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 km 2 3D4C 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 at https://doi.org/10.6084/m9.figshare.c.4812948