Application of synthetic aperture concepts to Towed Streamer EM data.

Abstract

In October 2012 a series of known oil & gas fields in the North Sea were surveyed with the newly developed Towed EM System. This is the first commercial Controlled Source EM (CSEM) system where both source and receivers are towed in a similar fashion as 2D seismic, and the technology is indeed also combinable with 2D seismic facilitating simultaneous acquisition from one vessel. One of the selected targets was an average size producing oil & gas field located 2,100 m below mudline, where approximately half of the recoverable oil has been produced, but with the gas cap still intact. The hydrocarbon column is approximately 52 m, with a 25 m thick gas cap overlying a 28 m oil column. The strength of the source was 1.2 MAm and it was towed at 10 m. The EM streamer was 8,700 m long and towed at a depth of 50 m. The resulting electric field was measured at 23 offsets ranging from 500 to 7,500 m. The towing speed was 4 kn, and the water depth was 110 - 125 m in the survey area. An anomaly of 8 % above background was detected, and with the newly introduced concept of synthetic aperture processing, the sensitivity can be strongly enhanced, and the bathymetry effects can also be more effectively suppressed. Additional strengths of the system include the ability to estimate vertical and horizontal resistivity of the rock volumes surrounding the reservoir. This allows us to build quite accurate resistivity & anisotropy models for the background, which is very important information to narrowly constrain the inversion of the EM data. This results in quite accurate quantitative estimates of the transverse resistance for the charged reservoir. Inversion and 3D modeling reveal the anomaly is consistent with the target depth.