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The study of structural style generally has been accepted as interpretation criteria in the assessment of hydrocarbon zones with respect to characteristics of velocities.
The study area is located in the Southern Indus Basin, Sindh Province, Pakistan, which has been known to prone for hydrocarbon evaluation. The research focuses on building and implementing a workflow for structural interpretation, velocity modeling and estimating physical properties (Shear Modulus, Shear Impedance and Vp/Vs ratio) of rocks using migrated seismic sections. In order to confirm the presence of hydrocarbons in the study area different types of velocities and physical properties of rocks from multicomponent seismic data are estimated.
Few specific issues in the use of seismic data for hydrocarbon evaluation based on modeling and real data are addressed that include distinguishing the source, reservoir and cap rocks, the effects of faults and folds and presence of hydrocarbons. Difference scheme for interpretation and modeling the seismic data by using softwares (SURFER 10, Rockworks 2006, and K-tron Vas) were implemented as 2D and 3D imaginary surfaces maps for this purpose.
The study of structural interpretation and modeling of the given seismic data have been carried out for the identification of traps. Appropriate structural traps for hydrocarbons are interpreted in the Southeast and East and in the Northwest and Northeast the results demonstrate that. Locally normal faults are present which are resulted by extensional tectonics during Eocene (Aamer, et al., 2005).
The study of 2D and 3D velocity modeling provides many insights into, the velocity modeling of the horizons of interest indicate that North-Eastern and South-Eastern parts of study area comprises low velocity zones while North-Western and South-Western parts comprise relatively high velocity zones. This variation in velocity is due to lithological variation in formations.
The drilled well in the study area (Fateh-01 and Ichhri-01) were found dried. The reason might not be the presence of suitable traps in case of Ichri-01 Well and in case of Fateh-01 Well; it drilled below the oil-water contact or due to hydrocarbons needed more time for maturity.
From the results of applied workflow for physical properties of rocks it is determined that the study area has low values of shear modulus and shear impedance in the Eastern, Northeastern and Southeastern parts, and the relatively high values of in the Western, Northwestern and Southwestern parts.
The resulted models clearly show the presence of horst and graben structures in the study area which is interpreted by low values of shear modulus and shear impedance. The Vp/Vs models illustrate the decreasing value of Vp/Vs; which indicates potential zone for hydrocarbon in this study area.
For more than a century, progress in petroleum exploration techniques has allowed us the continuous discovery of new hydrocarbon fields as new domains become open for exploration. This expansion of the research area is coming to an end, and it is becoming necessary to concentrate the exploration efforts in already studied areas. We must search for fields with underestimated potential, or fields for which the complex geometry could have limited the exploitation.
The development of more and more sophisticated imaging techniques answers this need, and gives access to a better understanding of the petroleum systems under study, as well as to the detection of smaller prospects, situated in complex areas. The complexity of these new exploration and exploitation techniques require from the user are the assumption of an important amount of modeling hypotheses, and strategic choices in the selection of the methods used to image the substratum.
Estimating subsurface lithological properties in the geosciences is a challenging problem, always subject to uncertainty. The main contributing causes for this uncertainty are limited measurement resolution, insufficient measurements relative to the subsurface complexities, a limited understanding of the physical and geological phenomena, and natural variability of the target rock properties. Each geoscience discipline brings different information often complementary, but sometimes contradictory about the subsurface heterogeneities. Therefore, integrating different types of geological and geophysical information can better constrain predictions of the subsurface. The challenge is to combine quantitatively these various kinds of statistics into a reliable outcome. The main objective of this thesis is to address this challenge by designing a methodology for structural and velocity modeling of the subsurface and applications of these models in hydrocarbon evaluation. In the present work, an attempt has been made to develop an integrated structural model for Jurassic to Cretaceous successions including Chiltan Limestone, Lower and Upper Goru Formation (Figure 1.1) of Bitrism Block. This model will be developed by correlating the results obtained from the seismic data.
The secondary objective of this study is to define the hydrocarbon traps.
1.2 Location of the Study Area
Bitrism Block covers an area of approximately 18,197 Km2 and lies in the Southern Indus Basin just South of Sukkar Rift. The coordinates of the study area are 26Â° 16' to 26Â° 29' N and 68Â° 54' to 69Â° 05' E and covering Nawabshah, Khairpur, and Sanghar districts of Sindh Province as shown in figure 1.2. The location of Bitrism area falls within Jacobabad Khairpur high in Lower Indus Basin of Pakistan (Ahmad, et al., 2006). Oil and Gas Development Corporation Limited (OGDCL) carried out a number of lines of 2D seismic survey previously in 1996, 1997, and 2000 (Ahmad, et al., 2006).
1.3 Base Map
A base map is constructed by using the data given in Appendix II. The map contains basic information and further interpretations are carried out on it. A base map characteristically includes concession boundaries, well locations and seismic survey points, with geographic references. Base maps are also used to construct surface geological maps. Normally shot point maps are used in geophysics, on which the direction of seismic lines and position of the shot points are shown. This map then used to present the interpretation of provided data. The base map of the study area is presented in figure 1.3.
1.4 Seismic Data
The acquisition of data needed for this research work was a major factor to be considered. All the data required for this research work has been received with gratitude from Directorate General of Petroleum Concessions (DGPC), Islamabad, Pakistan and Land Mark Resources (LMKR), Islamabad, Pakistan.
The navigation data used was converted into geodetic transformations using Everest 1830 ellipsoid and Universal Transverse Mercator (UTM) Zone 42 projection system. A list of seismic lines is given in Appendix I.
