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 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 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.

Abstract Identification and exploration drilling of the Volund Field as part of a sand injection complex is the first example of deliberate targeting of sandstone intrusions in oil exploration. Outcrop data were an important element in the process of constraining the uncertainty associated with reservoir presence and connectivity. A strong lobby against the relevance of sand injectites as exploration targets, and significant uncertainty associated with sub-surface sand injectite analogues associated with existing oil fields, combined to discourage and down-grade Volund’s prospectivity. Few outcrop studies provided data of relevance to exploration of sandstone intrusions and original outcrop data were utilized in evaluation of Volund. Sills and saucer-shaped sandstone intrusions are the most common reservoir units observed at outcrop and similar features were identified in the 3D seismic across the Volund Field prospect. A large-scale sand injection origin rather than a depositional origin was proposed. Outcrops of sandstone intrusions demonstrated excellent reservoir quality in composite sandstone bodies that cross-cut depositional bedding. High-quality reservoirs with excellent vertical and lateral connectivity are observed and used to support the prediction of similar quality reservoirs in the Volund prospect.

Abstract Three distinct analytical approaches are embraced in mineral–chemical stratigraphy: mineralogy, whole-rock geochemistry and single-grain geochemical analysis. Mineralogical studies identify and quantify the clastic components of sandstone, even though any clast category may be geochemically diverse. Whole-rock geochemical studies (sometimes referred to as chemostratigraphy), by contrast, quantify the abundance of major and trace elements in sandstone, but provide no information on the distribution and location of the elements in minerals. These approaches are linked by single-grain geochemical analysis, which enables further characterization and subdivision of individual mineralogical components, and identifies sites where specific major and trace elements reside. In this paper, we consider the relationships between minerals, mineral chemistry and whole-rock composition, before exploring the value of mineral–chemical stratigraphy for lithostratigraphic correlation and evaluation of sediment provenance, using published examples from the North Sea region, where the great majority of such studies have been undertaken. We conclude by discussing the important role that alluvial basins play in controlling mineral–chemical signatures.

Abstract Detrital garnet suites have been demonstrated to be reliable indicators of the mineralogical and lithological characteristics of sediment source areas. This study applies garnet analysis to the Paleocene to Eocene Sele Formation deep-water sandstone units of the central North Sea. These stratigraphic units are economically important as they represent one of the main hydrocarbon reservoir intervals in this mature basin. The routing of turbidity currents into the Central Graben has been demonstrated to be related to axial fans (ultimately sourced from Lewisian and Moine basement rocks and Triassic sandstones to the NW) and lateral fans (ultimately sourced from the Dalradian basement rocks to the west). Garnet analysis suggests the majority of samples can be attributed to the axial fan system and that the lateral system contributed little to sandstone deposition east of the Gannet Fields. This contradicts previous seismic mapping work, which suggested that the lateral fan system dominated sedimentation as far east as the Merganser Field. This reinterpretation is potentially important for our understanding of sediment routing and its impact on the distribution of reservoir quality, particularly as this is believed to relate directly to proximity to the shelf.

Abstract The Alikayasi Canyon Member of the Tekir Formation occurs in a thick sequence of deep-water slope deposits on the northern margin and center of the lower-middle Miocene Maras foreland basin in eastern Turkey. The canyon was one of at least four major sediment-bypass systems that sourced from a narrow shelf otherwise occupied by thick, coeval carbonate reefs. What remains of the source hinterland indicates that thick fan deltas propagated directly into the heads of the deep-water canyons that characterize these bypass systems. The Alikayasi Canyon is exposed as an almost completely exhumed sediment body in an area of sparse vegetation, where the contemporaneous shelf margin is still largely intact, and it represents the youngest of these four systems. It forms a 7-km (4-mi)-long, up to 300-m (984-ft)-high, and up to 1-km (0.6-mi)-wide sediment body, dissected once by a river, which is now drowned by an artificial lake behind the Menzelet Dam. The exposure is complete apart from a 1.5 km (0.9 mi) section through its most proximal reaches, and a 2 km (1.2 mi) section in its most distal reaches where it feeds into a series of sandy lobes. The canyon fill is characterized by stratified conglomerates and pebbly sandstones in its lower part, stratified conglomerates and braid-plain-style conglomerates and pebbly sandstones in its middle part, and steeply dipping fan-delta conglomerate clinoforms in its upper part. The axial area of the canyon is dominated by these coarse-grained deposits, although locally remnants of intracanyon shales, in the form of floating rafts, shale blocks, and clasts,

