Abstract
Production from the Ekofisk Chalk Field in the North Sea is believed to be significantly influenced by the presence of a connected fault and fracture network. In the current study, we create a 3D seismic discontinuity cube which is representative of this network within the southern part of the Ekofisk Field. This is done using a multiscale workflow which integrates seismic fault and fracture detection with borehole image log interpretation from three horizontal well sections. The results show that faults and fractures are prevalent in the Ekofisk Formations. Within the study area, faults are mainly organised in three orientations: 1) WNW-ESE, 2) NNE-SSW and 3) NNW-SSE. Smaller E-W striking faults are also observed. The interpreted fractures show a similar pattern and are organized in four orientation groups: NW-SE, WNW-ESE, ENE-WSW and NE-SW. The analysis of seismic discontinuity data (i.e. faults and fractures detectable on seismic) indicates that most small-scale discontinuities occur in proximity to large faults, and that the Lower Ekofisk Formation is characterized by more widespread – and a higher intensity of small-scale seismic discontinuities. It is also demonstrated that along each studied well section, the extracted seismic discontinuities show a qualitative correlation with the image log interpretation. This correlation suggests that the 3D seismic discontinuity cube can serve as a proxy for the fault and fracture network in the southern part of the Ekofisk Chalk Field. Following from our key findings, we conclude that the presented workflow and results could provide a starting point for future studies assessing the impact of natural fractures in the Ekofisk – and other complex reservoirs.
Highlights
Natural fractures play an important role in estimating the effective permeability in tight or low permeability rocks
This study focuses on the southern part of the Ekofisk Chalk Field, outside of the Seismically Obscured Area (SOA) (Fig. 2a)
The locality of the clustered WNW-ESE fault zones is consistent with previous interpretations of the Ekofisk fault network (e.g. Toublanc et al, 2005) (Fig. 2a)
Summary
Natural fractures play an important role in estimating the effective permeability in tight or low permeability rocks. Predicting the geometry, location and intensity of natural fractures has become an integral component of characterizing structurally complex reservoirs. These characterization studies often consider a multitude of scales and data types (Freeman et al, 2015; Quinn et al, 2014; Williams et al, 2017). A methodology commonly used in subsurface case studies is geostatistical modelling (Gauthier et al, 2002) This method generally integrates the seismic – and borehole input data using 1) statistical distributions (e.g. fracture length or orientation), 2) large scale probability maps, and/or 3) correlations between the different datasets, in order to create large scale representations of subsurface reservoirs This method generally integrates the seismic – and borehole input data using 1) statistical distributions (e.g. fracture length or orientation), 2) large scale probability maps, and/or 3) correlations between the different datasets, in order to create large scale representations of subsurface reservoirs (e.g. Gauthier et al, 2002; Toublanc et al, 2005)
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