1.5 Aims and Objectives
The key outcome of this research program is the documentation of structural traps and hydrocarbon prospect evaluation of the area. An important aspect is the interpretation of seismic data in terms of terms of structural traps. The results of this study will also be more helpful to understand the structural settings of the area. These results can be applied in future as a predictive tool to identify seal, source and structural traps in the un-explored areas. The main objectives of this research work are outlined as follows;
To Mark the structurally trapped horizons in Jurassic to Cretaceous succession as source, reservoir and cap rocks.
Identification of faults on seismic sections to define suitable location for hydrocarbon accumulation.
To interpret the velocities of seismic lines and to construct the seismic velocities models of studied horizons by using average velocity function against shot points and time of horizons.
To prepare 2D and 3D structural contour maps for Jurassic-Cretaceous successions on the basis of time, velocity and depth.
Comparison of velocity models of the Jurassic-Cretaceous succession prepared in two different softwares, i.e., Rockworks 2006 versus Surfer 10. This comparison will lead to a more robust lithological and structural interpretation.
To determine the rockworks parameters (poison ratio, shear modulus, young modulus etc.) for interpretation of hydrocarbon accumulation.
Correlation of wells in the study area for the interpretation of thickness, lateral variations and geological behavior of the studied succession.
Twenty three 2D migrated seismic lines and a composite suite of logs are interpreted in this research work. For structural interpretation the reflectors are marked on the basis of continuity of reflectors and with the help of synthetic seismogram. Faults are also interpreted from the vertical seismic sections provided. The processing, analysis, and visualization of velocity data to create useful velocity distribution models. This data is derived by primary information (Root Mean Square Velocity and times) given on seismic sections provided. This velocity data is also used in processing, interpretations, and rock physics applications. For data interpretation different softwares are used such as K-tron Vas, Surfer 10, Rocworks 2006 and Strater 2.0 developed by Golden Softwares Incorporation.
1.7 Structure of the Thesis
This thesis contains six chapters which are as follows:
Chapter 1-Introduction: This chapter describes the objectives and structure of the thesis, as well as an introduction to the study area including the methodology of the research work.
Chapter 2-Geology and Structural Settings of the Area: A chapter which gives an overview of the Geology of the study area of Bitrisim, as well as an outline of Petroleum System (source rock, reservoir rock and seal/cap rock) of the area.
Chapter 3-Seismic Data Structural Interpretation: A chapter which gives the details of the dataset as well as an introduction to the seismic method and the interpretation tools used in this thesis.
Chapter 4-Seismic Velocities Modeling: This chapter presents a detailed description of seismic velocities modeling, methodology, results and conclusions.
Chapter 5-Rock Physics: This chapter gives details of different physical properties of rocks, research methodology, results and conclusions discussed in terms of what they indicate for hydrocarbon evaluation.
Chapter 6-Results and Discussions of the Research: A short summation of the most important conclusions of this research.
Geology and Structural Settings of the area
GEOLOGY AND STRUCTURAL SETTINGS OF THE AREA
For accurate interpretation of seismic data the geological information of the area acts as significant part. The structural and stratigraphy changes in subsurface apear in different reflections on seismic section, so as if we donot know the exact surface geological informations of area we donot recognize the different reflections appearing in the seismic section.
The Indus Basin comprises the eastern two-thirds of the country and contains the Precambrian to Recent sedimentary successions. The basin axes generally runs parallel to the course of the River Indus (Kingston, 1986). As reported by Malkani (2010), "the Axial Belt (Suture Zone) seperates the Indus Basin (Southern Earth) from the Balochistan and Northern areas of Tethyan and Laurasian domains (northern earth). The Indus Basin (situated in the North-western part of Indo-Pakistan subcontinent) is located in the central and eastern part of Pakistan and further subdivided in to upper (Kohat and Potwar), middle (Sulaiman) and Lower (Kirthar) basins. The Sulaiman Basin is the largest basin of main Indus Basin and consists of about 170 thousand Km2 (which is almost 40% area of Indus Basin), while the Southern Indus Kirthar basin shows 120 thousand Km2 and Kohat and Potwar basin shows about 100 thousand Km2 (Malkani, 2010). The division of Indus Basin in three basins is mostly due to diverse lithostratigraphic style and tectonomorphic scenarios". The study area is located in the Southern (Lower) Indus Basin (Figure 2.1). The Southern Indus Basin is a large basin, it is bounded by the Central Indus Basin to the north, the Sulaiman Fold Belt in the northwest, the Kirthar Fold Belt in the southwest and Arabian Sea to its south (Figure 2.1).
The sedimentary succession of the Southern Indus Basin is dominated by "marine marls, turbidites, and shales, along with shelf carbonates and clastics" (Robison, et al., 1999). The Precambrian basement is exposed in the southeastern corner of the Basin in Thar- Nagarparkar Region. Significant unconformities are present on the basal part of Permian and Tertiary successions (Yousafzai, et al., 2010 and Suhaib, 1982).
Tectonically, the Southern Indus Basin is an extensional basin, overlain by a thick sedimentary succession (Kazmi and Jan, 1997). The extension resulted as a consequence of the northward drift of the Indo-Pakistan subcontinent from the Gondwana during early Paleozoic (Kazmi and Jan, 1997).
The strike-slip faults and normal faults (horsts and graben structures) are comonly present in the Indus Basin (Zaigham and Mallick, 2000). The cover sequence of the Indus Basin is deformed in Tertiary (Jaswal et al, 1997 and Kemal et al, 1992). The recommended geological models of Southern Indus Basin illustrate the possiblity of suitable environments for the presence of hydrocarbons and appropriate structures for existane of petroleum system (Zaigham and Mallick, 2000).
