Seal rock deformation by polygonal faulting, offshore Uruguay

Abstract

Due to a lack of borehole data, reservoir and seal rock presence in frontier basins is typically inferred from seismic reflection data. However, analysis of the geometry and kinematic development of polygonal fault systems (PFS), which almost exclusively form within very fine-grained strata, provides another, largely untested method to infer reservoir and seal rock presence. We here use very high-quality 3D seismic reflection data from offshore Uruguay and a range of quantitative GIS-based techniques to document the planform and cross-sectional characteristics of a basin-scale (>6400 km2) PFS, and to investigate the role that stratigraphic variations in the Tertiary deep-water host-rock have on its geometrical variability and kinematic development. We demonstrate that a series of likely sandstone-rich deep-water deposits occur at the base of and within the main PFS tier. The geometric characteristics and throw distribution on individual polygonal faults suggest these sandstone-rich deep-water deposits represent a mechanical barrier to fault propagation, thus influencing fault height and areal density and, in some cases, strike. We argue that in largely unexplored, deep-to ultra-deep water basins, such as those characterizing offshore Uruguay, the distribution and geometric attributes of PFS can be used to delineate sandstone-rich reservoir bodies. Furthermore, these characteristics may help exploration geoscientists better understand seal heterogeneity and quality in data-poor basins.