Within the Jurassic Broad Forteen Basin (North Sea), 31 vents built of 58 smaller structures have recently been recognized within a seismic cube thanks to seismic attribute analysis. Within the surveyed volume, well cuttings from Zeichstein salt structures contain nephelinic basalts dating of 100 Ma. Strong amplitude anomalies present within the salt are good indication of the presence of igneous intrusions. The vents are imaged as sub-transparent agglomeration of sub-vertical pipe structures emanating above the seismic amplitude anomalies. The vents were analyzed through distinct categories: size, shape, upper part shape and source bed. Most of the vents originate from the Lower Germanic Triassic Group (Bundsandstein). They are conical in shape and have an eye-shaped upper dome. At the top, numerous domes have been eroded. A few domes have jacked up the overburden, sometimes including lateral intrusion (sandstone sill). Almost all vents terminate at the Upper Jurassic layers. Based on the relationship with the overlaying deposits, we have constrained the time span of venting to be from pre- Mid Hauterivian times (eroded/partly eroded upper parts of the vents) to Albian times for the latest (onlaps on the vents, bended reflections, igneous intrusion dating from well cutting).
Vents formation and subsequent fluidized sandstone injection and expulsion are interesting from petroleum exploration perspective. Most of the vents punctured through basins primary source rock – the Posidonia shales. The breaching most probably created new pathways for fluid migration, connecting the source with the overlaying sandstone units with sand injectites /fracturation around the pipes. Sand injections in the vents could serve as both migration route and reservoir units, under the condition that fluids did not conceal the pore-space. The occurrence of hydrothermal activity could also create sweet spots where good maturity occur at shallow depth because of the associated localised heat-flow. In this new play, the seal is similar to the traditional one in the basin, the Vieland Claystone Fm, which drape and onlap on hydrothermal vents domes that are natural stratigraphic traps.
In addition, this discovery illustrates the complexity of transfer of fluids and solids associated with igneous intrusions within sedimentary basins. The zone was not considered particularly volcanic except for the isolated Zuidal volcano situated onshore (50 km East and older). Besides the economic impact of this discovery, one could ask what is the trigger of this activity? Was it so discreet in the seismic images that it is more widespread than thought in the basin? The relation with the tectonic context is still unclear since, at sedimentary scale, a large time-lag could exist between the disequilibrium of conditions triggering the melting and its actual emplacement within the basins. Nevertheless, early inversion of the Jurassic structures or regional doming are the best candidate at the origin of this discovery.