Subsurface Structures on the Basis of Seismic Reflection Data Research Paper
In this dissertation, focus is placed on the structural interpretation of the Khipro block in order to demarcate the probable zone for the accumulation of hydrocarbons. This thesis work includes preparation of synthetic seismogram of Bilal North-01 well. Analysis of geophysical borehole logs provides one of the best approaches to characterizing rocks within boreholes. So Facies analysis is also done in order to identify lithologies. Another important tool called Gassmann Fluid Substitution is used which provides a tool for fluid identification and quantification in reservoir.
For the interpretation of the seismic lines, two reflectors are marked by correlating synthetic seismogram on seismic section. As the area of study lies in the Lower Indus Basin, horst and graben geometry in this region is common which is confirmed by fault polygon and time and depth contours made from time and depth grid respectively.
Facies modeling is one of the reliable tool for the confirmation of lithologies. In this dissertation, with the help of facies analysis of Naimat Basal-01 well, we came to the result revealing sand as the reservoir lithology.
At the end Gassmann’s theory of fluid substitution is also applied on Siraj South well to get the response of logs in the presence of different fluids, which is helpful in 4-D analysis.
The purpose of this dissertation is to interpret the subsurface structures on the basis of seismic reflection data. We try to transform the whole seismic information into structural or strati graphical model of the earth through the seismic interpretation. As seismic section represents the geological model of earth, we try to find out the final zone of anomaly by interpretation. According to Sheriff (1999) it is very rare that we can make sharp boundary of correct or incorrect interpretation because actual geology is rarely well known. The test of good interpretation is consistency rather than correctness. Good interpretation be consistent with all the seismic data but it is also important to know all about the area, including gravity and magnetic data, well information, surface geology as well as geologic and physical concept (Sheriff, 1999).
Facies analysis is done in order to confirm the reseroir lithology. Different Seismic parameters are used in facies analysis in order to get information other than stuctural. Chopra (2005) describes the seismic facies unit can be defined as a sedimentary unit which is different from adjacent units in its seismic characteristics. Seismic facies analysis should take the parameters under consideration are: reflection amplitude, dominant reflection frequency, reflection polarity, interval velocity, reflection continuity, reflection configuration, abundance of reflections, geometry of seismic facies unit, and relationship with other units. Paleoecology is always helpful in understanding the depositional environment in interpretation. Porosity and permeability in carbonates are affected by depositional textures. In sequence stratigraphic interpretation, lateral nad vertical spread of facies is very helpful. A review of possibilities and usefulness of seismic facies analysis is given in oil exploration (Chopra, 2005).
Gassmann’s fluid substitutions also done to observe the characteristics of the reservoir in the presence of different fluids (oil, gas. Fluid substitution is an important part of seismic rock physics analysis (Kumar, 2006) (e.g., AVO, 4-D analysis) which provides a tool for fluid identification and quantification in reservoir. This is commonly performed using Gassamann’s equation (Gassamann, 1951).
The main objectives of this dissertation based on interpretation of seismic section are:
- Structural and stratigraphic interpretation to find out the structural traps and horizons of the formation.
- Generating Synthetic seismogram in order to mark horizons.
- Facies analysis to investigate the depositional environment.
- Gassmann Fluid Substitution to observe characteristics of reservoir rock in the presence of different fluids.
Seismic reflection, well and navigation data which consist of following formats respectively;
A SEG-Y file, whether on tape or disk, is composed of a 3200 byte EBCDIC-format header, followed by a 400 byte binary header, which in turn is followed by individual data traces. Each data trace is composed of a 240 byte trace header followed by the 4 byte SEG-Y samples comprising the actual data for that trace.
Geophysical logging results are stored in the industry-standard Log ASCII Standard (LAS) file format.
While navigation file provides the exact geographical coordinates of the study area, even the exact location of every shot point
All data sets used were provided by Directorate General of Petroleum Concession (DGPC), Government of Pakistan for the research purpose.
