10.1 Petroleum geophysics and seismic exploration
8 min read•august 14, 2024
Petroleum geophysics and seismic exploration are key tools in finding oil and gas. These methods use sound waves to create images of underground rock layers, helping geologists spot potential hydrocarbon traps.
Seismic data interpretation is crucial for understanding reservoir properties and planning drilling operations. By analyzing seismic reflections and integrating well log data, geophysicists can map out promising areas for oil and gas exploration.
Seismic Methods for Petroleum Exploration
Principles of Seismic Reflection and Refraction
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- and refraction are geophysical methods used to image subsurface geological structures and identify potential hydrocarbon traps
- Seismic waves are generated by controlled sources (vibroseis trucks or explosives) and propagate through the Earth's subsurface
- Seismic reflections occur when seismic waves encounter interfaces between layers with different acoustic impedances, causing a portion of the energy to be reflected back to the surface
- Acoustic impedance is the product of and seismic velocity of a rock layer
- Reflection strength depends on the contrast in acoustic impedance between layers
- Seismic refractions occur when seismic waves encounter layers with higher velocity, causing the waves to bend and travel along the interface before returning to the surface
- Refraction allows for determining the velocity structure of the subsurface
- Snell's law describes the relationship between the angles of incidence and refraction at an interface
- The two-way travel time of seismic waves and the velocity of the subsurface layers are used to calculate the depth and geometry of geological structures
- Two-way travel time is the time taken for a seismic wave to travel from the source to a reflector and back to the surface
- Velocity models are built using a combination of refraction and well log data
Applications and Advantages of Seismic Methods
- Seismic reflection is more commonly used in petroleum exploration due to its higher resolution and ability to image complex structures (faults, folds, and stratigraphic traps)
- Reflection seismic can provide detailed images of the subsurface up to several kilometers deep
- High-resolution seismic surveys can image thin beds and subtle stratigraphic features
- Seismic refraction is used for determining the velocity structure of the subsurface, particularly in areas with complex near-surface geology or for deep crustal studies
- Refraction surveys are often used to complement reflection data by providing velocity information for static corrections and depth conversion
- Seismic methods are non-invasive and can cover large areas efficiently, making them cost-effective for petroleum exploration
- Advancements in seismic acquisition, processing, and interpretation technologies have significantly improved the accuracy and resolution of subsurface imaging
Interpreting Seismic Data for Reservoirs
Seismic Data Interpretation Techniques
- Seismic data interpretation involves analyzing seismic sections, which are visual representations of the subsurface geology based on the recorded seismic waves
- Seismic sections display the two-way travel time of seismic reflections as a function of distance along a survey line
- Seismic sections can be displayed in various formats (wiggle trace, variable density, or color-coded)
- Hydrocarbon traps are geological structures that can accumulate and store oil and gas (anticlines, fault traps, and stratigraphic traps)
- Anticlines are folded structures where hydrocarbons can accumulate in the crest
- Fault traps form when permeable reservoir rocks are sealed by impermeable rocks due to faulting
- Stratigraphic traps result from changes in rock type or pinchouts of permeable layers
- Seismic reflections can indicate the presence of hydrocarbon traps by displaying characteristic patterns:
- Bright spots are high amplitude anomalies that can indicate the presence of gas or light oil
- Flat spots represent fluid contacts (gas-oil or oil-water) within a reservoir
- Dim spots or gas chimneys are zones of reduced amplitude caused by the absorption of seismic energy by gas-bearing rocks
- Seismic attributes, such as amplitude, frequency, and phase, can provide additional information about the subsurface geology and fluid content
- Amplitude attributes can highlight changes in lithology or fluid content
- Frequency attributes can indicate changes in bed thickness or fluid type
- Phase attributes can help identify stratigraphic features and discontinuities
Integrated Interpretation and Uncertainty Reduction
- Seismic facies analysis involves interpreting the spatial and temporal variations in seismic reflection patterns to identify depositional environments and reservoir properties
- Seismic facies are defined by their distinct reflection patterns, amplitude, frequency, and continuity
- Seismic facies can be related to depositional environments (channels, fans, reefs) and lithology (sandstone, shale, carbonate)
- Seismic interpretation is often integrated with well log data, core analysis, and other geological and geophysical information to reduce uncertainty and improve the understanding of the subsurface
- Well logs provide detailed information about the rock properties and fluid content at specific locations
