Abstract Observation of basin-scale networks of sandstone intrusions are described from subsurface studies and outcrop locations. Regional scale studies are prevalent in the volume and two new regionally significant subsurface sand injection complexes are described. Higher resolution studies, both outcrop and subsurface, show the small-scale complexity but high level of connectedness of sandstone intrusions. Discordance with bedding at all scales is diagnostic of sandstone intrusions. The propensity of hydraulic fractures to develop and fill with fluidized sand in a broad range of host rocks is demonstrated by examples from metamorphic and magmatic basement, and lignite. Terminology used to describe sandstone intrusions and other elements of sand injection complexes is diverse.

Abstract Subsurface sediment remobilization phenomena, including sand injection complexes and mud volcano systems, have been recognized to play a significant role in basinal fluid flow and in some cases are an important part of petroleum systems. Seismic-scale wing-like anomalies, interpreted as sand injections based on their similarities with North Sea examples, have been identified within the upper Paleocene sediments of the Great South Basin, New Zealand. The structures were observed on three-dimensional seismic data at about 2.5–2.7 s two-way time beneath the seabed. The sand injections occur below a well-developed polygonal fault system within the uppermost Paleocene sediments. The wing-shaped sand injections are often located near the downward extent of polygonal fault tips, possibly implying that the injections affected polygonal fault formation. This is the first time that a seismic-scale sand injection complex has been described in the southeastern hemisphere. The study adds to an emerging realization that sand injection complexes occur in many deep-water basins and has direct implications for basin evolution and hydrocarbon exploration in the Great South Basin.

Abstract Recent exploration activity in the Austral-Magallanes Basin revealed the presence of sand injection complexes of Upper Cretaceous–Paleocene age. The discovery of these large-scale sandstone intrusions documents the first onshore example of sand injection complexes as hydrocarbon exploration targets and confirms a more widespread phenomenon than previously known. Integration of regional understanding, three-dimensional seismic reflection data, exploration wells, and whole core have allowed the interpretation of large sand injectites associated with a deep-water depositional system in the Austral-Magallanes Basin. These injectites are characterized seismically by circular to elongate amplitude anomalies with cross-cutting and discordant stratigraphic relationships. Additionally, detailed sedimentological analysis from exploration wells and core confirm the presence of facies consistent with injected sand commonly associated with deep-water depositional systems. The presence of giant sand injection complexes in the Austral-Magallanes Basin records a previously undocumented period of pore-fluid overpressure that led to large-scale hydraulic fracturing of the overburden and subsequent fluidization of sand derived from a deep-water depositional system. The discovery of intrusive traps such as sand injection complexes defines a new play with significant exploration potential; however, additional evaluation is required to understand the nature and degree of primary sandbody geometrical modification and the subsequent impact on reservoir distribution, trap geometry, migration pathways and seals.

Abstract The Norwegian–Danish Basin is characterized by a depositional environment governed by incised valleys, localized sand deposition and rapid progradation of thick sediment packages during the Paleogene and Neogene. Subsurface sand injectites occur at five locations, with distinctive geometries and injection styles. Such vary from short-distance injectites where injected dykes and sills are in direct contact with the parent sand, to a long-distance sand extrudite >1 km vertically above the proposed parent sand. These endmembers establish the Norwegian–Danish Basin as a dynamic environment for subsurface sand remobilization. Although formation models exist for the injectites at each of the five locations, general considerations of sand injection mechanisms in the Norwegian–Danish Basin remain untested. This study presents a synthesis of geometrical characteristics, sand injection style and controlling factors for sand injection in the Norwegian–Danish Basin. By reviewing the existing literature and including new data, the study qualitatively addresses the effects of parent-sand distribution on injection style at the five locations, the overpressure generating mechanisms and the importance of faulting as a trigger mechanism. The results show that short-distance injectites typically occur in areas with a confined parent sand and that salt-related faulting is locally important as a trigger mechanism for sand remobilization. The main overpressure generating mechanism is differential loading and lateral transfer of pressure. The case studies also show a trend between larger depocentres required for remobilization of more distal parent sands relative to the depocentre. The results confirm the prevailing consensus that local conditions related to parent sand distribution and burial depth partly control injection geometry and style.

