Seismic Coffecent Rock And Rock Density Analysis Biology Essay

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The general theory of seismic reflection and transmission of waves that incident obliquely at an interface between two fluid-filled porous rocks is similar to that for two elastic non dissipative media. According to Biot's theory, an isotropic homogeneous porous rock and its pore-filling fluid are treated in the manner of two interpenetrating elastic continuation, (Dutta, et al., 1983).

The first use of amplitude information as hydrocarbon indicators was in the early 1970s when it was found that bright-spot amplitude anomalies could be associated with hydrocarbon traps (Hammond, 2011). Increasingly quantitative work after Ryseth et al., (1998) who used acoustic impedance inversions to guide the interpretation of sand channels, and Zeng, et al., (1996) used forward modeling to improve the understanding of shallow marine facies from seismic amplitudes. Neri, (1997) used neural networks to map facies from seismic pulse shape.

Two-dimensional and three-dimensional seismic surveys are the current primary exploration tool used for deepwater sandstone reservoir delineation Sandstone and shale properties vary widely in various areas of world. Reservoir sandstones have velocities that may be higher and lower than encasing shale, which can produce a positive, negative or no reflection (Batzle, et al., 2000).

A multiplicity of factors influences seismic reflection and transmition coefficients and the observed gravity of typical sedimentary rocks. Reflection and transmission coefficients can be defined in several ways, one of which is the ratio of energy fluxes. Another way is to define these coefficients as the ratio of matrix displacement amplitude of the appropriate Boot's wave and the incident amplitude (Nietzsche, 2005).

The velocity and density of the Rocks depend upon the mineral composition and the granular nature of the rock matrix, cementation, porosity, fluid content and environmental pressure. Depth of burial and geologic age also has an effect. Lithology and porosity can be related empirically to velocity by the time-average equation. This equation is most reliable when the rock is under substantial pressure and is saturated with brine and contains well-cemented grains. For very low porosity rocks under large pressures, the mineral composition can be related to velocity by the theories of Voigt and Reuss (Gardner, et al.,1974).

The main aim of this chapter is to interpret the lithologies of Miocene succession in the subsurface. Geologically meaning of seismic reflection is simply an indication of an acoustic boundary where we want to know that whether this boundary makes a stratigraphic contact with any other boundary. This chapter describes the seismic properties (such as acoustic impedance, reflection and transmition coefficient) as well as determines the lithologies of the Miocene succession. The seismic properties and lithologies are determined with the help of Gardner's (1974) of velocity-density relation in rocks of different lithology.

4.2 METHODOLOGY AND RESULT

The research methodology and results includes the acoustic impedance, seismic coefficients, formation velocities and densities calculations that are discussed as follows;

4.2.1 Acoustic Impedance

The acoustic impedance of a rock is the product of its density (ρ) and its wave velocity (v), which is a basic physical property of rocks; mathematically it is given as under;

…………………. 4.1

Where, Z is acoustic impedance, ρ is density (g/cc) and v is velocity (m/s). Reflections arise at boundaries where the acoustic impedance changes, impedance does not change even if lithology changes. The greater the difference in the acoustic impedance the stronger is the reflection.

To find the acoustic impedance, the velocities from the selected seismic sections were calculated. These velocities were then used in contouring program Surfer 10.0 for modeling, the final output from the surfer 10.0 program is given in Figure 4.1. The variation in color indicates high acoustic impedance values occur mostly on South-Western region of the study area, while most of the studied area comprises rocks having acoustic impedance value range from 7000 to 9000 g/m2.s which are the moderate values and usually results from loose materials such as soil, alluvium and sand.

In a comparison of 2D model (Figure 4.1) with 3D model (Figure 4.2) it is noted that the high acoustic impedance occurs in the South-West region of the study area (yellow color region, acoustic impedance ranges from 1,000 to 12,000 g/m2.s) this indicates the presence of compacted lithology deposited, in the slope fan depositional setting. The presence of sand bodies (as obvious from the velocities of seismic lines calculated in chapter 03) represented by dark blue color in Figure 4.1, further strengthens the interpretation of potential reservoir interval in the study area.

4.2.2 Reflection Coefficient

For a specific case of normal incidence there is a simple equation relating the incident amplitude to the reflected amplitude. The ratio of reflected amplitude to incident amplitude is called the "reflection coefficient" and it is given as under;

………………… 4.2

Where V1 is the interval velocity and ρ1 is the rock density of the first studied reflector (top of Miocene), and V2 is the interval velocity and ρ2 is the density of second studied reflector (bottom of Miocene), R is reflection coefficient between top and bottom of Miocene age. There are three types of interfaces which are as follow;

Solid + solid

Solid + perfect fluid

Free surface

e. image map .jpg

Figure 4.1 Acoustic impedance 2D map of the study area

showing high, moderate and low zones of acoustic

impedance in the study area.

