Broadband Benefits for Interpreters
Broadband seismic data boasts of enhanced resolution and sparkling detail, but what benefits does it offer the interpreter and is the extra effort in recording and analyzing the data justified?
Seismic interpretation is a fundamental step in the hydrocarbon exploration and production workflows. As our focus is increasingly on finding and developing subtle traps, we need higher quality data that enables precision mapping and seismic characterization of the subsurface. Having a seismic dataset that is relevant throughout the entire process creates an efficient and effective workflow, making it an ideal solution for the exploration and production life cycle.
3 Key Benefits of Broadband
- Broadband data provides a more precise foundation for exploration and production. The dataset is relevant for regional exploration, prospect hunting, near-field exploration and targeted field-scale interpretation for field development, and also for 4D surveying
- The wide frequency-spectrum results in a high-fidelity dataset that has advantages for structural and stratigraphic interpretation, mapping depositional models, analyzing seismic facies and reservoir characterization
- The stable seismic character inherent in the data enables high levels of confidence in the automation of more routine mapping freeing time for interpretation and analysis
Broadband data looks different because it is different. The geology is displayed in unprecedented detail and realism, with fewer geophysical artifacts due to the removal of the ghosting reflections and the reduction of unwanted wavelet sidelobes that created false events. The added value goes beyond the uplift in resolution and depth penetration achieved by broadband data over conventional data that is well documented and clearly shown in the image below from Aasta Hansteen in the Norwegian Sea.
Although the basic interpretation techniques remain the same, the increased amount of information captured within broadband data means that it may not always seem easier to interpret, but the final result will be more accurate and more detailed. Whether an interpreter is performing a regional structural interpretation, identifying subtle stratigraphic traps or producing geological models, for example, broadband seismic data can enhance their insight. Although they are looking for different clues in each of these scenarios, higher resolution, increased seismic signal content and reduction of unwanted noise will always be of benefit.
Traditional interpretation methods combine manual picking by the interpreter and picking based on auto-tracking algorithms. Both methods rely on data quality, continuity and similarity of the seismic character-wavelet to track a particular event. The horizon shown below was automatically picked from one manually selected seed point within a shallow zone of polygonal faulting. The consistency in the reflectors given by the stable and precise broadband data wavelets enables the interpreter to have confidence in the auto-picker to track horizons avoiding misties and cycle skips and correlating across faults. This speeds up the picking process, enabling the interpreter to focus on extracting further information and understanding from the data.
Good structural interpretations depend on trackable events with consistent seismic character. One of the key aspects of broadband data is that the wavelet is a sharp feature (due to the additional high-frequency content) with reduced side-lobe artifacts (due to the richer low frequencies) resulting in the consistent seismic character of the reflectors across the section. This is clear to see around the large scale fault blocks in the center of the Aasta Hansteen seismic example, where the horizons are truncated cleanly and the fault plane itself is imaged in parts. The consistency of the seismic character of the reflectors along the section is necessary for the interpreter to accurately read the section and extract an understanding of the structural history. For example, three closely spaced parallel reflectors are highlighted in the example. An interpreter can trace these easily across the large scale fault planes and have good confidence in their decision. This provides vital information on the timing of the structural development of the region which may have an impact on the prospectivity of the area. There is particular value to this in frontier areas where there may be little well-control. Having high confidence in the regional interpretation can be key to identifying new hydrocarbon provinces.
‘Feel’ the Detail
When assessing rocks in an outcrop, the geologist relies on erosion patterns generated by rocks of different competencies to assist with the overall interpretation and understanding of the different lithologies. The interpreter is able to apply the same visual assessment when working with broadband seismic data. The seismic line is a representation of the subsurface, primarily used to identify where there maybe hydrocarbon potential. The broadband seismic line provides a better approximation of the subsurface. The richer low frequencies acquired by GeoStreamer technology and inherent in broadband data give the seismic line a visual texture, where the line appears to have a 3D shaded relief.
Depending on the maturity of the basin, prospect hunting goes beyond identifying large structural closures. Even in qualitative interpretation, the interpreter utilizes amplitude variations and the overall seismic response to help identify more subtle leads. Broadband texture gives the data a realism that helps to draw the eye to initial areas of interest and to increase confidence in the decision-making process relating to identifying prospectivity. It enables the interpreter to quickly identify hard and soft packages. For example, the polygonal faulting layer shown in the image from the North Sea. Here the well clearly shows a sandy section that correlates with a homogenous package with a white (peak) top and black (trough) base that appears to stand proud of the page. It is reasonable to assume that other features with the same character may also be sandy. At this level of interpretation, it is only possible to predict this sand occurrence qualitatively, but it guides the interpreter to areas of interest for further quantitative interpretation or prospectivity studies.
Combining this texture with other broadband reflector properties, such as seismic amplitude, frequency, geometry, and continuity, results in more reliable seismic facies characterization. The broadband detail of reflector terminations is key for stratigraphic interpretation of pinch-outs, defining channel edges, and resolving on-lapping surfaces, all of which enable the seismic facies and depositional environments to be more accurately determined.
Correlation of the seismic line with known lithologies and its correct placement in space crucially depends on a good well tie. Well ties provide the principal means of relating the seismic waves to the stratigraphy and rock properties of the subsurface. The shape, consistency and frequency content of the broadband seismic wavelet enables a good well correlation and enhances the reliability of full stack and prestack data. In addition, a general reduction of geophysical artifacts in broadband datasets enables the interpreter to more confidently assign the reflector to the well top.
Read about: Optimizing Well Ties Using Broadband Data
Many subsurface features can mask the areas of interest, these are often directly related to the prospectivity of the area. Complexities, such as salt bodies, shallow gas clouds and ooze bodies can all degrade the seismic data quality. Modern processing techniques developed to utilize the full bandwidth and wavefield acquired with GeoStreamer data have enabled high-quality imaging beneath these complexities. This, in turn, may enable long-hypothesized petroleum plays to be opened up, enhancing the prospectivity of overlooked areas. The figure below is an example of this, where the sediments beneath a thick salt package are imaged with excellent definition, enabling the sub-salt prospectivity potential to be properly defined.
Seismic Data You Can Drill On
The superior imaging in the shallow of some GeoStreamer datasets has been shown to be suitable for preliminary site surveys and shallow hazards determination.
The seismic image shown below is a section located in deep water, off the west coast of Africa. Excellent reflectivity is seen at every level, with a clear definition of thin reflectors in the shallow, crisp imaging of the polygonal faults and clear imaging of the salt diapirs, even along the steep salt walls and intervening mini-basins, including their internal geometry. The seismic anomalies that stand proud of the screen in the shallow are interpreted to be shallow gas bodies, which are likely to be related to the salt diapirs. The precise imaging of seismic character available on broadband data enables a robust interpretation to be made even in areas where there are high levels of geological complexity with many shallow anomalies.
Broadband datasets can be also useful in field development and production. The level of detail displayed makes the broadband dataset an ideal candidate for a baseline survey for 4D seismic monitoring. The local variations within subsequent seismic surveys can potentially represent fluid movement between wells.
The Bottom Line
Broadband data offers the interpreter the chance to interrogate a dataset that closely represents the subsurface, with more detailed information about local facies variations and reservoir compartmentalization. Ghost and sidelobe removal result in fewer false reflectors and modern processing techniques mitigate or minimize the seismic artifacts. It is this potential of more precise and reliable subsurface information and the impact it can have on hydrocarbon exploration and exploitation decisions that present the true value of any investment in modern broadband seismic data. This is especially relevant in these cost-conscious times.