Delineating fractures in the sub-seismic domain through seismic and image log analysis: a North Sea case study

Abstract

Fault and fracture studies for fractured reservoir characterization have been an essential stage of prospect generation and field development from seismic data and well data. Fracture characterization studies often utilize cores and image logs to map fracture intensity and orientation (e.g., Trice, 1999). These results are then scaled up and applied on a reservoir scale. A similar process is applied to faults in seismic data, which are downscaled in an attempt to identify fracture networks for future well planning. Upscaling and downscaling fractures and faults is an attempt to bridge the seismic-to-well data gap (e.g., Barr et al., 2007; Fossen and Hesthammer, 2000). The biggest ambiguity with either of these processes relates to scale. The trends seen at the centimetre scale within cores and image logs may or may not be similar to regional fault patterns identified by surface seismic (e.g., Emsley et al., 2007). Outcrop analogues may help to bridge the gap between well and seismic observations. However, suitable outcrop analogues are not always available or adequate to build fracture models. Furthermore, comparisons between outcrops and subsurface reservoirs should consider differences between tectonic histories, subsidence and stress conditions. Fractures may play an important role in estimating the amount of recoverable reserves for a prospect/field in terms of porosity and permeability, alternatively the presence of fractures may act as baffles to fluid flow. Therefore, the aim of this publication is to show a methodology to improve understanding of fracture trends between well and seismic data, and secondarily to improve fracture pattern identification and delineation in the gas field E17a, offshore Netherlands (Block E), Southern North Sea (Figure 1). Two independent studies were performed; a well driven fracture and fault analysis and a seismic driven fault analysis, and the two results were compared. The seismic fault study was undertaken using Cognitive Interpretation workflows to illustrate the potential of small-scale faulting and fracture lineaments. The well study was focused on interpreting image logs to identify fracture orientation and intensity, and in situ stress conditions. The two sets of results were analysed together to see whether there is overlap in the faults/fractures being identified and to identify whether modern seismic acquisition and interpretation techniques are able to bridge the well-to-seismic scale gap.