A number of plays are proven but un-explored. One of these is Lower Cretaceous Lower Goru play. Significant remaining potential in the Lower Goru play has been highlighted in the previous works of Zaigham & Mallick (2000) and Ahmad et al. (2004, 2006). In the Southern Indus Basin the well known source for the generation of hydrocarbon are Early Cretaceuos sequence (Quadri, 1980 and Wandrey et al., 2004). The proved reservoirs in the Basin are the clastics and carbonates of Cretaceeus-Eocene (Shah, 1977 and Iqbal & Shah, 1980). Sealing intervals are present for all potential reservoirs in the area, especially intra-formational shale for Lower Cretaceous reservoirs (Annual Technical Report, 2010). Strucutral traps in the Southern Indus Basin and in the Jacobabad-Mari-Kandkot High areas contains the horsts and grabbens produced due to normal faults (Zaigham and Mallick, 2000). Stratigraphic traps are also present (Ahmad et al., 2006). The shales of Sember and Lower Goru formations are considered to be as source in the study area (Ahmad, et al., 2006). In the study area "Lower Goru Formation is acted as reservoir rock while, the shale and marl of Upper Goru Formation considered as cap rock" (Ahmad et al., 2006).
2.2 Tectonic and Stratigraphic Settings
The tectonic and stratigraphical settings of the Bitrism Block, Southern Indus Basin are discussed below;
2.2.1 Tectonic settings of the Bitrism Block
The location of Bitrism area falls within Jacobabad Khairpur high in Lower Indus Basin of Pakistan (Ahmad, et al., 2006). The origin of theses crustal features has been recommended by numerous suggestions, but these basements up warps keep on confusing.
The Jurassic-Early Cretaceous rifting of Indian Plate controlled the structures and sedimentology of the Southern Indus Basin. Northeast-Southwest rift systems are possibly produced by Jurassic-Early Cretaceous rifting (Annual Technical Report, 2010). This separation in turn caused uplift, erosion, extrusion of the Deccan flood basalts and probably the NNW-striking normal faults (Quad Report, 1994). The gentle folds in the Indus Basin are caused by Palaeocene-Eocene emplacement of the Bela Ophiolites. Fault reactivation during the Oligocene to present-day Himalayan collision caused the sinistral flower structures in the western part (Annual Technical Report, 2010).
2.2.2 Stratigraphic Settings
Infra-Cambrian to Recent clastics and carbonates are present in the Southern Indus Basin. During Late Cretaceous, the Southern Indus Basin comes to be part of the complex suture between the Indian Plate and the Afghan Block (Kazmi and Jan, 1997). From east to west the stratigraphic changes occurred and thickness of th sediments increases. In the southeastern corner of the Basin the Precambrian rocks are exposed (Kazmi and Abbasi, 2008 and Shah, 1977).
The oldest formation penetrated in the study area is the Jurassic Chiltan Formation of Shah (1977). The Chiltan Formation is overlain by the Lower Cretaceous Sember and Goru formations. The Goru Formation has confirmable contact with overlaid Parh Limestone of Late Cretaceous which is covered unconformably by the Ranikot Group. The Ranikot is capped unconformably by the Laki Formation of the Eocene age and the same is reported in Humaira et al. (2011). During the Oligocene the Proto-Indus River began to deposit Nari/Gaj sediments from the north into shallow embayment in the Karachi area (Shoaib and Salma, 2004). At the end of the Gaj depositional cycle, the eastward movement of the Afghan Plate along the Murray Ridge began and the Indus River course was shifted eastward by the compressional uplift in the Karachi area. The continental, Pliocene sediments of the Manchar Formation reflect that the Indus River course was in the east of the Surjan-Sanbak Ridge (Shoaib and Salma, 2004). Finally the continued eastward movement of the Afghanistan Plate and the consequent uplift in the Karachi area moved the Indus River eastward again to its present course in the Pleistocene (Shoaib and Salma, 2004). The fluvial Siwaliks were deposited during the Pleistocene time (Shoaib and Salma, 2004). Stratigraphy of the study area has established by using VSP and well log data.
The detailed stratigraphy of the Bitrism Block is discussed as follows and stratigraphic column of the Southern Indus Basin is shown in figure 2.2.
I. Chiltan Limestone
Stratigraphic Committee of Pakistan in 2002 named the Takatu Formation of Williams (1959) as Chiltan Limestone on the type locality Takatu Range in the Northeast of Quetta. Typically, Chiltan limestone is thick-massive bedded dark limestone, but shows color and texture variations. The color differs from dark-light grey, brownish grey, bluish grey, black and rarely white. Locally pisolitic beds are present. The texture differs from fine-grained, sub-lithographic to oolitic, reefoid and shelly. The Chilton limestone is extensively dispersed in the area.
Thickness of Chiltan limestone varies from place to place. The Chiltan limestone has no recognizable fossils though poorly preserved fragmentary remains are found rarely. Chiltan Formation is supposed to be the source of hydrocarbons in the study area (Ahmad et al., 2006).
II. Sember Formation
The name of the Formation was familiarized by Williams (1959), the type locality of the Formation is Sember Pass, Marri Hills, Kohlu District (Baluchistan). It comprises
black silty shale with interbeds of black siltstone and noduler rusty, argillaceous limestone beds or concretions and glauconite.In the lower part of the Formation pyretic and phosphatic nodules and sandy shales are present. Thickness of Sember Formation is ranging from 133-262 m.
The formation contains foraminifera, but mostly Belemnites are reported. The the late Jurassic age is assigned to Sember Formation (Williams 1959).