- Core analysis provides direct measurements of reservoir properties (porosity, permeability, and fluid saturation)
- Geological models and regional knowledge can guide seismic interpretation and help validate the results
- Uncertainty in seismic interpretation can be reduced by using multiple attributes, integrating different data types, and applying advanced techniques (seismic inversion and AVO analysis)
- Seismic inversion converts seismic data into rock properties (acoustic impedance or velocity)
- AVO (Amplitude Versus Offset) analysis studies the variation in seismic amplitude with distance from the source to detect changes in fluid content or lithology
Reservoir Characterization with Logging and Attributes
Well Logging for Reservoir Properties
- Well logging involves measuring various physical properties of the subsurface formations along the length of a borehole, providing detailed information about the reservoir properties
- Logging tools are lowered into the borehole to record continuous measurements of rock properties
- Modern logging techniques include wireline logging, logging while drilling (LWD), and measurement while drilling (MWD)
- Common well logs used in reservoir characterization include:
- Gamma ray logs measure the natural radioactivity of rocks, helping to distinguish between shale and non-shale layers
- Density logs measure the bulk density of the formation, which is related to porosity and lithology
- Neutron porosity logs measure the hydrogen content of the formation, providing an estimate of porosity
- Resistivity logs measure the electrical resistivity of the formation, which is sensitive to fluid content and saturation
- Sonic logs measure the velocity of sound waves in the formation, providing information about porosity and mechanical properties
- Well log data can be used to identify the lithology, porosity, permeability, and fluid content of the reservoir rocks, which are essential for estimating hydrocarbon reserves and planning field development
- Lithology is determined by combining gamma ray, density, and neutron logs
- Porosity is estimated using density, neutron, and sonic logs
- Permeability can be estimated from porosity and other log-derived properties using empirical relationships
- Fluid content and saturation are inferred from resistivity logs and other measurements
Seismic Attributes and Reservoir Characterization
- Seismic attributes are quantitative measures derived from seismic data that can provide additional insights into the subsurface geology and reservoir properties
- Attributes are calculated from the seismic trace data using mathematical algorithms
- Attributes can be extracted along horizons, time slices, or volumes
- Examples of seismic attributes include:
- Amplitude attributes (RMS amplitude, instantaneous amplitude) highlight changes in acoustic impedance and can indicate variations in lithology or fluid content
- Frequency attributes (instantaneous frequency, dominant frequency) can reveal changes in bed thickness or fluid type
- Phase attributes (instantaneous phase, cosine of phase) can help identify stratigraphic features and discontinuities
- Coherence attributes measure the similarity between seismic traces and can highlight faults, fractures, and other discontinuities
- Curvature attributes measure the degree of folding or bending of seismic reflectors and can indicate structural or stratigraphic features
- Seismic inversion is a technique that converts seismic reflection data into a quantitative representation of the subsurface rock properties, such as acoustic impedance or velocity, which can be correlated with well log data
- Deterministic inversion uses a single input model and produces a single output model
- Stochastic inversion uses a range of input models and produces multiple realizations of the reservoir properties
- The integration of well log data and seismic attributes allows for a more comprehensive characterization of the reservoir, enabling better estimation of hydrocarbon reserves, identification of sweet spots, and optimization of field development strategies
- Well logs provide high-resolution vertical information at discrete locations
- Seismic attributes provide spatially continuous information about the reservoir properties and geometry
- Geostatistical methods (kriging, co-kriging) can be used to integrate well and seismic data and create 3D reservoir models
3D and 4D Seismic in Field Development
3D Seismic Surveys and Interpretation
- 3D seismic surveys involve acquiring seismic data in a dense grid over the area of interest, providing a three-dimensional representation of the subsurface geology
- 3D surveys are designed with closely spaced receiver lines and source lines to provide high fold coverage and dense spatial sampling
- 3D seismic data is processed using specialized algorithms to enhance signal-to-noise ratio and image quality
- 3D seismic data allows for more accurate imaging of complex geological structures, such as faults, channels, and pinchouts, which can be crucial for identifying hydrocarbon traps and planning well locations
- 3D migration techniques (Kirchhoff, wave-equation) accurately position reflectors in their true subsurface locations
- 3D visualization tools enable interpreters to view and analyze the data in different orientations and perspectives
