Potential Seismic Interpretation Pitfalls

Seismic data interpretation plays a crucial role in the exploration and production of hydrocarbons in the oil and gas industry. However, there are several pitfalls that interpreters may encounter, which may impact the accuracy and reliability of their interpretations.

Understand your interpretation software abilities and limitations. Most packages today all do the same function, just with different button clicks - Honestly from what I have realized - that the main thing is to understand what the buttons do what - they are different for each but all get you the same end result - a reliable interpretation (hopefully). I personally have, in the past, worked with Petrel (SLB) - Seisworks (Landmark) - SeisX (Paradigm - dating myself now), and am currently learning OpendTech (@dGB Earth Science ), as well as Oasis Montaj (Seequent) for various Interpretation projects that I am currently working on and upskilling myself on. Familiarize yourself with the software before embarking on a large project. Interpretation is the main foundation on which all Oil and Gas projects are built, be it Geological Modeling - Resource Assessments - Structural Analysis - Decision Making etc...

Below I describe some common pitfalls junior (and some senior) interpreters could fall into. These are just the ones that come to the top of my head today, there are many other pitfalls, I just don't describe them here. Add to as you see fit or describe where I am imagining pitfalls....

Complex Geological Settings: Interpreting seismic data becomes challenging in areas with complex geological settings such as salt bodies, unconformities, and fault zones. These features can distort seismic reflections, leading to misinterpretations. Understand the geology before embarking on interpretation.

Incorrect Horizon Picking: Horizon picking involves identifying and marking seismic reflections corresponding to geological features and complex geological setting. Pitfalls in this process include picking noise as horizons, missing subtle but important reflections, and misinterpreting amplitude variations as horizons.

Ambiguity in Amplitude Interpretation: Amplitude variations on seismic data can be indicative of hydrocarbons, but they can also be influenced by other factors like lithology and fluid changes. Misinterpreting amplitude anomalies can lead to false positives or negatives. Watch for polarity reversals.

Over-Interpretation: Over-interpreting seismic data involves inferring complex geological features or structures based on limited data or ambiguous seismic signatures. This can lead to unrealistic geological models and incorrect drilling decisions. Don't interpret something that isn't there.

Resolution Limitations: Seismic data resolution may not be sufficient to accurately identify small-scale features or thin beds or unconformities leading to truncations. This limitation can result in overlooking important geological details.

Noise and Artifacts: Seismic data may contain various types of noise, such as acquisition-related noise, multiples, and processing artifacts. Failure to properly distinguish between signal and noise can lead to misinterpretations. (Work with your processor here is vitally important).

Velocity Model Uncertainties: Incorrect or poorly constrained velocity models can significantly affect the accuracy of depth conversion and the positioning of geological features in the subsurface. Seismic data interpretation heavily relies on accurate velocity models for depth conversion and imaging. Errors in velocity analysis or using an incorrect velocity model can lead to depth conversion errors, which in turn affect structural interpretations. (Working with your processor here is also vitally important as well with this step).

Incorrect Time-to-Depth Conversion: Converting seismic data from time domain to depth domain involves applying velocity information. Errors in this conversion, such as using the wrong velocity model or neglecting velocity variations, can lead to inaccurate depth interpretations.

Inadequate Well Calibration: Integrating well data with seismic data is crucial for accurate interpretation. Inadequate calibration, mismatched depths, or poor well ties can introduce errors in the interpretation. Proper petrophysics analysis is important here.

Data Quality Issues: Poor data quality, including irregularities in acquisition geometry, inadequate coverage, or low signal-to-noise ratio, can compromise the reliability of seismic interpretations. Low-quality seismic data, such as data with high levels of noise, low signal-to-noise ratio, or inadequate coverage, can result in misleading interpretations. It's essential to perform quality control checks and preprocess the data effectively. Older data usually suffers with poor data quality.

Pre-stack vs. Post-stack Interpretation: Misinterpreting seismic attributes in pre-stack or post-stack data can lead to different conclusions. Understanding the appropriate context for interpretation is essential.

