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
Top images from around the web for Principles of Seismic Reflection and Refraction
Frontiers | Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the ... View original
Is this image relevant?
SE - Seismic reflection data reveal the 3D structure of the newly discovered Exmouth Dyke Swarm ... View original
Is this image relevant?
SE - Fault interpretation in seismic reflection data: an experiment analysing the impact of ... View original
Is this image relevant?
Frontiers | Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the ... View original
Is this image relevant?
SE - Seismic reflection data reveal the 3D structure of the newly discovered Exmouth Dyke Swarm ... View original
Is this image relevant?
1 of 3
Top images from around the web for Principles of Seismic Reflection and Refraction
Frontiers | Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the ... View original
Is this image relevant?
SE - Seismic reflection data reveal the 3D structure of the newly discovered Exmouth Dyke Swarm ... View original
Is this image relevant?
SE - Fault interpretation in seismic reflection data: an experiment analysing the impact of ... View original
Is this image relevant?
Frontiers | Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the ... View original
Is this image relevant?
SE - Seismic reflection data reveal the 3D structure of the newly discovered Exmouth Dyke Swarm ... View original
Is this image relevant?
1 of 3
Seismic reflection 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 density 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