Seismic migrated, un-stacked and unfiltered migrated stack sections of the seismic line from the study area are listed in the table 1.1.
Table 1.1: Seismic reflection data used for base map
|LINE NAME||NATURE||LINE ORIENTATION||WELLS|
|2000KH-08||Dip Line||W-E||Naimat Basal-01|
|2000KH-39||Strike Line||S-N||Bilal North01, Bilal-01|
|2000KH-44||Dip Line||S-N||Siraj South-01|
Fourteen lines are provided to us. Nature of lines and their orientation is given in the Table: 1.1 along with the well names lies on the survey lines.
Information of the well data which has been provided to us for the dissertation. Information of two wells Bilal North-01 and Naimat Basal-01 are listed in table 1.2 and table 1.3 respectively.
Table 1.2: Information of Well Bilal North-01 well
|Total Depth||3153.1600 m|
|Sand Above Talhar Shale||2929.0|
Information of Bilal North-01 well is given in the above table 1.2 along with formation tops. Thickness of any formation can be calculated by this table.
Table 1.3: Information of Naimat Basal-01 Well
|Total Depth||3621.6300 m|
|Sand Above Talhar||3389.2|
1.4 Introduction to Study Area
The area of interest is laying in Southern Indus Basin. Normal faults are more prominent structures of the area (Horst and Graben geometry), which form the structural traps. The identification of these traps is one of the main task for the Geoscientists in exploration of hydrocarbons. To observe these structures the seismic lines 2003KH-44, 2001KH-30 and wells Bilal North-01, Bilal-01, Siraj South and Naimat Basal are provided by the department of Earth Sciences Quaid-e-Azam University Islamabad, in order to interpret the seismic section along the seismic lines. Geographical coordinates of the area is located in Figure 1.1.
Figure 1.1: Showing the location of the study area
A base map is a map on which primary data and interpretation can be plotted. A base map typically includes location of concession boundaries, wells, seismic survey points and length of seismic spread, longitude and latitude of the study area. Following 2-D reflection seismic lines are used to construct the Base map of 2-D seismic survey for given study area. Base map of the study area shown in Figure 1.2.
Figure 1.2: Showing the base map of the study area
1.5 Previous Exploration History
OPII, as the Operator, had achieved five Oil & Gas discoveries out of fifteen exploratory wells drilled in this concession with a success ratio of 1:3. Now the Operator ship stands transferred to British Petroleum along with its entire WI. OPII, as the Operator, had made six Oil & Gas discoveries out of the fourteen exploratory wells drilled so far.
Gas production of Khipro Concession is sold to Sui Southern Gas Company Limited under a long term Gas Sale and Purchase Agreement dated 22 July 2006, whilst oil/condensate is delivered to National Refinery and Pakistan Refinery at Karachi.
Seismic reflection data of the Lower Indus Basin of Khipro area is given to us in order to interpret the sub surface structures. In this practice, software Kingdom 8.4 was used. After uploading the seismic lines data on kingdom, synthetic seismogram was generated, from one of the given wells then the faults and horizons were marked and faults on seismic section. Facies modeling is also done for the confirmation of the lithologies in the prospective zone. Gassamann’s theory is also applied on the well at different water saturation level, which helps us in the drilling of the development wells in the same zone or formation. Brief methodology of work flow is shown in the Figure 1.3.
Figure 1.3: Showing work flow of dissertation.
1.7 Software Tools and Applications
SMT Kingdom 8.4
- Synthetic Seismogram
- Structural Interpretation
- Stratigraphic Interpretation
- Facies Analysis
- Gassamann’s Fluid Substitution.
