Fluidization and Associated Soft-sediment Deformation in Eolian Sandstones: Hopeman Sandstone (Permian), Scotland, and Rotliegend, North Sea
Published:January 01, 2007
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Ken Glennie, Andrew Hurst, 2007. "Fluidization and Associated Soft-sediment Deformation in Eolian Sandstones: Hopeman Sandstone (Permian), Scotland, and Rotliegend, North Sea", Sand Injectites: Implications for Hydrocarbon Exploration and Production, Andrew Hurst, Joseph Cartwright
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On the southern shore of the Moray Firth, Scotland, the foreshore and cliffs east of Hopeman Harbor display a wide variety of soft-sediment deformation structures formed in unconsolidated Late Permian eolian sands. These include flows of water-saturated sand containing rip-up clasts that overturned the underlying dune sand; dune bedding that is now vertical; subvertical pipes and swirls of partly dilated sand; sand dikes; widespread partial to complete homogenization of dune sand; and a vertical escape structure some 20 m (66 ft) high. The driving force behind the deformation is believed to be widespread heavy rain over the northern edge of the Grampian highlands, causing slumping of the southward-migrating dune sands and possibly slight local northward sliding subparallel to the regional top Devonian erosion surface; this could have induced major increases in internal hydrodynamic pressure. Because the pores in the dune sands were filled with air prior to flooding, some of the vertical deformation structures may have been formed by the upward escape of air through rain-dampened dune sand driven by the hydrodynamic increase in water pressure. Probable coeval deformation, but of a different style, has been seen in cores recovered from oil and gas fields of the central and southern North Sea.
Sand injection is typically associated with the fluidization of sand in an aqueous medium during burial (Lonergan et al., 2001; Hurst et al., 2003>). Where upward and lateral injection of sand occurs, it appears that fluidization is triggered by a buildup of overpressure in the pore fluid below a low-permeability sealing lithol- ogy that fails (probably by hydrofracture, Cosgrove, 2001), allowing the overpressure to dissipate and sand to be transported in a fluidized state into the sealing strata. In this chapter, we describe features formed by the upward movement of fluidized sand in eolian dunes within which it has been inferred that the process occurred when the surface of the dunes was at the Earth's surface (Glennie and Buller, 1983). In addition, we describe an extrusive sand body that is fed by small, subvertical dikes in an interdune lacustrine environment.
Deformation caused by the upward mobilization of sand during fluidization was interpreted from Weis- sliegend Permian eolian sandstones from the North Sea reservoirs and outcrops of the Hopeman Sandstone of the Moray Firth coast, Scotland (Glennie and Buller, 1983). Strömbäck and Howell (2002) confirmed the general occurrence of deformation features in the Late Permian in a regional study of borehole data from the southern North Sea. Granulation seams and associated fluidization features have been described from Rotlie- gend eolian sandstones from the United Kingdom central North Sea (Ardmore field), which are associated with enhanced permeability (Heward et al., 2003) but are dissimilar in appearance and association from those described by Glennie and Buller (1983). We are unaware of many other published examples of outcrops of eolian sand injectites (Netoff, 2002; Chan et al., 2007) that can be used to validate subsurface interpretations; hence, in subsurface studies, they remain enigmatic. In common with sand injectites in other strata, the significance of sand injectites in eolian sand on reservoir characteristics may be underestimated.
During the Late Permian (Lopingian epoch: Wu- chiapingian and Changsingian stages; Jin et al., 1997>), the Hopeman area lay near the southern margin of the inner Moray Firth basin (Figure 1). The upper age limit for the exposures of this area was largely confirmed by the discovery of the mold of a reptile skull (Dicynodon) of undoubtedly latest Permian age in undeformed dune sand near the top of the Hopeman Sandstones at Clashach quarry (Clark, 1999; Hopkins, 1999; Glennie, 2002). The deformation structures, first described by Peacock (1966) and Peacock et al. (1968), are spread intermittently along the present foreshore. They form some of the lowest stratigraphic sequences in the area and, thus, may represent the oldest Permian sands of the basin (Findhorn formation of the Rotlie- gend; Cameron, 1993), which unconformably overlie the Devonian Old Red Sandstone (Figure 2).
