Useful characteristics of shallow and deep marine CSEM responses inferred from 3D finite-difference modeling

摘要

Hydrocarbon reservoirs can be mapped if sufficient resistivitycontrasts exist between them and their confining layers, but prac-tical problems remain in target discrimination in deep and shal-low waters, especially in the presence of heterogeneous overbur-den. We have developed an efficient 3D staggered-grid finite-dif-ference controlled-source electromagnetic (CSEM) modelingcode that enables study of the physics underlying some practicalproblems. We undertook a comparative analysis of reservoir de-tection in 2000-m- and 300-m-deep waters using the simulatedelectric and magnetic field responses of a simple 3D reservoir.We examined the effect of two types of near-surface heterogene-ity (mimicking disconnected gas clouds and/or patchy geochem-ical alteration halos) on the 3D reservoir response. We found thatsmall-scale, shallow heterogeneities cause distortions that are al-most independent of the source frequency. These persist at allsource-receiver offsets in the electric amplitude response indeep and shallow waters and phase response in shallow water.They decrease in magnitude with increasing offset in deepwaterphase response. Large-scale near-surface heterogeneities distortthe horizontal electric field response more significantly than thesmall-scale ones, but the near-surface response gets smaller inamplitude as the offset increases. The distortions in shallow wa-ter are much smaller in magnitude than those for the deepwatercase, so that the reservoir signatures still are visible on the re-sponse profiles. This might be considered as a positive feature forshallow-water inline electric field exploration. The magneticfield responses for the orthogonal direction provide diagnostictarget signatures that are similar to the inline electric field re-sponses in deep water but that are different in shallow water. Themagnetic responses are affected by the airwave in a differentmanner from the electric field, suggesting that combined 3Delectric and magnetic field analysis might be vital for handlingthe airwave problem.