Incident Angle and Offset Relationship in Geology

AVO analysis = Exercise essential during seismic interpretation to gain confidence in anomalies Forward modelling = Method to determine seismic responses for different fluid types Reflectivity = Magnitude and sign of individual reflection creating seismic response Acoustic impedance = Calculated for each layer using Vp and Density well logs

Calculate reflectivity for each boundary = Convolution model Create a synthetic angle gather = Using elastic properties and Zoeppritz equations Model the top and base of a simple sand = Aki-Richards equation Generate synthetic gather for Class 3 sand = Aki-Richards equation

Class 1 = Increase in P-impedance change and change in polarity on synthetic Class 2 = Small P-impedance change but large amplitude change on synthetic Class 3 = Decrease in P-wave velocity and density in gas sand All three classes combined = Comparison showing picked amplitudes at top and base of gas sand

Incident angle = Affects amplitude of seismic waves P-velocity, S-velocity, and density contrast at interface = Factors affecting seismic wave amplitude VP/VS ratio decrease to 1.5 = Expected in clean gas sand Surface seismic-reflection data recorded as a function of offset = Dependent on fixed offset between source and receiver

Forward modeling = Process to create synthetic seismic data based on a known model of elastic rock properties Quantitative Interpretation = Workflow where forward modeling can serve different purposes Geological model = Model used to generate synthetic data in forward modeling Seismic data = Real recorded data that is compared with synthetic data in forward modeling

Agreement between synthetic and real seismic data = Indicates plausible geological model Disagreement between synthetic and real seismic data = Sign of incorrect geological model or inaccurate seismic data Gas saturated reservoir = May result in weaker amplitudes on synthetic trace Sonic logs measuring velocity in washout zone = Record a higher value due to influence of drill fluid

Checking consistency between seismic data and geological model = One of the main purposes of using forward modeling Generating synthetic data for comparison = Allows comparison with real recorded seismic data Identifying need for fluid substitution = May be necessary to achieve agreement between synthetic and real seismic data Validating geological model accuracy = Done by comparing synthetic and real seismic responses

AVO forward modelling = Part of the workflow for interpretation Reviewing different approaches to forward modelling = Helps understand various methods in AVO modeling Application of AVO forward modelling = Explained as part of the presentation Discussing AVO modelling workflow = Covers the overall process for AVO interpretation

Ray tracing = Computing the ray path through a model from a source to a receiver Elastic Wave Modeling = Exact solution for a plane wave propagating through a series of layers Primary reflection = Shown by solid lines in modeled events Shear-waves = Shown by dashed lines in modeled events

tan theta = x / (travel time * velocity) = Relationship between offset and incident angle derived by trigonometry Aki-Richards equation = Used with ray tracing to generate primaries-only synthetic gather Zoeppritz equations = Can be used for all propagation modes including direct waves, converted waves, etc. Elastic wave equation = Used to generate all propagation modes in elastic wave modeling

Creating a stack of N layers using well logs = Multi-layer modeling in AVO program Calculating both angles of incidence and arrival times = Using ray tracing in modeling Transforming CMP gathers from offset domain to incident angle domain = Performing ray tracing with velocity model Specifying a frequency range affecting run-time in modeling = Elastic Wave Modeling process

Zoeppritz equations = Fast and accurate for relatively thick layers modeling Primaries-only synthetic gather with Aki-Richards equation = Quick and convenient way of looking at modeled gathers Elastic wave equation = Generates all propagation modes including multiple reflection waves and converted waves Ray tracing-based modeling = Calculates angles of incidence and arrival times

Primary reflection modeling = Works well for relatively thick layers Shear-waves representation by dashed lines = Illustrated in modeled events Creating stack of N layers using well logs = First step in multi-layer AVO modeling Frequency range specification in modeling = Affects run-time in Elastic Wave Modeling

Elastic Wave Modeling = Should be more accurate than Zoeppritz Modeling if input parameters are correct Zoeppritz Modeling = May be more sensitive to log editing problems

Anisotropy in rock formations = May lead to misinterpretations of AVO responses Anisotropic behavior = Can help in characterizing fractured reservoirs Tuning thickness of seismic data = Occurs when reservoir reaches a critical thickness where amplitudes increase

Event A (Zoeppritz) = Single P-wave primary event modeled using Zoeppritz equations Event B = Primaries plus single leg shear waves Event C = Primaries with both single and double leg shear waves Event D = Adds in multiples

Wedge Modeling = Uses 2D model of soft sand within hard shale to determine tuning thickness Tuning Thickness Analysis = Defined by lambda divided by 4

Mode conversion at top of oil sand layer (Image C) = Converted shear wave passes through layer, reflects at base, and converts back to P wave at top Reverberation of primary P wave (Image D) = Shows multiples within oil sand layer Double leg shear wave (Image C) = Mode conversion at top, passes through layer, and reflects back converting to P wave at top Primary reflections from top and base of oil sand layer (Image A) = Shows primary reflections from top and base of oil sand layer

Preparation of well logs = Includes quality control and correction of Vp, Vs, and Density logs Rock Physics study = Creation of a set of porosity, fluid, and lithology replacement well logs Synthetic seismic gathers generation = Creating synthetic seismic data for various scenarios AVO attribute extraction and analysis = Extraction and analysis of appropriate AVO attributes

Forward modeling purpose = To determine how seismic responses change for different reservoir properties Typical modelling algorithms = Include multiples, conversion, anisotropy modeling Effect of fluid replacement on synthetic offset gathers = Illustrated in Intercept vs Gradient crossplot Resolvability limit calculation = Tuning thickness within reservoir which is λ divided by 4

Understanding pore fluid relationships = Indication of hydrocarbon reservoir or relevant lithology Analysis of lithologic relationships with seismic response = Understanding mineralogy relationships Selecting suitable attributes for AVO analysis = Based on model results to ensure successful analysis Determining velocity from sand wedge model or well data = Fundamental part of the interpretation process

λ determination = Determine the resolvability limit or tuning thickness within reservoir Intercept vs Gradient crossplot = Illustrates the effect of fluid replacement on synthetic offset gathers Full elastic wave modeling = A more robust and complicated forward modeling approach Anisotropy modeling = One of the effects that could be modeled in AVO analysis