Because of their basin-margin location, the Hope- man Sandstones may have been deposited sporadically as dunes that migrated (mainly south-southeast) toward and climbing onto the northern flank of the Grampian highlands under the influence of prevailing north and northwest winds, enhanced by convection over the mountains (Glennie, 2002). This occurred throughout the Late Permian and, thus, included the youngest Rot- liegend sands, as well as those of latest (late Zechs- tein) Permian age. Concurrently, and prior to the Zechs- tein transgression, a Late Permian sea expanded to the southwest between Norway and Greenland (Stemme- rik, 2000; Glennie et al., 2003), which, together with a prevailing north wind, resulted in a general increase in humidity over the area of Rotliegend deposition in the North Sea area (Pennington, 1975; Heward, 1991; Heward et al., 2003). Hence, at the time of their deformation, the eolian Hopeman Sandstones along the Moray Firth coast formed a wedge, thinning to the south and southwest (and perhaps also to the north), which were wetted by rainwater and ingressed from the north and northeast by a rising water table and from the south and southwest by both ground and surface water flowing northward from the Grampian highlands.
Some moisture is essential for the preservation of deformation structures in eolian sands. During deposition, eolian sands flow freely under the action of a driving wind or under gravity down the avalanche slope of a dune, and the characteristics of these modes of deposition are preserved. Postdepositional deformation structures in eolian sediments are preserved only if the sands were sufficiently moist (but not fully water saturated) at the time of deformation to possess enough capillary strength to hold the grains together. Homogenization into structureless or part structureless sand will occur if the moisture content is too high for effective grain cohesion.
Because eolian dune sands are essentially dry and their pores are filled with air prior to deformation, water must be added to the system in the form of rainfall, dew, or by a rise in the water table to preserve deformation. Heavy rain may cause downslope slumping and overthrusting of rain-wetted surface sands of dunes (e.g., Dem- berelyin et al., 1995; Loope et al., 1999; Glennie, 2005).Floodwater may cause erosion and slumping or sliding of dune flanks. Alternatively, provided that a capillary seal is created on the dune surface by rain or dew moistening (wet grain contacts and air-filled pores), a rise of the water table will raise the air pressure in the dune, perhaps leading to a dramatic upward escape of air and fluidized sand when the temporary surface seal fails (Glennie and Buller, 1983).
We suspect that internal downslope sliding of the lower beds of a dune sequence that had become partly water saturated could lead to shortening of wedges of low-angle bedding by rotating it to the vertical. Such movement could lead to dune collapse and to large local increases in pore pressure in a dune, the upward injection of entrapped water into air-filled dune sand, and the creation of high-relief air-escape structures. Deformation can occur any time after sediment deposition provided that lithification has not occurred.
In the Leman gas field (UK Blocks 49/26, 49/27; Hillier, 2003), cores show that deformation is present in Rotliegend sandstones about 2 m (6.6 ft) below the base Zechstein Kupferschiefer and extends downward intermittently for more than 30 m (98 ft) until undisturbed dune bedding is reached (Figure 3). Glennie and Buller (1983)> assumed that rising water during the Zechstein transgression would be rapid enough (perhaps 30 cm/day [12 in./day]) to trap air inside the dune, a top seal being formed by the capillary rise of water up and over the flanks of the dune being flooded; this explanation is now deemed less likely.
Here, we investigate the range of deformation features that are preserved in the Hopeman Sandstones and differentiate at least two categories of deformation structures, one clearly formed by sand fluidization in an aqueous medium, and a second one probably resulting from sand fluidization by air.