2 Geology and Stratigraphy of the Area
2.1 Introduction to Area
General geology and geological history of an area is very important for exploration of oil and gas. A geological history of basin can be compiled by considering basin forming tectonics and depositional sequence (Kingston et al., 1993)
Rifting and breaks up Gondwana land in Jurassic period is responsible for the formation of Khipro block baisan. East Gondwana plate (India, Antarctica and Australia) separated from the west Gondwana plate (Africa and South America) in the Cretaceous period. In Aptian time (120 Ma), the Indian plate separated from east Gondwana. Powell (1979) defines in article “A speculative Tectonic history of Pakistan and surroundings” that at the end of Cretaceous, Seychelles and Madagascar separated from India with associated faulting resulting in basaltic flows (Deccan Volcanism) in the southern part of lower Indus basin. After Paleocene there were continuous oblique convergence of Asian plate and Indian plate throughout Tertiary time and the collision results in tilting of the entire region. Deposition during the rifting shown by the presence of Jurassic rocks in the area. Due to rifting normal faulting and horst and graben structures are formed. The famous among these structures include “Sukkur Rift”. However, this localized rifting phase was unable to continue after the Paleocene-Eocene time (Powell, 1979; Smith et al., 1994).
2.2 Structural Settings of Study Area
Normal faults are generated as a result extensional tectonics, forming horst and graben structures with former being of great exploratory importance. The extensional tectonics during the Cretaceous time created tilted fault blocks over a wide area of the Eastern lower Indus basin (Kemal et al, 1991). According to Zaigham (2000), Lower Indus basin is characterized by passive roof complex type structure and a passive back thrust along Kirthar fold belt, passive roof thrust forming a frontal culmination wall along the margin of the fold belt and the Kirthar depression and out of syncline intra-molasses detachment in the Kirthar depression sequence. Kirthar and Karachi depression contain several large anticlines and dome sand some of these contain small gas fields e.g. Sari, Hundi, Mazarani, but in eastern part of it, there are several faults and tilted blocks which form structural traps containing small oil and gas fields e.g. Sinjhoro, Khipro, Sanghar. On the northern side, there is Sukkur Rift zone bearing large anticlimax structures and contains Khandkot and Mari gas fields (Zaigham, 2000).
As the Khipro area lies in the foreland side of the Himalayas, so in that region normal faulting is common phenomenon. Faults are generally of low throw. Conjugate faulting is also sometimes present on small scale. As such no great structural variation found in Khipro although book shelf geometry is present.
The structural style present in the area is due to result of a normal block faulting on west dipping Indus Plain. Kadri (1995) describes as the fault planes act as migrating paths for hydrocarbons from underlying shaly source sequence. Trends of faults and contours are mapped utilizing wells and seismic control for the field. Seismic interpretation creates basis for structural interpretation as no surface outcrops are found over the field. Study area characterized by a series of horst and graben structures present almost below the base Paleocene within the cretaceous age Producing reservoir Basal Sand is bounded on east and west by regional extension faults dipping to west and east and trending NW-SE (Kadri, 1995). Structural boundaries of the Khipro block are shown in the Figure 2.1.
Figure 2.1: Showing Structural boundary of the study area (Kadri, 1995)
2.3 Petroleum play of the Study Area
The below stratigraphic chart shows the generalized stratigraphy of the Khipro area
Figure 2.2: Showing the general stratigraphy and petroleum play of the study area (Zaigham et al., 2000).
Stratigraphic column (Figure 2.2) Samber formation and Lower Shales are marked as the source rock and Basal Sands act as reservoir rock. Upper Goru Shale is identified as the seal rock. Chiltan limestone is also one of the good source rock in this area. In the third chapter, Lower Goru and Basal Sands is mark on the seismic section as reservoir rocks and further seismic studies applied.
3.1 Seismic Surveying
In seismic surveying, seismic waves are created by a controlled source and propagate through the subsurface. Some waves will return to the surface after refraction or reflection at geological boundaries within the subsurface. Instruments distributed along the surface detect the ground motion caused by these returning waves and hence measure the arrival times of the waves at different ranges from the source. These travel times may be converted into depth values and, hence the interfaces of subsurface lithologies can be marked (Kearey et al., 2002).