Abstract The Tinker channel is exposed in a series of dip and strike sections to the east of Hasret Mountain, near Elazig, in eastern Turkey ( Figure 1A ). The exposures are part of the exhumed northern margin of the northeast-to-southwest-oriented Elazig Basin, which has almost continuous exposures for 75 km (46 mi) in the high eastern Anatolian badlands ( Cronin et al., 2005 ). The outcrop allows study of a series of time-equivalent stratigraphic intervals through a clastic system that propagated from the elevated middle Eocene hinterlands and narrow shelves to the north, toward the deep basin axis to the south and east. The Kirkgecit Formation is interpreted as a predominantly low net-to-gross, deep-water, slope-environment succession, which has infilled a topographically irregular basin margin, created during basin formation in a rapidly subsiding back-arc setting. Incised and entrenched slope-channel complexes contain most of the coarser grained, deep-water clastic sediment within the Kirkgecit Formation. The Tinker channel ( Cronin et al., 2000b ) is one of a series of such channel-complex exposures that allow detailed examination of the fill and overbank stratigraphic architecture. The main Tinker channel exposures are to the east of Hasret Mountain, 15 km (9 mi) east of the city of Elazig. The channel is located 3 km (1.8 mi) downdip of the inferred contemporaneous slope break (Karadag, Figure IB). It is the most proximal channel of a series of four slope-channel complexes ( Figure 1C ) that occur within the same stratigraphic interval. The Tinker channel was documented by Cronin et al. (2000b) and the architecture and chronology of the enveloping deep-water slope succession by Cronin et al. (2000a).

Abstract A series of entrenched, deep-water slope-channel complexes form a planform tributary network of conduits on the northern slope of the Eocene-Oligocene back-arc Elazig Basin in eastern Turkey (Figures 1—3). The lower part (60%) of the channel complex fills are characterized by coarse-grained sediments and the upper 40% by heterolithic facies ( Figure 4 ). The heterolithic facies drape a synsedimentary faulting phase of activity that saw a reactivation of the channel complexes after filling. The lower part of this facies, which thickens over the channel-complex axes, is dominated by slumped shales and local, thin-bedded turbidite sandstones and siltstones. The upper part comprises sheet and lenticular pebbly sandstones and sandstones, interbedded with rippled sandstones and siltstones, associated with channelized geometries ( Figure 5 A, B). This interval is also characterized by lateral accretion surfaces, where beds have an asymptotic and shingling character, an association with sedimentary structures such as trough cross-bedding, and where paleocurrents suggest sinuosity of the channel elements. The sinuous channels are up to 3 m (10 ft) thick and 10-40 m (33-131 ft) wide. Interleaving shales have abundant and diverse deep-water and shelfal ichnofacies, and deep-water benthic/plank-tonic foraminiferal assemblage ratios. The sand-prone portions of these sinuous channel bodies are dislocated and usually confined to the accretionary margins and channel floors. The channel elements can be tracked for 7 km (4 mi) in a series of sections above four separate, entrenched, deep-water slope-channel complexes. These late-stage sinuous channel elements and their association with larger, longer-lived, and persistent (i.e., entrenched), deep-water slope-channel complexes are documented for the Main channel.

Abstract Sand injectites are described 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 intrareservoir flow units that create fieldwide vertical communication through depositionally extensive, low-permeability units. Because 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 as well as mitigating timing and rate of hydrocarbon migration. Injected sand bodies form intrusive traps, which are distinct from structural or stratigraphic traps. Reservoir quality is typically excellent, with a high level of connectivity between sand bodies of all sizes. In a production context, sand injections enhance sweep efficiency but may cause more rapid-than-expected water breakthrough if wells are placed too near injectite complexes. Despite experience from the North Sea, recognition of sand injectites and their significance in hydrocarbon basins globally are at an early stage.