Distribution of Fluidization and Deformation Structures
Fluidization and deformation structures are present along an approximately 6-km (3.7-mi) stretch of coastline between Hopeman Harbor and Covesea Skerries lighthouse (Figure 1); they are separated by largely undeformed Permian eolian sandstone. The deformation structures are exposed mostly along the foreshore, covering areas from about 2–3 to 20 m (6.6–9.8 to 66 ft) in width (at low tide) and as much as about 300 m (984 ft) parallel to the shore; many of the foreshore structures exhibit preserved relief of possibly 3 –4 m (9.8–13 ft). A few other structures are in cliff exposures that may extend some 20 m (66 ft) or more above sea level, where structureless sandstone can be present.The youngest dune sands (Clashach quarry) have no known deformation structures.
Numbers refer to localities shown in Figure 1.
- 1)An upstanding block 3–4 m (9.8–13 ft) high comprises water-lain strata containing rip-up clasts; it overlies overturned and distended dune sand (Figure 4a, b), indicating that the water-lain strata flowed to the north (McKee et al., 1962).
- a)An extruded sand unit, as much as approximately 30 cm (12 in.) thick, was fed by a series of dikes that cut through deformed and undeformed, ripple-laminated sandstones that accumulated in shallow (interdune) pools of water (Figure 6a–f). The extruded sand is moderately well sorted and medium grained with occasional low-angle (<15°) lamination that dips away from inferred vents (Figure 6a). It has a blotched weathering texture that has no superficial relationship to mineralogical or grain size variation. 0ccasional, roughly equant, angular clasts of ripple-laminated sandstone (clasts typically <10 cm [<4 in.] thick) occur within the sand matrix (Figure 6b) and are interpreted to be derived from the underlying lacustrine units. Linguoid ripples and streaming lineations record south to north sediment drainage into standing lacustrine water (at about 8° at present, postdepositional regional tilt possibly of the order of 4–6°), together with uncommon single-grain horizons of granules and small pebbles of probable fluvial origin. Subvertical sandstone dikes (<5 mm to 40 cm [<0.2 to 15 in.] wide) cut the laminated lake floor at irregular intervals (Figure 6c, d), commonly less than 1 m (3.3 ft) apart. They are planar, but lateral exposure is poor with signs of lateral thinning. Some faint saucerlike internal laminations are, in places, present, but generally, the sand fill is structureless. Small horizontal steps (sills) are sometimes present in the dike profiles. At least six dikes are identified along the southern (landward) margin of the outcrop. The source sand body from which the dikes emanate is unexposed. The exposed length of the dikes is at least 50 cm (19 in.). A possible vent (sand volcano) is exposed at the southeastern end of the outcrop (Figure 6e); the extrusion, however, is clearly fed by several dikes and vents.
Seaward and underlying the extruded sand unit (Figure 6c) is an extensive area (some 20 × 30 m [66 × 98 ft] or more and minimum thickness of 1 m [3.3 ft]) of weakly bedded to structureless sandstone that is deformed into swirls formed about steep to vertical axes (Figure 6f); this unit is, in part, age equivalent with the nearby rippled horizon.
A little farther seaward, the extrusion structures are overlain by low-angle eolian sandstone, which is truncated by another flow (slurry?) of sand containing rip-up clasts.
Below and immediately to the west of the coastguard tower (at Gow's Castle) are two separate zones of deformation.
- a)Beds of eolian sandstone in, and flanking, large caves in which the bedding has been tilted as much as 50–70°; the bedding is locally dilated to the extent that original laminae are no longer preserved. These structures grade westward into possibly oversteepened dune sand that climbs the cliff at angles of about 20–25°, but flattens near the top and westward to subhorizontal (i.e., the windward slope of a climbing dune). Immediately to the west is a bay, some 200–300 m (656–984 ft) across, which is flanked at sea level by mostly homogeneous (structureless) sandstone; in occasional boulders on the beach, subrectilinear slabs as much as 20 cm (8 in.) long of bedded sandstone are preserved within the structureless matrix (Figure 7a).
The western part of the cliff is remarkable for a complex soft-sediment structure that extends vertically for some 20 m (66 ft) (Figure 7b). The eastern part of this structure bends sharply from subhorizontal to vertical. The western part is more complex, with beds angling into the structure at 30–40°; near the base of this slope, the bedding takes on a much more complex character, which includes vertical bedding and the development of concertina structures, suggestive of vertical collapse following uplift. At sea level beneath the cliff are clasts showing evidence of brittle fracture in an otherwise almost structureless sandstone.