3.2 Seismic Methods
Seismic Methods deal with the use of artificially generated elastic waves to locate hydrocarbon deposits, geothermal reservoirs, groundwater, archaeological sites, and to obtain geological information for engineering. It provides geophysical, borehole and geological data, and with concepts of physics and geology, can provide information about the structure and distribution of rock types in the subsurface (Kearey et al., 2002).
There are two types of seismic methods:
- Seismic refraction method
- Seismic reflection method
3.2.1 Seismic Reflection Method
In seismic reflection surveys seismic energy pulses are recorded which are reflected back from the subsurface interfaces. The travel times are measured and can be converted into estimates of depths to the interfaces (Kearey et al., 2002). Depth of reflecting interfaces can be estimating from the recorded time and velocity information that can be obtain either from reflected signal themselves or from surveys in well.(Dobrin & Savit, 1988). Velocity may also vary horizontally, due to lateral lithological changes within the individual layers (Kearey et al., 2002).
3.3 Seismic Data Acquisition
In Seismic data acquisition, continuous seismic signals from seismic stations are gathered and recorded properly. In data acquisition, earth response caused due to seismic source is recorded. Kearey (2002) describes geophones are used to record seismic waves, in case of marine survey hydrophone is use. The basic field activity in seismic surveying is the recording of seismic data which may be defined as analog or digital time series that record the amplitude of ground motion as a function of time during the passage of seismic waves. The Acquisition starts from shot and ends at recording the seismic events through various steps. Different energy sources are used to produce seismic waves and array of geophones are used to detect the resulting motion of earth. The resulting data, combined with assumptions about the velocity of the waves through the rocks and the density of the rocks, are interpreted to generate maps of the formations. Fundamental purpose of seismic data acquisition is to record the ground motion caused by a known source in a known location. First step in seismic data acquisition is to generate a seismic pulse with a suitable source. Second is to detect and record the seismic waves propagating through ground with a suitable receiver in digital or analogue form. Third is the registration of data on a tape (Kearey et al., 2002).
3.4 Seismic Interpretation
Seismic interpretation is the technique to get the information about the subsurface using available seismic data. This information may contribute towards the general information of the area, tells the interpreter about the favorable prospects or it is used for further development of already discovered oil and gas field.
Sheriff (1999) describes that it is rare that the correctness or incorrectness of interpretation can be firmed because the actual geology is rarely known in well manner. A good interpretation is consistency with all of the available data. In oil and gas exploration, our focus is on finding an interpretation that is more favorable for hydrocarbon accumulation. As with many scientific investigations, interpretations are almost always non-unique (Sheriff, 1999).
According to Coffeen (1986), most common activity in interpretation is picking a horizon and marking faults. A horizon is a reflection that appears on the seismic section for a considerable geographical extent. The reflections are picked i.e. marked at shot points, along the vertical section over the area. The picks are timed by reading the reflection times. These times can then be plotted on a map and contoured, to show locally high places or other features that may be prospects for drilling.
3.5 Types of Seismic Interpretation
There are two types of seismic interpretation
- Stratigraphical Interpretation
- Structural Interpretation
3.5.1 Stratigraphical Interpretation
According to Telford et al., (1990) stratigraphy analysis involves the delineating the seismic sequences, which present the different depositional units, recognizing the seismic facies characteristic with suggest depositional environment and analysis of the reflection characteristic variation to locate the both stratigraphy change and hydrocarbon depositional environment. 3-D work is especially important in recognizing the stratigraphic feature with distinct shape. The amplitude, velocity, frequency or the change in wave shape indicates hydrocarbon accumulation. Variation of the amplitude with the offset is also an important hydrocarbon indicator. Unconformities are marked by drainage pattern that help to develop the depositional environment. Reef, lenses. Unconformity is example of stratigraphic traps (Telford et al., 1990).
