The Mechanics of Clastic Intrusion: Implications for Deep Water Clastic Reservoirs1

THE MECHANICS OF CLASTIC INTRUSION: IMPLICATIONS FOR DEEP WATER CLASTIC RESERVOIRS1

Lidia Lonergan, and Richard J.H, Jolly* T.H. Huxley School, Imperial College, London, UK *Now at Golder Associates UK Ltd, Maidenhead, England [email protected] intrusions. However, our work suggests that large-scale Abstract (several hundred metre) sandstone intrusions within Several productive Paleogene deep water sand- Paleogene deep marine successions of the North Sea, stone reservoirs in the North Sea (e.g. Alba, Forth/ require the presence of fluids migrating from deeper Harding, Balder, Gryphon) show evidence of having within the basin (e.g. gas charge) to drive the injection. undergone major post-depositional remobilisation and clastic injection, which can result in disruption of primary reservoir distribution. Remobilization features, Introduction range from centimetres (e.g. core-scale) to hundreds of metres (e.g. seismic-scale). The scale of the clastic in- Cubic-kilometre sized clastic intrusions and remo- trusion and remobilisation has significant impact on res- bilized sandstones are increasingly recognised as an ervoir architecture and production performance, includ- important component of the deepwater play within the ing changes in (a) original depositional geometries; (b) latest Paleocene and early Eocene of the North Sea ba- reservoir properties; (c) connectivity, (d) top reservoir sin. Sediment deformation and intrusion are not just lo- surface structure, (e) reservoir volumetrics, and (f) re- calized processes which have affected one or two fields covery/performance predictions. in the North Sea, but within the late Paleogene interval, There are two prerequisites for sandstone in- remobilization and intrusion of sands has directly af- trusions to form: the source sediment must be fected the identification, definition, and understanding uncemented, and the ‘parent’ sand body must be sealed of at least 10 important hydrocarbon reservoirs and pros- 2 such that an overpressure with a steep hydraulic gradi- pects within an area of ~500,000 km in the Central and ent can be generated. The seal on an overpressured sand Northern North Sea (e.g. Forth/Harding, Alba, Balder, must then be breached for the sand to fluidize and in- Gryphon fields; Alexander et al. 1993; Jenssen et al., ject. The stress state within the basin, burial depth, fluid 1993; Newman et al. 1993; Newton & Flanagan, 1993; pressure and the nature of the sedimentary host rock all Timbrell, 1993; Dixon et al. 1995; Lonergan & contribute to the final style, geometry and scale of in- Cartwright, 1999; Lawrence et al. 1999; MacLeod et al. trusion. At shallow depths, within a few meters of the 1999). Figure 1 illustrates an example of a kilometer – surface, small irregular intrusions are generated, more scale intrusion, consisting of a dike and sill imaged on commonly forming sills, whereas at depth dikes and sills 3D seismic data within Eocene strata, from the North- form clastic intrusion networks. We use field examples ern North Sea. The intrusion cross-cuts 200 m of stratig- from the western Ireland, and Santa Cruz and Panoche raphy, and the presence of sand in this intrusion has been Hills in California to illustrate the control of burial depth/ verified by drilling. (More examples of both core, and stress on intrusion scale. The cohesivity of the host sedi- seismic-scale intrusions from the North Sea are described ment, and the flow velocity of the intruding sands ap- in Lonergan et al. submitted). pears to control whether the intrusion is emplaced by Traditionally, the approach adopted when interpret- stoping (the incorporation of host rock material as rafts ing Tertiary deep-water sandstone reservoirs in the North in the intrusion), or dilation (the forceful pushing apart Sea, has been to assume that the current reservoir ge- of the host rock to create space), resulting in diverse ometry and heterogeneity is largely a function of an origi- styles of intrusive geometry. nal primary depositional origin. We argue, that there is Earthquake induced liquefaction, tectonics and now a mounting body of evidence which suggests that build up of excess in situ pore pressure are the most com- this paradigm be reassessed and that in other deepwater monly cited explanations for the occurrence of clastic settings, the possibility that observed sandstone geometries might be a function of post-depositional

1This contribution includes material from two papers currently in review: Jolly, R.J.H. & Lonergan, L. The mechanisms of clastic sill and dike intrusion,, GSA Bull. and Lonergan, L. et al. Remoblisation and Injection in Deepwater Depositional Systems- Implications for reservoir architecture and prediction, GSSEPM- Deep Water Reservoirs of the World Conference Publication Stanford Rock Fracture Project Vol. 11, 2000 C-1 Dyke Balder Marker Sill

SAND 100 m

~1 km

Figure 1. Seismic section illustrating a kilometer-scale dike and sill complex in the North Sea, within Eocene strata.

remobilisation, needs to be evaluated. To further 2a): Remobilisation leads to fundamental understand and predict geometries of intrusions changes in reservoir architecture e.g. that form in deepwater sedimentary environments steepening of original depositional geom- we need an improved theoretical basis from which etries (e.g. Balder Field, Jenssen et al. to consider the process of clastic intrusion and 1993; Rye-Larsen, 1994), development of the resultant geometries. The mechanical control pod-like sandbodies (e.g Balder and Alba on intrusion geometry, size and intrusion mecha- Fields), intrusion of clastic dikes and sills nism is the main topic of this paper. above the reservoir (e.g. Forth/Harding, Frigg, Gryphon and Alba Fields; Dixon et al., 1995; Newman et al. 1993; Newton & Remobilisation and the effects on res- Flanagan, 1993) and laterally along the reservoir margins (e.g. Alba Field, ervoir geology Lonergan & Cartwright, 1999; MacLeod As a direct result of remobilisation and injec- et al. 1999). tion, many of the North Sea Paleogene reservoirs (b) Changes in reservoir properties (Figure have complex geometries. This, coupled with their 2b): Remobilization often homogenizes subtle expression on seismic data, and with their reservoir properties (e.g. by clay lack of primary depositional characteristics com- elutriation) and eliminates original sedi- bine to make them challenging prospects for ex- mentary structures leading to a massive ploration and appraisal. Sand remobilisation pro- sandstone facies. These facies are often cesses can affect reservoir geology in a number considered an original depositional facies of ways: and then interpreted as the deposits of ei- (a) Changes in reservoir architecture (Figure ther sandy grainflows or debris flows.

C-2 Stanford Rock Fracture Project Vol. 11, 2000 A. Change in reservoir geometry

B. Change in reservoir properties Massive blocky sandstone

intra-reservoir shales Channelised No internal turbiditic sandstones clay breaks

C. Change in connectivity Dike

OWC Sill Isolated channel sands

D. Change in top reservoir surface & in reservoir volumetrics Producer No Yes

OWC OWC

Figure 2. Schematic diagram illustrating the potential effects of clastic intrusion and remobilization on reservoir geology.

