14-DHI&AVO - Lecture notes 14 PDF

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Direct Hydrocarbon Indicators& Amplitude variation withOffset1 IntroductionWe have discussed effects of different environmental and material properties on seismic velocities in the last lecture. Today we will talkabout two concepts that will take us towards an early interpretation of seismic...

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Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

Direct Hydrocarbon Indicators & Amplitude variation with Offset 1

Introduction

We have discussed effects of different environmental and material properties on seismic velocities in the last lecture. Today we will talk about two concepts that will take us towards an early interpretation of seismic data. These are methods that are not formally interpretation, though. Amplitude variation with offset (AVO) uses amplitudes at oblique incidence to extract additional information from the data.

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AVO

We have discussed that the reflection coefficient at normal incidence is given by the ratio of the impedance difference and the impedance sum over the interface. α2 ρ2 − α1 ρ1 R= (1) α2 ρ2 + α1 ρ1 For normal incidence we do not get wave conversions, i.e. an incident P-wave is reflected and converted as P-wave. At oblique incidence we observe phase conversions, i.e. an incident P-wave produces both reflected and transmitted P-waves and S-waves. The amplitudes of these are described by the Zoeppritz equations or in a slightly different formulation through Knott’s energy equations. The Zoeppritz equations are rather complex, although solvable with computers. Nonetheless, we often use an approximation, valid for small incidence angle, to describe the P-wave reflection coefficient: RP (θ) = A + B sin2 θ 1

(2)

Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

with A being the normal incidence reflection coefficient and the constant B is a simple function of α1 , β1 , rho1 , α2 , β2 , and rho2 . theta is the incidence angle and the approximation is valid for incidence angles less than 30 degrees. Equation ?? is called Shuey’s equation and can be used to better understand the amplitude behavior of reflected P-wave energy. The factor A and B are defined as R(θ) = A + B sin2 θ   1 ∆ρ ∆VP · + A= ρ VP 2 2 ∆V V 2 ∆VS V ∆ρ P + − 4 S2 B = −2 S2 2VP VP VS VP ρ

(3)

with VP the average velocity across the interface and ∆VP the difference across the interface with the same for the S-waves and density ρ. So where can we get information on the reflection coefficient from? It is contained directly in the CMP data as they represent different incidence angles at the interface. The traces with increasing offset are showing increasing incidence angle. Thus, if we display the amplitude of a reflection within a CMP, carefully corrected for geometric spreading and anelastic attenuation due to the different path lengths, we can analyse the amplitude information in the CMP with respect to Shuey’s equation. We can convert distances to incidence angle through using a velocity model. Plotting the amplitudes versus sin2 θ then allows us to form a regression line to determine B as the gradient of the regression and A as the y (Amplitude) intercept. Please note that this requires careful processing so that amplitudes are regained through all processing steps. So not processing that alters amplitudes (e.g. gain recovery and normalisation) is allowed. The process is done on specific reflectors. From the CMP display we extract the individual traces of the CMP (the number of the traces is the fold of the CMP). We extract the amplitudes of arrivals, often the arrivals from the top and the bottom of a layer. Displaying the amplitudes against sin2 θ allows us to extract the gradient and intercept of the reflection according to eq. ?? (Fig. ??). The AVO information is then used in AVO cross plots (Fig. ??) displaying the AVO attributes against each other that can help to identify pore fill anomalies. This is often done in profiles along interesting features of the 2

Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

Figure 1: Extraction of intercept and gradient from amplitude analysis by offset.

Figure 2: Example of AVO crossplots showing the parameters A and B from Shuey’s equation for different reservoir fill and lithologies.

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Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

Figure 3: Profiles across potential reservoir showing AVO attributes over anticline (top) and related two-way-time (bottom). Profiles through AVO attributes can help with identifying pore fluid anomalies.

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Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

Figure 4: Schematic of hydrocarbon traps.

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Direct Hydrocarbon Indicators

Direct hydrocarbon indicators (DHIs) are features in the seismic data that might indicate the existence (or not) of hydrocarbons in a reservoir. DHIs use the different velocities between the different pre fills, i.e. the difference in velocity between brine and oil/gas.

3.1

Hydrocarbon Traps

Hydrocarbon reservoirs consists of a source rock that is creating the hydrocarbon, a reservoir rock to store the hydrocarbon (good porosity and permeability) and a cap-rock that prevents escape of the hydrocarbon to the surface. We can differentiate traps into structural traps which are formed through structural changes and stratigraphic traps which are formed through changes in the stratigraphy. Fig. ?? shows examples of traps. A classical trap is an anticline. These were the first traps used for exploration with anticlines created by salt plume deforming. Another possibility is a fault that seals the 5

Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

reservoir through the throw on the fault. Stratigraphic traps can form through a change in stratigraphy such as a pinchout (e.g. a paleao beach overlain by a cap rock), an unconformity (again the reservoir sealed by a cap) or a paleao reed covered in younger sediment.