Below low cliffs of low-angle eolian bedding is an isolated structure involving the local deformation to the vertical of north-dipping (˜20°) eolian sandstone (Figure 7c). This structure measures about 2 m (6.6 ft) across and high and is underlain by a thickened pad of partly homogenized subhorizontal bedding.
- 6)About 400 m (1300 ft) east of Covesea (locality 7), a headland cliff exposes dune bedding that dips consistently to the south at about 25°. At the coast, the dune bedding passes up abruptly into two pods of highly deformed sandstone separated by structureless sandstone, the whole being overlain near the cliff top by subhorizontally bedded eolian sandstone (Figures 8 and 9). South of the cliff exposures, dune sand can be traced inland for about 100 m (330 ft), with no sign of overlying deformation; thus, deformation is confined to the coastal cliffs. Footprints referred to Cheirotherium (known in Germany from lateral equivalents of the Kupferschiefer; C. Hopkins, 2000, personal communication) occur on south-dipping eolian foresets (Figure 9) 50 m (164 ft) west of this locality.
Shortly before the Late Permian Zechstein transgression, there appears to have been a time of increased precipitation and associated flooding (Trewin et al., 2003), which, in the case of the Hopeman area, seems to have been the driving force behind the deformations and sand fluidization. The deformation structures range from very large (20 m [66 ft] high; Figure 7b) to very small (millimeter-wide sand dikes; Figure 6c). Each structure demands a separate interpretation of its origin, but collectively, they are all involved with the presence of water and/or moisture. What are the main possibilities?
Two distinctive styles of deformation that can be attributed to sand fluidization are identified, small- scale dikes and a sand extrusion (Figure 6) and a large- scale deformation of beds (Figure 7, where substantial volumes of dunes are involved). The former is unequivocally related to overpressured, water-saturated fluidization of sand. The latter may involve both air and water in the deformation and fluidization that has occurred irrespective of dune type, our preference for climbing dunes or Clemmensen's (1987) star dunes.
Sand Dikes and Extrusion
Subvertical sandstone dikes and the structureless sand associated with extrusion into a shallow, ephemeral lake (Figure 6a–e) is unequivocal evidence for sand fluidization. We infer that at the time of the sand injection and extrusion, the water table was at, or slightly above, the surface onto which the extrusion occurred. The presence of low-angle lamination (Figure 6a), a possible small sand volcano (Figure 6e), and numerous dikes are entirely consistent with the simultaneous activity of several vents from which sand extruded onto a lake floor. Although the parent bed from which the dikes emanate is unidentified, it is unlikely to be more than a few tens of centimeters below the present-day exposed surface.
In contrast with deep-water clastic environments in which sand dikes commonly intrude mudstones or shales (Lonergan et al., 2001; Hurst et al., 2003), these dikes intrude ripple-laminated or thinly bedded, finegrained sands. Even when water saturated, these laminated sandstones are expected to have had several orders of magnitude higher permeability (probably darcy scale) than mud (probably <0.01 md) when at the Earth's surface. The implication is that the fluidization of the sand and subsequent intrusion must have occurred very rapidly, allowing no time for the pressure to dissipate. Because the volumes of sediment concerned are relatively small (probably <1500 m3 [<3310 ft3] in the area of exposure) and ephemeral lakes in deserts may not cover large areas, we infer that the extrusion was a local event. In view of the abundance of larger scale deformation features that underlie and are adjacent to the extrusion (Figure 6c, f), the trigger for the rise in overpressure and dike emplacement is likely to be associated with these. In the context of the flood events recorded in this area, in combination with the regional rise in water table, we believe that the overpressure was likely to be generated by a rapid influx of flood water into the dune system that caused large-scale collapse of water-saturated volumes of dune, which loaded the underlying and down- dip strata. Sudden loading of porous sand by denser strata of lower permeability is a widely documented mechanism for generating overpressure and sand injection in deep-water settings (Gill and Kuenen, 1957; Strachan, 2002).
