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

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers

<p>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</p> Signup and view all the answers