Subsurface sediment remobilization and polygonal faulting in the northern Dutch offshore

Abstract

In this research project 3D seismic data are examined for the characterization of fluid flow expressions in Lower Tertiary sediments of the northern Dutch offshore. The study area is located in quadrants A-G, with focus on the D- and western E- quadrants. The availability of new 3D seismic data in this region allowed accurate mapping and quantification of the fluid flow features present. These features might represent migration pathways for ascending hydrocarbons and can potentially be related to dry wells drilled in the study area. In total, a number of 135 mounded structures have been recognized on the Mid Miocene Unconformity. The focus of this research is to investigate the relationship between these mounded structures and the underlying polygonal fault system present in the Paleogene sediments. This relationship is examined via quantitative analysis on the polygonal faults, which include azimuth, throw and fault trace length measurements. Results show that areas where fault throw is largest, mounded structures are generally present. Furthermore, a positive relation exists between the mound height and magnitude of the underlying fault throw. These relationships combined with the characteristics found on 3D seismic data, indicate that the mounds most likely originate from remobilized sediments. Fluids dewatering from the underlying polygonal faulted sequence resulted in the entrainment and expulsion of these sediments on the paleo seafloor. The lithology of the mounds however, is uncertain as no wells have been drilled through these structures. Finally this research discusses the implications for the petroleum industry, as the observed fluid flow features are known migration pathways and might represent drilling hazards.