Advanced 3D imaging of complex geoelectrical structures using towed streamer EM data over the Mariner field in the North Sea

Towed streamer electromagnetic (EM) data acquired at the Mariner heavy oilfield in the UK sector of the North Sea have been inverted using a fast and efficient 3D anisotropic inversion code. The towed streamer EM system was towed from a single vessel. The system consisted of a horizontal bipole source and electrode sensors housed in a streamer cable. This enabled a densely sampled grid of data over the subsurface volume of interest. The purpose was to estimate the resistivity structure in a volume including the Maureen and Heimdal reservoir structures in the Mariner complex. The 3D inversion algorithm is based on the contraction integral equation method and utilizes a re-weighted regularized conjugate gradient technique to minimize an objective functional (e.g., Zhdanov et al., 2014). A moving sensitivity domain approach is introduced to handle the large amount of data over the large area (Zhdanov, 2010; Zhdanov and Cox, 2012; Zhdanov et al., 2014; Cox and Zhdanov, 2014). This inversion method is proven to be fast and efficient and is here shown to be suitable for towed streamer EM data from complex geological environments such as the Mariner area. In this case, the final 3D resistivity cube after inversion and with a corresponding normalized misfit of 5.4% correlates well with the expected structure from seismic data and well logs. In particular, the 3D inversion was able to extract an anomaly with vertical and horizontal resistivities of 8-10 and 4-5 Ohm-m, respectively, corresponding to the Maureen and Heimdal reservoirs close to the resistive chalk and basement. The run time on a PC cluster was only seven hours for the full 3D inversion with data from all survey lines covering the area of interest.