III. Goru Formation
"The term Goru Formation was introduced by Williams (1959). The type locality of the Formation is near Goru village on the Nar River in the southern Kirthar Range". Goru Formation consists of about "three thick marl units and two shale units. The shale is calcareous in nature and color is grey to khaki while the marl is thin bedded to thick bedded and color is cream white" (Malkani, 2010).
Thickness of the Formation in type locality is 536m, but drops to 60m at places. Foraminifera and belemnites have been reported from Formation and Early Cretaceous age has been assigned to it (Williams, 1959). This Formation is further divided into the following lithological units;
a) Lower Goru
The inter-beds of sandstone and shale in diverse extents are the lithology of Lower Goru Formation. The Lower Goru sandstones produced oil in many areas Khaskeli and Kadanwari (Petroconsultants, 1996). From southeast to northwest the percentage of sand decreases progressively. The reservoir of the study area is considered to be Lower Goru Formation. The Lower Goru horizon has been divided into five parts based on lithologies; "the Basal Sand unit, Lower Shale, Middle Sand unit (which has a good reservoir potential), Upper Shale and Upper Sand" (Shah, 1977).
b) Upper Goru
The Upper Goru is mainly consists of shale or clay and marl, rarely with inerbeds of silt and limestone. It has no reservoir potential but it forms a thick shield cover to act as cap rock for oil and gas reservoirs of Lower Goru.
IV. Parh Limestone
The first name given to the Formation by Blanford (1879) at the type locality Parh Range. Later in 1959, Williams introduces "a limestone between the Goru and Mughal Kot formations". It is very different limestone unit. It is hard and has different colors; light grey, white, cream. It is thin-medium bedded, argillaceous. In the type locality the thickness of the Formation is 268m but it varies from 300-600m at different places. The Formation contains foraminifera and the Late Cretaceous age is assigned to the Formation.
V. Ranikot Group
The name "Ranikot Group" was introduced by Blanford (1867) at the type locality Lakhi Range, Sind.
The Group consists of three formations, which, in stratigraphic order are, Khadro Formation (Cardita beaumonti beds of Blanford, 1867), Bara Formation (Lower Ranikot) and Lakhra Formation (Upper Ranikot).
a) Khadro Formation
Williams (1959) introduced the name Khadro Formation to the Cardita beaumonti beds of Blanford. It is extensively dispersed in the Southern Indus Basin and adjacent region. At the type locality Bara Nai in the Laki Range, the basal part of the Formation is limestone comprising oysters and reptiles bones. This is followed by the interbeded sandstone and shale. Basaltic lava flows are also present. The color of sandstone is yellowish brown, olive, green and grey. The nature of the sandstone is soft medium-grained, calcareous and ferruginous. Both the sandstone and limestone are reported to be fossiliferous. Top of the Formation is typically marked by volcanic flow. The average thickness is 170m but varies from 140m to 180m. Foraminifera are also reported from the Formation, the age assigned to the Formation is early Paleocene.
b) Bara Formation
The Bara Formation of Shah, 1977 is well exposed in the study area. It includes dominantly sandstone with lesser amount of shale and minor volcanic debris. Sandstone varies in color, and is fine-coarse grained, soft and crumbly. Massive looking beds of a few cm to 3m thickness are commonly present. Both shale and sandstone are carbonaceous in nature. Thickness of the Formation is 450m at the type locality, 600m at Ranikot. No fossils have been reported, except for some oysters, reptile's remains and carbonized leaf impressions.
c) Lakhra Formation
Lakhra Formation of Shah, 1977 is exposed at Lakhra anticline, at Laki Range, Lower Indus Basin. The Formation comprises mainly limestone, and the color of the limestone is yellowish grey, brown and buff. Texturally the limestone is nodular and a brecciated texture. Some of the fossiliferrous beds are coquina like. This thin to thick bedded limestone is extensively distributed in the basin. At the type locality it is 242m thick and varies at different places in the basin. Foraminifera are reported from the Formation and Late Paleocene age has been assigned to the Formation.
VI. Laki Formation
The Formation is located near Meting, Mari Nai in the northern Laki Range. Hunting Survey Corporation has divided the lower part of the formation into the Sonari member, Meting Limestone and Shale Member (Hunting Survey Corporation, 1961).
The Formation comprises dominantly of cream-grey colored limestone but marl, calcareous shale, sandstone and lateritic clay are the significant lithologies of the Formation. Thickness is about 240m locally but may vary up to 600m at different places. The Formation is composed of rich fossil assemblages including; foraminifera, gastropods, bivalves, echinoids and algea. These fossils indicate an early Eocene age.
2.3 Structural Setting
2.3.1 Structural settings of Bitrism Block
The Bitrism and its surrounding areas are covered by an extensive alluvial plain (Zaigham et al., 2000). The structures drilled in the Bitrism were delineated on the basis of provide data. The prospectivity reports of the structures drilled in Bitrism show that all the structures are fault-bounded small horsts and are oriented in the Northwest and Southeast direction (Zaigham et al., 2000).
"The dominant structure in the study area is resulted by normal faulting on the west dipping Indus Plain" (Ahmad et al., 2006). The migration of hydrocarbons might be occurred through these fault planes. The interpretation of the provided data establishes the structural models of the field. The reservoir of the study area is bounded by the normal faults in this area (Ahmad et al., 2006).
Structural history of area is characterized by extensional regime. Which produced normal faulting, and basement related structure within late Paleozoic to Quaternary sediments (Kazmi and Jan, 1977). These sediments were deposited on the peneplained Precambrian basements along the stable margin of Indian shield (Kazmi and Jan, 1977). A series of extensional events during the late Paleozoic to cretaceous as well as middle Tertiary collision between Indian and Eurasian plate reactivated old faults and produced new faults in the area (Tectostrat, 1992). N-W oriented main structural features of Talhar fault zone are the results of this extensional tectonics (Tectostrat, 1992).