3.2

DHI

For the DHIs we use the lower velocities of oil or gas compared to the brine that typically fills rocks. This is only valid for P-waves as the S-waves are not sensitive to either fluid (or gas). A large number of DHIs have been identified and are used (and sometimes misused) in interpretation. These are: 1. Structural crest of against a fault Trapping location 2. Local increase in amplitude

Bright Spot

3. Local decrease in amplitude

Dim Spot

4. Discordant flat reflector

Flat Spot

5. Local waveshape change

Polarity reversal

6. Reservoir limits consistent

Consistent model

7. Polarities consistent

-ve over rock/gas

8. Low frequencies underneath

Attenuation shadow

9. Time sag underneath

Velocity sag

10. Lower amplitudes underneath

Amplitude shadow

11. Increase in amplitude with offset

AVO anomaly

12. P-wave but no S-wave anomaly

S-wave support

13. Data deterioration

Gas chimney

For all DHIs the velocity structure of the reservoir and cap rock (in relation) are essential. Existence of hydro carbons always reduces the velocity of the reservoir. But depending on the velocity contrast to the caprock this might mean that the contrast gets larger, smaller or disappears. 6

Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

We will discuss a subset of these now. More examples of seismic sections showing these features are given in the lecture so please refer to the lecture slides for additional data and synthetic examples.

3.3

Bright Spot Bright spots are amplitude highs in a seismic section along a reflector. They are associated with hydrocarbon accumulations due to the increase in reflection coefficient at top and bottom of a reservoir caused by hydrocarbon in the pore space. For a bright spot to develop we require a negative velocity jump from the cap rock (e.g. shale) to the brine filled reservoir which is exacerbated when brine is displaced by hydrocarbons (Fig. reffig:bright).

3.4

Dim Spot

If the rock overlying the reservoir Figure 5: Schematic velocity structure (cap) has a velocity appreciably for bright spot (top) and schematic lower than that of the reservoir itself seismic section (bottom). (e.g. carbonate reservoir capped by shales), the effect of hydrocarbon is to decrease the contrast in acoustic impedance and reduce the reflection coefficient producing a ’dim spot? (Fig. ??).

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Dr Sebastian Rost

3.5

SOEE2550 - Lecture 16 - DHI & AVO

Flat Spot Where a well-defined fluid contact is present (esp. gas/oil or gas/water) the contrast may be large enough to give a fairly strong flat reflection that may stand out on the seismic records. The fluid contact is always following gravity and therefore appears completely flat. Dim spots can be seen at the contact points between brine and oil/gas in the sandstone in Figs. ?? and ??.

3.6

Polarity Reversal

Where the rock overlying the reservoir has a velocity slightly smaller than that of the reservoir rock, lowering the reservoir rock by hydrocarbons may invert the sign of the reflection, producing a polarity reverFigure 6: Schematic velocity structure for dim spot (top) and schematic seis- sal (Fig. ??). mic section (bottom).

3.7

Shadow Effects

The lowering of the velocity in a hydrocarbon accumulation will affect the travel times from deeper reflections by increasing travel-times to cause a reflection sag. This is an apparent deeper reflection beneath the hydrocarbon filled reservoir. If oil is replacing the brine this effect is typically small. The high amplitudes of a bright spot are often processed in such a way as to cause a lower amplitude shadow zone above a bright spot as well as below. We can also observe changes in the attenuation structure of the reservoir which translates to deeper reflectors. These are difficult to resolve with ’normal’ data, but can be studied using special processing approaches (e.g. 8

Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

differential spectra).

3.8

Gas Chimney A gas chimney is formed through subsurface leakage of gas from a poorly sealed hydrocarbon accumulation. The gas percolates upwards through the overlying rocks lowering their velocities in the process. Gas chimneys are visible in seismic data as areas of poor data quality or pushdowns of underlying reflectors due to the lower velocity (Fig. ??).

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Summary

A wide variety of features in seismic sections have been used in interpretation to point towards the existence of hydrocarbons in a reservoir. They can be useful tools for easy interpretation but have the potential to be misleading. The most Figure 7: Schematic velocity struc- common ones are bright and dim ture for polarity reversal (top) and spots, flat spots indicating fluid conschematic seismic section (bottom). tacts and polarity reversals over potential reservoirs. A good knowledge of the velocity structure of reservoir and cap is essential for useful interpretation. Although seismic processing often destroys amplitude information, the amplitudes of reflections carry information about the structure of the reflecting interface. Using careful amplitude processing we can use the amplitudes to better understand the structure of the reservoir and particularly resolve changes in pore fill. The method used is called amplitude variation with offset. Cross plots and profiles of AVO attributes can be used in parallel 9

Dr Sebastian Rost

SOEE2550 - Lecture 16 - DHI & AVO

Figure 8: Seismic section showing velocity sag and quality decease due to gas leaking from reservoir. with ’traditional’ interpretation. For the rest of the term we will look at the processing of a marine seismic dataset using industry standard software. We will pick up the interpretation of seismic data at the beginning of the second semester.

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