Three-Dimensional Computer Modeling of Realistic Marine CSEM Earth Models in the Flemish Pass Basin

Abstract

The Flemish Pass Basin located offshore Newfoundland and Labrador has seen a significant increase in exploration activity over the past decade and the prospectivity in this region continues to grow. The first well drilled in the Mizzen area, L-11, hit minor oil in one of three Tithonian aged sands, with underlying sands encountering brine. Recent seismic acquisition over the prospect shows AVO signatures in all three Tithonian aged sands up-dip from where the L-11 well was drilled. The purpose of this study is to improve the prospect knowledge at Mizzen L-11 and support, or refute the hydrocarbon potential hypothesis shown by the AVO signatures. Assessing the potential in these sands would typically be done using fluid substitution techniques, but this study uses a marine controlled-source electromagnetics (CSEM) forward modeling approach with a comparison to recently acquired CSEM data in the region. Industry often uses finite-difference algorithms on rectilinear meshes to simulate CSEM responses and this is not optimal or accurate for complex scenarios (i.e. curved surfaces and topography). However, a finite-element algorithm considered by this study allows earth models to be represented more realistically through the flexibility of tetrahedral meshes. The type of earth model required for CSEM forward modeling is a resistivity model and the most logical method was to use seismic data and well logs from the Flemish Pass Basin. Surfaces separating geochronologic intervals (i.e. base Tertiary, Cretaceous, etc.) were derived from the seismic data and these surfaces delineate the subsurface structure. The well logs were used to assign a resistivity value for each of the geochronologic intervals determined by the seismic. This builds the skeleton of the model and it becomes complete when the Tithonian reservoir sands are included. The resistivity model is then filled with tetrahedral cells and transformed into a tetrahedral mesh. The finite-element code is used at this stage to compute what the marine CSEM response would be based on the tetrahedralized resistivity model. Since measured data is available, this allows for a basis of comparison. Forward data can be simulated with Tithonian sands up-dip in structure containing hydrocarbons or brine. Whichever case best matches the measured data will likely be the most plausible scenario. This study will not only improve our knowledge of the CSEM forward modeling science, but also in using CSEM to de-risk reservoirs in an offshore exploration setting.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016