Integrating towed streamer EM and 3D data: From anisotropic resistivity to net-to-gross and total hydrocarbon volumes in place

Abstract

Summary The integration of Towed Streamer EM data with 3D seismic data offers unprecedented possibilities to extract quantitative estimates of the most important reservoir characteristics. The workflow is initiated using depth converted 3D seismic to build a sparse horizon model to constrain the inversion of towed streamer EM data. The total volume of the assumed reservoir is also estimated based on the seismic data. The EM inversion results yield, crucially, an estimate of anisotropy, here treated as simply the ratio of vertical (RV) and horizontal (RH) resistivity for the reservoir interval. By introducing estimated values for RV and RH of the shales that are inter-bedded with the reservoir sands, a range of net-to-gross (N/G) values can be defined that satisfies the inverted RV and RH for the entire reservoir. Based on pre-stack inverted seismic data, the N/G value can be further refined, and the porosity of the sands can also be estimated. With the improved knowledge of N/G together with the shale resistivity, it is possible to resolve the RV and RH for the sands, hence also including their effective anisotropy. The sand anisotropy arises as effective anisotropy when the hydrocarbon charged reservoir sands are layered in beds with different grainsizes, resulting in hydrocarbon saturation and resistivity that increases with grain-size. The final results offer estimates of the total reservoir volume, N/G, sandstone porosity, volumetric distribution of sand-beds by grain-size, each with their characteristic hydrocarbon saturation, and total hydrocarbon volumes in