Lack of Collaboration: Seismic interpretation is often a collaborative effort involving geophysicists, geologists, and engineers. Lack of collaboration or communication gaps between team members can lead to misinterpretations or missed opportunities to integrate different perspectives.

Over-Reliance on Automation: While automation tools can aid interpretation, over-reliance on them without proper validation or interpreter oversight can lead to errors. Interpreter expertise is still crucial in complex geological settings. Watch for horizon skipping in auto-pick mode.

Lack of Integration: Failing to integrate seismic interpretations with other geological and geophysical data, such as gravity, magnetic, and well data, can result in an incomplete understanding of the subsurface. Look for additional data to help your understanding of your area, and incorporate it all

Assumption of Homogeneity: Assuming geological homogeneity across the entire seismic volume may lead to oversimplified interpretations. Geological complexities and variations should be considered.

To mitigate these pitfalls, interpreters should engage in continuous quality control, collaborate with multidisciplinary teams ( ie your Geologist and Petrophysicists), utilize advanced interpretation techniques, and stay updated on industry best practices. Regular validation against well data and geological knowledge is essential to ensure the accuracy and reliability of seismic interpretations.

When starting seismic data interpretation, beginners often encounter various challenges, and one of the most common errors is the tendency to oversimplify or overlook critical aspects of the interpretation process. Here are some other mistakes made by individuals new to seismic data interpretation:

Failure to Understand Data Acquisition and Processing: Ignoring or not fully grasping the principles behind seismic data acquisition and processing can lead to misinterpretation. It's crucial to have a basic understanding of how seismic data is collected, processed, and migrated. Most companies have a graduated program in their organization for junior geophysicists to move around and learn the various aspects of these to build a foundation for interpretation.

Inadequate Familiarity with Geological Setting: Lack of geological knowledge about the study area can result in misinterpretation. Understanding the regional geology, tectonic history, and depositional environments is essential for accurate seismic interpretation.

Over-Reliance on Software Automation: New interpreters might rely too heavily on automated interpretation tools without understanding the underlying geology. It's important to use automation as a supportive tool rather than a replacement for geological expertise. Don't just "blast" or auto-pick a horizon without the basic understanding of what the automation is doing. Does it pick based on correlation to the next trace etc... Review your Horizon Maps after - look for "Bullseyes", where the auto picker may have jumped horizons and you may have to manually go re-pick it.

Neglecting Data Quality Control: Failing to perform thorough quality control on seismic data, including checking for artifacts, noise, or inconsistencies, can lead to misinterpretation. It's crucial to ensure the reliability of the data before initiating interpretation.

Ignoring the Importance of Velocity Models: Underestimating the significance of a proper velocity model for depth conversion can result in inaccurate subsurface structure positioning. Beginners may not fully grasp the impact of velocity on seismic images. Always get the Velocity model used in the migration algorithm as well as the stacking velocity stacks - check for inconsistencies.

Misinterpreting Seismic Attributes: Misunderstanding the significance and limitations of seismic attributes can lead to incorrect geological interpretations. Beginners may not be aware of the complexities associated with certain attributes and their geological meaning. I will have a small Post on Seismic Attribute Pitfalls coming up in the next week or so.

Skipping Well-to-Seismic Ties: Neglecting the integration of well data with seismic data is a common mistake. Well ties are essential for calibrating seismic interpretations and ensuring consistency between the two datasets. Understanding how the Well to Seismic Tie works for your software package is KEY. You need to know where you are in your section, be it the Oligocene or the Base Tertiary - or Top Cretaceous or Top Jurassic as well as knowing where your "Economic" Basement is - is crucial.

Inappropriate Scale of Interpretation: Failing to interpret seismic data at the appropriate scale can lead to oversights. Beginners may not realize the importance of adjusting the interpretation scale based on the geological features of interest.

Ignoring Rock Physics and Petrophysics: Not considering the rock physics and petrophysical properties of the subsurface can result in inaccurate predictions of lithology, porosity, or fluid content. Beginners may overlook the importance of integrating these aspects into their interpretations.