Although the extrusion is inferred to have been into a shallow lake, we have no direct evidence to support this; however, the extrusion lies between water- lain strata. The preservation potential of subaerial extrusions is likely to be less than for subaqueous extrusions simply because of erosion. Despite the very different environment of deposition, the scale and appearance of features in the dikes and the extruded sand are very similar to the well-known deep-water sand extrusions in County Clare (Gill and Kuenen, 1957; Jonk et al., 2007).
When observed in isolation from features such as sandstone dikes (Figure 6c, d) and sand volcanoes (Figure 6e), for example, in a borehole section, it may be difficult to differentiate extrusive sands from depo- sitional facies. The following criteria may be useful when making a differentiation. (1) Angular sandstone intraclasts (Figure 6b) formed by in-situ brecciation could be confused with clasts derived from collapse of the margins of an ephemeral channel (partially water- saturated dunes). In common with angular mudstone clasts in deep-water injectite complexes (Duranti and Hurst, 2004), their angularity is attributed to very minor transportation. (2) Low-angle lamination (Figure 7a) should have an approximately radial pattern (around dike vents) unlike sedimentary lamination caused by aqueous flow, which will tend toward uniaxial. (3) If extrusive sand in eolian environments is associated with ephemeral lakes and the fluidization of small volumes of sand, it is unlikely that extrusions will exceed a few tens of centimeters thickness, and that their lateral extent will be widespread (but no larger than the approximate area of the lake). The possible genetic association with lacustrine facies may be particularly useful when identifying extrusive sands in borehole sections, although fluvial and wadi facies may have a similar association.
The extrusion structures of locality 3 are overlain by low-angle bedded eolian sandstone capped by another flow (slurry) of sandstone containing rip-up clasts. The time interval represented by the deposition of this eolian sand is not known, but it could have been days or weeks (cf. figure 49 in Glennie, 1970).
Large-scale Deformation of Hopeman Dunes
First described by Peacock (1966) and Peacock et al. (1968), these spectacular deformation structures have previously been attributed to the fluidization of sand by overpressured air trapped between a rising water table and a temporary seal created by moisture on the dune surface (Glennie and Buller, 1983). The former presence of air deep below the structure seen in Figure 7b is apparently confirmed by clasts showing evidence of brittle fracture in a matrix of structureless water- saturated sand. Such fractures have been preserved because of the capillarity between damp grain contacts and air-filled pores.
Some of the deformation structures at Hopeman are clearly associated with thick flows of wet sand (containing rip-up clasts) that caused local overturning to the north-northwest of the bedding over which they flowed (McKee et al., 1962). As the Hopeman Sandstone is located on the northern flank of the Grampian highlands, it is suggested that the water involved in the flooding was dammed behind dunes sufficiently long for internal deformation structures to develop and for some sand to collapse where the dune sands became saturated.
Partial saturation of dune sands causes them to increase in density to the point where they may become gravitationally unstable and slide down dune slopes. Such processes have been inferred by Loope et al. (1999) for the catastrophic burial of Cretaceous vertebrates in Outer Mongolia and have been recorded by Glennie (2005) in dunes of the Wahiba Sands, Oman, where slides resulted in local overthrusting after only 12–15 mm (0.47–0.59 in.) of rain; such sheetlike flows and brittle fracturing of the sheets indicate that the sands were cohesive.
Hence, two related models are envisaged in which the interaction of a rising water table and flood water creates the conditions for sand fluidization. As postulated by Glennie and Buller (1983), air may have been trapped in dunes between a rising water table and a temporary seal on a moistened dune surface. In flood, the water level rises rapidly, causing the dune to inflate as the air pressure increases. When the seal fails and air and fluidized sand escape, a volume reduction occurs within the body of the dune that causes grain consolidation and the formation of dish structures and consolidation laminae. Equally important, and possibly more widespread, is the effect that the increased water saturation had on gravitational stability of dune sand during periods of higher precipitation and flooding. Many of the deformation structures are attributed to sliding of parts of water-saturated dunes that, during motion and when they come to rest, would load underlying sediment and potentially create overpressured pockets of water and/or air.
