Abstract Late Jurassic sandstones form important potential reservoirs within the West Shetland Basin. These strata are thin (4–84 m) and broadly equivalent in age to the Kimmeridge Clay Formation. They were deposited predominantly in deep marine environments although subaerial and shallow marine facies are also present. The Rona Sandstone forms the lowermost of three sandstone units within the Upper Jurassic succession. Data from 23 exploration wells show, however, that it is a heterogeneous unit ranging from coarse-grained pebbly sandstones to mudstones and, because of this wide lithological variation, the Rona Member is considered a more appropriate name. The facies identified (cohesionless debris flow, high and low density turbidites, upper and lower shoreface) and their vertical transitions are considered typical of fan delta deposition, initially in a subaerial environment and later under progressively more marine conditions. Coarse-grained sediments (debris flow and high density turbidites) of the Rona Member are restricted to isolated topographic lows within Lewisianoid basement whereas finergrained sediments (low density turbidites) have a wide distribution across the basin and cover much of the Rona Ridge. Previous models have suggested that the coarse-grained sediments were derived from active fault scarps. However, to explain the facies distribution it is suggested that footwall uplift on the Rona and Judd Faults (Rona Ridge and associated basement highs) was responsible for the supply of coarse-grained material to isolated fan deltas. Transgression eventually resulted in: flooding of the basement highs, cut off of sediment supply and deposition of finer-grained sediments. Deposition of the Rona Member is interpreted to have been controlled by the gradual transgression of residual islands of Lewisianoid basement.
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.
Organization is recognized in the forereef–deep water slope–submarine fan system of the Burdigalian‐Langhian Kaplankaya Formation. A basinwards transition from a prograding shelfal reef complex, through forereef talus, deep‐water slope and laterally encroaching bypass deep‐water clastic system is described, although the deep‐water slope makes up the bulk of the succession. Considerable thickness variations occur between the reef and deep‐water clastic complexes; these are controlled by sea‐floor topography, carbonate foreslope gradient and degree of mass wasting off the platform and foreslope. The vertical and lateral heterogeneity of the Kaplankaya deep‐water slope system is described from a number of localities along a 40‐km‐long and up to 3‐km‐wide exposed section of the northern margin of the Miocene Adana Basin, a foreland basin setting resulting from thrust sheet loading from the north during the Tauride Orogeny. Detailed field mapping is supplemented with vertical sedimentary logs, photomosaics, palaeontological and petrological data to investigate stratal variation, diagnostic architectural elements, controls on slope progradation, differential timing of basinward encroachment of the reefal complex and lateral onlap of the deep‐water clastic system onto the slope. Three‐dimensional models are presented showing the vertical and lateral facies associations in different parts of the deep‐water slope system, and provide a basis for architectural prediction of geometry and relative position in such environments.
ABSTRACT Consolidation laminae and dish structures form high-density zones that are interpreted to have formed by disruption of primary structures during gravitational collapse of the grain framework during water escape and consolidation. They are not associated with higher contents of clay-size material than adjacent units, but the chloritic clay minerals associated with consolidation laminae and dish structures have a different microtexture than observed elsewhere. Clay mineral texture has a direct effect on petrophysical characteristics, in particular water saturation and conductivity in hydrocarbon-saturated intervals. CT-scans identify consolidation laminae and dish structures as zones of high density that correspond to tighter packing of sand grains and are unrelated to clay distribution. Dish structures may form independently of consolidation laminae or by further modification of fragmented consolidation laminae.