In addition, the major faults in the area would have been responsible for providing vertical migration pathways for hydrocarbon for Sember source. These faults might have acted as seals also (Ahhmad et al., 2012). Indeed some faults particularly of low intensity and minor throw may have not provided effective sealing to hydrocarbons at certain stages and as a result the hydrocarbons would have escaped laterally in the up dip direction along the carrier pathways (Ahhmad et al., 2012).
The structures in the area covering Bitrism Block may have been modified by the re-activation of the faults after the migration of the hydrocarbons have occurred in the up dip direction towards north and south. Alternatively, the existing faults system may not have provided effective sealing in the present traps both at Basal and Massive sand level (Ahmad et al., 2004).
2.4 Hydrocarbon Potential
Lower Goru Formation is considered as potential reservoir rock in Bitrism Block. The presence of gas in the study area signifies the possibility of oil from the Middle Jurassic Chiltan limestone, which acts as a source rock as well as reservoir. The shale unit of Upper Goru provides the regional seal for Cretaceous Lower Goru reservoir zone (Ahmad et al., 2006).
2.4.1 Source Rocks
The rocks provided the hydrocarbons are known as source rocks. The oil and gas are generated by the organic compound which is present in the source rocks. The formations, which act as source rocks in the study area are as follows;
Chiltan Formation: The main source rock in the Bitrism block is Chiltan Formation (Ahmad et al., 2006).
Goru Formation: The Lower Goru sand is also reported as source rock in Bitrism block (Ahmad et al., 2006).
2.4.2 Reservoir Rocks
The proven reservoir in this region is Cretaceous sandstone of Goru Formation. The principle reservoirs are the sandstone of the Goru Formation in the Southern Indus Basin (Ahmad et al., 2006). Potential reservoirs are as thick as 400m (Ahmad et al., 2006). The depth of these reservoirs is about 2400m according to the formation tops data of well Fateh-I (Ahmad et al., 2006).
In the study area known seals are predominantly includes the shales which caped the reservoir. In Southern Indus Basin, the reported seals are thin shale beds of variable thicknesses. The shales of Upper Goru Formation and interbedded shales of Sui Main Limestone are performing as a seal in Bitrism block (Ahmad et al., 2006).
The traps in study area are mainly structural. These traps provide the significant trapping system along tilted fault blocks and negative flower structures. But the most prominent traps are associated with transtensional products (negative flower structures) forming closures in the forms of Highs (Ahmad et al., 2006).
Seismic Data Structural Interpretation
SEISMIC DATA STRUCTURAL INTERPRETATION
Seismic data interpretation is the key skill that is used commonly in the hydrocarbon industry. The seismic information in interpreted to create useful structural model of the subsurface. The resulted models are not helpful unless an interpreter knows about all about the area which includes geophysical surveys, well log data and surface geological informations.
The interpretation of seismic data is commonly applied in hydrocarbon exploration to obtain useful information about structural traps. The most common structural targets associated with oil entrapment are anticlines and faults.
This chapter presents the structural interpretation of the seismic data. The interpretation has been done in a way to be able to answer a set of the following questions about study area; 1) What type of structures are present? 2) What is the depth of horizons of interest? And 3) are there any faults zones located in the study area?
In this chapter methodology of the identification of faults, their dip/strike direction and generation of time and depth contour maps will be discussed in order to answer the above questions. The seismic structural interpretation in this study is done in following steps;
Main seismic reflectors are identified, which include; the Goru Formation (Cap Rock), Basal sand of the Goru Formation (Reservoir Rock), and top of the Chiltan Limestone (Source Rock).
Fault networks are recognized.
2D and 3D structural contour maps are constructed.
3.2 Research Methodology
The research methodology used in this chapter is discussed as follows;
3.2.1 Seismic Horizons
Normally the horizons are marked on the basis of the check shot survey and on the basis of Vertical Seismic Profile (VSP) data. To some extent the synthetic seismogram can also be used to mark the horizons accurately. But marking of the horizons can also be done on the basis of well summery sheet. The available VSP data and well log data are used in this chapter for marking the horizons. In total the following three horizons have been mapped throughout this study;
3. Upper Goru Formation (H-1)
2. Massive Sand of the Lower Goru Formation (H-2)
Chiltan Limestone (H-3)
These three horizons are the target horizons in this study as well as in a number of hydrocarbon exploration projects in the study area (e.g., in Bitrism block licensed to OGDCL).
The above discussed horizons are marked on seismic section and their times across shot points are noted. By using the following relationship the depth of each horizon is calculated;
"" â€¦â€¦.â€¦. (3.1)
Where; "D is Depth of reflector in meters, Vave is Mean average velocity in m/s and T is One way travel time in seconds".
The well log data of Fateh-01, located on shot point (sp): 215 on line 20017-BTM-02 gives the tops of reflectors are given in Table-3.1.
Table 3.1: Time and depth given at the top of each horizon.
3.2.2 Picking and correlation of reflectors
Reflectors are identified in order to tie the seismic sections to the well data. The depth tops of the different formations are defined by using the composite logs, while the one way travel time of these tops is determined by sonic logs and well velocity survey data. Accordingly two-way times are calculated, which is used to describe the reflecting formation tops on the seismic sections. The time and depth data for the tops of the reflectors given in table 3.1 are calculated through the process explained above.