Abstract Sand injections are overrepresented in deep marine deposits and other settings with slope instabilities. We have examined seismic data from the North Sea to reveal their main trigger mechanisms. This was performed by inspection of local seismic observations in the Oligocene to Miocene sediments above the Utsira High and analyses of regional distribution of mounds at the top Hordaland Group surface in the northern North Sea. We make the observations that (1) the uplift of jack-up folds is in most cases larger than the sand thickness within the mounds, (2) onlap to mounds is present in periods with sand deposition, but no such onlap is seen in mudstones that were deposited in periods with little or no sand input, (3) onlapping to several Oligocene surfaces is seen above the Utsira High, (4) rim synclines are not present adjacent to Oligocene and Miocene mounds and (5) mounds that affect the top Hordaland Group surface are present in basin flanks and at basin-flank transitions but not in basin centres. We suggest that triggering by incipient slab sliding can explain these observations.

Abstract The Balder Field reservoir sandstone has been interpreted as remobilized from a Mesozoic parent bed. This paper seeks to address the questions raised about this unusual origin. Research and broadly analogous processes are reviewed, leading to the proposal that the parent beds may have been fluidized by bedding-parallel retrogressive entrainment of Statfjord Formation sands by a connected larger source of overpressured fluids. These fluids are identified as most likely derived by lateral migration from the Viking Graben, initiated in response to early Eocene basin inversion related to North Atlantic rifting. The event probably involved breaching of the topseal in multiple places over a large section of the Utsira High. The geometry of large sills formed from small breach points and the internal differentiation seen may show that the sills inflated by lateral accretion from a medial active flowing zone of turbulent, transitional or laminar flowing suspension. It is suggested that sequence-stratigraphic and structural context should be considered as additional criteria to discriminate between depositional and intruded sands.

Abstract Sand remobilization often occurs in deltaic depositional environments as a result of sediment load and de-watering during sediment compaction. Three-dimensional seismic data from a shelf and fan delta environment in the Central North Sea are interpreted using a colour processing technique which gives clues for reconstructing the depositional environment and remobilizing the sand. The shape and thickness of the sand remobilization features appear to be influenced by the depositional environment of parent sand. In the shallow part of the fan delta, irregular injectite geobodies are observed. In the proximal fan delta, large round saucer-shaped injectites indicate that the pressure release largely occurred in a crater-like shape. With increasing distance from the shelf break, the thickness of the sediment load and, thus, the pressure increases. Thin and extensive sheet-like injectite structures develop along faults. Some of these appear to reactivate pre-existing flanks of giant pockmarks.

Abstract Three-dimensional seismic reflection data provide a means to assess the impact of injection on parent sands, and to quantify the character of the resulting injectite networks. The morphology of a series of large injectite structures hosted in the Paleocene Lower Lista Formation were mapped using broadband 3D seismic data from the North Sea to investigate their relationship with parent sands. Fourteen bowl-shaped structures were identified within the Lista Formation in the study area (60–85 m in height, and 200–900 m in width). Sand is absent (below resolution) below these large-scale bowls, suggesting that the parent sand is the underlying Maureen Formation and that sand ‘welds’ formed, rather than sand-prone channelized deposits within the Lista Formation. Identification of injectite networks can be ambiguous, which impacts geological model development. Observations from exhumed systems and core offer high-resolution insights into the complexity of injectite networks. To advance our understanding of this scale gap, we argue for injectites being scale invariant in their shape and grain size. This permits the application of outcrop-scale knowledge to seismic-scale interpretation. The demonstrable depletion of parent sands, and their scale invariance, can be applied to basin-fills worldwide to reduce uncertainties of the impact of sand injectites on hydrocarbon reservoirs.

Abstract Reservoirs in the Volund Field are all sandstone intrusions with wings on three sides forming the main reservoir volumes. The southern wing was the target of exploration and appraisal wells, which led to the field development. Identification of three smaller intrusions proves the southern wing to be a composite intrusion, similar to outcrop analogues. Identification from core and borehole logs shows that it comprises sandstone-, mudstone- and mudstone-rich intervals, including mudstone clast breccia. Mudstone clast breccia constitutes a significant missed pay candidate. Breccia is porous and has a sand-supported matrix, which gives it excellent reservoir quality. This may be missed pay using analysis of borehole logs. Well data, largely borehole logs, show consistently uniform sandstone porosity distribution within the intrusions, independent of depth. Significantly, at about 100 m from the depth at which the wing emanates from sills, porosity has a broader spread of values. The spread of values is attributable to mudstone clast breccia and thin-bedded sandstone and mudstone. Porosity derived from borehole logs does not differentiate breccia from siltstone, but inference is possible using calibration of logs with core.

