mCSEM data interpretation for hydrocarbon exploration: A fast interpretation workflow for drilling decision

Abstract

In a hydrocarbon exploration workflow, marine controlled-source electromagnetic (mCSEM) data are usually acquired after seismic interpretation for prospect identification and close-to-the-drilling decision making. Therefore, the mCSEM interpreter must provide quick answers to the asset teams in a way that the EM interpretation can add value to that decision. To achieve that goal, Petrobras developed a fast-track mCSEM interpretation workflow that consists in identifying anomalies in the mCSEM data set by frequency normalization, and then performing 1D CMP inversions followed by 2.5D polygonal inversions. The proposed workflow was successfully applied to several mCSEM surveys offshore Brazil. We evaluated an application in a complex geologic setting where the reservoir dips toward allochthonous salt. The reservoir appears as a flat spot in the seismic section, but with no significant amplitude variation with offset response. The mCSEM analysis confirmed the seismic anomaly and extended it northward. Two drilled wells corroborated the mCSEM interpretation.

Delineation of Anisotropic Layers Through 1d Inversion of Marine Csem Data

“…This error can be reduced by constraining the responses using a priori information, for example from a well log, as the one exhibited in Figures 9 and 10. As the study shown here doesn't include any such a priori information, the results reproduce the common behavior, and the maximum resistivity in the resulting models is located at about 500 m above the peak values found in Buonora et al (2014).…”

“…2-Above 4000 m, the curves show a strong indication of a resistive zone, approximately in the correct depth as indicated by Buonora et al (2014). Whereas in the previous results this resistive regions can be interpreted as being composed of two distinct zones, in our results only the horizontal resistivity curve of the global smoothness inversion shows any indication of such division.…”

“…The results are illustrated in Figures 9 and 10, overlaid on a section obtained from the inversion method described by Buonora et al (2014), which doesn't take the anisotropy of the medium into account. Two distinct behaviors can be observed in the results, in two depth zones: 1-At depths greater than about 4000 m the horizontal resistivities appear higher than the vertical resistivities.…”

“…The data are part of a large multiclient acquisition campaign covering several Brazilian offshore basins. The particular set to which we have applied the 1D inversion has already been successfully used by PETROBRAS in an interpretation workflow (Buonora et al, 2014).…”

“…This line crosses a successful well (referred to as W1), which intercepted a 100 m thick hydrocarbon layer at a 3720 m depth, with an average resistivity of 70 ohm-m. The line is laid between two salt structures which appear prominently in the seismic section, as shown in Buonora et al (2014). We chose to work with data from the receiver closest to the well, located approximately at 1.5 km from the flank of the southeast salt body.…”