(c) Change in connectivity of originally iso- bedding (sills) (Figure 3). Jolly and Lonergan (sub- lated reservoirs (Figure 2c): Clastic intru- mitted) review the extensive published literature sions alter the transmissivity of the reser- spanning a century and more, on the occurrence voir and connectivity between previously of clastic dikes and sills. While clastic intrusions isolated reservoir units can be established. have been documented from all depositional en- Vertical or steeply dipping dikes will not vironments, they have been most frequently re- be imaged on seismic data, so connectiv- corded in deepwater depositional settings. Clas- ity between apparently separate sand bod- tic intrusions documented in outcrop are typically ies may not be evident. small, rarely reaching tens of metres thickness in (d) Changes in top reservoir surface and in dimension, but have never been observed at the reservoir volumetrics (Fig 2d) (e.g. Alba scale of those interpreted from seismic data in the Field, MacLeod et al., 1999). North Sea. The largest known outcrop examples occur within tectonically active basins, such as the large oil-bearing intrusions exposed near Santa Clastic injection and intrusion trigger- Cruz in California within the Miocene Santa Cruz mudstone (see Thompson et al. 1999 for a recent ing mechanisms description) in the Santa Cruz/La Honda strike slip The emplacement of remobilised clastic sedi- basin along the San Andreas fault, or from thrust ment into the surrounding strata, can either form belt/accretionary prism settings (e.g. Winslow tabular bodies of sediment that are discordant to 1983; Scott, 1966) bedding (dikes), or sheets largely concordant with The majority of previously published work

Stanford Rock Fracture Project Vol. 11, 2000 C-3 m. (see references cited in Obermeier 1996). Thus, ? ? when considering earthquake liquefaction as a sill length potential trigger for clastic intrusion it is impor- T tant to consider the scale of the intrusion, the depth Step at which the intrusions formed, and the likelihood dyke that earthquakes greater than magnitude 5 may height Jog w L have occurred at the time of intrusion (i.e. was the basin in an active plate tectonic setting?). Source bed Fluid- induced liquefaction, where the fluid is not the in-situ pore fluid, but migrates into the sealed sand body from elsewhere in the basin, has rarely been cited as a mechanism for triggering Figure 3 Diagram illustrating principle features of clas- clastic intrusions. Jenkins (1930) recognised that tic sills and dikes intruded in the subsurface. W= dike the migration of hydrocarbon fluids potentially width, L= dike length (strike length if exposed in out- played an important role in the formation of the crop), T= sill thickness. Note the blunt termination at large number of dikes found in the oil-producting one end of the illustrated sill and the thin fingers (2D); basin of California. Thompson et al. (1999) sug- sheets (3D) at the other end. gest that the large Yellow Bank Creek intrusion west of Santa Cruz, initiated as a dike forming considers that there are three principal triggers that along a fault, and that horizontal break-out of water lead to the formation of clastic intrusions (Figure saturated sands ahead of a migrating hydrocarbon 4a): (a) tectonic stress (e.g. Winslow, 1983) (b) front increased the size and complexity of the in- earthquake induced liquefaction (e.g. Fuller, 1912; trusion. Fluidized sediments may even have Obermeier, 1989) and (c) localised excess pore erupted onto the seafloor as a sand volcano. In fluid pressures in the sediments resulting from the central North Sea, Brooke et al. (1995) have depositional processes, such as local loading due identified gas-bearing, circular, mounded sand to mass movements and slumping (e.g. Truswell bodies, 1 km in diameter at depths of ~500m as- 1972), the passage of storm waves (Allen 1985; sociated with gas conduits. These authors suggest Martel & Gibling, 1993) or channel switching (e.g. that gas migrating upwards (possibly along faults) Hiscott, 1979). In general, depositional event trig- entered into a laterally-continuous, deposited sand gers for clastic intrusions tend to be responsible body with minor irregularities in its top surface, for small-scale (metre and less) intrusions domi- and liquefied the sand. Migration of the gas and nated by sills (see for instance the examples from entrained sand grains towards the subtle highs, the Ordovician of Quebec and Western Ireland generated mounds with the withdrawal of sand documented by Hiscott (1979) and Archer (1984) from the flank areas. respectively). Cyclical shear stresses that occur during shallow focus earthquakes are an effective mechanism for liquefying near-surface water saturated sands, Mechanics of Clastic Intrusion but data from the earthquake engineering litera- Injection of high pressure fluid (with entrained ture shows that liquefaction only occurs for earth- sand grains) into the surrounding sediments re- quakes with magnitudes greater than 5 (e.g. quires firstly a sealed, unlithified, source sand- Ambraseys, 1988; Obermeier, 1996; Figure 4b). body, in which an excess-fluid pressure has built Even after large earthquakes (>mag 6), studies of up. The seal on the overpressured sand-body must recent-Pleistocene sediments show that although then be breached (suddenly) in order to generate earthquake-induced dikes can occupy fissures that the fluid velocities necessary for fluidization and are several hundreds of metres long in plan view, mobilization of the sediment. A sustained pres- sand volcanoes up to 40m in diameter can form, sure differential between the fluid in the propa- and at most dikes travel 10 m vertically from their gating fracture and the fluid in the pores of the source beds. The maximum recorded dimensions sedimentary rock is required, so that the fracture of earthquake induced intrusion are dikes up to can remain dilated and the sand-fluid mixture can 2m in width and sills with thicknesses up to 0.5 flow through the fracture forming an injection.

C-4 Stanford Rock Fracture Project Vol. 11, 2000 (b) (a) 10

9 Tectonic Activity /Seismic shaking

Build up of 6 excess pore Moment Magnitude (M) pressure due to depositional events (e.g. 5 channel Addition of a fluid loading, slumps) 4 1 5 10 50 100 500 1000 Maximum distance to furthest surface evidence of liquefaction induced ground failure (km) shallow focus earthquakes

Figure 4. (a) Reported intrusion trigger mechanisms. Dots refer to clastic intrusion localities cited in the literature (see Jolly & Lonergan for full cited references). Points not located within a cluster at an apice, specify more than one mechanism operating. Grey dots are examples where earthquake induced liquefaction was specifically mentioned. (b) Lique- faction occurrences according to earthquake magnitudes for historical shallow focus earthquakes. Note the lack of liquefaction for events smaller than magnitude 5 (data from Ambraseys, 1988).

