Seismic Interpretation
Rainforest, Costa Rica.
Seismic interpretation is the foundation of our consultancy. At Lago we take a multi-disciplinary approach to seismic interpretation - using state-of-the-art technology - in all of our projects. Each interpretation project is treated uniquely, and customized to meet the specific technical challenge at hand. Lago staff draw on a wealth of collective experience working basins throughout the world to design workflows suitable for any particular geologic setting or subsurface challenge. The results - maps, cross-sections, and fully integrated earth models – are used for prospect generation, wellbore planning, field development, economic evaluation, reservoir simulation, frac modeling and many other applications.
Each interpretation begins with an assessment of data quality and identification of any artifacts or abnormalities in the data that may compromise the integrity of the interpretation. These problems are mitigated via data conditioning, using interactive processing tools for signal enhancement, filtering, and amplitude scaling. All 3D surveys and/or 2D seismic lines are tied together in an analytical fashion, using line-tie and grid balancing techniques, to ensure that we are using a dataset that is internally consistent in time, phase, amplitude, and frequency.
Synthetic seismograms and vertical seismic profiles are utilized to characterize the phase and frequency content of the inherent seismic wavelet, and integrate well tops and fault picks. For wells without any velocity data, nearby wells can be used to build a local velocity function so that fault picks and formation tops may be visualized properly as overlays on interactive seismic displays, and used as control for the interpretation.
Initial seismic interpretations are performed along proper orientations in order to understand the true genetic nature of structures and the relationships between faults and fault sets. The inline and cross-line orientations of a 3D survey are generally not orthogonal to the longitudinal axis of a geologic structure, nor are they orthogonal major fault trends. Because we seek to visualize true, rather than apparent, structural dips and fault displacements, interpretation must begin on arbitrary lines along geologically correct orientations. These orientations are most easily defined via time slices, dipmeter/image log data, and regional geology. A particularly complex area may require several different arbitrary line directions to satisfy all imaging goals. Horizons and faults are correlated and mapped to develop a detailed image of the subsurface. The interpretation will rely on arbitrary lines, inlines, cross-lines, and time slices, and performed in a disciplined way, along regularly spaced transects. The majority of faults will be named and assigned, so that fault planes can be created and visualized to ensure 3D consistency during interpretation. In short, Lago’s workflow has been developed to allow full integration between geoscientists.
All formation tops and fault picks in wells must be accounted for in the interpretation, and all interpreted horizons and faults must be corroborated in the well correlations. As such, employing software packages with fully integrated databases and applications is a must, as is the inclusion of database managers who rigorously maintain the integrity, security and processing of data and files for every project – regardless of size. Lago uses interpretation systems which enable a thorough geophysical assessment that is integrated with all available geologic and engineering data. Our geophysicists have considerable experience with all of the major seismic interpretation platforms in use within the petroleum industry today.
Once the integrated interpretation work is complete throughout a fine grid through a 3D volume, each horizon is converted into a continuous surface using either linear or smart interpolation schemes – depending on the noise content of the data, and the type of horizon interface. The software used in this work must allow for the presence of fault boundaries, and must stop the interpolation at those boundaries. Again, depending on the noise content of the data and the nature of the horizon at hand, it may also be possible to employ volume interpretation packages for this work.
Every seismic interpretation project requires the performance of velocity analysis and time-depth conversion. The methodology Lago uses depends on the:
· Nature of the Geologic Area
· Velocity of the Formations Above the Zone of Interest
· Nature of the Reflections at the Top, Base and Within the Zone of Interest
· Nature of the Surface
· Density of Well Control.
A layer-cake approach would be used where we encounter spatially variable layers above the zone of interest, or a thick but highly-variable salt section. Depth grids are ultimately created by a stacking or downward continuation approach. Polynomial functions are used in areas characterized by clastic sedimentation, where compaction / depth of burial dictates rock velocity. Average velocity mapping is often used in development projects with high well densities and dense seismic control. Geostatistical approaches may be performed for large seismic survey area that contain few wells. Usually it is justifiable in light of complex geology and little well control to be used in layer-based calculations.
Depth maps and horizons are always re-calibrated to well control via variance mapping to ensure that formation picks are honored. Lago also has proprietary methodologies to ensure consistency with all dipmeter and image log data. Our contention is that if a map agrees with a well pick, then time/velocity values are consistent. Taking integration to a higher level, if a map agrees with dip data, then time/velocity gradients are consistent. Velocity volumes are then built to allow for the conversion of various seismic, inversion, and attribute volumes from time to depth. These are especially useful tools for geosteering horizontal wells in unconventional plays.