in a rock will be modified by the fluid pres- formation of randomly oriented extension frac- sure to an state o-2 -p and o3 -p tures and the generation of breccia textures and the Mohr circle will be moved to the left by (Fig. 7b(iv)). It is clear from the above discussion an amount equal to the fluid pressure. The that the expression of hydraulic fracturing can three stress states represented by the solid vary, ranging from randomly oriented exten- Mohr circles shown in Fig. 8 are all stable sional fractures through aligned extensional stress fields in that they do not touch the failure fractures to shear fractures. envelope and therefore will not cause failure. Having briefly considered brittle failure, the However, all three circles can be driven to the relationship between stress and fractures and left by a fluid pressure until they intersect the the process of hydraulic fracturing, we can pro- failure envelope, when hydraulic fracturing will ceed to consider the factors that affect the state Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

6 J.W. COSGROVE

ff'i 17 l I: = C+ glSn i

b) c) / l \ ~n ~l

q;2+4T(Yn -

Fig. 5. (a) The Navier-Coulomb/Griffith brittle failure envelope. The two Mohr circles represent stress states that would give rise to extensional failure (the smaller circle) and shear failure. (b) and (e) show the relationship between the principal stresses and extensional failure and shear failure planes respectively. of stress in a sedimentary and the type the main source of stress is due to the over- and orientation of the hydraulic fractures that burden. If the boundary conditions are such might develop in them. that horizontal strains are prevented by the con- straints of the rock mass surrounding the area of interest, then it can be shown (e.g. Price 1966) Stress variation with depth that the vertical and horizontal stresses are related as follows: In this section the state of stress in sediments being buried in a basin is briefly considered in ~rH = av/(m- 1) (1) order to predict the type and orientation of where m is Poisson's number, the reciprocal of hydraulic fractures that might form. The stress Poisson's ratio. The vertical stress will be crI state will depend on the material properties of and its magnitude given by: the sediment or rock and upon the boundary crv = cr I = zpg (2) conditions. Consider the relatively simple boundary conditions which affect sediments in where z is the depth, p the average density of the a tectonically relaxed basin, i.e. one in which overlying rocks and g the acceleration due to

• " " :i.-::i.i

Fig. 6. Line drawing of incipient boudins formed in a sandstone layer in the Carboniferous turbidites at Millook, north Cornwall. Some boudins are separated from each other by a single extensional gash (i) and others by an array of extensional fractures (ii). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

STRUCTURAL GEOLOGY AND RESERVOIR CHARACTERIZATION 7

0.1 (i) (ii)

> G3 G3 < 0"3 0"3 ~ 1

G1 0.1 GI 0.1 O'n (iii) (iv) I~, )~1 0"3 ~ ~~ > O'3 0.3 > (Y3

a) b)

151 O"1

Fig. 7. (a) Four Mohr circles that represent four stress states that will give rise to extensional failure (i.e. o-1 - c~3 < 4T). (b) (i)-(iv) show the organization of the extensional fractures for each stress state. gravity. These equations show that if the over- differential stress is greater than 4T will be burden has a constant density and Poisson's shear fractures dipping at around 60 ° and strik- number does not change with depth, then the ing normal to o-3 . vertical and horizontal stresses increase linearly Thus when the vertical stress is the maximum with depth (Fig. 9). principal compression and when the two hori- It can be seen from Fig, 9 that the differential zontal principal stresses are not equal, the frac- stress (o-v- o-H) will also increase linearly with ture patterns generated by hydraulic fracturing depth. From the discussion of brittle failure in the upper and lower zones indicated on Fig. given earlier it follows that the type of hydraulic 9 will be as shown in Fig. 10a and b. In the fractures that will form in the upper section of upper zone where extensional failure occurs, the sedimentary pile, where the differential the fractures will be vertical and will be aligned stress is less than four times the tensile strength normal to o-3. In the lower zone where shear fail- of the rock, will be vertical extensional fractures ure occurs, conjugate fractures will form dipping opening against the least principal stress o-3 and at 60 ° and intersecting (and striking) parallel to the fractures that form at depths where the o-2. It can be seen from Fig. 10a and b that in