Once the source of the excess pressure is removed, muds adjacent to the sand body are able to con- the remaining fluid in the clastic intrusion will tinue consolidating relative to the sand body, and bleed into the surrounding porous sedimentary so form a low permeability seal. Eventually when layers equilibrating the pressure, and terminating the seal fails, the resulting pressure differential fracture propagation. This process has three steps: allows rapid fluid flow, fluidising the sand body (1) the build of an excess fluid pressure in a sand and mobilising the sand grains into the escaping body, (2) failure of the seal and (3) subsequent fluid. fluidisation of the sand. The most common method of producing an Overpressure excess fluid pressure in a sedimentary basin is by An excess fluid pressure (known as overpres- simple burial, where the burial rate exceeds the sure) is generated by having a sealed pocket of ability of the fluid to escape from within the pore sediment in which the pore fluid pressure is greater spaces between the sediment grains. With increas- than hydrostatic pressure (e.g. Maltman 1994). ing burial depth the pore fluid pressure initially This typically occurs in a depositional environ- follows the hydrostatic gradient when the system ment where a sand body is enclosed in low per- is unsealed (Fig. 5). When the sediment body be- meability mudstones. The mudstones immediately comes sealed, the pore fluid pressure gradient de- surrounding the sand body prevent the expulsion viates away from the hydrostatic gradient, and an of pore fluids, and as a result, the overburden load excess fluid pressure begins to build up. There is is partially supported by the excess pore fluid pres- often a steep increase in fluid pressure gradient sure, causing the sand body to be due to the pore fluids from the surrounding muds underconsolidated. The remaining component of draining into to the sand body. Once the excess the overburden load is that sustained by the sedi- fluid has drained into the sand body the fluids are ment particles and is called the effective stress, trapped and the overpressure gradient parallels the σ’ (Terzaghi, 1936): lithostatic stress with increasing burial (Fig. 5). ( 1) An influx of remote fluids such as migrating hy- σ'= σ Lith − Pf drocarbons or fluids released during geochemical where is the overburden or lithostatic σLith reactions, may also cause an increase in pore pres- stress and P is the fluid pressure (Figure 5). The f sure in porous sandstones (e.g. Swarbrick and

Stanford Rock Fracture Project Vol. 11, 2000 C-5 Osborne, 1998). fied, injection of overpressured sand will mani- fest itself in terms of pipes (and sand volcanoes) Mechanisms of Seal Failure rather than planar dikes/sills, or soft-sediment Intrusion of a clastic dike requires that the host fluid-like mixing will occur between the overpres- rock fails in the brittle regime, by a fracture which sured sandy fluid and the fluid-rich surrounding is filled with the injecting sediment. A propagat- muds and silts (Nichols 1995). ing dike (whether of igneous or clastic origin) is considered an example of an opening mode (mode I) fracture that propagates as a tensile crack in a Brittle failure and dike orientation plane normal to the least compressive stress di- By considering the state of stress in the sedi- rection (e.g. Delaney et al. 1986). For mode I fail- mentary section and the response of the sediments ure, the host rock must have tensile strength. to brittle failure we can, in some circumstances, Cohesion of the host rock is, therefore, critical if predict the orientation of a clastic intrusion. We clastic intrusions are to form. Close to the sur- consider three different ways in which the host face, muds have low cohesive strength, and fail- sediment might fail: ure in this regime is dominated by shear failure, plastic deformation and density inversions lead- (a) When the pore pressure (P ) within the ing to local soft-sediment mixing, and ductile-de- f sands exceeds the minimum principal stress (σ ) formation textures (Lowe 1975; Nichols, 1995). 3 and the tensile strength (T) of the host material, The porosity reduction with increasing burial, al- brittle failure will occur (Fig. 6a). lows a closer packing of the grains leading to a greater strength (mainly due to an increase in elec- Pf > σ 3 + T (2) trostatic charges between mineral grains). This For a sedimentary basin, without imposed tec- suggests that there is a minimum depth at which a tonic stresses the maximum principal stress is typi- fracture and thus a dike can form, and this depth cally vertical (due to gravitational loading) and will be a function of both the tensile cohesive the minimum principal stress is in the horizontal strength, (T) of the host sediment and the differ- plane. If such a sedimentary basin was filled with ential stress. Where these conditions are not satis- isotropic, homogeneous sediments then vertical dikes would form (Fig. 6a).

Pressure (MPa) (b) When there are faults or fractures within 10 20 30 the host sediment, the pore fluid pressure (Pf) need only exceed the resolved normal stress (σn) across the older fracture for dilation to occur (Fig. 6b). Seal onset (3) 500 Pf > σ n Under these circumstances non vertical dikes Fluid pressure Pf may form. As the fluid pressure increases a larger

Depth (m) overpressure range in fracture orientations will be able to dilate 1000 effective and be intruded by the injecting sands (Delaney stress σ' et al. 1986; Baer et al., 1994; Jolly and Sanderson, 1995). The photograph in Figure 7a illustrates two 1500 oblique dikes intruding flat lying Miocene Santa Hydrostatic Lithostatic Cruz siliceous mudstones, that may have formed gradient gradient σv by this mechanism. The dikes dip at ~60° and intrude along a set of pre-existing fractures within Figure 5. Pressure versus depth for a sedimentary suc- the mudstones. cession within a basin sedimentary; relationship between overburden load (lithostatic pressure, σ ), pore v (c) An increase in fluid pressure can cause pre- fluid pressure (Pf), overpressure and effective stress (σ’) shown. When a sedimentary unit seals, with continued existing fractures to shear (Barton et al., 1995). burial the fluid pressure will deviate from the hydro- The geometry of the pre-existing fractures can be static fluid gradient, as shown by the solid thin black such that areas of overlap between shearing frac- line.