Underestimating Uncertainty and Ambiguity: New interpreters might struggle with embracing uncertainty and ambiguity in seismic data. It's important to acknowledge the limitations of the data and interpretations and to communicate uncertainties in the final results.

Failure to Update Interpretations: Geological interpretations should be updated and refined as new data becomes available or as drilling results are obtained. Failure to update interpretations can lead to outdated models that do not reflect the current understanding of the subsurface.

To overcome these common errors, individuals starting seismic data interpretation should invest time in acquiring a solid foundation in geophysics, geology, and the specific methods used in seismic data processing. By being aware of these pitfalls and employing rigorous quality control measures, cross-validation techniques, and collaboration among multidisciplinary teams, seismic interpreters can mitigate errors and enhance the reliability of their interpretations.

ONE IMPORTANT NOTE:

When it comes to seismic interpretation in the oil and gas industry, both SEG (Society of Exploration Geophysicists) Normal and SEG Europe conventions play crucial roles. However, there are some pitfalls associated with each convention that interpreters should be aware of.

SEG Normal Convention Pitfalls:

Reflection Sign Convention: One of the primary differences between SEG Normal and SEG Europe conventions is the reflection sign convention. SEG Normal convention assumes that velocity increases with depth, leading to upward reflections indicating positive polarity. This can be a pitfall when interpreting seismic data acquired in areas with anomalous velocity structures or in regions with significant lateral velocity variations.

Velocity Model Assumptions: SEG Normal convention assumes a simple velocity model where velocity increases with depth. In reality, geological structures can lead to complex velocity variations, causing misinterpretations if not properly accounted for.

Wavelet Considerations: Wavelets used in seismic data processing may not always match the assumptions of the SEG Normal convention. This can lead to challenges in properly correlating seismic events and identifying geological features accurately.

SEG Europe Convention Pitfalls:

Reflection Sign Convention: SEG Europe convention assumes that velocity decreases with depth, resulting in downward reflections indicating positive polarity. This can be a pitfall when integrating data processed with SEG Normal convention, leading to polarity inconsistencies.

(NB: The SEG (Society of Exploration Geophysicists) Europe convention does not explicitly assume that velocity decreases with depth. Instead, it follows a reflection sign convention where downward reflections are considered positive polarity. This convention is commonly used in seismic data processing and interpretation within the European geophysical community. The interpretation of downward reflections as positive polarity is a convention that simplifies seismic data analysis and visualization. It does not necessarily imply a specific velocity-depth relationship such as velocity decreasing with depth. Velocity-depth relationships are determined separately through velocity modeling and analysis, which are essential steps in seismic data processing and imaging.)

Integration Challenges: When integrating data from different sources or processed using different conventions (such as SEG Normal and SEG Europe), interpreters may face challenges in aligning seismic events and maintaining consistent interpretations, especially in areas with complex geological structures.

Cross-Training Issues: Geoscientists and interpreters trained predominantly in one convention may find it challenging to switch between SEG Normal and SEG Europe conventions seamlessly, potentially leading to interpretation errors and inconsistencies.

To mitigate these particular pitfalls, interpreters should:

• Understand the underlying assumptions and conventions of both SEG Normal and SEG Europe.
• Account for geological complexities and velocity variations specific to the study area.
• Use advanced seismic processing and interpretation techniques to reconcile data from different sources or processed using different conventions.
• Collaborate effectively with multidisciplinary teams comprising geophysicists, geologists, and engineers to validate interpretations and mitigate potential errors.

Overall, while both SEG Normal and SEG Europe conventions offer valuable frameworks for seismic interpretation, interpreters must be aware of their respective pitfalls and take necessary precautions to ensure accurate and reliable geological interpretations.

Pitfalls in 3-D Interpretation 2005 - Alistair Brown (Did a course from Alistair back in 2002 or 2003 - helped me immensely) https://csegrecorder.com/articles/view/pitfalls-in-3-d-interpretation

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