, .. ', .{. S/ ~3 i/} 0") oi co (o3- P) (o~- p)

Fig. 8. The influence of an increase of fluid pressure on three stable stress fields i, ii and iii (see text). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

8 J.W. COSGROVE

Stres____2 2 i' ~" Typical fluid I ~\~ pressure gradient /

(~v - or,) = (~, - a.,)

l \ ", \ ~ ..... (a, - or3) = 4T ___/__i ......

~3 G3

Fig. 9. Plot of variation of vertical and horizontal stress and fluid pressure with depths according to Eqns 1 and 2 which assume that the stresses are generated by the overburden in a tectonically relaxed basin. The expression of the hydraulic fractures is determined by the differential stress which increases with depth. At the depth when it exceeds 4T the fractures change from extensional to shear. the upper zone characterized by extensional fail- between the true dip of around 60 ° and zero ure, the dip of the fractures will be vertical on any (Fig. 10d). vertical face. In the lower shear fracture zone, the In the above discussion the state of stress at apparent dip of the fractures will vary between any depth has been assumed to be determined 60 ° and 0 °. However, on any particular plane by Eqns 1 and 2 and it was assumed that the the apparent dip will be constant. density of the rock or sediment and Poisson's The three-dimensional array of fractures that number remained unchanged with depth. These would form in the extensional and shear fracture assumptions are clearly unreasonable (e.g. Price zones if the two principal horizontal stresses, o2 (1958) and Eaton (1969) for a discussion of the and o-3, are equal are shown in Fig. 10c and d. change of m with depth) and direct measure- In the upper zone, because the horizontal stresses ments of the stress state in several present- are the same in all directions, there is no horizon- basins show that the stresses change in a non- tal direction of relatively easy opening and there- linear manner with increasing depth (Fertl fore no tendency for the alignment of fractures in 1976; Breckels & van Eekelen 1982). any particular direction. The result would be the formation of vertical fractures with random strikes. If these fractures are spaced closely Field evidence for hydraulic fractures enough to interfere, they would generate a poly- gonal array (Fig. 10c). It is clear from this figure The arguments outlined above indicate that as that the dip of the fractures would be vertical on sediments undergo burial and diagenesis in a any vertical face. basin, conditions of stress and fluid pressure are Similarly, in the zone in which shear fractures likely to be encountered that will to the form, because the horizontal stress is the same in formation of hydraulic fractures. Indeed, these all directions the strike of the shear fractures fractures would provide a transient increase in would also be random. Thus if they were permeability that would facilitate the dewatering spaced closely enough they too would interfere of relatively impermeable horizons. Unfortu- to form a polygonal array. The apparent dip of nately these fractures are generally not preserved, these fractures on any vertical face would vary forming as they do at relatively shallow depths Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

STRUCTURAL GEOLOGY AND RESERVOIR CHARACTERIZATION 9

I0"1 > 0"2 > 0"3} ~, ~,

((~'1- (~3) < aT f f (G"I- (~'3) > aT a) er, b) e~,

(~"1 > G"2 = 1~31 O'h O-1

(J'3 --.....~

(~1- ~3) < 4T t f (G'I- ~s) > 4T | c) O"t d) O'h

Fig. 10. The three-dimensional organisation of hydraulic fractures: (a) when 0-1 - 03 < 4T and the two horizontal principal stresses are unequal; (b) when 0-1 - 0-3 > 4T and the two horizontal principal stresses are unequal; (e) when 0-1 - 0-3 < 4T and 0-2 = 0-3; (d) when 0-1 - 0-2 > 4T and 0-2 = 0-3.