Chiltan Formation has good contrast on the seismic sections, due to limestone and exhibits the best continuity. The marked tops of the three horizons are selected to express the development of structural elements affecting the Jurassic-Cretaceous succession. This is conceded, first, in order to tie the VSP with the 2D seismic sections and determine the two-way travel time on the top of each horizon. When the reflectors are identified the time of studied horizon was picked up through seismic lines. The discontinuity of the reflectors along the seismic gaps is dissolved by using the folding process. If reflectors are either moved vertically or disappear; this disturbance is due to the faults or pinch-out in the study area. The variation in amplitude of the reflectors may result due to the following factors; variation in acoustic contrast and thickening or thinning of lithologies. It is probable to compare across small faults by using strike lines in order to complete the interpretation.
3.2.3 Detection of Faults
Faults of large vertical displacements are easily predictable, particularly from the abrupt change of reflections. Faults with small displacements are marked on the basis of Campbell's (1965) criteria in the current study. The criteria for fault identification are as follows;
Relationship of distinctive reflectors.
Identification of reflection gaps.
Prediction of shallow faults with associated reflections to deeper levels.
Distinctions between drilled holes.
Geological and geophysical dips.
Mis-closures around a guide unit of seismic control further than the possible limits of precision.
Dip pattern along several lines of control and diffraction as a mask and clue to faulting.
The faults identified are normal faults in the study area.
3.3 Interpretation and Results
3.3.1 Time and Depth Contouring
Construction of the time and depth structural maps and results obtained through these maps are discussed in the following sections;
3.3.2 Interpretation of Seismic Section
There are two main approaches for the interpretations of a seismic section, these are;
(a) Structural Analysis and (b) Stratigraphic Analysis (this approach is out of the scope of the present study and therefore not discussed here)
(a) Structural Analysis
The tectonic settings of an area considered to be very important to understand the structural behavior of the study area. In the analysis of structural behavior of an area the structural traps have significant importance. The type and correlation of structural traps with each other is governed by tectonic settings. So to understand the structural behavior and to trace the traps of an area the tectonic settings are very supportive. The major types of structural traps are "the folds (anticline), faults, pop up and duplex structures".
The study area of Bitrism Block lies in an intense extensional regime, so generally structures of normal faults were identified in the study area (e.g. Zaigham et al., 2000 and Ahmad et al., 2004). In the area the tensile forces are responsible for the generation of normal faults.
Many normal faults marked on the seismic section some of which are the intra-formational faults. These faults are identified on detecting the changes in the continuity of the reflectors (2D and 3D Structural maps are shown in Figures 3.1-3.14).
3.3.3 Methodology used in Interpretation
Some of the basic steps used in the seismic interpretation of the Bitrism area are given below:
Calculation of the coordinates for each shot point of the given seismic dip/strike lines from the base map of Bitrism area (see Appendices I). All these coordinates are given in the Appendix II.
Preparation of the base map in software SURFER 10 for the studied area, with the help of navigation data (Appendix II) calculated in earlier step.
Calculation of the interval velocities and the mean average velocities using Dix equations. Velocity (Vave) versus time (T) of studied horizon graphs is prepared by using software K-tron Vas in order to determine the time of each reflector with its mean-average velocity. Hence, the depth of Goru Formation, Massive Sand and Chiltan Limestone is calculated by using the mean-average velocity and the time of studied reflectors (for details see section 4.1.1, Chapter 4).
Structural feature like faults are marked with the help of reflectors trend and discontinuity (discussed in detail in section 3.2.3 above).
After marking of the reflectors is done, the names are given to the reflectors on the basis of general stratigraphy (details given in section 3.2.2 above). All the three reflectors are represented by different colors in order to distinguish them from one another (e.g., Upper Goru Formation-green, Massive Sand-red and Chilton Limestone-yellow).
The points of equal time of horizon with faults data are contoured (details given in sections 3.3.4 and 3.3.5 below).
Using the contouring software SURFER 10 an imaginary 3D structural map and 2D contour maps were also prepared (see Figures 3.1-3.14).
3.3.4 2D and 3D Structural Contour Maps
Contouring is the main tool used in the seismic interpretation because the contouring enables us to interpret the structure present in the area. After marking the horizons and the faults on the section, the next step is to generate the structural maps of the formations. There are three horizons selected for the purpose of constructing structural contour maps which are discussed as earlier. The 2D and 3D structural maps of time and depth are constructed to delineate the depth of each horizon, faults and folds mechanism of the studied area. The purpose to generate these structural maps is comparing the subsurface structures of each horizon to confirm the interpretation.
3.3.5 Time Contour Maps
The SURFER 10 software is used to create the time contour maps of the three horizons. To generate the time contour maps, the two-way times of the Horizons 1, 2 and 3 were noted to create the data files (Appendices II-V). This data file is a file which contains three columns of data values which are the latitude and longitude for the data points and the two-way time. The details of each horizon for time contouring are described as follows;
From the time contour maps of each horizon shown in figures 3.1-3.6, the low values of time show the uplift of each horizon between two normal faults indicating Horst structure where as high values of the time show the presence of Graben structures. The figures 3.2, 3.4 and 3.6 also show the presence of normal faults in the study area by breaking of contour lines. In the study area total seven faults are identified which are normal faults in nature. The presence of normal faults confirms the horsts and grabens present in the study area which indicates the important zone for the accumulation of hydrocarbons. It is obvious from the existing literature (e.g. Wandrey, et al., 2004 and Zaigham & Mallick, 2000) that the presence of hydrocarbons in the Southern Indus Basin is controlled by fault generated structures. The studied block has faulted anticlines (horsts and grabens). Ahmed et al. (2004) reported the presence of combined stratigraphic-structural traps in the Birtrism Block.
