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

• Published in: Basin Research
• Main Authors: Prada, Manel, Lavoué, François, Saqab, Muhammad Mudasar, O'Reilly, Brian M., Lebedev, Sergei, Walsh, John J., Childs, Conrad
• Other Authors: Science Foundation Ireland
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2018
• Subjects: North Atlantic
• Online Access: http://dx.doi.org/10.1111/bre.12308; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fbre.12308; https://onlinelibrary.wiley.com/doi/pdf/10.1111/bre.12308

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 ( V p ) 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 V p structure of the postrift sequence along a ~130‐km‐long EW profile across the northern Porcupine Basin. A new V p –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 V p 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 V p 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 ...