where they are unlikely to be preserved as most convincing demonstrations of hydraulic systems and where the rock properties are such fracturing in sedimentary successions. that barren fractures would tend to heal once Recently, Cartwright (1994) has reported the the excess fluid pressure has been dissipated. occurrence of large polygonal fracture arrays in The author has examined a number of quarries Early Cainozoic mudrock-dominated sequences and other of low permeability shales from the North Sea. The fractures have been thought to have been overpressured during mapped using regional two- and three-dimensional their burial. Remarkably little evidence was seismic data. They occur in the stratigraphically found of such fracturing. Occasionally, thin (1- bounded tiers in deep-water sequences bounded 2mm thick) bedding-parallel veins of fibrous by regionally condensed sequences (seals). The calcite occur, with the fibres oriented normal to faults are organized into cellular networks com- the bedding fractures, as for example in the prising polygonal prismatic and pyramidal forms Kimmeridge shales at Kimmeridge Bay in and Cartwright suggests that these faults are the Dorset. 'Chicken wire' texture has been recorded result of hydraulic fracturing. The polygonal cells in cores from shales known to have been pre- are between 500 and 1000m in diameter and a viously overpressured (Powey 1990, pers. synoptic block diagram illustrating the structural comm.). These are polygonal arrays of fractures framework of the faults is shown in Fig. 11. along which very thin veins of calcite have been Despite the impressive seismic images of precipitated. Hydraulic fractures are sometimes hydraulic fracture patterns presented by Cart- preserved as sedimentary dykes in the wright (1994), evidence for the occurrence of around sandstone bodies in shales if the fluid hydraulic fractures in low-permeability rocks are sufficiently high to fluidize the known to have been overpressured at some . Sedimentary dykes provide one of the in their , is remarkably scanty. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

10 J. W. COSGROVE

STRIKEOF N PALEO.ILOPE' 41 ....

uPPER~...... Z.2~"30

R 500,'n500 ,,n

t COMPLEX CONJUGATE INTERSECTIONS Fig. 11. A structural synopsis of a three-dimensional seismic survey of block 30/19, located in the basin centre, showing a polygonal network of small extensional faults affecting the lower Tertiary (from Cartwright 1994).

Even when fractures are found in such rocks it is under a variety of boundary conditions can be difficult to demonstrate convincingly that they studied with relative ease. are the result of hydraulic fracturing. For example, the barren fractures frequently exhib- ited by shales at often form polygonal The association of thrusts and high fluid arrays when viewed on the bedding plane and pressures are normal to bedding. Exposures of such rocks are often characterized by polygonal prisms. In the above discussion the build-up of fluid pres- This is a fracture pattern that might be expected sure and the formation of hydraulic fracturing in to form during the burial and overpressuring of a tectonically relaxed basin was considered. Such the rock (Fig. 10c). Nevertheless it is difficult to a fluid pressure build-up will also occur in determine .whether these fractures represent tectonic regimes, particularly those associated ancient hydraulic fractures generated during with compressional . In this section the burial and dewatering which have been reopened association of thrusts and high fluid pressures as a result of the release of residual stresses are considered. during , or whether they represent The recognition of overthrusts, large blocks of desiccation fractures resulting from the drying rock up to l Okra thick and with lengths and out of the shales on exposure. widths often in excess of lOOkm, which had This similarity in the geometric organization been transported tens of kilometres along sub- of desiccation fractures and extensional hydrau- horizontal planes, presented lic fractures reflects an underlying similarity in with the challenge of understanding the mechan- the mechanism of formation of both structures. ism by which these blocks were moved. An This can be exploited when hydraulic fracturing early study of this problem was carried out by is studied using analogue models. Smoluchowski who represented the overthrust The experimental investigation of the initia- block as a single rectangular prism caused to tion and development of hydraulic fractures in move over a fiat, dry surface by the application overpressured shales and semi-lithified rocks is of a horizontal push from one end. By selecting extremely difficult. However, if it can be argued the appropriate values for the coefficient of that the process is directly analogous to the sliding and rock strength it can be formation of cooling fractures or desiccation shown that overthrusts with lengths greater fracture, then the formation of these fractures than a few tens of kilometres cannot be moved Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