- 3D seismic interpretation techniques, such as volume rendering, horizon slicing, and attribute analysis, enable a more detailed understanding of the reservoir geometry and properties
- Volume rendering displays the 3D seismic data as a semi-transparent volume, allowing interpreters to visualize the spatial relationships between different geological features
- Horizon slicing involves extracting seismic attributes along interpreted horizons to map lateral variations in reservoir properties
- Attribute analysis can highlight specific features of interest, such as faults, channels, or fluid contacts
4D Seismic Monitoring and Reservoir Management
- 4D seismic, also known as time-lapse seismic, involves repeating 3D seismic surveys over the same area at different times during field development and production
- 4D surveys are typically acquired at intervals of several years, depending on the field's production history and management objectives
- 4D seismic data is carefully processed to ensure consistency between the surveys and to minimize non-production-related changes
- 4D seismic data can monitor changes in the reservoir over time, such as fluid movement, pressure depletion, and compaction, which can help optimize production strategies and improve recovery rates
- Fluid substitution (water replacing oil or gas) can cause detectable changes in seismic response
- Pressure depletion can lead to compaction and subsidence, which can be monitored using 4D seismic
- 4D seismic can help identify bypassed or undrained reserves, guiding infill drilling locations
- 4D seismic interpretation techniques, such as difference volumes and time-shift analysis, can highlight areas of the reservoir that have undergone changes due to production or injection, allowing for better management of the field
- Difference volumes are created by subtracting the baseline survey from the monitor survey, revealing changes in seismic response
- Time-shift analysis measures the travel-time differences between the baseline and monitor surveys, which can indicate velocity changes due to pressure depletion or fluid substitution
- The integration of 3D and 4D seismic data with reservoir simulation models can improve the understanding of reservoir behavior, optimize well placement and production strategies, and reduce the risks associated with field development
- Reservoir simulation models use seismic-derived properties (porosity, permeability) to predict fluid flow and production behavior
- 4D seismic data can be used to calibrate and update reservoir models, improving their predictive accuracy
- Integrated workflows combining seismic, well, and production data enable data-driven reservoir management decisions and optimize field performance
Key Terms to Review (18)
Amplitude Variation with Offset (AVO): Amplitude Variation with Offset (AVO) is a seismic analysis technique that examines how the amplitude of seismic waves changes as the distance between the source and receiver increases. This phenomenon is crucial in petroleum geophysics because it provides insights into the presence and characteristics of subsurface materials, helping in identifying hydrocarbon reservoirs and understanding their properties.
Anticline: An anticline is a type of fold in rock layers characterized by an arch-like shape where the oldest layers are at the core and the younger layers dip away from the center. This geological structure is significant in petroleum geophysics as it often acts as a trap for hydrocarbons, creating ideal conditions for oil and gas accumulation beneath the earth's surface.
Carbon capture: Carbon capture is a technology designed to prevent carbon dioxide (CO2) from entering the atmosphere by capturing it from sources like power plants and industrial processes. This process is vital for reducing greenhouse gas emissions, especially in the context of fossil fuel energy production, and plays a critical role in mitigating climate change while still allowing for the use of fossil fuels.
Cretaceous: The Cretaceous is a geological period that lasted from about 145 to 66 million years ago, marking the final segment of the Mesozoic Era. This period is characterized by significant geological and biological changes, including the widespread dominance of dinosaurs and the appearance of flowering plants. The Cretaceous is vital for understanding petroleum geophysics as it is often associated with substantial hydrocarbon deposits formed in marine sedimentary basins during this time.
D. J. H. Johnson: D. J. H. Johnson is a significant figure in the field of petroleum geophysics, known for his contributions to seismic exploration techniques. His work has had a lasting impact on the methods used to locate and assess petroleum reserves, combining geophysical data with geological insights. Johnson's innovations in seismic interpretation and modeling have helped to refine the exploration processes, making them more efficient and effective in identifying hydrocarbon deposits.
Density: Density is a measure of mass per unit volume, typically expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). This property is crucial for understanding the composition and structure of Earth, as it influences how materials behave under different conditions and plays a significant role in various geophysical processes.
