Overpressure transmission through interconnected igneous intrusions

Abstract

In situ overpressures in sedimentary basins are commonly attributed to disequilibrium compaction or fluid expansion mechanisms, although overpressures in shallow sedimentary sequences may also develop by vertical transfer of pressure from deeper basin levels, for example, via faults. Mafic sill complexes are common features of sedimentary basins at rifted continental margins, often comprising networks of interconnected sills and dikes that facilitate the transfer of magma over considerable vertical distances to shallow basinal depths. Here, we document evidence for deep sills (depths >5 km [>16,000 ft]) hosting permeable, open fracture systems that may have allowed transmission of overpressure from ultradeep basinal (>7 km [>23,000 ft]) levels in the Faroe-Shetland Basin, northeast Atlantic margin. Most notably, well 214/28-1 encountered overpressured, thin (<8 m [<26 ft]), and fractured gas-charged intrusions, which resulted in temporary loss of well control. Although the overpressure could reflect local gas generation related to thermal maturation of Cretaceous shales into which the sills were emplaced, this would require the overpressures to have been sustained for unfeasibly long timescales (>58 m.y.). We instead suggest that transgressive, interconnected sill complexes, such as those penetrated by well 214/28-1, may represent a previously unrecognized mechanism of transferring overpressures (and indeed hydrocarbons) laterally and vertically from deep to shallow levels in sedimentary basins and that they represent a potentially underrecognized hazard to both scientific and petroleum drilling in the vicinity of subsurface igneous complexes.

The Rosebank Field, NE Atlantic: Volcanic characterisation of an inter‐lava hydrocarbon discovery

“…No clear‐cut intrusions are interpreted within the main Rosebank Field; however, two seismically resolved intrusions within the Cretaceous are penetrated by the 213/27‐2 borehole and display typical high densities and box‐shaped wireline profiles (see also Schofield et al., 2020).…”

Linking surface and subsurface volcanic stratigraphy in the Turkana Depression of the East African Rift system

“…In general, logging of volcanic lithologies is an uncommon occurrence within petroleum exploration, with rare exceptions where igneous rocks act as reservoirs to hydrocarbons. This is because wells avoid penetrating igneous rocks, due to issues with seismic imaging (Ziolkowski et al 2003), slow drilling rates and drilling hazards associated with fractured rocks (Millett et al 2016;Schofield et al 2020). This aspect has led to a general lack of exposure of wellsite loggers and petrophysical interpreters to volcanic lithologies within the subsurface, which can lead to a radically different interpretation of volcanic sequences.…”

Seismic, petrophysical and petrological constraints on the alteration of igneous rocks in the Northern Carnarvon Basin, Western Australia: implications for petroleum exploration and drilling operations

“…Understanding igneous lithologies and issues related to the intersection of igneous rocks is crucial for a comprehensive appreciation of magmatic impacts on both petroleum systems and drilling. For example, igneous intrusions can overmature source rocks at a local scale and compartmentalise hydrocarbon reservoirs (Holford et al 2013;Senger et al 2017); transgressive intrusions can act as conduits for overpressure, risking blowout if penetrated accidentally (Schofield et al 2020); whilst volcanic ash altered to bentonite clay can also present drilling difficulties (Watson et al 2020).…”