STRUCTURAL GEOLOGY AND RESERVOIR CHARACTERIZATION 11 by this mechanism unless the applied stress In contrast, the migration of fluids in associa- exceeds the strength of the rock. tion with the development of folds has received Alternative mechanisms for the movement of comparatively little attention. It is, however, large overthrusts were presented. For example, possible to determine the stress gradients within (1921) suggested that movement on a and around a fold from the equations that thrust plane may not be synchronous and that govern the buckling behaviour of anisotropic decoupling and displacement may occur locally bodies such as a sedimentary sequence. They and migrate along the thrust plane in a caterpil- show that there is a difference between the lar-like manner. This mechanism would allow mean stress inside and outside a fold and that the overthrust block to move forward without this difference (i.e. the stress gradient) changes having to overcome the total frictional resistance as the fold amplifies. For a box fold the gradient of the block to movement at any one time. In this is initially such that fluids are drawn into the fold way, it was argued, large overthrust blocks could from the surrounding . However, beyond a be moved at stresses below the strength of the certain amplification the gradient is reversed and rock. Despite such innovative thinking it was fluids are driven out of the fold. This process can not until 1959 that a widely accepted solution be inferred from the geometric changes that appeared to the mechanical problems associated accompany the amplification of a kink-band in with the movement of large-scale thrusts. a layered material where the layers remain a con- In their classic paper, Hubbert & Rubey (1959) stant thickness during the deformation (Fig. argued that the existence of high fluid pressures 12a-c). Initially there is an increase in volume would reduce the effective normal stress across a within the kink-band. This continues until the potential thrust plane and thus reduce the layering inside the kink-band is normal to the horizontal compressive stress necessary to move kink-band boundary, i.e. a~= 0 (Fig. 12b), the thrust to below the brittle strength of the when it reaches a maximum. Up to this point rock. This idea represented a major step forward fluids will be drawn into the fold from the sur- in the understanding of the generation and move- rounding unfolded region. As the kink-band ment of large overthrusts and, following this work, amplifies beyond this point, the volume of the the association of overthrusting and high fluid kink-band is reduced and fluids will be expelled pressures became almost axiomatic. More recently from the fold. When co = 20 (Fig. 12c), the the overthrust 'paradox' has been reassessed (Price volume of the kink-band is the same as it was 1989) and this association questioned. before the fold was initiated. Any further ampli- Although Hubbert & Rubey's work on high fication would require the layering inside the fluid pressures provided a great insight into the kink-band to thin and if there is no mechanism role that fluid pressures may play in thrust initia- by which this thinning can be achieved, the fold tion, it says nothing about the migration of fluids locks up. during thrusting. The magnitude of the stress gradient between the fold and its surroundings has been quantified by Summers, who shows that the difference in Faults, folds and fluid migration mean stress inside (Oi) and outside (#e) a fold is given by: The migration of fluids along major faults during and immediately after reshear has been discussed by Sibson et al. (1975), who introduced the idea of seismic pumping. This concept sprang from observations of hydrothermal vein deposits ( N / Q + Y) - -(N-FQ - ~--~-ss-4--(O~-~) found in the upper, brittle regions of ancient (3) fault zones. The textures of these deposits usually indicate that mineralization took place episodi- where 0 and co are defined as in Fig. 12d, N and Q cally and it was suggested that episodic injection are measures of the resistance to compression of hydrothermal fluids could be accounted for by and shear respectively, in the direction of the the dilatancy-fluid model for applied maximum principal compressive stress, release in shallow proposed by and Tem is the maximum shear stress in the Scholtz et al. (1973). In later papers, Sibson layering adjacent to (i.e. outside) the fold. The et al. (1988) and Sibson (1990) proposed two variation in mean stress gradient as the fold other mechanisms, the suction-pump and fault- amplifies can be clearly seen by expressing Eqn valve mechanisms which also predict periodic 3 graphically (Fig. 12e). As the fold begins to variations in fluid pressure along faults asso- amplify, there is an increase in the stress differ- ciated with fault reactivation. ence which would tend to draw fluids into the Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