C-6 Stanford Rock Fracture Project Vol. 11, 2000 (a) No fractures present hydrostatic fluid pressures, and the fluid flow velocity would never reach the minimum fluidisation velocity. σh For spherical particles, Richardson (1971) has shown that there is a minimum fluid velocity (Umf) necessary to cause fluidisation of the particulate Pf > σh+ Τ source bed. The minimum fluid velocity is de- pendant on the particle diameter (d), the densities of the fluid (ρf) and the particles (ρs), acceleration due to gravity (g), and the dynamic viscosity of the fluid (µ) (b) Pre-existing fractures 2 0.() 00059dgρρs − f U = (4) mf µ σn According to equation (4) the most favourable conditions for fluidisation of a sediment horizon P > Pf < σn f σn should be sediments with a low density, a small particle size, a particle shape approaching spherical, and a small particle size range (Richardson, 1971). A well sorted fine-grained sandstone should Figure 6. Mechanisms by which seal will rupture, al- therefore be most prone to fluidisation. lowing a seal body to fluidise and then inject into overlying sediments. (a) No pre-existing fractures; dykes will intrude in a plane perpendicular to the least prin- Determining the Geometry of an Intru- ciple stress. (b) Pre-existing fractures exist; to dilate sive System the fluid pressure (P ) must exceed the normal stress on f Sills or dikes and the effect of burial the fracture. depth In trying to understand the particular stress tures are dilated at jogs that are then infilled by conditions that favour the formation of dikes or fluidised sand. This is illustrated in Figure 7b sills and predicting when a dike or a sill, or both, where four fracture segments dilate and in the will form we will use some simple mechanical overlap injected sand can be seen. It is likely that concepts to provide qualitative insights into this this mechanism for host sediment failure would complex problem. To do this we apply simple occur in conjunction with the dilation of other frac- boundary conditions and geometries and thus we tures as described in (b). make the following assumptions: (a) There are no remote tectonic stresses applied to the sedimen- Fluidization of the Clastic Horizon tary succession in question; (b) The maximum A simple increase of the pore pressure within principal stress is vertical and is due to overbur- a sealed system does not remobilise the sediment den load; (c) Due to the anisotropy of bedding, particles. For there to be movement of the par- the sedimentary rocks will have different tensile ticles, and hence flow, the pore fluid must have a strengths in orientations parallel and perpendicu- velocity, causing a pressure differential and im- lar to the bedding. In a well-laminated mudrock posing a drag force on the particles (Davidson and or shale sequence the tensile strength in the direc- Harrison, 1971; Nichols 1995). Within a sealed tion normal to bedding (T ) is likely to be less than v unit there has to be some venting mechanism (e.g. that parallel to it (Th) (Cosgrove, 1995); (d) The failure of the seal) that allows loss of fluid from bedding is horizontal; (e) And we neglect any vis- the body, and therefore generating fluid flow cous drag effects that will tend to decrease the flow within the body. This must be a catastrophic or velocities at the margins of dikes or sills. sudden event in order to generate the fluid velocities necessary for mobilisation of the sediment. For a dike to propagate the fluid pressure (Pf) Diffuse fluid flow from a horizon would maintain must exceed the horizontal stress (σh) and the ten-

Stanford Rock Fracture Project Vol. 11, 2000 C-7 So

Bedding

sill sill

dyke fracture with sill no intruded sand Bedding dyke

sill

dyke

Figure 7. Sandstone dykes and sills in the Miocene Santa Cruz mudstone, in roadside outcrops along Highway 1, North of Santa Cruz California. A series of dipping fractures occur throughout the outcrop of the flat lying mudstones (a) Dipping dikes have formed where the injecting fluidised sand was able to dilate existing fractures, rather than propagating a new fracture and making a vertical dike. (b).The four fractures illustrated here were dilated by the higher fluid pressures of the injecting sandstone slurry, and at the tip/overlap of the fractures, dilational jogs formed that were infilled with intruding sand forming short sills sub-parallel to bedding.

C-8 Stanford Rock Fracture Project Vol. 11, 2000 sile strength of the host sediment parallel to the metres of the surface, differential stresses are typi- bedding (Th) (e.g. Delaney et al., 1986; Price and cally low (the overburden stress, σv, is very small Cosgrove, 1990; Chapt.3), and likely approaches the magnitude of the minimum horizontal stress, σh ). In this case, the dif- P T (7) ferential stress may not be significantly greater f > σ h + h than the differential tensile strength and the bedding /fabric anisotropy will favour the formation For a sill to form the fluid pressure must exceed sills as it will be more difficult for a dike to propa- the vertical stress ( ) and the tensile strength σv gate against the anisotropy: perpendicular to the bedding (T ) (Price and v (9b) Cosgrove, 1990): σσv − h < TTHV− (8) Pf > σ v + Tv Bedding or layering forms a closed planar discontinuity along which there is lower tensile and Pressure data from boreholes and reservoirs shear strength, making it easier for material to in- in most sedimentary basins worldwide, show that trude along the discontinuity. Pollard (1973) con- rock fails before the pore fluid pressure reaches ducted some analogue experiments of this process the lithostatic or overburden pressure (in this ex- using grease and gelatin to show how a dike turns ample σv). Typically the rock fractures at values into a sill when it encounters a discontinuity. He of 0.7-0.8 σv (Lorenz et al., 1991). This implies also investigated the effect of different material that when the seal trapping the overpressured sand properties of the layers in a layered material body fails by fracture, sills will not form, because (analogous to interbedded lithologies for instance) the fluid pressure is less than the vertical stress. on the propagation direction of an intruding fluid. Or put another way, the vertical stress is larger The change in elastic modulus of the gelatin used than the horizontal stress and it is easier for fail- for the experiments at a layer interface was also ure to occur perpendicular to the direction of the found to force a dike to turn into a sill. minimum horizontal compressive stress σh. At significant differential stresses, dikes will always Burial depth and intrusion scale be preferentially formed because the differential stress will be greater than the differential tensile Next we consider the very simple case of sedi- stress. ments being buried within a basin without any applied tectonic stress, as might be the case in the σσ− > TT− (9a) v h HV post-rift infill stage in an extensional basin or intracratonic basin. We also restrict our discus- Therefore, when the seal fails, initially a dike sion to shallow burial depths (e.g. first 1 km) where is more likely to form (vertical in the case of a the sediments are consolidating, and show no evi- vertical maximum principal compressive stress dence for significant cementation or pressure so- (e.g. overburden pressure); and at an oblique angle lution. If we further assume that the source of the if other fractures are present, as discussed previ- fluid in the sand body is the original pore fluid, ously. Because the dike initially propagates in the then a sealed sand that is breached at very shal- vertical plane there is a decrease in depth, with a low depths is likely to form a compact sill and relatively small decrease in the fluid pressure as- dike complex. The initial pressure differential will sociated with volume change. In the case of a faster be small and the dike will only propagate a small rate of reduction of the vertical stress than fluid distance vertically before the fluid pressure equals pressure, a point will be reached shallower in the the vertical pressure and a sill develops (see path section where the fluid pressure will exceed the a, a’, a’’ on figure 8). The limiting case is where vertical stress. At this point equation (8) is satis- the source sand body is small with only a small fied and the dike will turn into a sill. For this to amount of sealed fluid, restricting the amount of occur there has to be a significant initial fluid sand that can be mobilised before the pressure dis- pressure head. If not, and the fluid pressure dissi- sipates, and fluid pressure drops. pates rapidly, the intrusion will ‘freeze’ as a dike before it ever gets shallow enough to form a sill. Next, let us consider the case where a sand At very shallow burial depths, within a few