Where the consolidation processes extend into the water-saturated part of the dune, the bedding may deform plastically and create local pockets of overpressure, which is released by formation of upward- intruding, small-scale sand dikes (Figure 6c). The flooding of a wadi on the upstream side of a dune system that is blocking its path may result in the local saturation of the dune and induce the downstream collapse of part of the dune (Djofra graben, Libya: figure 49 in Glennie, 1970), leading to a slurry of wet sand flowing downslope.
The conceptual sketch (Figure 10) tries to depict the environment in which the Hopeman eolian sands were deposited and deformed. With a few feet of intervening pebbly sandstone west of Elgin, the Permian dune sands were deposited unconformably over the Devonian 0ld Red Sandstone on the northern flank of the Grampian highlands. These sands were deposited largely under the influence of prevailing northerly winds to form what were possibly climbing dunes (cf. figure 9.29 in Glennie, 2005) with well-developed north- dipping topset beds (see east end [left side] of Figure 7b). Shortly before the Zechstein transgression of the North Sea area, the highlands were the site of torrential rain, which not only soaked the upper beds of the dunes, but also formed a temporary lake (or lakes?) on their southern side. Locally, lake water may have spilled over a low area between dunes and caused rapid downcutting. Under the influence of hydrostatic pressure, flood water may also have slowly penetrated the lower bedding of a dune horizontally, eventually causing collapse and a northward-flowing slurry of wet sand such as is recorded in Figure 4a and b. It is under such conditions of flooding that the lower parts of the dunes may have become saturated with water and have allowed parts of the overlying heavy rain-wetted dune sand to creep or slide short distances down the regional slope. This is inferred to have generated the increase in hydrostatic pressure that is thought to have driven the creation of the 20-m (66-ft)-high escape structure (Figure 7b; such low-level water saturation could also have been responsible for the sand dikes and sand volcanoes that extruded sand into interdune temporary lakes). The whole process from the start of rain to the end of deformation could have taken from a few days to several weeks to complete.
Deformation structures with inferred similar origin are reported from approximately equivalent chrono- stratgraphic units in the Auk and Argyll (Ardmore) fields of the central North Sea (Pennington, 1975; Heward, 1991, 2003). Presumed similar deformation (no core available) is reported from well 12/29-2 in the northeast Moray Firth (Cameron, 1993). If creation of the late Rotliegend deformation structures of the Southern Permian basin (Glennie and Buller, 1983; Strömbäck and Howell, 2002), including the Leman field (Figure 4), also correlate temporally with those at Hopeman and Auk (this is very possible because they all underlie the Kupferschiefer or its approximate lateral time equivalent), the deformation structures constitute evidence for widespread torrential rainfall that caused flooding in such disparate areas. In the Leman Bank and other southern North Sea fields, alternations of partly preserved sequences of dune sand, deformation structures, and homogenized sands occur in the same vertical sequence; individual occurrences presumably were controlled by differences in internal bedding geometry and permeability paths in the dune during the internal rise of the water table.
In common with their deep-water clastic analogs, the small-scale injectites and extrusions are below seismic resolution. They are significant, however, in terms of assigning appropriate reservoir geometry and architecture to units identified in borehole sections. We infer that extrusive sands will most often be confused with debrites. Dikes will create strong, localized enhancement of vertical permeability. Large-scale features that exceed 20 m (66 ft) in height in the study area (Figure 7b) may be resolved seismically and, because of their subvertical orientation, may produce broken or diffuse reflections. It is possible that when viewed in core, they will be confused with features of tectonic origin or possibly dune collapse features, as opposed to fluidization features.
The Hopeman Sandstone is unusual because it is an eolian setting located between two areas of increasing humidity: to the north, fluvial sandstones and increasing proximity to an ocean; and to the south, the Grampian Mountains. Following heavy rainfall, the water table rose, and floods affected the area. We have documented the importance of water in a desert environment in the generation of two styles of deformation, both associated with sand fluidization.
