Improved Visualization of Structural Deformation on the Kraka Structure (Danish Central Graben) with Color-Processed Seismic Data

This study shows a workflow to improve the visualization of fault-related heterogeneities from seismic data using a combination of color-processing (eXchromaSG Petrel plug-in) and ant-tracking leading to higher vertical resolution than wavelet-based methods. The reason is that only seismic amplitude differences between adjacent time slices matter, rather than trace similarity, and thus leans on the concept of seismic detectability rather than resolution (1/16th of the dominant wavelength vs. 1/4th). The workflow is applied to a producing hydrocarbon field in the Chalk Group on the Kraka salt dome in the Danish Central Graben. The results show structural lineations in 2D sections, having typical lengths of 200–1000 m, are generally inclined at quite shallow dips to bedding (typically 10°–45°), and terminate often at specific stratigraphic horizons. They form diffuse zones surrounding low-throw faults seen from seismic amplitude sections. In plan view on stratigraphic horizons, sets of parallel or near-parallel lineations can be seen, with individual segments often linked together to form 200–1000 m long segments. At connecting junctions, the eXchromaSG ant-tracked data shows highest values. Three orientations are observed: (1) NE–SW; (2) NW–SE; (3) concentric. Comparison to borehole image logs shows no clear relationship to the attribute in terms of fracture intensity, however, the orientations of both open and cemented fractures are strikingly similar. Given that amplitude changes within the chalk are mainly the result of porosity changes, it is proposed that the structural lineaments from this attribute reflect diffuse zones of lower or higher acoustic impedance surrounding faults, e.g. fault damage zones. We propose three processes for creating fault-related changes in acoustic impedance in chalks: (1) fault damage zones with open fractures having lower acoustic impedance; (2) fault damage zones with cemented fractures having higher acoustic impedance; (3) fault damage zones that have led to improved drainage of burial fluids, causing enhanced compaction and high acoustic impedance. They therefore are diagenetic zones surrounding faults and are as such related to areas of faulting. The three observed orientations are likely reflecting reactivation of structural weaknesses related to Permo—Triassic rifting (NE–SW) and Late Jurassic rifting (NW–SE), and concentric to salt doming. Importantly, these data provide high-resolution information on how faults interact at junctions, which provides valuable insights for modelling discrete fracture networks and reservoir compartmentalization. The results can thus be used as input for refining static and dynamic models of the main reservoirs but also for seals, within the context of Carbon Capture Storage, geothermal and hydrocarbon production.