Geology & Exploration

Coherence (seismic)

Coherence: Unveiling the Secrets of the Subsurface in Oil & Gas Exploration

Coherence, in the context of oil & gas exploration, is a powerful seismic attribute that acts like a detective, helping us understand the complex structure and features of the earth's subsurface. It's a crucial tool for identifying potential hydrocarbon traps and optimizing drilling decisions.

What is Coherence?

Imagine seismic data as a series of images of the subsurface created by sending sound waves into the earth and recording their echoes. Coherence, simply put, measures the similarity of these echoes. Areas with high coherence indicate smooth, continuous geological features like fault planes, layers of rock, or even buried channels. Conversely, areas with low coherence or incoherence point towards abrupt changes, fractures, or chaotic zones.

How Coherence Works:

The concept behind coherence is rooted in cross-correlation, a mathematical process that compares the similarity between two sets of seismic data. By applying this comparison across various seismic traces, coherence analysis helps identify:

  • Faults: Discontinuities in the earth's structure, often representing pathways for hydrocarbons to migrate.
  • Channels: Ancient riverbeds or other erosional features, which can act as potential reservoirs.
  • Salt Diapirs: Dome-like structures of salt, which can trap hydrocarbons.
  • Sandstone Bodies: Sedimentary layers that are often good reservoir rocks.

Coherence: A Versatile Tool:

The beauty of coherence lies in its versatility. It can be applied to different types of seismic data, including 2D and 3D surveys, and various processing methods. This allows geophysicists to:

  • Visualize complex structures: Coherence maps provide a clearer representation of subsurface features compared to conventional seismic data.
  • Identify subtle features: Even small changes in geological structure can be highlighted through coherence analysis.
  • Improve seismic interpretation: By combining coherence with other seismic attributes, geophysicists gain a more comprehensive understanding of the subsurface.

Coherence: A key for better decision making:

Using coherence, oil & gas companies can:

  • Reduce exploration risk: Identifying potential traps with higher accuracy.
  • Optimize well placement: Targeting drilling locations in areas with high reservoir potential.
  • Improve production efficiency: Understanding reservoir geometry for better production strategies.

Incoherence: The Other Side of the Coin:

While coherence emphasizes similarity, incoherence highlights dissimilarity. It is often associated with:

  • Fractures: Breaks in the rock, which can enhance permeability and allow hydrocarbons to flow.
  • Fault zones: Areas of tectonic activity, often associated with complex geological structures.
  • Unconventional reservoirs: Highly fractured or porous rocks that can hold hydrocarbons but are more challenging to extract.

Coherence and Incoherence: Complementary tools:

Coherence and incoherence are not opposing concepts but rather complementary tools. Together, they provide a holistic view of the subsurface, guiding exploration and production decisions.

Conclusion:

Coherence has revolutionized seismic interpretation, offering a powerful and versatile method for understanding the earth's subsurface. By revealing subtle geological features and highlighting potential hydrocarbon traps, coherence empowers oil & gas companies to make smarter and more informed decisions, maximizing their chances of success in exploration and production.


Test Your Knowledge

Coherence in Oil & Gas Exploration Quiz

Instructions: Choose the best answer for each question.

1. What is coherence in the context of oil & gas exploration?

a) A type of seismic data acquisition technique. b) A measure of the similarity between seismic echoes. c) A geological formation that traps hydrocarbons. d) A method for predicting oil prices.

Answer

b) A measure of the similarity between seismic echoes.

2. What does high coherence indicate in seismic data?

a) Fractures and faults. b) Chaotic geological zones. c) Smooth and continuous geological features. d) Areas with low reservoir potential.

Answer

c) Smooth and continuous geological features.

3. Which of the following is NOT a typical application of coherence analysis?

a) Identifying faults. b) Mapping channels. c) Detecting volcanic activity. d) Analyzing salt diapirs.

Answer

c) Detecting volcanic activity.

4. How can coherence help improve oil & gas exploration?

a) By providing a clearer understanding of the subsurface. b) By reducing exploration costs. c) By predicting the future price of oil. d) By eliminating the need for drilling.

Answer

a) By providing a clearer understanding of the subsurface.

5. What does incoherence indicate in seismic data?

a) Areas with high reservoir potential. b) Smooth and continuous geological features. c) Discontinuities and abrupt changes in the subsurface. d) The absence of seismic data.

Answer

c) Discontinuities and abrupt changes in the subsurface.