3.5.2 Structural Interpretation
Seismic data interpretation is mainly done on the basis of available information and stratigraphy of the area. Seismic is correlated with the formation tops penetrated in the wells using well tops if available. In this study, seismic interpretation is done by picking horizons in Kingdom suit and reflector is continued in all other seismic lines. Major faults are picked on the dip lines and their parts are correlated across the strike lines to map the structures throughout the area. Two way time (TWT) maps are generated using fault polygons in order to describe the structural inclination at different levels. The study area is in extensional regime, horst and graben structures are present in the area. The horizons which are marked on seismic section show normal faults.
3.6 Seismic Interpretation Workflow
Procedure adapted for interpretation is given in Figure 3.1. Base map is prepared by loading navigation data and Seg-Y in software Kingdom 8.4. Horizons of interest are marked manual with help of synthetic seismogram. In this process faults are identified and also marked on the seismic section. Faults polygons are generated and horizons are contoured to find out structural highs and lows. The different important steps involved in the interpretation workflow are discussed in the Figure 3.1.
Figure 3.1: Workflow for seismic data interpretation
3.7 Synthetic Seismogram
Synthetic seismograms are artificial seismic traces use to establish correlations between local stratigraphy and seismic reflections. To produce a synthetic seismogram a sonic log is needed. Ideally, a density log should also be used, but these are not always available. With the help of Bilal-01 well, we construct the synthetic seismogram (Figure 3.3) in order to mark the horizons.
Synthetic seismograms provide a crucial link between lithological variations within a drill hole and reflectors on seismic profiles crossing the site. In essence, they provide a ground-truth for the interpretation of seismic data. Synthetic seismograms are useful tools for linking drill hole geology to seismic sections, because they can provide a direct link between observed lithologies and seismic reflection patterns (Handwerger et al., 2004). Reflection profiles are sensitive to changes in sediment impedance, the product of compression wave velocity and density. Changes in these two physical parameters do not always correspond to observed changes in lithologies. By creating a synthetic seismogram based on sediment petro-physics, it is possible to identify the origin of seismic reflectors and trace them laterally along the seismic line (Handwerger et al., 2004).
Figure 3.2: Showing Synthetic seismogram of Bilal-01 well
3.8 Picking of Horizons and Fault Identification
Primary task of interpretation is the identification of various horizons as an interface between geological formations. For this purpose, good structural as well as stratigraphic knowledge of the area is required. Thus during interpretation process, I mark both, the horizons and faults on the seismic section (McQuillin, et al., 1984). Using well data of wells Bilal-01, horizons are marked on dip lines 2001-KH-30, 2003-KH-44 and strike lines 2003-KH-39, 2003-KH-35. Bilal-01 is drilled on shot point 686 of line 2003-KH-39 and ties all available dip lines and horizons are continued on all lines.
Finally, marked horizons are named as Basal Sands and Top of Lower Goru formation with the help of synthetic seismogram of Bilal-01 well data on the strike line 2003-KH-39 (Figure 3.3).
Figure 3.3: Showing Synthetic Seismogram on Seismic section of line 2033-KH-39
Study area lies in extensional regime dominated by normal faults and associated horst and graben structures. The identification of faults was difficult to some extent due to data quality. The average throw of the faults is observed to be about 15 – 20 ms. On the basis of discontinuity in time, seven normal faults have been marked on the seismic sections of the line 2001-KH-30 (Figure 3.4) forming the horst and graben features. Clues of normal faulting exist on all of interpreted seismic lines.
Figure 3.4: Showing normal faulting on Interpreted seismic dip line 2001-KH-30
Here is the Seismic section shown (Figure 3.4) on which seven faults are marked on the basis of continuity of reflectors and two formation Basal sand and Top of Lower Goru is marked with the help of synthetic seismogram. After marking Horizons and faults, we got good horst and graben geometry structures, which with matches our previous geological information.
Figure 3.5: Seismic Section of dip line 2003-KH-44 showing horst and graben geometry
Seismic section of line 2003-KH-44 shown in Figure 3.5 also shows the book shelf geometry as the previous section (Figure 3.4) shows. As the extension of this line is comparatively smaller than the others so only five faults are marked on the section.