Stanford Rock Fracture Project Vol. 11, 2000 C-9 body is sealed early on, but the seal is not breached vertically in 1-dimension, or uniaxially. This stress until greater burial depths (path b, b’ on Figure 8). path has been investigated in soil mechanics and In this instance a greater pressure differential is is known as the state of “earth at rest”, or K con- 0 established, higher flow rates ensue, and a larger dition (e.g. Lambe and Whitman, 1979; Jones, volume of sediment can be fluidised. Obviously 1994) and is defined as the ratio between the prin- thicker and larger volume clastic intrusions can cipal horizontal and vertical effective stresses. form. Because of the greater depth of initiation According to Jones (1994) in all sedimentary ba- the dikes will propagate further before the fluid sins without significant surface relief, the sedi- pressure exceeds the maximum overburden pres- ments are likely to be under stress conditions close sure and a sill forms (path b’-b’’, Figure 8). The to K at shallow burial depths. With increasing 0 depth of failure, therefore, produces distinctive depth, the presence of non-gravitational tectonic styles of intrusion. stresses will move the sediment away from the K 0 stress path. Data from laboratory sediment con- With a knowledge of the state of stress in the solidation experiments shows that under K con- 0 sedimentary succession, e.g., knowing the appro- ditions the ratio of the horizontal and vertical priate lithostatic and hydrostatic gradients, we can stresses remains constant. The magnitude of the quantify the distances a-a’ and a’-a’’ and make K ratio depends on the type of sediment. Gener- 0 estimates of the depth of seal failure and the dis- ally the finer grained and more clay-rich the sedi- tance above the source bed that a dike will be- ment the larger the K value (Jones, 1994). For mud 0 come a sill. The problem is formulated in Figure and clays, K , is typically ~ 0.7 or higher, but can 0 9(a). be as low as 0.4 for sands. (Lambe and Whitman, 1979). Some consolidation experiments conducted During continuing sedimentation and burial the by Karig and Hou (1992) to stresses higher than maximum applied stress can be considered verti- those routinely achieved in soil mechanics tests cal and the minimum principal stress will be hori- showed that the K ratio remained remarkably 0 zontal. If the boundary conditions on the basin are constant at a value of 0.63 for a silty clay up to such as to prevent lateral strain in the sediments, effective vertical stresses of 35 MPa (equivalent then the sediments are constrained to compact to depth of several kilometres). As a first order approximation, we can assume that at the burial cm - 10 m depths of interest for the formation of clastic intrusions in sedimentary sequences of a single lithology, the ratio of the vertical to horizontal ef- Pressure (MPa) 10 20 fective stresses remains constant, and for a suc- C cession dominated by shales K will be ~0.6-0.7. 0 If we know an appropriate K value for the a'' 0 sedimentary succession and (the lithostatic pres- 10's m σv a b'' sure), using equation (1) we can calculate the prin- a' ciple horizontal stress, σ as follows: b h

σ′h σ h− P hydr 500 Ko = = and (10) σ′v σ v − Phydr

b' ∴σσh = KPPov()− hydr+ hydr Depth (m) 100m + where P is the fluid pressure (hydrostatic hydr gradient in this case). We define K as the ratio of 1000 the principle vertical and horizontal stresses: Lithostatic Hydrostatic σhor σ . K = h Figure 8. A simple model for how the scale of clastic σ v Thus using equation (10) and knowing we sill and dyke complexes is a function of the depth of σv seal onset and depth at which the seal ruptures, valid can calculate K for a sedimentary section that is for the case of a basin where the maximum principal stress is vertical (gravitational loading). C-10 Stanford Rock Fracture Project Vol. 11, 2000 P1 P2

= density of pore fluid (water) ρw ρ = density of sedimentary succession H s 1 = K σh /σv z g = acceleration due to gravity H = depth to sealing of sand body H 1 H2= depth from point of sealing to failure Depth (h) H = distance from source bed to sill H2 Z= depth to seal failure (H1+H2)

Phydr =ρ .h.g = .h.g w σh = σv ρs .h.g) Κ (ρs Figure 9. (a) Method to determine depth of seal failure, distance from source bed to sill formation and initiation of injection as a function of original sealing depth of an isolated sand body, for a sedimentary succession of one lithology (e.g. all muds) and assuming a constant lithostatic gradient (i.e. neglecting the changes in bulk sediment density due to porosity reduction over burial depth of interest).

not overpressured (where the fluid pressure is pre- burial depths up to ~ 1.5 km. dominantly hydrostatic). At K = 0.6, K = 0.7; and 0 at K = 0.8, K=0.95; this brackets the range con- 0 Assuming that pore fluid at a hydrostatic pres- sidered reasonable for silty-clays and clays. sure is sealed into the sand body at a depth H 1 and no other fluid enters the sand body before it Let H be the distance from the source bed to fails, the depth at which the seal ruptures with the point where a dike becomes a sill, and z is the respect to the seal formation depth (H ) can be 2 depth to seal failure (Figure 9a). The pressure at calculated as a function of the seal formation point P , where a dike turns into a sill can be cal- 1 depth (H ) (Figure 9a). At point P , when the seal 1 2 culated as: fails, the pressure can be estimated as: PzHg() 1 = ρs − PHgHg21=+ρρws 2 and P1 = Kρρsw zg− Hg and PKHHg=+ρ () therefore: 212s and therefore: ρρ− ()1− K ρ zH= sw or Hz= s ()1− K ρs ρρsw− ()K ρρsw− (12) (11) HH21= ()1− K ρs Where ρ = bulk density of the sediments, and ρ s w For a shale section, with a K ratio of 0.7, = density of the porefluid (water). 0 (K=0.85) and an average bulk sediment density In the case where the sedimentary section of 2000 kg m-3 , the seal will rupture at a depth of consists of muds, with a K value of 0.7, (equiva- 0 H + 2.33 H . lent to K=0.85) and assuming a bulk sediment 1 1 Equation (11) (Figure 9b) is likely to be a density of 2000kg m-3, the depth of failure, z, is more useful relationship than equation (12), be- 3.33H, the complex height. Or a dike will become cause it is independent of how the overpressure a sill at a distance above the source bed = 0.3z. in the sealed sand body is achieved. It should be This relationship is illustrated graphically in Fig- valid even if the sand is overpressured by fluid ure 9b, for a range of K and ρ values considered o s migrating from deeper depths within the basin likely for a muddy sedimentary succession at

Stanford Rock Fracture Project Vol. 11, 2000 C-11 Sill-dyke dimension & seal failure depth

1500.0

1400.0

1300.0

1200.0

1100.0

1000.0

900.0

800.0

700.0

600.0

500.0

Depth of seal failure (Z) 400.0

300.0 Ko=0.65, rho=2200 200.0 Ko=0.65, rho=1500 Ko=0.8, rho=2200 100.0 Ko=0.8, rho=1500 0.0 0100200300400500600700800 source bed- sill height (H) Figure 9 (b) Source-bed to sill distance plotted versus depth of seal failure for an isolated, small, over-pressured sand body in a hydrostatically pressured sedimentary succession. Solutions shown for a bulk sediment densities of 2000 – 1500 kgm-3 and K = 0.65-0.8. These values are representative of a clay dominated succession (e.g. using an exponential o decay porosity-depth relationship for shales, shale density varies from 1500 kgm-3 at the depositional surface to 2200 kgm-3 at 1 km burial). This simple relationship can only be considered appropriate for conditions of shallow burial in a basin (e.g. <1.5 km), where the sediments undergo 1D consolidation with no lateral strain, and no other compaction processes (such as cementation, pressure solution) operate.