Coherence in Oil & Gas Exploration Exercise

Scenario:

You are a geophysicist working for an oil & gas exploration company. Your team has acquired 3D seismic data from a potential exploration area. After analyzing the data, you notice a distinct zone with high coherence that appears to be a channel feature.

Task:

  1. Explain how the high coherence zone in the channel feature could be a potential target for oil and gas exploration.
  2. Discuss potential risks associated with exploring this channel feature.
  3. Suggest additional data or analysis that could be used to further evaluate the potential of this channel feature.

Exercice Correction

**1. Potential Target for Oil & Gas Exploration:** * **Reservoir Potential:** Channels often represent ancient riverbeds or other erosional features, which can be filled with porous and permeable sediment, creating favorable reservoir conditions. * **Trap Formation:** The channel feature, especially if it is buried and sealed by overlying layers, could act as a trap for hydrocarbons. The high coherence suggests a well-defined structure that could hold trapped hydrocarbons. * **Migration Pathways:** Channels can serve as migration pathways for hydrocarbons from source rocks to potential traps. The high coherence could indicate a channel that has been effective in transporting hydrocarbons. **2. Potential Risks:** * **Uncertainty in Channel Geometry:** While high coherence suggests a well-defined channel, there could be complexities in its geometry, such as changes in thickness or unexpected bends, which might affect the hydrocarbon accumulation. * **Seal Integrity:** The presence of a seal, such as a layer of impermeable rock, is crucial to trap hydrocarbons. Additional analysis is needed to confirm the seal's presence and integrity. * **Hydrocarbon Presence:** High coherence indicates a structural feature, but it does not guarantee the presence of hydrocarbons. Further exploration and analysis are required to confirm hydrocarbon presence and quality. **3. Additional Data & Analysis:** * **Seismic Attributes:** Combine coherence with other seismic attributes like amplitude, impedance, and inversion to obtain a more comprehensive understanding of the channel feature. * **Well Logs:** If available, analyze well logs from nearby wells to gain insight into the lithology, porosity, and permeability of similar channels. * **Geological Modeling:** Construct a geological model of the channel feature based on seismic and other data to better understand its geometry, reservoir properties, and potential hydrocarbon accumulation.


Books

  • Seismic Attributes for Exploration and Reservoir Analysis by Alan F. Gassmann, Robert S. Smith and John D. O'Brien (2018) - This book covers a wide range of seismic attributes including coherence, providing detailed explanations and practical applications in hydrocarbon exploration.
  • Understanding Seismic Attributes by Martin Landrø and Lasse Amundsen (2010) - Offers a comprehensive overview of seismic attributes, with specific chapters dedicated to coherence and its application in reservoir characterization.
  • Interpretation of Three-Dimensional Seismic Data by John C. Bancroft (2007) - A classic text that explores various aspects of seismic data interpretation, including coherence analysis and its role in identifying structural features.

Articles

  • Seismic Attributes for Reservoir Characterization: A Review by J.D. O'Brien and S.L. Hart (2003) - This article offers a comprehensive review of seismic attributes, including coherence, highlighting their use in reservoir characterization and production optimization.
  • Coherence Cube Interpretation in the Deep Water Gulf of Mexico by R.F. Sigismondi and S.C. Marfurt (2005) - This article focuses on the application of coherence analysis in deep water exploration, demonstrating its effectiveness in identifying subtle structures.
  • Applications of Coherence in Seismic Interpretation by J.F. Claerbout (2013) - This article provides an insightful overview of the history, theory, and practical applications of coherence analysis in seismic interpretation.

Online Resources

  • SEG Wiki - Coherence - https://wiki.seg.org/wiki/Coherence - Provides a concise definition and explanation of seismic coherence, along with links to related resources and publications.
  • Search "Seismic Coherence" on the Society of Exploration Geophysicists (SEG) website - https://www.seg.org - This website offers a vast collection of articles, presentations, and research related to seismic interpretation and coherence analysis.
  • "The Impact of Coherence on Exploration & Production" - Schlumberger - https://www.slb.com - Schlumberger's website features articles and case studies highlighting the role of coherence in optimizing exploration and production operations.

Search Tips

  • "Seismic coherence" + "oil and gas" - This search will provide results related to the application of coherence specifically within the oil and gas industry.
  • "Coherence attribute" + "interpretation" - This search will focus on how coherence is used for interpreting seismic data and identifying subsurface features.
  • "Coherence analysis" + "case studies" - This search will lead to case studies demonstrating the practical implementation of coherence analysis in various geological settings.

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