3.9 Construction of Fault Polygons
Construction of fault polygons are very important as far as time contouring of a particular horizon is concerned. Fault polygons shows the extension of faults on the base map which me mark on seismic lines. So it gives us a quick view about the structural disturbance in the area. Any mapping software needs all faults to be converted into polygons prior to contouring. The reason is that if a fault is not converted into a polygon, software doesn’t recognize it as a barrier or discontinuity, thus making any possible closures against faults represents a false picture of the subsurface structures.
Figure 3.6: Fault polygon at Top of Lower Goru level showing normal faulting
In Figure 3.6, the color variation along both sides of polygon depict the change in time of seismic reflected waves due to presence of faults. If the time is noticed along the boundaries of faults polygons, it came to the result as the normal faulting the area also confirms the horst and graben geometry.
Figure 3.7: Fault polygon at Basal Sand level
Fault polygon formed at Basal Sand level shown in Figure 3.7 also shows alternate different color due to time change along both sides of polygon, showing normal faulting pattern. Shape in nature of fault polygons almost similar with polygons formed at Top of Lower Goru level.
3.10 Contour Maps
Contouring is the essential constituent of the interpretation of the seismic data. Seismic interpretation actually displays the most essential information extracted during interpretation in the form of time and depth contour maps. The contours are the lines of the same time or depth roving about the map as dictated by the data (Coffeen, 1986).
3.10.1 Time Contouring
After completing horizons and fault interpretation time contour maps are constructed. There are some reasons for making time maps. The times are read directly from the sections and are immediately available for mapping. The pattern of Time Contour map confirms the shape of the subsurface structure. Time contour maps of these formations show 2D-variations with respect to time and the hydrocarbons probably accumulate at those places where contour values are low.
Figure 3.8: Time contour at Top of Lower Goru level showing change in time on the base map
Top of Lower Goru grid (Figure 3.8) shows alternate color along the boundaries of fault polygons. If we notice the time values, we can clearly see the presence of normal faulting with horst and graben geometry. Zone surrounded by the seismic line 2003-KH-39 is the shallower zone in the base map, so there is maximum chance of accumulation of hydrocarbon in that elevated zone.
Figure 3.9: Time contour at Basal Sand level showing color variation in time along polygons
Time contour map of Basal Sand also shows the same pattern of variation in time through the base map as the Lower Goru formation. This alternate color variation is the sign of horst and graben geometry and time variation proves that it is normal faulting. N-E region is found to be uplifted area as compared to S-W region.
3.10.2 Depth Contouring
When we read the time of a horizon from the section it tends to show the structure of the horizon in the subsurface, it does not show us the structure directly. Depth conversion and depth contour maps are constructed to see the horizons in the subsurface at their true positions. Depth must be calculated from time to make a map that is more truly related to the subsurface shapes, because structure is a matter of depth. The idea of converting the times into depths is very reasonable in case of showing the subsurface structures.
Figure 3.10: Showing Variation in depth of the Basal Sand in the base map
As Basal sand is recognize as a good reservoir rock of the Lower Indus Basin, depth contour map of this formation is constructed shown in Figure 3.10. Contouring pattern shows N-E region as relatively uplifted area. Structural pattern of the area also confirm by contouring as the horst and graben geometry. Region shown in blue color is the shallowest horst with low values of depth might be the good zone for the accumulation of hydrocarbons because hydrocarbons move towards low pressure area.
A rock or stratified body distinguished from others by its appearance or composition.
It can be defined as: The characteristics of a rock or series of rocks reflecting their appearance, composition, and conditions of formation (Lucia. 1995).
4.2 Facies Types
4.2.1 Sedimentary Facies
Sedimentary facies are bodies of sediment recognizably different from adjacent sediment deposited in a different depositional environment
4.2.2 Metamorphic Facies
The sequences of minerals that develop during progressive metamorphism define a facies series.
