Across‐axis variations in petrophysical properties of the North Porcupine Basin, offshore Ireland: New insights from long‐streamer traveltime tomography

Abstract

The Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin, SW of Ireland, in which a postrift phase of extensional faulting and reactivation of synrift faults occurred during the Mid–Late Eocene. Fault zones are known to act as either conduits or barriers for fluid flow and to contribute to overpressure. Yet, little is known about the distribution of fluids and their relation to the tectono‐stratigraphic architecture of the Porcupine Basin. One way to tackle this aspect is by assessing seismic (Vp) and petrophysical (e.g., porosity) properties of the basin stratigraphy. Here, we use for the first time in the Porcupine Basin 10‐km‐long‐streamer data to perform traveltime tomography of first arrivals and retrieve the 2D Vp structure of the postrift sequence along a ~130‐km‐long EW profile across the northern Porcupine Basin. A new Vp–density relationship is derived from the exploration wells tied to the seismic line to estimate density and bulk porosity of the Cenozoic postrift sequence from the tomographic result. The Vp model covers the shallowest 4 km of the basin and reveals a steeper vertical velocity gradient in the centre of the basin than in the flanks. This variation together with a relatively thick Neogene and Quaternary sediment accumulation in the centre of the basin suggests higher overburden pressure and compaction compared to the margins, implying fluid flow towards the edges of the basin driven by differential compaction. The Vp model also reveals two prominent subvertical low‐velocity bodies on the western margin of the basin. The tomographic model in combination with the time‐migrated seismic section shows that whereas the first anomaly spatially coincides with the western basin‐bounding fault, the second body occurs within the hangingwall of the fault, where no major faulting is observed. Porosity estimates suggest that this latter anomaly indicates pore overpressure of sandier Early–Mid Eocene units. Lithological well control together with fault displacement analysis suggests that the western basin‐bounding fault can act as a hydraulic barrier for fluids migrating from the centre of the basin towards its flanks, favouring fluid compartmentalization and overpressure of sandier units of its hangingwall.

Predicting fluid pressure in sedimentary basins from seismic tomography

“…A detailed P-wave velocity tomographic image (Fig. 2b) of the shallow post-rift structure of the northern Porcupine Basin has been derived using 10-km-long streamer data by Prada et al (2018). This study demonstrated the application of quantitative seismic tomography of long-streamer controlled source data in providing insight into fluid and fault interaction within post-rift sedimentary systems.…”

“…Tectonic extension in the basin led to a dramatic crustal thinning that increases from north to south (Reston et al 2004;O'Reilly et al 2006;Watremez et al 2016;Prada et al 2017;Chen et al 2018). A significant post-rift phase of extensional faulting and reactivation of syn-rift faults occurred during the Mid-Late Eocene (Worthington & Walsh 2016;Prada et al 2018).…”

“…Recent advances in the acquisition of multichannel seismic (MCS) data sets using long offset streamers has permitted the development of detailed seismic velocity models of the structure of sedimentary basins. This paper uses results from a previous study in the Porcupine Basin (Prada et al 2018) to develop a methodology for predicting pore pressure variations at a basin-wide scale (10 2 km) from such tomographic models. The methods developed are generic and applicable to a variety of tectonic settings, where gravitational compaction of sediments into solid rock is important.…”

“…Over basin-wide scales, the degree of fluid overpressure is controlled by the balance between the creation of pore pressure by sediment loading and its dissipation, controlled by sediment permeability (Hantschel & Kauerauf 2009;Marín-Moreno et al 2013a). This is ultimately related to the development and breaching of lithological permeability barriers during basin development and sedimentation (Osborne & Swarbrick 1997;Swarbrick et al 2002;Tingay et al 2009;Prada et al 2018).…”

“…mentary basin is investigated, given the recent results on long-offset streamer traveltime tomography of the post-rift sequence in the basin (Prada et al 2018).…”