dike and sill complexes which formed at different and getting trapped by a top seal in the sand body. ends of the depth/differential stress spectrum. A Equation (12) assumes that the overpressure is Cretaceous dike and sill complex exposed in the entirely generated by burial disequilibrium, and Panoche Hills in the San Joaquin valley, northern that there is no contribution to excess fluid pres- California (Fig. 10a) (Smyers and Peterson, 1971) sures in the source sand body from either diage- represents an example of trajectory b-b’-b’’ in Fig- netic reactions, hydrocarbon generation, nor in- ure 8. The exposed cliff section is about 80 m high flow of fluid from elsewhere in the basin. Over- and the main sill is at least 80m above the source pressure development due to burial disequilibrium sand. In comparison Figure 10b, shows a metre- is unlikely to build-up in as simple and predict- scale sill-dominated clastic intrusion from Ordovi- able a fashion as we have shown in the scenarios cian deepwater sedimentary rocks on the Rosroe illustrated in figures 8 and 9 (where the pore flu- Peninsula, Western Ireland. The succession at ids seal at one depth and then the fluid pressure Rosroe has 10-15 sill-dominated clastic intrusion increases, paralleling the lithostatic gradient). complexes (Archer, 1984; Lonergan & Jolly, un- However, with the approach outlined above, some published data) which formed at very shallow simple, order of magnitude constraints, can be burial depths triggered by loading of overlying placed on scaling relationships for the formation of clastic sill and dike complexes. channel and debris flow events. Insights from studies of igneous intru- Examples sions In figure 10 we show two field examples of Clastic intrusions that form by upward intru-

C-12 Stanford Rock Fracture Project Vol. 11, 2000 sion of fluidised sand, appear to be predominantly fed by tabular dikes as opposed to cylindrical pipes. This lack of pipe-shaped feeders is not re- stricted to clastic intrusions, but is also a feature of intrusive igneous environments. Using con- tinuum-mechanics models of host rock deformation, Delaney and Pollard (1981) show that tabular dikes are a more efficient means of supplying magma when compared with cylindrical pipes. At a given driving pressure a tabular crack (or dike) dilates to accept greater volumes of magma, and for a given dilation less mechanical work is done on the host rocks. Furthermore, the pressure required for dike propagation decreases with increasing dike length. Although the host sediments during clastic intrusions are not likely to behave entirely elastically as assumed by Delaney and Pol- lard for the brittle deformation of igneous host rocks at shallow crustal levels, it is probable that the processes are broadly analogous, and we suggest that intrusion efficiency explains why pipes are uncommon in clastic intrusion complexes.

The formation of sills: dilation versus Figure 10. Clastic intrusion that formed at opposite ends stoping of the burial-depth spectrum. (a) Hillside in the Panoche Many of the features of clastic dikes and sills Hills, western margin of the Great Valley, Northern are analogous to those observed in igneous intru- California. Cliff is ~80m high. (b) An example of a small sions (see Delaney et al., 1986; Nicholson & Pol- sill, very short feeder dike and source bed, from the lard, 1985; Pollard et al., 1978.) Dikes have steps, Ordovician Rosroe Formation, Rosroe, western Ireland. jogs, en-echelon segments, and horns. Sills are often stepped and jump levels in the stratigraphy. around the intrusion tips. Locally the bending of Both dikes and sills often contain entrained blocks the host sediment is accommodated by small-scale or clasts of the host sediments. These inclusions faulting and folding (Figure 11). The margins of can range from cm-sized angular clasts of the host dilational sills tend to be sharp and linear (unless mudrocks to larger blocks of stratified parts of the exploiting an irregular fracture system). Stoping host sediments. on the other hand, produces multiple thin sheets Two distinct processes of formation have been (fingers in cross-section) at the tips of the intru- observed for igneous dikes: dilation (Delaney et sion with irregular and stepped margins as the sill al. 1986) and stoping and both of these mecha- follows the bedding fabric in the stoped blocks nisms can be observed operating in the formation and the surrounding host sediments (Figure 12). of clastic sill intrusions resulting in different ge- Stoped intusions will often contain abundant rafts ometries. Dilation is the emplacement of the sands of host sediment. A single intrusion can display by jacking open the shales, and for this to occur both dilational and stoping geometries. the fluid pressure of the fluid mobilising the sand We think that there are two principal factors must exceed the overburden pressure. Stoping which probably determine which style of intru- occurs where blocks of the host sediment are bro- sion is adopted by the injecting sand: the cohesivity ken off and become rafts within the fluidised sand; of the host sediment and the flow rate (velocity) the fluidised sand then fills the remaining space. of injecting fluid. A cohesive sediment will resist Intrusions that form by dilation tend to have breaking off into blocks- so dilation predominates, blunt-ended tips and the host sediments are flexed whereas less cohesive sediment will break into

Stanford Rock Fracture Project Vol. 11, 2000 C-13 blocks more easily and stoping will be preferred. stoping. Layered host sediments, where thin sands or silts For clastic intrusions forming at greater depths, are interbedded with muds are also likely to pro- where the host sediment is lithified or cemented, mote stoping, because sills will propagate along then brecciation at the margins, or intersections, bedding interfaces, dislodging chunks of bedded of sills and dikes during flow is likely to be a more sediments. Muddy sediments increase their important process favouring the formation of cohesivity as they are buried and they compact, stoped clasts and blocks. It may be possible to dis- thus stoping is likely to be a more common pro- tinguish between shallow, low cohesivity stoping cess at shallower intrusion depths. Lateral varia- and deeper brecciation induced stoping by exam- tions in host sediment cohesivity due to a change ining both the angularity of the stoped material in facies, grainsize or local stress state, will mean and the contacts between the stoped blocks and that intrusion style can change from one end mem- the intrusive sands. In our experience, stoped rafts ber to the other in the same sill complex. At higher included with intrusions that formed at shallow flow rates there will be a greater viscosity con- burial depths tend to have very diffuse-fluid trast between the injecting fluid and the host sedi- boundaries showing mixing between the host sediment and thus higher flow rates will favour dila- ment and the intrusive sandstone. tion. At lower flow rates, or during waning flow, The photographs in Figure 12 illustrate fea- the creeping or diffusion of thin sheets of sandy tures of an exposure within Ordovician rocks in fluid into surrounding sediments will favour western Ireland (Rosroe Formation) that allows

(a) local faulting blunt sill tip

sill

flexure of host sediment sharp intrusion fabric margins

(b) balls, disrupted sand & mud at top of sill shale rafts granule channel irregular bed coarse sand

laminated ssd 1m NE 1.5 m Dilational sill, fed 4 m by dyke. W V:H = 2:1 E

Figure 11. (a) Sketch of a sill that forms by a dilation. Note the blunt tip, host sediment layering flexed in the vicinity of the sill tip and the local, small-scale structures that occur in the tip region to accommodate the dilation. (b) an example of a dilational sill in Ordovician rocks from the Rosroe Peninsula, Western Ireland. Sketch from field photographs. Some stoping also occurs at the west end of this intrusion but the sill has predominantly intruded by dilation.