12 J. W. COSGROVE

~0,, wavelength on to these layers and, as they accommodate themselves to the multilayer wavelength, they often develop -order i structures known as accommodation structures (a) (b) (c) (Ramsay 1974; Price & Cosgrove 1990, pp. :0/ 319-321). These structures, which form pre- b,0 /~' .<.~ -- / dominantly in the hinge region of folds, may be either ductile or brittle (Fig. 13a-d). In addition /g'--~ ~ ,,// to accommodation structures, the high stresses generated in the inner and outer arcs of the hinge region often lead to localized brittle failure. These fractures, combined with the (d) ////~ .. brittle accommodation structures shown in Fig. / 13b-d, can dramatically increase the permeabil- ity of the hinge regions and become channels of easy fluid migration. Thus, during the expulsion of fluids from a fold during the late stages of its amplification in response to the stress gradients L illustrated in Fig. 12e, the fluids are likely to be I \ ]f°" " ; 'External] expelled along these zones of high fracture- 2 induced permeability.

1 It can be seen from the above discussion that in a deforming sedimentary succession, both fold- , 0 30 60 m 90 ing and faulting can be expected to cause episodic -1 variations in pore pressure and stress. The expul- sion of fluids along faults and out of folds as they -2 amplify and lock up may help to generate new -3 structures in the adjacent, relatively undeformed ~ parts of the succession. This insight into the fluid -4 100 dynamics associated with the formation of -5 (e) Fig. 12. (a)-(d) Volume changes that accompany the amplification of a kink-band. During the early stages the volume increases and fluids are drawn into the fold (a-b). However, as the structure amplifies beyond the stage where a~ = 0 (b), the volume decreases and fluids are expelled. (d) Detail of (b) showing the dilation within the kink-band. (e) The graphical expression of Eqn 3 showing the relationship between the difference in mean stress inside and outside the kink-band and the (b) ~ Accommodation orientation w of the within the kink-band. (a) ~ thrust Graphs for materials with (N/Q) of 5 and 100 are shown ((e) after Summers). /I/S~'- Sadf01o fold. The stress difference rises sharply until the layering inside the kink-band is normal to the kink-band boundary (co = 0). The gradient then drops dramatically and reverses so that a large gradient is established which will tend to (c) (d) drive fluids out of the fold as it continues to amplify. Fig. 13. (a)-(d) Various accommodation structures that develop during multilayer buckling as layers with During the buckling of a complex multilayer, anomalous thicknesses adjust to fit into the overall that is a multilayer made up of a variety of wavelength and amplitude of the multilayer buckles. different rock types and layer thicknesses, the These may considerably increase the permeability of individual layers will attempt to develop their the hinge region of the fold ((a) and (b) after Ramsay own characteristic wavelengths. However, the 1974; (c) after Price & Cosgrove 1990; (d) after Herman multilayer will impose its own characteristic 1923). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