4.3 Walther’s Law of Facies
Walther’s Law of Facies, or simply Walther’s Law, states that the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when a depositional environment “migrates” laterally, sediments of one depositional environment come to lie on top of another. A classic example of this law is the vertical stratigraphic succession that typifies marine transgressions and regressions. However, the law is not applicable where the contact between different lithologies is non-conformable (Lucia 1995).
4.4 Facies Analysis
Fundamental to all subsurface geologic studies is an analysis of depositional facies. Lucia (1995) describes development of a facies classification scheme is a particular challenging interplay between capturing enough information for environmental interpretation yet remaining simple. Particularly important is the characterization of facies such that their recognition criteria relate to critical environmental thresholds such as sea level, normal wave base, and storm wave base. These physical environmental zones regulate sedimentary textures and biotic assemblages. A good understanding of paleoecology always strengthen the interpretation and such studies should be included as part of all depositional facies studies. Depositional textures in turn affect porosity-permeability in carbonates. The vertical and lateral organization of facies is an exercise essential to sequence stratigraphic interpretations. (Lucia 1995).
4.5 Facies Analysis Procedure
From the KINGDOM software main window menu bar, choose Tools > Cross plot > New to open the Select Data dialog box. Window will appear shown in Figure 4.1.
Data for each axis
X =Rho b
Figure 4.1: Facies models showing different cluster points of lithologies
Figure 4.2: Facies model showing the Sand and Shaly Sand in Naimat Basal-01 well
4.6 Results of Facies Analysis
There are two clusters of data points in the Figure 4.2. The high resistivity values and corresponding low gamma ray values indicate clean sands. High gamma ray values associated with low resistivity values indicate shale.
Fluid substitution is an important part of seismic rock physics analysis (e.g., AVO, 4-D analysis) which provides a tool for fluid identification and quantification in reservoir. This is commonly performed using Gassamann’s equation (Gassamann, 1951). Many authors (Batzle and Wang, 1992; Berryman, 1999; Wang, 2001; Smith et al., 2003; Russell et al., 2003; Han and Batzle, 2004) have discussed the formulations, strength and limitations of the Gassmann fluid substitution (Kumar 2006).
5.2 Gassmann’s Fluid Substitution
Saturated porous reservoir rock contains fluid and rock matrix. Porous rock is called dry rock, when there is no fluid in pores. Production of oil from a reservoir affects the fluid part, and does not affect solid part. Kumar (2006) describes these fluctuations affect as the elastic constants (bulk modulus (K), shear modulus ( ) and density ( ) etc), which affect the velocity of seismic wave. K of dry and water saturated rocks ( and ) are more sensitive to water saturation than P wave velocity ( ) under same condition of pressure (Batzle and Wang, 2004)). Furthermore, water saturation has slight effects on μ, as shear modulus of fluid ( ) is zero.
Gassmann’s model calculates and of saturated rock by computing K, μ and ρ for saturated reservoir rock, in order to efficiently replicate effects of fluid substitution. The objective of fluid substitution is to model the seismic properties (seismic velocities) and density of a reservoir at a given reservoir condition (e.g., pressure, temperature, porosity, mineral type, and water salinity) and pore fluid saturation such as 100% water saturation or hydrocarbon with only oil or only gas saturation (Kumar, 2006). Seismic velocity of anisotropic material can be estimated using known rock moduli and density. P- and S-wave velocities in isotropic media are estimated as,
respectively, where and are the P- and S-wave velocity, K and are the bulk and shear moduli, and is the mass density. Density of a saturated rock can be simply computed with the volume averaging equation (mass balance). Other parameters required to estimate seismic velocity after fluid substitution are the moduli and which can be computed using the Gassmann’s equations.