The resulted time contour maps also predict that the low values of time are in the Southern and Southeastern portion of the study area and high values of time in the areas of Northwestern and Northeastern portions. It is observed by studying previous literature (Ahmad et al., 2004) that the presence of oil and gas in Lower Indus Basin is in the Lower Goru Formation which acts as reservoir rock of the studied area. It is also observed that the source rock of oil and gas in Lower Indus Basin is the Chiltan Limestone.
3.3.6 Depth Contour Maps
The data obtained by time contour maps and the velocities, additionally from the velocity windows, for each shot point at the Goru Formation (H-1), Massive Sand (H-2) and Chiltan Limestone (H-3) are noted. Then by using the relation (Equation 3.1) the depth at each shot point is calculated (Appendix II). The procedure used for the generation of time contour maps is also applied to create depth contour maps. The 2D and 3D structural maps of depth are prepared to confirm the promising zone so the 3D structural map also confirms the structures which are normal faults in the studied area. The details of each horizon for depth contouring are described as follows:
The depth of the formation is determined and contoured as discussed earlier (3.2.1 Seismic Horizons). The abrupt change in the elevation is represented by the closely spaced contours. Depth contour maps of each horizon are shown in the figures 3.7-3.12. The figures 3.7, 3.9 and 3.11 show the color variation of depth of each horizon in the area while figures 3.8, 3.10 and 3.12 show normal faults in the area which also confirmed the results obtained by the interpretation of time contour maps.
3.3.6 3D Structural Maps
The 3D structural maps provide an impressive three dimensional display of seismic data.Â The 3D structural maps are also generated by using the SURFER 10; this software uses shading and color to emphasize the structural features present in the area. The 3D block diagrams were constructed by overlaying 3D structural maps of all the three horizons (Figures 3.13 and 3.14). The 3D structural maps of time and depth for all
the three horizons were prepared to further strengthen the interpretation of the structures (drawn from the above 2D maps) present in the Bitrism Block. Zaigham et al., 2000 reported the presence of normal faults in the subsurface of the studied area. The primary purpose of constructing 3D structural maps are many folds; 1) identify the faults in subsurface of the studied area to confirm the results obtained by 2D structural maps, 2) determination of their geometry and dimensions, 3) explanation of the fault type and 4) confirmation of the subsurface structures.
(a) Interpretation of 3D Time structural Maps of the Studied Horizons
The 3D maps are prepared to confirm the same structures identified on the 2D time contour maps of studied horizons. The 3D structural maps of studied horizons show that normal fault and the horsts and grabens structures are present in the study area. The 3D time structural is shown in figure 3.13.
(b) Interpretation of 3D Depth structural Maps of the Studied Horizons
3D depth surface maps show the depth variation of studied horizons. As it is confirmed by 3D depth surface maps that normal faults (horsts and grabens structures) are present in the studied area. The prepared 3D depth surface maps are shown in the figure 3.14.
The purpose of the chapter is two folds; 1) stratigraphic interpretation and 2) structural interpretation of the Bitrism Block.
On the basis of provided data (seismic lines, VSP and well log data) the following three horizons are marked;
(a) Horizon-1: Upper Goru Formation (Depth ranges from 2000 to 2200m)
This is the upper most horizon of interest. It is identified as Upper Goru Formation in the study area which is also reported by Ahmad et al. (2004) and structurally the Formation is faulted in the area.
(b) Horizon-2: Massive Sand (Depth ranges from 2700 to 3200m)
This horizon is identified as the Massive Sand bed of Goru Formation. It might be a good reservoir in this area which is also faulted more densely then Upper Goru Formation.
(c) Horizon-3: Chiltan Limestone (Depth ranges from 3700 to 4500m)
This horizon referred to third zone of interest. It is identified as Chiltan Limestone. This horizon is more compacted, faulted and denser then Massive Sand because of its depth and lithology.
2D and 3D structural maps of time and depth were prepared from the seismic lines of the Bitrism Block. With the comparison of all the above discussed maps, the results deduced are; locally normal faults are present in the study area. These faults contain local scale horst and graben geometries which are capable of hydrocarbon accumulation.
Seismic Velocities Modeling
SEISMIC VELOCITIES MODELING
The role of seismic velocities in geophysical exploration is very important. The seismic velocities are used in different areas of seismic data processing, seismic stratigraphic interpretation and in structural geology and subsurface lithology (Li and Stewart, 1991).
"Velocity is a fundamental property of rocks that depends on density and elastic moduli. It varies laterally as well as vertically due to the physical and geological variations of rocks. Sediment velocities generally increase with depth due to increased overburden pressure" (Gardner et al., 1974). "Fluids within pores tend to make the rocks less compressible and lead to higher interval velocities for P-waves" (Gardner et al., 1974).
In order to prepare the seismic velocity models, three horizons which were identified in Chapter 3 were selected, these are; H-1(Upper Goru Formation), H-2 (Massive Sand of Lower Goru Formation) and H-3 (Chiltan Limestone). The analysis was done using two dimensional and three dimensional velocity models and these models were prepared by using the following softwares; SURFER 10, K-tron Vas and Rockworks 2006.
In this chapter a work flow is presented for processing, analysis, and visualization of velocity data to create useful velocity distribution models. A workflow is demonstrated for Mean Average Velocity estimation from travel time of seismic data. Khan et al. (2009) introduces a "velocity database along with an array of other geo-scientific databases". The seismic velocity information is used in different aspects of "processing, interpretations and rock physics applications".
A lateral and vertical velocity comparison was made to identify the subsurface structural behavior of the selected horizons for hydrocarbon accumulation. This chapter describes the methods, processing techniques and results of the seismic data velocities modeling.