STRUCTURAL GEOLOGY AND RESERVOIR CHARACTERIZATION structures such as folds and faults, which are DUBEY, A. K. & COBBOLD,P. R. 1977. Non-cylindrical potential hydrocarbon traps, provides a fertile flexural slip folds in and . area of research which will doubtless have an , 38, 223-239. important impact on the role that structural EATON, B. A. 1969. Fracture gradient prediction and its application in oilfield operations. Journal of geology plays in hydrocarbon exploration. Technology, Paper 1353-60; Trans., AIME, 246. FERTL, W. H. 1976. Abnormal Formation Pressures. Conclusions Elsevier, 382 pp. HERMAN, H. 1923. Structure of the Bendigo field. Knowledge of the three-dimensional geometry Bulletin of the Geological Society, Victoria, 47. and spatial organization of structural traps has HUBBERT, M. K. & RUBEY, W. W. 1959. The role of for many been one of the most powerful fluid pressure in the mechanics of overthrusting. tools in hydrocarbon exploration. Nevertheless, Bulletin of the Geological Society of America, 70, 115-166. these models of structure geometry and organiza- KIDAN, T. W. & COSGROVE, J. W. 1996. The deforma- tion are essentially static and do not consider the tion of multilayers by layer-normal compression. interaction of fluids and structures during defor- Journal of Structural Geology, 18, 461-474. mation when the structure is being initiated and OLDHAM, R. D. 1921. Know your faults. Quarterly amplified. However, recent advances in the Journal of the Geological Society, London, 77, understanding of the role of fluid pressure in 77 92. the initiation of faults and folds and the sub- PHILLIPS, W. J. 1972. Hydraulic fracturing and miner- sequent influence of these structures on the alisation. Journal of the Geological Society, migration and concentration of fluids, has London, 128, 337-359. PRICE, N. J. 1958. A study of rock properties in opened a new chapter in the role of structural conditions of triaxial stress. Proceedings of a geology in reservoir characterization. Conference on the Mechanical Properties of Non-metallic Brittle Materials, Butterworth, London. References --1966. Fault and Development in Brittle and Semi-brittle Rock. Pergamon, Oxford. ANDERSON, E. M. 1951. The Dynamics of Faulting, -- & COSGROVE, J. W. 1990. of Geological Oliver and Boyd, Edinburgh. Structures. Cambridge University Press. BLAY, P. K., COSGROVE, J. W. & SUMMERS, J. M. 1977. PRICE, R. A. 1989. The mechanical paradox of large An experimental investigation of the development overthrusts. Bulletin of the Geological Society of of structures in multilayers under the influence of America, 100, 1898 1908. gravity. Journal of the Geological Society, RAMSAY, J. G. 1974. Development of folds. London, 133, 329-342. Bulletin of the Geological Society of America, 85, BRECKELS, I. M. & VAN EEKELEN, H. A. M. 1982. 1741-1754. Relationship between horizontal stress and depth SCHOLZ, C. H., SYKES, L. R. & AGGARWAL, Y. P. 1973. in sedimentary basins. Journal of Petroleum Tech- prediction: a physical basis. Science, nology, Technical Paper SPE 11282, 2191-2199. 181, 803-809. CARTWRIGHT, J. A. Episodic basin-wide hydrofrac- SIBSON, R. H. 1990. Conditions for fault-valve turing of overpressured Early Cainozoic mudrock behaviour. In: KNIPE, R. J. & RUTTER, E. H. sequences in the North Sea Basin. Marine and (eds) , and , 11, 587-607. Tectonics. Geological Society, London, Special COSGROVE, J. W. 1993. The interplay between fluids, Publication 54, 15-28. folds and thrusts during the deformation of a sedi- --, MOORE, J. McA. & RANKIN, A. H. 1975. Seismic mentary succession. Journal of Structural Geology, pumping - a hydrothennal fluid transport 15(3-5), 491-500. mechanism. Journal of the Geological Society, 1995. The expression of hydraulic fracturing in London, 131, 653-659. rocks and sediments. In: AMEEN, M. S. (ed.) Frac- , ROBERT, F. & POULSEN, K. H. 1988. High angle tography: fracture topography as a tool in J'racture reverse faults, fluid pressure cycling and meso- mechanics and stress analysis. Geological Society, thermal gold-quartz deposits. Geology, 16, 551- London, Special Publication 92, 187 196. 555.