3 d wire frame.jpg

Figure 4.2 3D map of study area showing high acoustic impedance

on south-west region in the study area.

Reflection coefficient can also be described as the ratio of amplitude of reflected and incident waves. Now if Z1 is the acoustic impedance of first layer and Z2 acoustic impedance of second layer then the reflection coefficient is positive provided Z1<Z2 and negative if Z1>Z2. Generally values of the reflection coefficient range from -1 to +1, instead of describing reflection in term of positive and negative. Society of exploration Geophysics (SEG) defines the normal polarity as: "A positive signal produces an upward initial motion on the geophone and recorded as a negative number on the tape, a negative deflection on the monitor record and a trough (white) on the seismic section, so reflection boundary appears to be trough for Z1<Z2 and peak for Z1>Z2".

For Dynamite

(B) (C) (D)

For Vibroseis

(A) (B) (C) (D)

A = Normal polarity C = Normal polarity

B = Reverse polarity D = Reverse polarity

In order to interpret the reflection phenomenon in the study area, seven migrated stack seismic sections were interpreted and the reflection coefficient of studied Miocene succession rock was noted.

The results indicated that in the west and north side of the study area there was high

reflection map.jpg

Figure 4.3 2D map of reflection coefficient showing the high reflection

zone in west region of the study area.

N

3D wire fram .jpg

Figure 4.4 3D reflection coefficient map of study area which showing moderate

zone in south- west region of the area.

values of reflection coefficient occurs while eastern side the values of reflection coefficient were relatively low but on moving towards the south-west region mostly values of reflection coefficient is relatively low. The variation in the value of reflection coefficients is graphically shown in the right side of the Figure 4.3.

In order to interpret the behavior of Miocene rocks in the subsurface of the study area 3-D model of reflection coefficient was generated, (Figure 4.4). This model clearly shows that the value of reflection coefficient is low on south-west side of the study area which indicates that there is soft rock (shale), where as in the west region of the study area shows the high values of the reflection coefficient, which indicates the compacted layer (compacted such as limestone).

4.2.3 Transmission Coefficient:

An equation for transmission coefficient is similar to that for the reflection coefficient, and is derived by using the same assumptions, gives the normal incidence transmission coefficient.

The ratio of transmitted energy in a P-wave (Et) to incident energy (Ei) is:

Et / Ei = {(2V1* ρ1) / (V2* ρ2 + V1* ρ1)} 2 …………….. 4.3

The square root of equation 4.3 is termed as transmission coefficient (T). The transmission Coefficient also expresses the relative amplitude of the transmitted to the incident waves.

T = At / Ai = {(2V1* ρ1) / (V2* ρ2 + V1* ρ1)} ……………. 4.4

For displacement and particle velocity, it is equal to 1-R, while for pressure it is equal to 1+R. Transmission coefficient can have values from 0 to 2. For any value greater than 1 the transmitted amplitude is larger than the incident amplitude.

From the Figure 4.5 drawn for the transmission coefficient it is noted that values of the transmission coefficient were relatively high in the south west region of the study

Figure 4.5 2D map of transmission coefficient which showing the moderate and low zone in south-west region of the study area

transmision map.jpg

3 d wire fram.jpg

Figure 4.6 3D map of transmission coefficient showing high value in

South-west region of the study area.

area and very low values occur in west region of the study area (blue color indicates the low transmission coefficient zone).The high values of transmission coefficient might tend to be the soft surface (such as shale or sandstone) layers while low values indicate the surface might be hard (limestone or sandstone). The comparison of 2-D and 3-D model of the transmission coefficient shows same result.

The relationship between 2D and 3D models (Figure 4.5 and 4.6), shows that south-west zone comprises relatively high values of transmition coefficient as compared to the values of north-west and south- east region of the study area.

4.2.4 INTERVAL VELOCITY:

If two reflectors at depths 'd1' and 'd2' give reflections having respective one way times of 't1' and 't2', the interval velocity 'Vint' between the 'z1' and 'z2' is defined simply as:

………………….4.5

The individual layer velocities are called interval velocities because they indicate the specific reflectors.

In order to interpret the thickness of Miocene succession in the studied area the interval velocity values were calculated and these values were modeled two dimensionally and three dimensionally to identify the zone of high velocities and low velocities which gave the thickness of the Miocene lithologies. On the north-western region, the 2-D model indicates the presence of high interval velocity zone and on the western margin it shows low interval velocity values which indicate that at the western margin mostly hard layer were present. On the other side, the north east region of the study area has low velocity values which are compared with 3-D model of the interval velocity shown in Figure 4.7 and 4.8.

