Constraining the depth and timing of large-scale conical sandstone intrusions

Abstract

Kilometer-scale sandstone intrusions-a recently recognized end member of the spectrum of soft-sediment deformation structures-are important components of many deepwater sandstone reservoirs. They result from the remobilization of considerable volumes of sand during early stages of burial. This paper uses three-dimensional seismic data, calibrated to nearby wells, to describe an extensive system of large downward-tapering conical sandstone intrusions from the Faeroe-Shetland Basin, on the UK Atlantic margin. The intrusions are derived from a middle Eocene submarine fan and are injected vertically upward through overlying, highly faulted claystones to form conical bodies that interconnect across the ϳ700-m-thick claystone interval to feed a set of shallow intrusions located within a few hundred meters of the paleoseabed. The forcible intrusion of these shallow sandstone bodies produced complementary folding of the contemporaneous seabed. The subsequent onlap of these seabed domal folds took place immediately after the intrusion event, and by correlation to biostratigraphic data from nearby wells, we date these intrusions as late Miocene. This is the first time that both the age and emplacement depth range of a system of large-scale sandstone intrusions have been constrained.

Injection of solids to lift coastal areas

“…In the Faeroe-Shetland basin, for example, these features are 500-1000 m in diameter and up to 20-30 m thick (Shoulders & Cartwright 2004). Detailed interpretation of seismic data indicates that these bodies were formed at depths of 200-300 m below the sea floor and lifted their overburden when they were emplaced (Shoulders & Cartwright 2004). Similar features are observed in seismic data from the North Sea, where they are tens of metres thick and more than 1 km in diameter (Huuse & Mickelson 2004).…”

“…Sand injectites range in thickness from less than a metre to several tens of metres and extend laterally from hundreds of metres to a kilometre or more (figure 6a). In the Faeroe-Shetland basin, for example, these features are 500-1000 m in diameter and up to 20-30 m thick (Shoulders & Cartwright 2004). Detailed interpretation of seismic data indicates that these bodies were formed at depths of 200-300 m below the sea floor and lifted their overburden when they were emplaced (Shoulders & Cartwright 2004).…”

“…For example, laboratory experiments using silty clay showed that hydraulic fractures could be created (figure 4a) by injecting fluid into samples with water contents ranging from 0.17 to 0.33 (Chang et al 2003;Chang 2004;Germanovich et al 2007). The shape of (c) resembles sand diking (Shoulders & Cartwright 2004). (Murdoch 1993).…”

“…Injectites indicate that sand has been injected in nature at the scales and geometries of the SIRGE process. It is certainly possible that the formation of sand injectites involves processes, such as fluidization caused by seismicity (Shoulders & Cartwright 2004), which would be difficult to duplicate in an engineering setting. Nevertheless, the similarity between the forms of engineered hydraulic fractures and sand injectites suggests that the scale difference between these features may not necessarily introduce fundamentally different mechanisms.…”

“…To address this issue, we turn to sand sills, or sand injectites (e.g. Shoulders & Cartwright 2004). Sand injectites range in thickness from less than a metre to several tens of metres and extend laterally from hundreds of metres to a kilometre or more (figure 6a).…”