C-14 Stanford Rock Fracture Project Vol. 11, 2000 A wings stepped margins (interbedded thin fine-grained sandstones and muddy siltstone) can be clearly identified incor- porated within the sill. The main sill has branched into a number of thin sheets around the incorpo- rated rafts, towards the margins of the intrusion. fingers at tip host rock rafts rotation of fabric The lack of distortation or deformation in the stoped rafts suggests that they have been detached B locally from the host sediment by the sheets of fluidised sand injecting within the stratigraphy. sill In comparison, the example illustrated in Fig- ure 11, also from the Rosroe Formation, shows a sill sill that has formed by dilation. At this locality sill sill both the source bed and feeder dike can be seen. sill The three-dimensional exposure allows the stratig- sill sill raphy to be matched around the sill tip and it can be clearly seen that the dilational space for the Stoped rafts of intruding sand was made by forcing apart adja- host sediment cent muddy layers. In this example, the adjacent muds are flexed around the sill, but there has been C little internal deformation of the shales. Other examples typically show local, cm-scale, folding or sill faulting in the host rock near the intrusion tips, accommodating some of the dilational deformation. sill

Large-scale Paleocene-Eocene intru- sill sions of the UK North Sea When considering the formation of very large clastic intrusions that are hundreds of metres in size, it will be easiest to remoblise large volumes Figure 12 (a) sketch of a sill that forms by a stoping. of sand under conditions of significant applied Note the stepped margins, the fingers (sheets in 3D), tectonic stresses (e.g. fold and thrust belts, accre- and small wings at the intrusion margins. Large stoped tionary prisms, or strike-slip basins), resulting in blocks of bedded host sediment occur within the sill. high differential stresses. Otherwise, for a basin The rafts are more commonly aligned parallel to sill where the maximum stress is the vertical over- margins, but they can also be rotated. (b) and (c) an burden pressure, the sand body will have to be example of a sill that has intruded by stoping in Or- sealed early, when it has high porosity, and buried dovician rocks from the Rosroe Peninsula, Western Ire- deep enough (>1km?) to allow a high enough over- land. Photographs illustrate part of a 20m long intru- pressure to build up that will fluidise large vol- sion. Thin sheets of intruded sand are separated by large stoped blocks of the host sediment. (c) is detail of area umes of sand when its seal is breached (e.g. Fig- outlined in (b). ure 8). In order to inject large volumes of sand the flow would have to be sustained for a significant period of time. For the cubic-kilometer scale North an intrusion dominated by stoping to be investi- Sea intrusions, it is very difficult to appeal to high gated in 3-dimensions. The extensive sills, that tectonic stresses as the causal mechanism for sand can be traced for a total distance of 20 m in one remobilisation. By Late Cretaceous times the plane and a further 8 m in the orthogonal dimen- North Sea was no longer actively rifting and was sion are predominately bed parallel. The sills vary subsiding in its post-rift thermal phase. Aside from in thickness from 1 cm to 30 cm and stoped rafts localised salt-domes that continued to be active (up to 1.5m in size) of the surrounding facies throughout the Tertiary, there was no active tec-

Stanford Rock Fracture Project Vol. 11, 2000 C-15 tonic faulting affecting the basin. The lack of ma- bodies being deposited (Timbrell, 1993). Narrow jor Tertiary faults also suggests that there was no elongate channel or gully-filled sands (i.e. non- large magnitude seismicity (> magnitude 5) in the leveed channel systems), and isolated sand-rich region. However it is the large-scale of the intru- mounds (e.g. ‘ponded’ sand bodies) encased in sions, two orders of magnitude bigger than the claystones are those that are most susceptible to scale of intrusion observed even after the most remobilisation. Sand bodies located above basi- severe magnitude 7 historical earthquakes, that nal faults, which periodically appear to have acted really precludes earthquake-triggered liquefaction as vertical fluid escape pathways, were especially as a mechanism for the large-scale remobilisation. susceptible to remobilisation. Tertiary sand bod- Another mechanism, however, that would be ies above leaking Cretaceous or Jurassic reservoirs very effective in mobilising large volumes of sand, would also be susceptible. is the addition of another fluid, such as migrating hydrocarbons, into the sealed system (as suggested by Jenkins, 1930 and Brookes et al., 1995). Gas Conclusions that entered a sealed sand could generate high Previous work has tended to examine clastic in- pore-fluid pressures at much shallower depths, trusions in isolation with little consideration of the than if the original interstitial pore fluids were mechanics involved in their formation. Using trapped as shown by trajectory b-b’-b’’ in Figure simple mechanical concepts and drawing on the 8. In a North Sea context, maturity modelling stud- existing, more rigorous analysis of the formation ies have shown that the main source rock (within of tabular igneous intrusions (e.g., Delaney and the Jurassic Kimmeridge Clay Formation) was Pollard, 1981; Delaney et al., 1986; Nicholson & actively generating hydrocarbons in the deepest Pollard, 1985; Pollard 1975, Pollard et al., 1978, parts of the central and northern North Sea basin Pollard and Segall, 1987) useful insights can be during the Paleogene providing a fluid charge to gained about the formation of clastic intrusions. the basin (e.g. Cornford, 1998). It seems highly This paper has used theory to propose a number plausible that the high fluid pressures required to of factors that contribute to the formation and ge- remobilise the large volumes of intrusive North ometry of clastic intrusions, and illustrated these Sea sands came from hydrocarbon fluids migrat- results with field examples. ing to shallow levels, via salt structures and com- (1) Burial depth and the ratio of maximum to paction faults over tilted normal fault blocks. Once minimum principal stresses, plus their respec- the buoyant gas or oil entered isolated sand bod- tive orientations play a fundamental role in ies with relatively weak mudstone seals there controlling intrusion geometry and scale. would be enough fluid pressure to rupture the seal, (2) At shallow burial depths in basins (order of remobilise and inject large volumes of sand. The 10-20m ?) bedding anisotropy will favour the identification of Eocene-age gas pock-mark cra- formation of sills. In cases of a horizontal ters in the central North Sea (Cole et al., in press) minimum principal stress within a basin, dikes testifies to the fact that this is a viable mechanism, should form in preference to sills, until shal- and proves that large volumes of gas were mak- low levels are reached and the effect of bed- ing their way to the seabed at this time. ding anisotropy predominates. An important feature of a model that appeals (3) Sills can be observed to intrude by two mecha- to a remote-fluid driven trigger for the North Sea nisms- dilation and stoping, which we sug- intrusions is that the style of deepwater deposi- gest is a function of the cohesivity and layer- tion during the latest Paleocene-early Eocene. is ing of the host sediments and the flow veloc- also likely to have favoured sand remobilisation. ity of the injecting sediment. At the end of the Paleocene and into the Eocene (4) Intrusion style, whether by dilation or stoping, the large submarine fan complexes such as For- generates different intrusion geometries. Sills ties and Andrew, that had dominated the Pale- that form by dilation typically have blunt ter- ocene, gave way to a much muddier deep water minations and sharp, smooth sides. Sills ex- environment with only isolated sporadic sands hibiting a large degree of stoping tend to form making their way outboard of the shelf to deeper interconnected thin sheets, (or fingers when waters (Liu Xijin and Galloway, 1997), resulting viewed in a two-dimensional cross-section) in isolated, linear, gully-type or shoe-string sand