Abstract The Eocene age Brimmond Sand Fairway is situated along the north-eastern flank of the Paleocene Forties Field (UKCNS blocks 21/10 and 22/6). Located along the western margin of this Brimmond Fairway are well imaged remobilized sands that form the reservoir interval for the Maule and Tonto Fields and, along with deep-water channels, the Brimmond Field. These Eocene Brimmond sandstones are encased in the Horda Shale which provides the sealing lithology. The interpretation of these remobilized and injected sands is driven from geometries derived from 3D seismic and historic logging of thin sandstones in the Eocene interval. Conical shape features with sills and steep dykes are mapped, with seismic evidence of injection along active faults and fractures. The developments of the Brimmond, Maule and Tonto Fields has been successful due to impressive seismic imaging with inversion and Direct Hydrocarbon Indicator (DHI) volumes allowing the identification of hydrocarbon bearing remobilized sandstones, along with 4D data imaging un-swept areas.

Abstract The Tumey Giant Injection Complex (TGIC) is a regionally developed sandstone intrusion complex emplaced into the deep-water Kreyenhagen Shale (Eocene) in the San Joaquin Basin, Central California. Detailed geological mapping, stratigraphic reconstruction and outcrop description, supported by structural analysis, allowed the architectural characterization of the TGIC. The complex is described as two main stratigraphically constrained intervals: (1) a lower interval (250 m thick) emplaced into clay-rich mudrock, consisting dominantly of sills with stepped and multilayered geometry; and (2) an upper interval (200 m thick) characterized by injection breccia and large wing-like intrusions ( c. 600 m width × 100 m high) emplaced within predominantly biosiliceous mudrock strata. The intrusions in both intervals were derived from turbiditic channel fills intensely modified by sand fluidization. Sandstone intrusions and fractures affecting host strata are dominantly oriented sub-parallel to the basin axis striking between NW–SE and N–S, mainly dipping to NE and forming asymmetric saucer-shaped intrusions, suggesting structurally driven hydraulic fracturing and sand emplacement. The absence of a deep aquifer and potential sand sources underlying the complex suggests a lateral contribution of fluid flow. The TGIC occurs at a scale similar to injection complexes recognized in the subsurface and is a valuable reservoir analogue for hydrocarbon accumulations associated with sand injectites.

Abstract Sedimentary injectites hosted within basement rocks, where preserved on land, offer a means to investigate the geometry, extent, dimensions and spacing of fractures that form an interconnected network within faulted/fractured crystalline host rock. Hydrocarbon occurrences within fractured basement are of high interest following the success of basement-targeted exploration in the North Sea and UK continental shelf. In some cases, these settings lack direct access to the fractured basement that constitute the crystalline-rock reservoir, due to the presence of a thick sedimentary cover. For this reason, we investigated two regional-scale crystalline-rock-hosted systems of sedimentary injectites and sediment-filled fractures that occupy basement fracture arrays within granitoids. Localities are in the Serre massif of Calabria, Italy and the Front Range of Colorado (USA). The injected sediment within fractures acts as a natural ‘proppant’ that maintained open pathways for fluid migration or accumulation. Study of the arrays of sediment-filled fractures and faults advances our understanding of unconventional fluid migration pathways, controls upon the porosity and permeability, and the potential of crystalline basement rock to act as a hydrocarbon reservoir.

Abstract Sand injectites are reported from a Miocene-age lignite seam in the Lower Rhine Embayment, which is exploited in the Garzweiler open-cast mine, Germany. Owing to the ongoing mining of the seam, the sand injectite morphology and extent were documented over a 3 year period. The combination of fieldwork, geophysical and drilling well data, as well as a 3D reconstruction of the host unit, reveal that the sand injectites within the Frimmersdorf Seam (= host unit) are highly variable, and include sills, dykes, reticulate (i.e. an interconnected network of thin sand veins) and irregular-shaped sand bodies. The injectites are connected to the underlying Frimmersdorf Sand or the overlying Neurath Sand (= parent sand units), or can be distributed throughout the seam. The differentiation of sand injectites from syn-depositional sand bodies is based on the orientation (discordant/concordant) and the presence of primary sedimentary structures in the latter. The prevailing stress field across the Lower Rhine Basin, and the coeval nature of fault activity, controlled the formation of sand injectites in the region. While such structures have been noted for over 30 years, this study is the first to provide a detailed analysis of these economically important features, including their classification, extent and morphology.