4.2.5 Density Determination

Density of rock is a major property which describes the amount of solid part of the rock body per unit volume. Simply mass per unit volume is called density. The attenuation

V interval map.jpg

Figure 4.7 2D map of interval velocity showing high velocity

in south-west zone of the study area.

Figure 4.7 3D map of interval velocity showing high velocity in south-west

zone of the study area.

N3 d wire frame.jpg

in dense rock is relatively high. The case is reverse for the less dense rocks. Seismic velocity is inversely proportional to density. Direct estimation of density from seismic velocities has been done by using the following formula;

ρ = 0.31 * (Vint) 0.25 ………………. 4.7

Where

ρ = Density

Vint = Interval velocity in m/sec.

The velocity is inversely proportional to the square root of density, but it is common observation that velocity appears to increase with density. The reason is that compaction (density) reduces porosity and increases elasticity in such a way that it offsets the effect introduced by density increase.

Figure 4.8 shows the density model with base map of the area. In this figure, the less dense rock layers were identified in the south-western region of the study area shown by blue color and on the other hand north-west and south-east region of the study area comprises relatively more dense rocks shown by pale yellow color in the Figure 4.8. From the previous study it was also cleared that the zone of low density is best for hydrocarbon accumulation (Galpenergia, 2011).

The comparison of 2D and 3D models (Figure 4.9) shows that south-west zone comprises relatively low dense rocks as compared to rocks present in north-west and south- east region of the study area.

density+ base map1.jpg

Figure 4.8 3D density- base map of the study area showing

high density on north-east region of the study area.

N

3D wire fram-1 copy.jpg

Figure 4.9 3D density map of the study area showing low density

rock in south-western region of study area.

N

4.3 IDENTIFICATION OF LITHOLOGY OF DIFFERENT FORMATION

Every rock or material has a range of density, so by calculating the density, we can identify the type of rock. In this way the lithology of reflector can also be identified.

DENSITY OF COMMON ROCKS

Rock

Density Range(g/cc)

Shale

1.95-2.40

Limestone and dolomite

2.20-2.85

Sandstone

2.10-2.60

Soil and alluvium

1.65-2.20

Table 4.1 general values of the densities of different rock

Robinson, 1988).

4.3.1 Lithology identification a general procedure

There are two main parameter require to identify the lithology of different formation.

Interval velocity (Vint)

Density (ρ)

These two parameter were used to interpret the lithology by simple correlation of the average interval velocity and average density of the formation on a standard modified graph of Gardner, et al., (1974), in which bulk densities versus velocity for different lithology were plotted in Figure 4.10.

4.3.2 Velocity-Density-Lithology Correlations

In the present study interval velocities were calculated in feet/sec and density in gm/cm3. By using standard graph of Gardner, et al., (1974), a perpendicular line on the basis of average calculated value of density and a horizontal line on the basis of average calculated interval velocity value are projected for each contact. The point of intersection

Figure 4.10 Generalized graph of Velocity-Density relation in rocks of different lithology modified after Gardner, et al, (1974).

model .jpg

of these two lines gives the appropriate type of lithology which is shown in Figures

4.11 to 4.17.

An illustration of the wide range of P-wave velocities and lesser range of bulk densities for the more prevalent sedimentary rock types through a wide range of depths is given in figure 4.10

In the present study the lithology of Miocene succession is interpreted using seven seismic lines. The top and bottom time, depth of marked reflector gives the average interval velocity and using interval velocity the average density valued is calculated which is then used to find the density at the top and bottom of the reflector. In this manner the lithology was identified by supper imposing the lines drawn corresponds to density and interval velocity on graph developed by Gardner, et al., (1974). The Lithological interpretation of seismic lines is given below.

4.3.3 Lithology identification of Line No 86-9007

The interval velocity in feet/sec and bulk density in gm/cm3 were calculated for each shot point of reflector, the average of the bulk density and interval velocity were then obtained for studied reflector which is 10199 feet/sec and average bulk density is 2.3 gm/cm3. Then a perpendicular line was drawn corresponding to the density value of 2.3 gm/cm3 and a horizontal line drawn corresponds to the interval velocity 10199 feet/sec on standard graph of Gardner, et al., (1974). The point of intersection of these two lines gives the type of lithology of reflector, shown in Figure 4.11. The lithology identified for the reflector might be shale. The results were verified form well data located in the studied area (Appendix 3).

