Wed, 01/20/2016 - 11:21

Measuring wellbore resistivity has been a fundamental formation evaluation tool since the 1920s. Marine Controlled Source Electromagnetic (CSEM) surveying uses the same principles to illuminate resistive bodies, such as commercial-scale hydrocarbon reservoirs, from the seabed. When integrated with other data, 3D CSEM data helps distinguish prospects with high hydrocarbon saturation from prospects with low hydrocarbon saturation. When a CSEM survey is performed, a horizontal electric dipole (HED) source is towed above the seafloor. The dipole source emits a continuous, time-dependent EM signal that yields a specific frequency spectrum. The resulting EM fields are measured by receivers that are deployed on the seafloor. The receiver grid has a 3D configuration, which enables recording data from all azimuths. The water depth of operation for a CSEM survey ranges from 20-3,500m. The depth of penetration into the subsurface is between 0-3,500m, depending on the electrical properties of the overburden.

3D CSEM inversion is EMGS’s standard imaging product for CSEM data. The firm’s 3D CSEM inversion has been successfully used to image formation resistivities with CSEM data acquired in a variety of geological settings such as shallow water blocks, continental slopes, deepwater blocks, and salt basins, as well as for a range of E&P applications. The goal of CSEM inversion is to estimate a subsurface resistivity volume that explains the acquired CSEM data within the measurement accuracy and which is geologically meaningful.

To conduct a 3D CSEM inversion, EMGS creates an initial 3D resistivity model. This model is a simplified representation of the subsurface, often called a temperature-compaction model, typified by resistivity that gradually increases with depth. Using all the navigation parameters of the 3D CSEM survey, forward modeling is performed over the input 3D resistivity volume to generate a synthetic dataset, which is quantitatively compared to the acquired data. If the mismatch between the synthetic data and the observed data is larger than a given threshold (derived from the measurement uncertainty), the 3D resistivity model will be updated based on the mismatch. This process iterates until the synthetic data matches the observed data within the specified misfit threshold. The output of 3D CSEM inversion is a 3D resistivity volume, which can be loaded in any interpretation software environment, and thus can be correlated and integrated with other available geophysical information.

3D CSEM inversion provides a subsurface 3D resistivity distribution that delineates prospects and reservoirs, and places them accurately both laterally and in depth. The incorporation of full-azimuth data acquired in 3D grids enhances the spatial resolution and accuracy of the final 3D resistivity volume. Ultimately, when clients integrate the provided resistivity volumes with seismic, well log, and other data, they are able to upgrade their Probability of Success (PoS) and Volume in Place assessments. These improvements are often possible when CSEM data is part of the prospect evaluation workflow, whether integrated into a customer’s current practice or by using the tools offered by EMGS. By using CSEM data in combination with other subsurface measurements, users can increase their success in exploration, de-risk and polarize their prospect portfolio, and optimize their development programs.