Abstract The Kodachrome pipes were developed in a porous-sand host medium which is very unusual. The sub-vertical cylindrical sand pipes have previously been interpreted as injection structures with material derived from source beds near the base of the pipes. However, the large pipe formation (diameter >4 m) was probably caused by the disruption and partial liquefaction of the host sediments which have been broken into blocks by upward fluid streaming, with injection of only small amounts (<20% of pipe volume) of fluidized sand, derived from a source below. The strongest evidence for this mechanism is the sub-horizontal layering of large tabular blocks up to 4 m in length, which occurs in the largest pipes, and the preservation of traces of bedding extending from the host rock across the pipes. There is also a notable absence of rim synclines and upturned bedding around most of the pipes, which would be expected with local removal of highly viscous material containing large blocks derived from a source bed below the pipes.

Abstract The principle aim of this paper is to document well-preserved field examples of sandstone-filled faults in order to raise awareness of these poorly understood structures, and discuss their potential as fault seals within injection-prone, multilayered siliciclastic reservoirs. To achieve this goal, we have undertaken a detailed field survey in the Panoche and Tumey hills in Central California, which allowed us to recognize numerous faults filled with injected sand. In particular, sandstone-filled extensional, contractional and strike-slip faults are observed cutting the sandstone/mudstone successions. Sandstone-filled faults commonly display small offsets and apertures ranging from a few centimetres to some decimetres. Evidence of tectonic deformation is usually lacking, meaning that sand injection supported by overpressured fluids propped open the fault walls. In this paper we also describe the main mechanism leading to the emplacement of sand along a fault plane, and propose a predictive model of sandstone-filled fault distributions in different structural environments. Finally, we discuss the role of sandstone-filled faults, that although relatively small and not adding significant volume to the reservoirs, can markedly increase fluid transmissibility and thereby promote better reservoir connectivity.

Abstract Sandstone intrusions in giant injection complexes are characterized by texturally immature sand with common micro-fractured framework grains. Individual micro-fractures are distinctive in geometry and unaligned within or between grains, thus differentiating them from micro-fractures formed by shock metamorphism or tectonics. Individual grains preserve histories of multiple impacts. The geometry of micro-fractures and their textural association makes them diagnostic of high-energy inter-granular collisions during sand injection. Mudstone clasts have sand-propped micro-fractures associated with hydraulic fracturing and individual sand grains embedded in clasts by corrasion, which is diagnostic of high grain velocity. Heavy mineral assemblages record abrasion of apatite and hydrodynamic segregation of zircon (both relative to abundance of tourmaline) upward through the injection complex. Granular abrasion and hydrodynamic segregation are consistent with turbulent flow during sand injection. Collectively the petrographic and mineralogical data support the interaction of high-velocity grains in turbulent flow during sand injection in which the granular content is likely to be dilute.

Sand injectites form during shallow-crustal deformation. Short periods of elevated pore-fluid pressure, which developed regionally, triggered formation of hydrofracture networks into which sand was sometimes injected. Sand injection complexes preserve a record of this process and sandstone intrusions are significant reservoirs in many petroleum systems. Most known subsurface sand injection complexes are from offshore NW Europe and associated with Paleogene strata. Outcrop occurrence is global. Sand injection into unconventional host rocks, including granitoid and metamorphic basement and coal seams, raises awareness of the breadth of geological environments in which sand injection may occur. Discordance between sandstone intrusions and sedimentary hosts occurs on a scale from millimetres to kilometres and is a fundamental diagnostic of intrusions. Microscale textural characterization provides new opportunities to establish possible additional criteria for differentiating intrusions from depositional sandstone. The significance of sand injection complexes in shallow crustal evolution is exemplified by the wide range of lithological hosts and diverse tectonostratigraphic settings documented in this volume. Potential for original research still remains.