The presence of hydrocarbon-bearing sandstones within the Eocene of the Forties area was first documented in 1985, when a Forties field (Paleocene) development well discovered the Brimmond field. Further hydrocarbons in the Eocene were discovered in the adjacent Maule field in 2009. Reservoir geometry derived from three-dimensional seismic data has provided evidence for both a depositional and a sand injectite origin for the Eocene sandstones. The Brimmond field is located in a deep-water channel complex that extends to the southeast, whereas the Maule field sandstones have the geometry of an injection sheet on the updip margin of the Brimmond channel system with a cone-shape feature emanating from the top of the Forties Sandstone Member (Paleocene). The geometry of the Eocene sandstones in the Maule field indicates that they are intrusive and originated by the fluidization and injection of sand during burial. From seismic and borehole data, it is unclear whether the sand that was injected to form the Maule reservoir was derived from depositional Eocene sandstones or from the underlying Forties Sandstone Member. These two alternatives are tested by comparing the heavy mineral and garnet geochemical characteristics of the injectite sandstones in the Maule field with the depositional sandstones of the Brimmond field and the Forties sandstones of the Forties field. The study revealed significant differences between the sandstones in the Forties field and those of the Maule and Brimmond fields), both in terms of heavy mineral and garnet geochemical data. The Brimmond-Maule and Forties sandstones therefore have different provenances and are genetically unrelated, indicating that the sandstones in the Maule field did not originate by the fluidization of Forties sandstones. By contrast, the provenance characteristics of the depositional Brimmond sandstones are closely comparable with sandstone intrusions in the Maule field. We conclude that the injectites in the Maule field formed by the fluidization of depositional Brimmond sandstones but do not exclude the important function of water from the huge underlying Forties Sandstone Member aquifer as the agent for developing the fluid supply and elevating pore pressure to fluidize and inject the Eocene sand. The study has demonstrated that heavy mineral provenance studies are an effective method of tracing the origin of injected sandstones, which are increasingly being recognized as an important hydrocarbon play.
Abstract Post-depositional remobilization and injection of sand can significantly change the geometry of deepwater clastic reservoirs. Features associated with these processes are particularly well developed in the lower Paleogene of the South Viking Graben of the UK and Norwegian North Sea. Seismic scale sandstone intrusions can be grouped in two classes. Class 1 comprises low-angle (20-40 degrees) tabular sandstone intrusions emanating from steep-sided in situ sand bodies within the Balder Formation. The intrusions may be 5-30+m thick and crosscut 120-250+m of compacted stratigraphic section. They terminate at an unconformity at the top of the Frigg interval where they may have extruded onto the palaeo-seafloor. Class 2 comprises conical sandstone intrusions that emanate some 50-300+m upward from distinct apexes located 400-700+m above the nearest depositional sand body. The conical intrusions may have been sourced from underlying sand bodies by clastic blow out pipes. Both types of intrusions seem to adopt their particular geometry independently of (but occasionally exploiting) polygonal faults within the encasing mudstones. Sandstone intrusions may be highly porous and permeable and are thus important both as reservoirs and as plumbing within thick mudstone sequences.
Synthetic well tests have been produced using a 3D model of an outcropping turbidite sandstone unit from the Cingöz region in southern Turkey. The model contains realistic sand sheet, tongue, lobe and background sand facies architecture (i.e. geometry and stacking) mapped from an outcrop study. The geometric information is useful as an analogue for high net-to-gross turbidite oil fields. The facies have been assigned petrophysical properties from a subsurface analogue. There is little shale in this system. Well test responses were then derived from the high net-to-gross turbidite model using various architectural, porosity–permeability scenarios and completion strategies. The impact on well test derivatives of various sand body geometries and permeability contrasts could then be determined. Two completion strategies – partial penetration and fully perforated intervals – were assessed for their applicability in the high net-to-gross system. The geological model is effectively a sandbox, and shows a very uniform testing response from the rather uniform property distributions. However, when the level of permeability heterogeneity is increased by populating the model with varying contrasts of permeability and porosity, the sand body geometry can be seen to influence the well tests. Partial completions in sand bodies are particularly effective in detecting sand body geometry. The geometry controls the flow regimes in a well test response despite variations in the permeability contrasts. The effect of varying geometry is illustrated and an external linear flow regime is identified. Where there is sufficient sand body thickness, partial perforation results in spherical flow, from which a vertical permeability can be obtained. In the model, the vertical permeability thus obtained is a local (to the volume investigated) effective permeability of stacked isotropic facies. This work was undertaken to give guidance on the description of hydrocarbon reservoirs by well testing. If well testing is to be used in high net-to-gross turbidite systems for the purposes of reservoir characterization, then partial perforation of the system should be planned. Interpreted vertical permeabilities should be applied with careful consideration of the stacked pattern of sand bodies.
ABSTRACT Clay microporosity data are derived from image analysis of petrographic sections. These data allow estimation of clay-bound water and establish a quantitative relationship between clay texture and effective volume. Small quantities of highlymicroporous clay may cause drastic degradation of permeability with little effect on total porosity.
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