Upward injection of dikes is arguably the most commonly identified feature of sand injectites and is unequivocal evidence of fluidization of sand. In the study area, dikes reach the surface in ephemeral lakes and formed an extrusive sand unit. Breccias, low-angle lamination, and a sand volcano are documented, all of which typify injectite associations, but are untypical of eolian settings. Structureless sands and weakly laminated swirls about subvertical axes are associated with the dikes, which we interpret to have formed as (water-) fluidized sand was injected upward. Where air was trapped within the otherwise water-wet system, apparent brittle fracture evidenced the local retention of air in the sand. We attribute the fluidization and injection of the sands to the loading of adjacent strata, probably by dune collapse, which created overpressure by forcing water downslope into the area below the ephemeral lake into which sand eventually extruded.
The second and more spectacular deformation is interpreted to have formed by the fluidization of sand in air. Air was trapped in dune sands by the rising water table and surface wetness from rain; the overpressure and consequent fluidization caused by the downslope movement of partially water-saturated dunes that were degrading and collapsing following high rainfall loaded the areas in which fluidization occurred. Because the pores of these dunes were filled with air prior to wetting, some of the high-angle structures that penetrated up into undeformed sand may have been caused by air escape (no obvious homogenization is related to water saturation), possibly driven hydrodynamically by the foreshortening effects of a sliding mass of dune sand. At the base of the giant air-escape structure, clasts of bedded sandstone exhibit brittle-fracture margins in an otherwise homogenized matrix. Such clasts are believed to have been preserved because of the capillary attraction associated with only slightly moistened sand (wet grain contacts but air-filled pores); thus, they did not lose all their air content during flooding. These brittle-fracture clasts indicate that prior to water saturation and sliding at the base of the high escape structure (Figure 7b), their pores, and presumably also the pores of the sands that were homogenized, were originally filled with air.
The distribution of other probable coeval deformation structures (northeast inner Moray Firth, southern and central North Sea fields) indicates that the rainstorms must have been very widespread; differences from Hopeman in deformation style are possibly the outcome of differences in local dune-type and underlying relief, the vagaries of discovery by a narrow borehole, and perhaps the distance from highland areas. Certainly, deformation associated with downslope sliding as suggested above for the Hopeman area is less likely to have occurred in the dune fields of the central and southern North Sea, where flood-induced air- escape structures may still be the preferred mechanism for deformation, although rain-induced slides and slumps, together with slumps induced by dunes damming flood waters, are also likely in these lowland dune systems, where the Leman field discovery well has a vertical sequence of some 118 ft (36 m) of deformation; all that deformation is likely to have been within the dune itself at a level above that of the adjacent preflood water table.
Sand Injectites: Implications for Hydrocarbon Exploration and Production
Sand injectites are described in scientific literature as an increasingly common occurrence in hydrocarbon reservoirs, in particular in deep-water clastic systems, where they are known to influence reserves distribution and recovery. Seismically-detectable injected sand bodies constitute targets for exploration and development wells and, subseismic sand bodies provide excellent intra-reservoir flow units that create field-wide vertical communication through depositionally extensive, low-permeability units. As sand injectites form permeable conduits in otherwise low-permeability units they facilitate the expulsion of basinal fluids; hence they act both as a seal risk and mitigate timing and rate of hydrocarbon migration. Injected sand bodies form intrusive traps, which are distinct from structural or stratigraphic traps. Included in this publication are 10 chapters on subsurface examination of sand injectites, 1 chapter on theoretical considerations, and 13 outcrop analogs in reservoirs across the world. Captured in this volume is at least a taste of the global and stratigraphic distribution of sand injectites, and an attempt to introduce readers to sand injectites and their significance in the context of hydrocarbon exploration and production. The book is not intended as a complete review of the field-based literature, but emphasizes high quality case studies from the surface and subsurface. The geographic scope of the book is large, and illustrates the diversity of geological settings in which these fascinating and economically significant features are found.