Prospect ranking using multiphysics analysis of towed streamer CSEM data
Characterisation of sub-surface reservoirs can be challenging when only a single geophysical data type is considered. For example, seismic provides a sub-surface structural framework and, from AVO information, the possibility to derive P- and S-wave impedance volumes. These two valuable, independent measurements can be linked to porosity and lithology. However, the seismic data alone cannot distinguish between a commercial hydrocarbon saturation and a sub-commercial or residual saturation due to their similar AVO responses. This leaves a significant commercial ambiguity when ranking prospects.
In recent years the importance of combining diverse geophysical datasets in multiphysics workflows has become clear. Controlled Source ElectroMagnetic (CSEM) methods, which measure the electrical resistivity of the seafloor, are used to complement other geophysical approaches across a range of applications.
The towed streamer EM system uses a high powered horizontal electric dipole source towed behind a survey vessel to transmit EM fields through the seawater column and into the seafloor. The resulting signals are recorded by an array of electric fields receivers, also towed behind the survey vessel. This configuration provides a fast and efficient way of acquiring high resolution CSEM data at significantly lower cost and with lower environmental impact than other EM methods, paving the way for large area surveys in parallel with seismic programs.
Multiphysics workflows which combine electromagnetic data with other sources of information such as seismic, well logs or gravity/magnetics, can then lead to improved imaging and more robust characterisation of rock and fluid properties of the sub-surface. In the upstream oil and gas industry, approaches that integrate seismic and electromagnetic data have been used successfully to improve imaging in areas of complex geology (for example around basalt or salt), leading to greater understanding of structure and stratigraphy. Similarly, rock physics driven workflows combining pre-stack seismic and controlled source electromagnetic data have been used to distinguish commercial from sub-commercial hydrocarbon accumulations, something that is notoriously challenging using seismic alone, leading to better exploration decision making.
In this example seismic and towed streamer EM data were acquired simultaneously over an area in the Northern Barents Sea. Co-rendered resistivity and seismic data (upper panel) shows areas of higher resistivity (yellow-red colours in the seismically guided inversion shown) that could be indicative of significant hydrocarbon saturation.
Multiphysics analysis of these two datasets (lower panel) has been used to map saturation in the reservoir directly. In this case a rock physics driven petrophysical joint inversion approach was used, in which seismically derived porosity, lithology and litho-fluid facies were combined with CSEM derived resistivity information. This approach clearly distinguishes the commercial oil accumulations in the Western and central prospect, from the residual (non-commercial) saturation in the Eastern prospect, something that is not possible using seismic data alone.
Alvarez, P., Alvarez, A., MacGregor, L., Bolivar, F., Keirstead, R. & Martin, T., 2017. Reservoir properties prediction integrating controlled source electromagnetic, pre-stack seismic and well log data using a rock physics framework: Case study in the Hoop Area, Barents Sea, Norway, Interpretation,5 (2), SE43-SE60