Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 The role of structural geology in reservoir 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 structural geology 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 stress and fluid migration. Traditionally the role of structural geology in the an individual fold 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. Field 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 dome, structures which, in plan view, have deflections basin or saddle, 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 well 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 deformation & 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 present being carried out in an attempt to fering folds are off-set but the amount of off-set is understand the stress distribution and fracture 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 fractures. 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 mica 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 mountains. 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 reservoirs, it does not show extensional fractures and shear 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 times the tensile strength of together with the stress fields that generate the rock (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 diagenesis. The theory 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; event. 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 boudinage in a relatively thick of these concepts relevant to the ideas discussed sandstone layer in the Carboniferous turbidites 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- quartz veins; some of these are single extensional mined in part by the Navier-Coulomb criteria veins which cut 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.