Andrew Hurst

Sand injectites: an emerging global play in deep-water clastic environments

Although long-recognized features in the geological record, the hydrocarbon reserve potential of sand injectites has only recently become apparent. Advances in the quality and resolution of 3D seismic data allow the definition of a range of trap and reservoir geometries that have, in the past, not been deliberately targeted by exploration wells. Sand injectites form a new trapping style. They are intrusive, occuring as discrete traps and in combination with structural and stratigraphical features. Dykes, sills, emergent sills, scalloped tops and irregular bodies are identified as trapping styles. Reservoir quality is typically homogeneous and good, even when in an overall low net:gross system. The North Sea Palaeogene, in which at least 2.4 × 10 9 BOE reserves are associated with injectite fields, or fields modified by sand injection, is used as an example of sand injectite plays. Statistically, the additional reserve potential for sand injectites in the North Sea Palaeogene is high. Globally, the presence and significance of sand injectites is largely overlooked. As most sand injectite reservoirs of commercial significance develop during early burial and are associated with overpressure, play concepts are developed that combine mechanisms for overpressure development with sedimentology. Recognition of the presence of sand injectites will have a major affect on many deep-water and other plays both in terms of exploration and production.

Regional sand injectite architecture as a record of pore-pressure evolution and sand redistribution in the shallow crust: insights from the Panoche Giant Injection Complex, California

Abstract: Observations on outcrop of a regionally developed sand injectite are used to infer and estimate the pore-pressure conditions in the shallow crust that caused the fluidization and injection of tens of cubic kilometres of sand. The estimated pore-fluid pressures at the base of the injection complex (at 1500 m burial depth, below a regionally developed shale-dominated seal) are from 22.26 to 25.08 MPa, which respectively correspond to 0.81 and 0.95 lithostatic pressure. A theoretical basis for prediction of sand injection is defined and applied to the prediction of pore pressure at the time of sand injection, the depth at which seal failure occurred, and the density and granular content of the fluidized flow. Lateral variations in the style and abundance of sandstone intrusions are described and these all fit into a remarkably uniform tripartite division of parent units, an intrusive complex and an extrusive complex. A sill zone (intrusions are dominated by sills) occurs in a restricted stratigraphic interval 200–270 m thick. Location of the base of the sill zone is directly related to the thickness of the overburden, and an isobaric surface at the time of sand injection, the lithostatic equilibrium surface, is defined at the base of the sill zone. When the sills formed an extended period of supra-lithostatic pressure occurred within the sill zone.

Relevance of Sand Injectites to Hydrocarbon Exploration and Production

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.

Seismic Characterization of Large-scale Sandstone Intrusions

Postdepositional remobilization and injection of sand are important processes in deep-water clastic systems. Features resulting from these processes are particularly well documented in the Paleogene of the central and northern North Sea, where large-scale sandstone intrusions significantly affect reservoir geometries and fluid-flow properties of sand and mudstone intervals throughout large areas. Large-scale sandstone intrusions seen in seismic data from the Paleogene of the North Sea can be grouped into three main categories based on their size, morphology, and relation to their parent sand body: Type 1: Winglike sandstone intrusions are seen as discordant seismic anomalies that emanate from the sides and sometimes from the crests of steep-sided concordant sand bodies, which may be of depositional or intrusive origin. The intrusions may be as much as 50 m (164 ft) thick, and crosscut some 100–250 m (330–820 ft) of compacted mudstone section at angles between 10 and 35. Winglike intrusions may form regardless of preexisting structures, but commonly exploit polygonal fault systems in the encasing mudstones. Type 2: Conical sandstone intrusions are seen as conical amplitude anomalies that emanate some 50–300 m (164–1000 ft) upward from distinct apexes located a few meters to more than 1 km (0.6 mi) above the likely parent sand body. The intrusions may be as much as 60 m (196 ft) thick and are discordant to bedding along most of their extent, with dips ranging from 15 to 40. The nature of the feeder system is conjectural, but may comprise subvertical zones of weakness such as blowout pipes or polygonal fault planes, whereas the intrusions themselves do not appear to be controlled by preexisting fault systems. Type 3: Crestal intrusion complexes comprise networks of intrusions above more massive parent sand bodies. These intrusions are either too thin or too geometrically complex to be well imaged by seismic data. Despite the small scale of their component intrusions, crestal intrusion complexes may be volumetrically important. Large-scale sandstone intrusions commonly terminate at unconformities such as base Balder (uppermost Paleocene), top Frigg (lower Eocene), or base Oligocene, where they may have extruded onto the paleo-sea-floor. Because sandstone intrusions are commonly highly porous and permeable, they are important as reservoirs and as efficient plumbing systems in thick mudstone sequences. Because the intrusions occur in unusual stratigraphic positions not predicted by standard sedimentary facies models, they may constitute drilling hazards by hosting shallow gas accumulations or by acting as sinks to dense and overpressured drilling fluids. Predrill prediction of the occurrence of large-scale sandstone intrusions based on seismic data and predictive models is thus vital to successful exploration of deep-water clastic plays.

