Kilometre-scale sandstone intrusions in the Eocene of the Outer Moray Firth (UK North Sea): migration paths, reservoirs and potential drilling hazards

Abstract

Kilometre-scale sandstone intrusions have recently been documented from the Palaeogene deep-water sequences of the northern North Sea. The distribution of Eocene sandstones within a 126 km23D4C seismic survey located in the Outer Moray Firth was mapped using surface and volume based interpretation of converted wave seismic data calibrated to a number of boreholes. About 35 conical sandstone intrusions, a few tens of metres thick and 1–1.5 km wide, were mapped within the lower Eocene. The intrusions are inclined about 15–20° and crosscut some 100–200 m of lower Eocene mudstones, terminating upward at the Middle Eocene unconformity, below the oil-charged, upper Eocene sandstones of the Alba and Chestnut reservoirs. Individual sandstone intrusions may contain millions of cubic metres of sandstone and often have excellent reservoir properties. Previous studies failed to define a viable migration path for the oil in the Alba/Chestnut reservoirs because they appeared to be completely encased in poorly permeable Eocene mudstones. However, detailed mapping and volume visualization of the converted wave seismic data demonstrates that the upper Eocene reservoirs have suffered post-depositional remobilization and that the conical intrusions provide a viable migration path between the Alba/Chestnut reservoirs and the underlying Paleocene aquifers. Despite more than two decades of exploration activity in the Outer Moray Firth, the widespread occurrence of conical intrusions in the lower Eocene is still not widely recognized, and it is likely that the sandstones may pose a hazard when drilling for deeper targets. However, it is also possible that large conical intrusions that do not connect with overlying sandstones may contain economical quantities of hydrocarbons.Supplementary material:The movie referred to in the article is available athttps://doi.org/10.6084/m9.figshare.c.4812948

Miocene igneous activity in the Northern Subbasin, offshore Senegal, NW Africa

“…), and pipelike sediment injection features have been interpreted on 3D seismic data from offshore West Africa (Davies, 2003). They are imaged on seismic data as discordant, moderate to high amplitude anomalies with dimensions ranging from 50 to 300 m in vertical relief, a few hundreds to a few kilometres in lateral extent and covering areas of hundreds of m 2 to a few km 2 (Huuse and Mickelson, 2004;Huuse et al, 2005;Huuse et al, 2007). They are typically 'V'-shaped or appear as 'wing'-like discordant edges emanating from concordant reflections in cross-section (Huuse et al, 2007).…”

“…They have circular to oval to somewhat angular plan-view geometries and most terminate downward in discrete apexes, with only a minority terminating downwards at concordant amplitude anomalies (Huuse and Mickelson, 2004). The inclination of the anomalies relative to the host sediment typically ranges between 101 and 401 (Huuse and Mickelson, 2004;Huuse et al, 2005;Huuse et al, 2007).…”

Fluid flow features in hydrocarbon plumbing systems: What do they tell us about the basin evolution?

“…Subsequently, several observations of kilometer-scale sand injections have been made particularly in relation to Paleogene oil and gas fields of the North Sea (MacLeod et al, 1999;Duranti et al, 2002;Purvis et al, 2002;Hurst et al, 2003;Huuse et al, 2005). In seismic data the intrusions are recognized as high-amplitude reflections cross-cutting the surrounding strata giving rise to a variety of geometries including dykes, sills, conical, saucer-shaped and wing-like intrusions (Fig.…”

“…If the sand is water-saturated the acoustic contrast to the host decreases and the intrusions are harder to detect (Svendsen et al, 2010). Similarly, Huuse et al (2005) describe oil-saturated sandstone intrusions with a very low acoustic contrast to the encasing mudstones that could not be resolved by compressional wave seismic data, but was much better imaged by converted shear wave data.…”

Large-scale conical sandstone intrusions and polygonal fault systems in Tranche 6, Faroe-Shetland Basin

“…Understanding the factors controlling the geometry of large-scale sandstone intrusions is important because widespread sand intrusion may complicate the depositional geometry of hydrocarbon reservoirs (e.g., Dixon et al, 1995;MacLeod et al, 1998;Purvis et al, 2002;Bergslien, 2002) and conical sandstone intrusions may themselves form hydrocarbon reservoirs (Huuse et al, 2005;Huuse and Mickelson, 2004). The presence of high-permeability intruded sands within otherwise impermeable sequences may have important implications for vertical connectivity.…”