5.3 Gassmann’s Equations
Gassmann’s equations relate the bulk modulus of a rock to its pore, frame, and fluid properties as
where , , , and are the bulk moduli of the saturated rock, porous rock frame (drained of any pore-filling fluid), mineral matrix, and pore fluid, respectively, and φ is porosity (as fraction). In the Gassmann formulation shear modulus is independent of the pore fluid and held constant during the fluid substitutions. Bulk modulus ( ) and shear modulus (µ) at in-situ (or initial) condition can be estimated from the wire line log data (seismic velocities and density) by rewriting equations 1 and 2 as
5.4 Gassmann Fluid Substitution Wizard via Kingdom Software
The Gassmann Fluid Substitution Wizard uses Gassmann’s theory to create new velocity and density logs. Gassmann’s theory describes seismic wave propagation in a fluid-saturated porous solid using mathematical relationships between various properties of the rock skeleton (the solid material) and the fluid filling the pore space.
There are three steps to completing Gassmann’s wizard:
Select a well, borehole, and porosity source (Figure 5.1).
Figure 5.1: Window of Gassmann’s Substitution First step
As the Siraj South is producing well, so we select that well for Fluid Substitution.
Select the density/porosity and velocity logs, define the skeleton and fluid parameters, and digitize the substitution regions (Figure 5.2).
5.4.1 Gassmann Fluid Substitution Fluid Parameters as Water and Gas
Figure 5.2: Window of Gassmann’s Fluid Substitution second step
As mention earlier, in this step we input the density and velocity log of the Siraj South well. Then I mark the probable prospective zone (Lower Goru formation thickness) which act as reservoir for hydrocarbon accumulation. After that we define the parameters (density and bulk modulus) of skeleton and types of fluid present in reservoir rock.
By clicking<Next> in the windows, we get the computed density and velocity log (Figure 5.3).
Figure 5.3: Input and computed velocity and density logs (for fluid as water and gas)
Save the resulting density and velocity log curves.
5.4.2 Gassmann Fluid Substitution Fluid Parameters as Water and Oil
Figure 5.4: Window of Gassmann’s Fluid Substitution second step (for fluid as water and oil)
Second step of Gassmann fluid substitution for oil and water as fluids is shown in the Figure 5.4
Figure 5.5: Input and computed velocity and density logs (for fluid as water and oil)
Variation in density and velocity is shown in Figure 5.5 due to Gassmann Fluid Substitution theory.
5.5 Results of Gassmann Fluid Substitution
We can clearly see that after applying the Gassamann’s fluid substitution, there decrease in density and velocity in the marked zone. When fluid parameters are water and gas, there is more decrease in density and velocity (Figure 5.3) as compared when fluids are water and oil (Figure 5.5). This is because density of gas of gas is less than oil, so velocity of waves is low in gas medium. We can generate more computed curve by changing the water saturation level and other fluids in the reservoir zone can estimate the amount of hydrocarbon present in the reservoir at different water saturation level. These log curves helps in the drilling of development wells. These new logs can also be saved and used to generate a new synthetic seismogram, which shows better results.
Discussion and Conclusions
Seismic reflection surveying is the most widely used and well-known geophysical technique specially used for hydrocarbon exploration. By this technique we can extract the information of geological structures on scales from the top tens of meters of drift to the whole lithosphere. Part of the spectacular success of the method lies in the fact that the raw data are processed to produce a seismic section which is an image of the subsurface structure.
Reflectors of two formations Basal Sand and Top of Lower Goru are marked on seismic section, with the help of synthetic seismogram of Bilal-01 well. Time and depth contour maps show the presence of horst and graben structures in study area. These Uplifted horst structures may act as structural traps in the area and are favorable places for hydrocarbon accumulation.
For the confirmation of reservoir lithology Facies analysis is done of Naimat Basal-01 well which reveals the result as the reservoir lithology as sand.
After Gassamann Fluid Substitution, there is decrease in both density and velocity in the marked (reservoir) area. There is comparatively less decrease in density and velocity in the presence of oil as compared to gas, it is because of density contrast of oil and gas.
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