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.

Extrusive sandstones (extrudites): a new class of stratigraphic trap?

Abstract Extrusive sandstone bodies are identified as entirely stratigraphic traps associated with sand injectites. They may be difficult to recognize but have four-way dip closure and are invariably connected through underlying lower permeability strata to parent sandbodies by sandstone dykes or transgressive sills that belong to sand injectite complexes. Extrusive sandstones (extrudites) constitute an immature exploration target, which is largely untested by deliberate exploration wells. Using seismic data alone, the distinction between extrudites and intrusive sills, and between extrudites and depositional sands, may be problematic. Sedimentological criteria may make differentiation possible when core is available. Extrudites are likely to have been drilled and misinterpreted as conventional deep-water turbidites within deep-water systems affected by sand injection.

Successful Exploration of a Sand Injectite Complex: Hamsun Prospect, Norway Block 24/9

A large-scale sand injection complex, the Hamsun prospect, was the specific target of exploration well 24/9-7, drilled in 2004 offshore Norway. The well and subsequent sidetracks proved, as predicted, the presence of high-reservoir-quality sand injection facies. An oil column in excess of 100 m (330 ft) was demonstrated with a small associated gas cap. To our knowledge, this is the first deliberate exploration well to successfully target an injection complex.

Sandstone intrusions: detection and significance for exploration and production

Early recognition of sandstone intrusions is a key factor in maximising exploration and production success of the Paleogene deepwater sandstone reservoirs of the northern North Sea. Discordant sandstone intrusions are readily detected in cores, image logs and high quality seismic data by cross-cutting relations with the encasing shales. Many examples of “ratty” sands seen in borehole logs and “artefacts” or “channel margins” seen in seismic data have later proven to be sandstone intrusions, with significant implications for exploration and production. The effects of sand remobilisation and injection include increased connectivity between reservoir compartments, thief sands caused by brecciation and injection into the seal, and large-scale modifications of reservoir geometry, in particular top reservoir. Detailed case studies from the North Sea Paleogene and pilot studies including various other deepwater clastic successions indicate that sandstone intrusions could prove to be an important factor in the development of some highly prolific deepwater provinces such as the West African Atlantic margin. Early recognition of sandstone intrusions in such areas is important for optimal development planning. It requires that the appropriate borehole and seismic data are acquired, and that sandstone intrusions are incorporated in the interpreter’s mindset.

Variations in Sediment Extrusion in Basin-floor, Slope, and Delta-front Settings: Sand Volcanoes and Extruded Sand Sheets from the Namurian of County Clare, Ireland

An outcrop and microtextural study of sand volcanoes from the Namurian of County Clare is presented. Sand volcanoes occur on top of mud-rich slumps that are interpreted to have loaded the sediment pile and caused rapid compaction and fluid expulsion from the underlying units. Fluids migrated into the most permeable sand-rich bodies in the slump, and fluidized grains were then extruded at the sediment-water interface. In some cases, a laterally extensive extrusive sheet of silt and sand developed, with volcanoes located at focused sites of sediment expulsion. From microtextural studies, several (geologically short-lived) episodes of sediment and fluid expulsion are recognized as distinct, normally graded silt to sand-size beds separated by clay-rich beds. The clay-rich beds may either represent background sedimentation between expulsion events or may have been part of the extruding fluidized sediment itself.

Fluidization and Associated Soft-sediment Deformation in Eolian Sandstones: Hopeman Sandstone (Permian), Scotland, and Rotliegend, North Sea

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.