AVO Class 1 Sands in Seismic Data

Intercept (A) = Amplitude at zero offset or angle Gradient (B) = Change in amplitude with respect to sine squared of the angle of incidence Curvature = Higher order term of the AVO approximations Reflection Coefficients = Representation of seismic event amplitudes

Class 1 = Strong positive amplitudes, high impedance contrast Class 2 = Weak positive amplitudes, moderate impedance contrast Class 3 = Weak negative amplitudes, moderate impedance contrast Class 4 = Strong negative amplitudes, high impedance contrast

Explaining basic principles of AVO cross-plotting = Objective 1 Exploring four AVO classes appearance in different plots = Objective 2 Discussing practical application of AVO cross-plotting = Objective 3 Analyzing Intercept and Gradient in different ways = Objective 3

Extracting amplitudes for all time samples in offset gathers = Graph plotting as a function of sin²θ Linearized AVO equation approximation = Representation by Intercept and Gradient Considering higher order term as an AVO attribute = Term known as Curvature Dropping higher order term for smaller angles = Using first order leaner approximation for reflection coefficients

Intercept = Reflection amplitude at zero angle of incidence Gradient = Slope term value for each time sample Quadrant III = Class 3 sands in AVO analysis Quadrant IV = Class 1 sands in AVO analysis

Class 2 sands = Low normal incidence reflectivity from small impedance contrasts Class 3 sands = Lower impedance than overlying shales, forming bright spots Class 4 sands = Low impedance but reflection magnitude decreases with offset Class 1 sands = High impedance relative to overlying shales

Quadrant I = Not mentioned in the provided text Quadrant II = Associated with low impedance sands and decreasing reflection magnitude Quadrant III = Associated with lower impedance sands and bright spots Quadrant IV = Associated with high impedance sands relative to overlying shales

Background trend = Linear trend followed by brine-saturated sandstones and shales AVO effects classification = Based on intercept and gradient signs in the crossplot AVO classes classification = Based on deviation from the background trend in AVO analysis Background line rotation = Occurs as Vp/Vs ratio increases

Gas saturation effect on Intercept (A) = Becomes more negative compared to fully brine-saturated sand Gas saturation effect on Gradient (B) = Becomes more negative compared to fully brine-saturated sand Common assumptions about gas-sand amplitude increase with offset = Contradicted by various AVO behaviors observed Positive correlation between Intercept (A) and Gradient (B) = Possible at very high Vp/Vs ratios

AVO crossploting is a commonly used technique in AVO analysis because it is a simple and efficient way to identify a background trend and to reveal distinguishable AVO anomalies that lie off that background trend. = Summary point about the efficiency of AVO crossploting There are four AVO classes defined according to where the top of the gas sands response falls on the intercept-versus-gradient crossplot. = Summary point about the classification based on gas sands response Brine-saturated clastic rocks and shales, over a limited depth range and in a particular locality, usually exhibit a well-defined background trend in the intercept gradient cross-plot. = Summary point about background trends in brine-saturated rocks The interval chosen for crossplotting should not be too wide, because that would result in the superposition of a variety of background trends that correspond to different zones. = Summary point about the selection of interval for crossplotting

Partial gas saturation causes both the intercept and the gradient to be more negative than they are in the corresponding fully brine-saturated sand. = Effect of partial gas saturation on intercept and gradient The slope of the background trend depends primarily on the background Vp/Vs ratio. = Effect of Vp/Vs ratio on background trend slope When working with large data volumes, the identification of background trends may be challenging because when we plot a huge amount of points, the anomalies can be lost. = Challenge of identifying background trends with large data volumes The commonly used methods are to reduce the number of data points or to filter out points. = Strategies for dealing with large data volumes

Yellow and blue cross-plot polygons = Highlighting specific data points Colouring data points from yellow and blue cross-plot polygons in the same fashion at their respective positions in a cross-section = Confirming plausibility of hydrocarbon saturation Yellow data points sitting directly above blue data points in a cross-section = Geological consistency between zones Both yellow and blue data points lying within a plausible trap structure = Confirmation of potential hydrocarbon accumulation

Class 1 = Single point on the Intercept-Gradient crossplot for brine-filled sand overlain by shale Class 2 = AVO response for gas sand when water sand is harder than shale but hydrocarbon sand is still softer than shale Class 2p = AVO response depending on the difference between acoustic impedance of hydrocarbon sand and shale Class 3 = AVO response at the top of gas sand when hydrocarbon sand is significantly softer than shale

Zone 1 = Hydrocarbon sand significantly softer than shale, classic Class 3 AVO response at top of gas sand Zone 2 = Gas sand AVO response Class 2 when hydrocarbon sand is softer than shale but water sand is harder than shale Zone 3 = Water sand harder than shale, hydrocarbon sand also harder than shale, AVO response can be Class 2p or Class 1 Deep section = Hydrocarbon sand can become higher impedance than shale

Increasing porosity of sand in the model = Moves the point by decreasing both Intercept and Gradient values Increasing shale content = Lowers Intercept-Gradient values but trend is steeper than porosity trend Fluid substitution in sands of various porosity = Creates a separate data cluster that falls to the left of points for brine saturated sands Change in lithology = Can be modeled by changing porosity of sand or by changing shale content

Random noise = Creates oval distribution around real location of point on Intercept-Gradient crossplot Systematic noise = Not considered in this example Environmental noise = Not discussed in relation to crossplot trends Red line definition noise = Discriminator between gas and brine saturated clusters

Background trend on Intercept-Gradient time-window crossplot = Identified as grey zone with off-trend cluster points Off-trend cluster falling into Class 3 AVO anomaly classification = Yellow zone consistent with soft impedance gas reservoir

Class 1 = Higher impedance than encasing medium, positive reflection at zero offset, amplitude decreases with offset Class 2 = Near zero impedance contrast with overlying rocks, small amplitude response that becomes more negative with offset Class 3 = Lower impedance than encasing medium, strong negative reflections at near offset that become more negative with increasing offsets Class 4 = Low impedance gas sands, reflection coefficients are negative at zero offset and decrease with increasing offset

2p Class Anomaly = Positive intercept and negative gradient causing polarity change at far offsets 2n Class Anomaly = Both negative intercept and gradient, hence no polarity change Bright Spot Anomaly = Associated with class 3 gas sands, strong negative reflections at near offset Dim Spot Anomaly = Appear as dim spots on stacked seismic data, hard to detect in noise

Mature Highly Compacted Sand = Example of Class 1 gas sand with relatively low Vp/Vs compared to surrounding rocks Moderately Compacted Sediments = Where examples of Class 2 anomalies are usually seen Shallow Undercompacted Gas Sands = Typical examples of Class 3 gas sands Porous Sand Overlain by High-Velocity Unit = Example of Class 4 gas sand overlain by hard shale or tightly cemented sand

Class 1 Sand = Positive reflection at zero offset with decreasing amplitude as offset increases Class 2 Sand = Small amplitude response that becomes more negative with offset, appearing as dim spots or bright spots Class 3 Sand = Strong negative reflections at near offset that become more negative with increasing offsets, creating 'bright spot' anomalies Class 4 Sand = Reflection coefficients are negative at zero offset and decrease with increasing offset

Shale Impedance Trend = Increases in impedance with depth and age, higher impedance than sand in shallow section Water Sand Impedance Trend = Increases in acoustic impedance slower than compacted sand and crosses over shale trend Hydrocarbon Sand Impedance Trend = Has a lower impedance than encasing medium, forms strong negative reflections at near offset AVO Interface Property Trend = Depends on contrast between gas sand and overlying shale