This is a condensed version of an article published in First Break in May 2009:
3D CSEM inversion in a frontier basin, offshore West Greenland
Overview
The high risk of exploring in difficult environments with complex geology is substantially mitigated when data from non-seismic methods, such as electromagnetic, gravity and magnetic surveys, are integrated with the seismic interpretation. This project has shown significant progress towards high-grading a prospect portfolio for drilling locations by using marine controlled-source electromagnetic (CSEM) inversions constrained by seismic interpretations. This enhances the explorationist’s understanding of the target geology and assessment of pre-drill risk.
Area Prospectivity
In 2008, a joint venture (JV) led by the EnCana Corporation with partners Cairn and Nunaoil needed to develop solutions for ranking prospects in the offshore West of Greenland area (Fig. 1).
The JV had mapped a series of exploration prospects on two blocks in 350-1,800 meters of water about 120-200 km west of Nuuk, Greenland’s capital. Faced with the challenge of selecting the best drilling candidates on the two blocks with only mapped prospect closures and the nearest well control 120 km away, WesternGeco Electromagnetics was commissioned to conduct the CSEM surveys over the prospects in the Lady Franklin and Atammik blocks.
An integrated approach was followed where structural models, created from seismic interpretations and well log data were used as a starting point for the 3D CSEM inversions, to identify prospects with a resistive response. The results of this process helped EnCana and its partners to high-grade their prospect portfolio and increase the hydrocarbon charge confidence in the drilling candidates in one of the world’s most challenging wildcat frontiers.
CSEM Acquisition and Processing
Extensive 3D modeling was conducted over each prospect before conducting the survey to resolve questions of the prospectivity of Cretaceous reservoirs at up to 3,000 meters below the seafloor. Forward modeling and inversion of synthetic data were used to optimize the design for issues such as target sensitivity, volcanic cover, reservoir proximity to basement, signal waveform and the need to survey a vast area in a cost-efficient way. The resulting survey design consisted of 24 transmitter lines and 182 receivers, with towlines located such that they would generate data from multiple angles on the receivers. The CSEM electric and magnetic field measurements were processed to obtain amplitude and phase responses as a function of transmitter offset at each receiver location.
The 3D CSEM inversion process models the Earth response using Maxwell’s equations and iteratively modifies the starting model to yield a final modeled electrical and magnetic field response that best fit the field observations. Using EnCana’s seismic interpretations and well log data, 3D anisotropic electrical models were created as a starting point for the 3D inversion. These starting models did not include resistive bodies at the potential reservoirs. The resulting 3D inversions were numerically stable and yielded electrical models which are geologically consistent.
Working closely with the WesternGeco Electromagnetics team, EnCana produced inversion volumes from the EM datasets coincident with their seismic volumes for each prospect. To aid in the visualization of the data and integrated interpretation, each prospect was co-rendered using Petrel software (Fig. 2). Those showing high resistivity at the target level were tested to validate their inversion results. Effects of reasonable geologic variations to the starting model were tested by subsequent inversions. Speed of inversion solution convergence, repeatability of the final resistivity models, calculated sensitivity of the data to features of interest and consistency with seismic interpretations were criteria used to establish confidence in the inversion results and the interpretations’ validity.
Ranking the prospects
The 3D CSEM inversions clearly delineated resistive bodies on 8 of the 14 prospects surveyed in the two West Greenland blocks. Examples from one of the blocks that show a highly resistive response for 4 out of 8 prospects are shown in Fig. 3. Confidence in the resulting Earth models was buoyed when known Paleocene volcanics that were excluded from the starting models were imaged by the inversions. Further confidence was gained where the geometries of the resistivity anomalies produced by inversion coincided with structural closure.
One prospect yielded CSEM inversion results that showed all of the characteristics consistent with an interpretation of an oil-filled reservoir. Prior to the CSEM interpretation, the probability that this prospect was hydrocarbon charged was not supported by any data and assigned a nominal 50% probability of hydrocarbon charge. Post CSEM inversion, the estimated charge probability increased considerably.
Another prospect was given the same 50% probability of hydrocarbon charge prior to integrated analysis. The CSEM results, however, showed no sign of a resistive anomaly at the target depth. Based on the integrated interpretation, this target’s charge probability decreased.