Elastic Modulus: Elastic modulus is a fundamental property of materials that quantifies their stiffness, defined as the ratio of stress to strain in the linear elastic region of a material's stress-strain curve. This parameter is crucial in understanding how materials respond to applied forces, especially in the context of subsurface formations in petroleum geophysics and seismic exploration, where the behavior of rocks under stress impacts the propagation of seismic waves and influences reservoir characterization.
Fault trap: A fault trap is a geological feature where hydrocarbons, such as oil and natural gas, are contained due to the presence of a fault line that acts as a barrier to their migration. This occurs when the fault creates a structural trap, allowing hydrocarbons to accumulate in a reservoir rock above or adjacent to the fault plane, often enhancing the potential for successful extraction during petroleum exploration.
Full waveform inversion: Full waveform inversion (FWI) is an advanced seismic imaging technique that utilizes the complete seismic wavefield data to create high-resolution subsurface models. By minimizing the difference between observed and simulated seismic data, FWI enhances the accuracy of geological interpretations, particularly in petroleum exploration where understanding subsurface structures is crucial for identifying potential hydrocarbon reservoirs.
Geohazards: Geohazards are natural events or processes that pose risks to human life, property, and the environment, often resulting from geological or hydrological phenomena. They encompass a variety of events such as earthquakes, landslides, tsunamis, and volcanic eruptions, and their impact can be exacerbated by human activities. Understanding geohazards is essential in assessing risks and implementing effective mitigation strategies, particularly in industries like petroleum extraction and seismic exploration where geological stability is critical.
Hampson-Russell: Hampson-Russell is a software suite widely used in geophysics, specifically for seismic data interpretation and reservoir characterization. This toolset provides powerful capabilities for processing seismic data, modeling, and inversion, helping geophysicists better understand subsurface geology and optimize exploration efforts in the petroleum industry.
Jurassic: The Jurassic period is a significant geological time frame that lasted from about 201 to 145 million years ago, known for the dominance of dinosaurs and significant developments in Earth's climate and geography. This period saw extensive sedimentation and the formation of key petroleum reservoirs, making it crucial for understanding oil and gas exploration and extraction methods.
K. R. L. Hossack: K. R. L. Hossack is a prominent figure in the field of petroleum geophysics, known for his contributions to seismic exploration methods and techniques. His work emphasizes the importance of interpreting subsurface geological structures using seismic data to enhance hydrocarbon exploration and production efficiency. Hossack's research has influenced various methodologies in seismic analysis, promoting a better understanding of how geological formations affect the distribution of petroleum reserves.
Petrel: Petrel refers to a software application used extensively in the oil and gas industry for geoscience and engineering tasks, particularly in petroleum geophysics and seismic exploration. This tool enables users to integrate and analyze geological, geophysical, and engineering data to optimize the exploration and production of hydrocarbon resources. By providing a collaborative platform, Petrel supports multi-disciplinary teams in making informed decisions throughout the oil and gas lifecycle.
Refraction Seismology: Refraction seismology is a geophysical method that analyzes the refraction of seismic waves as they travel through different layers of the Earth. This technique helps in determining the subsurface structure, including the depth and type of geological formations. By measuring the arrival times of refracted waves, geophysicists can infer the properties of subsurface materials, making it a crucial tool in both data acquisition and exploration.
Seismic Reflection: Seismic reflection is a geophysical technique used to analyze subsurface structures by sending seismic waves into the ground and recording the waves that bounce back from different geological layers. This method helps in understanding the composition, properties, and depth of subsurface materials, making it crucial for applications like resource exploration, environmental assessments, and geotechnical investigations.
Time-lapse monitoring: Time-lapse monitoring is a technique used to observe and analyze changes in the subsurface over time by repeatedly collecting data at specific intervals. This method is particularly valuable in tracking fluid movements and reservoir behavior in petroleum geophysics, providing insights into the dynamics of oil and gas fields. By capturing seismic data at different times, it enables geophysicists to assess changes caused by extraction activities and understand how reservoirs evolve.
Wave propagation: Wave propagation refers to the movement of waves through different mediums, which can be solid, liquid, or gas. In geophysics, understanding how these waves travel is crucial for studying the Earth's structure and its material properties, as well as for exploring natural resources such as oil and gas through seismic methods. This concept helps in interpreting data related to the layers of the Earth and locating potential hydrocarbon reservoirs.