The Role of Polygonal Fault Mapping in De-Risking Deepwater Reservoir Presence: A 3-D Seismic Reflection Case Study from Offshore Norway

Jackson, Christopher A.; Mahlo, Seshane; Briggs, Omieari

Polygonal fault systems (PFS) are composed of relatively low-displacement (<50 m) normal faults that form in response to burial, compaction and dewatering of smectite-rich claystones. In many petroliferous sedimentary basins, PFS occur in the same stratigraphic interval as deep-water sandstone reservoirs, thus the geometry and distribution of PFS, which are both sensitive to the lithology of the host rock, should provide important insights into the distribution of deep-water reservoirs. There are, however, relatively few detailed studies that have focused on the utilisation of PFS mapping to de-risk deep-water reservoir presence, or that have outlined exploration workflows that may be implemented by industry geoscientists. In this study we use 3-D seismic reflection and borehole data from the Cretaceous-to-lowermost Tertiary succession of the Måløy Slope, offshore Norway to demonstrate that the distribution of polygonal faults can be used to de-risk the presence of a deep-water reservoirs. We first use a variety of seismic attribute mapping techniques to image the reservoir and PFSs, and then present a detailed, statistical analysis of polygonal fault geometry and distribution, both vertically (i.e. stratigraphically) and laterally. Our data indicates that the an Upper Cretaceous (Turonian) submarine fan reservoir, which is up to 120 m thick and pinches out into slope mudstone, is largely unfaulted, but that polygonal faults are abundant in the time-equivalent, mudstone-dominated slope succession; the polygonal faults thus define the areal extent of the reservoir at this particular stratigraphic level. Furthermore, polygonal faults below and above the reservoir tip out upwards and downwards, respectively, into the sandstone-dominated reservoir succession, thereby constraining its stratigraphic extent. The reservoir that forms the focus of this study is very well-imaged, but we suggest that the general workflow we describe here is equally applicable to the exploration for deep-water reservoirs that are poorly-imaged on seismic reflection data. We stress that polygonal faults are not just an academic curiosity, but that they may have economic applications by serving as a key deep-water exploration tool.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013