4.3.4 Lithology identification of Line No 86-9013

The average interval velocity calculated from seismic section line number 86-9013 is 10397 feet/sec and average bulk density is 2.4 gm/cm3. A perpendicular and horizontal line was drawn corresponding to 2.4 gm/cm3 and 12532 feet/sec respectively in the similar fashion as done for the previous line on standard graph of Gardner, et al., (1974). The points of intersection of these two lines lie near to shale line and by

line # 86-9007.jpg

Figure 4.11 Lithological interpretation of line # 86-9007 showing the shale is present in this region.

line # 9013.jpg

Figure 4.12 Lithological interpretation of line # 86-9013 is showing the presence of shale on the basis of velocity and density.

correlating with well log data (Appendix 3), the Miocene rock comprises mostly shale in this line, (see Figure 4.12.)

4.4.5 Lithology identification of Line No 86-9029

A similar method was applied for this line as stated above. A perpendicular and horizontal line was drawn corresponding to 2.2 gm/cm3 and 9880 feet/sec respectively on standard graph of Gardner, et al., (1974). The points of intersection of these two lines also lie near to shale line and by correlating with well log data (Appendix 3) the Miocene succession comprises mostly shale in this line, (Figure 4.13).

4.3.6 Lithology identification of Line No 86-9072

A perpendicular and horizontal line was drawn corresponding to 2.37 gm/cm3 and 11374 feet/sec respectively on standard graph of Gardner, et al; (1974). The points of intersection of these two lines lie near to sandstone line and by correlating with well log data the Miocene rock (Appendix 3), comprises mostly sandstone in this line, see Figure 4.14.

4.4.7.1 Lithology identification of Line No 86-9052

For this line a perpendicular and horizontal line was drawn corresponding to 2.30gm/cm3 and 9943 feet/sec respectively on standard graph of Gardner, et al., (1974). The points of intersection of these two lines also lie near to shale line and by correlating with well log data the Miocene rock comprises mostly shale in this line, see figure 4.15.

4.3.8 Lithology identification of Line No 86-9068

For this line a perpendicular and horizontal line was drawn corresponding to 2.2 gm/cm3 and 9876 feet/sec respectively on standard graph of Gardner, et al., (1974). The points of intersection of these two lines also lie near to shale line and by correlating with well log data (Appendix 3), the Miocene succession comprises mostly shale in this line, see Figure 4.16.

4.3.9 Lithology identification of Line No 86-9033

For this line a perpendicular and horizontal line was drawn corresponding to 2.31 gm/cm3 and 10052 feet/sec respectively on standard graph of Gardner, et al., (1974). The points of intersection of these two lines also lie near to shale line and by correlating with

well log data (Appendix 3) the Miocene rock comprises mostly shale in this line, see Figure 4.17.

line # 9029.jpg

Figure 4.13 Lithological interpretation of line # 86-9029also

showing shale in this region of study area.

Figure 4.14 Lithological interpretation of line # 86-9072 is showing sandstone in this region of the study area.

line # 9072.jpg

Figure 4.15 Lithological interpretation of line # 86-9052 which shows

the shale in the zone of the study area.

line #9052.jpg

Figure 4.16 Lithological interpretation of line # 86-9068 which shows the shale in the zone of the study area.

line # 9068.jpg

line # 9033.jpg

Figure 4.17 Lithological interpretation of line # 86-9033

is lie near the sandstone in the study area.

3.4 Conclusions

The seven seismic lines have been interpreted, which resulted in the construction of Acoustic Impedance on the basis of density and velocities from seismic line of the study area, Transmition and Reflection coefficient and interval velocity maps. The lithology of the different formation has been identified by using standard graph of Gardner, et al., (1974) in study area. All the results are prepared by using the Vertical Seismic Profile (VSP) data. From the interpretation of all seismic lines the results are:

The values of acoustic impedance are gradually increases, from 2000 to 17000 in south west region. And in Reflection coefficient map their value is varies from 0.3 to -0.4 (Figure 4.1 to 4.4). All these values form models, which show that the area of interest having soft surface might be shale or sand stone.

In the transmition coefficient which showing the values were increases from 0.7 to 1.4 gradually in south west region, whereas interval velocity also the high values in south west region varies from 200 to 6200. From the values of the transmition coefficient and in interval velocity it shows that this region also having shale of sand stone, see Figure 4.5 to 4.8.

In the present study the lithology of Miocene succession is interpreted using seven seismic lines. Time and depth of marked reflector gives the average interval velocity, by using interval velocity the average density valued is calculated which is used to determined the density at the top and bottom of the reflector. The identified lithology from the reflector might be shale, for line numbers 86-9013, 86-9029, 86-9052, 86-9068, 86-9033, and sandstone for line number 86-9072. The results were verified form well data located (Appendix 5) in the studied area.

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