C-16 Stanford Rock Fracture Project Vol. 11, 2000 that step from layer to layer within the strata. talline rock: Geology, v.23, p.683-686. (5) Very large volume (km3-scale) intrusions re- Bridgewater and Coe, 1970 quire high differential stresses for formation Baer, G., Beyth, M, and Reches, Z, 1994, Dikes and we predict that such stresses are only emplaced into fracured basement, Timna Igneous Complex, Israel: Journal of Geophysical Re- likely to arise under conditions of applied tec- search, v.99, p.24039-24051. tonic stresses (areas of active thrust or strike- Brooke, C.M., T.J. Trimble and T. A. Mackay, 1995, slip faulting), or when fluids, such as hydro- Mounded shallow gas sands from the Quaternary carbons, migrate into sealed source sand beds. of the North Sea: analogues for the formation of The role of seismicity in triggering clastic sand mounds in deep water Tertiary sediments? intrusions has possibly been overplayed in in A.J. Hartley and D.J. Prosser, eds., previous studies. Seismicity only appears to Characterisation of Deep Marine Clastic Systems: cause significant liquefaction of water-satu- Geological Society London Special Publication rated sands at depths less than ~10 m and for v. 94, p. 95-101. Cole, D., S.A.Stewart, and J.A. Cartwight, (in press) large earthquakes (M>5). We suggest, that in Giant irregular pockmark craters in the the absence of active tectonics, and the un- Palaeogene of the Outer Moray Firth Basin, UK likely occurrence of large magnitude earth- North Sea, Marine and Petroleum Geology. quakes in the North Sea during the Eocene at Cornfield, C. 1998, Source rocks and hydrocarbons this time, that the very large intrusions in early of the North Sea, in: Glennie, K.W., ed., Petro- Eocene deposits of the North Sea were trig- leum geology of the North Sea : basic concepts gered by hydrocarbons migrating up dip from and recent advances, 4th edition: Oxford, UK, deeper parts of the basin. Blackwells Science, p.376-462. Cosgrove, J.W., 1995, The expression of hydraulic fracturing in rocks and sediments. in: Ameen, Acknowledgements: We thank T. Bai, J. M.S., ed., Fractography: fracture topography as Cosgrove, D. Dewhurst, P. Eichhubl, H.D. a tool in fracture mechanics and stress analysis: Johnson, N. Lee, D. Pollard, and J. Wilkinson for Geological Society of London Special Publica- tion, v. 92, p.187-196. fruitful discussions. LL is funded by the Royal Davidson, J.F. and Harrison, D., editors, 1971, Flu- Society and RJHJ was funded by a NERC Ropa idization: London, Academic Press, 847p. award. Delaney, P.T., and Pollard, D.D., 1981, Deformation of Host Rocks and Flow of Magma during Growth of Minette Dikes and Breccia-bearing intrusions References near Ship Rock, New Mexico. United States Pro- Alexander, R.W.S., K. Schofield, and M. C. Williams, fessional Paper, No. 1202, 61p. 1993, Understanding the Eocene Reservoirs of the Delaney, P.T., Pollard, D.D., Ziony, J.I., and McKie, Forth Field, UKCS Block 9/23b. in A.M. Spen- E.H., 1986, Field relations between dikes and cer, ed., Generation, Accumulation and Produc- joints: Emplacement processes and paleostress tion of Europe’s Hydrocarbons III: Special Pub- analysis: Journal of Geophysical Research, v.91, lication, European Association of Petroleum Geo- p.4920-4938. scientists, p. 3-15. Dixon, R.J., Schofield, K., Anderton, R., Reynolds, Allen, J.R., 1985, Principles of Physical Sedimentol- A.D., Alexander, R.W.S., Williams, M.C., and ogy: London, George, Allen and Unwin, 272 Davies, K.G., 1995, Sandstone diapirism and clas- p.Archer, J.B., 1984, Clastic intrusions in deep- tic intrusion in the Tertiary Sub-marine fans of sea fan deposits of the Rosroe formation, Lower the Bruce-Beryl Embayment, Quadrant 9, UKCS, Ordovician, Western Ireland: Journal of Sedimen- in: Hartley, A.J. and Prosser, D.J. eds, Character- tary Petrology, v.54, p.1197-1205. ization of Deep Marine Clastic Systems: Geologi- Ambraseys, N.N., 1988, Engineering Seismology: cal Society of London pecial Publication, v.94, Earthquake Engineering and Structural Dynam- p.77-94. ics, v.17 p.1-105. Fuller , M.L., 1912, The New Madrid Earthquake: U. Archer, J.B., 1984, Clastic intrusions in deep-sea fan S. Geological Survey Bulletin, no. 494, 119p. deposits of the Rosroe formation, Lower Ordovi- Hiscott, R.N., 1979, Clastic sills and dykes associ- cian, Western Ireland: Journal of Sedimentary ated with deep-water sandstones, Tourelle forma- Petrology, v.54, p.1197-1205. tion, Ordovician Quebec: Journal of Sedimen- Barton, C.A., Zoback, M.D., and Moos, D., 1995, tary Petrology, v.49, p.1-10. Fluid flow along potentially active faults in crys- Jenkins, O.P., 1930., Sandstone dikes as conduits for

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