4.1.1 Relationships between Velocity Types
Prior to the processing and analysis of seismic velocities, the different types of velocities and their relationship are discussed as follows. The given velocity on seismic sections is the root-mean-square (RMS) velocity. 2D and 3D velocity models are prepared by using the mean average velocity, which is obtained by the workflow discussed in following section. Following are the relationship and definitions of different velocities calculated during velocity analysis.
(a) Root-Mean-Square Velocity (Vrms)
"The root-mean-square (RMS) velocity is defined as the velocity of a wave through subsurface layers of different interval velocities along a specific raypath, and is typically several percent higher than the average velocity. The stacking velocity and the root-mean-square velocity are approximately equal when source-receiver offset approaches zero and layers are horizontal and isotropic"
(b) Interval Velocity (Vint)
"Interval velocity is the constant velocity of a single layer". Dix equation (Dix, 1955) is used to calculate the interval velocity from root mean square velocity (Vrms).
The interval velocity is the velocity considered between two depths. It can be calculated from sonic logs or from seismic stacking velocities. The interval velocity is used for calculation of thickness of the layer,
Where Vint is interval velocity, t1 and t2 are travel-time of the first and second reflectors respectively and Vrms1 & Vrms2 are the root-mean-square velocities of first and second reflectors respectively
(c) Average Velocity (Vave)
"In geophysics, the average velocity is defined as the depth divided by the travel-time of a wave to that depth. Average velocity is commonly calculated by assuming a vertical path, parallel layers and straight raypaths, conditions that are quite idealized compared to those actually found in the Earth". The following relation is used to calculate the average velocity of the seismic wave;
Average velocity can also be calculated from the interval velocities (calculated from the root mean square velocities) by the Dix equation (Dix, 1955) which is as follows;
(d) Mean Velocity and Mean Average Velocity Graph
"The mean average velocity is calculated by dividing the sum of average velocities at constant intervals of time with the total number of observations" as shown in Appendix-II, the mean velocity can be calculate by using the following equation;
The obtained results of mean average velocities of the provided 2D seismic lines are given in Table 1, Appendix-II. Figures 4.1 to 4.2 shows the resulted graphs of mean average velocity potted against the time.
The below mention graphs in Figures 4.1-4.4 are drawn using K-tron Vas software, which shows that the velocity increases laterally and vertically. The result obtained by drawing these graphs is the time of reflector with its mean-average velocity. Hence, the depth of Goru Formation, Massive Sand and Chiltan Limestone is calculated by using the mean-average velocity and the time of studied reflectors, by using the following equation;
The calculated seismic velocities can also be used to determine the following;
"True depth determination"
"Stacking of seismic data"
"Migration of seismic data"
"Possible lithology determination"
"General interpretation purposes"
"Reflector's dip determination"
4.2 Research Methodology
Stacking velocity information (interval velocity derived using Dix's formula from the RMS velocity) is the prime seismic velocity information obtained from a multichannel seismic (MCS) reflection survey. However, stacking velocities are useful in data processing (they allow stack correction and enable data migration), they are used as interval velocities to predict the sub-bottom depth of a target reflector. The average velocity is used to construct the contour maps and 3D structural models of selected horizons. The research methodology used for constructing the contour maps and models are explained in the following sections.
4.2.1 Seismic Velocity Contouring of Studied Horizons
Seismic velocity maps are useful supplement to structural-stratigraphic studies therefore, such maps will also add to the interpretations of structures drawn in Chapter 3. In order to study the variation of velocities of the horizons of interest 2D and 3D models of P-wave velocity were generated. A velocity contour map has lines that show elevations but in this case contour lines help to understand the velocity of the studied area. Each line represents the velocity at specific depth.
In this study the velocity contour maps (both 2D and 3D) were generated and superimposed on the base map (shown in Chapter 1, Figure 1.1). The models were generated using software SURFER 10.0 and Rockworks (2006) by using a the following set of data; shot points of seismic lines, longitudes/latitudes of shot points and the mean average velocity (as discussed in section 4.1.1). The models of studied horizons are shown in figures 4.5 and 4.6 for Goru Formation, figures 4.9 and 4.10 for Massive Sand and figures 4.11and 4.12 for Chiltan Limestone. The 3D velocity contour maps of the studied horizons are shown in figure 4.13.
4.3 Results and Interpretation
The results obtained by the interpretation are discussed as follows;
4.3.1 2D Velocity Contour Maps
(a) Goru Formation (Upper Goru, Cap Rock)
The studied Horizon-1 of interest is identified as the Upper Goru Formation in Chapter 3 which is considered as cap rock in the study area. Two types of velocity contour maps were generated from the data of this Formation, one shows lateral color variation of velocity (Figure 4.5) and the second shows the structures such as faults in the study area comprises low velocity zones while North-Western and South-Western parts comprise relatively high velocity zones. This variation in velocity is due to lithological variation in Goru Formation.
Lithologically the low velocity zones (less than 2500 meter/miliseconds) indicate the presence of shale and marl while the relatively high velocity zones (more than 2500 meter/miliseconds) indicate the presence of siltstone and sandstone which is confirmed by the provided VSP and well log data and also reported by Zaigham et al. (2000). Shale, an abundant sedimentary rock of extremely low permeability, is considered a natural barrier to the migration of oil and gas (Zaigham et al., 2000). It is interpreted that the Southeast, East, Northwest and Northeast parts have horst and graben geometries which are suitable locations for the presence of hydrocarbon. The same structural styles are interpreted in Chapter 3 by using time and depth structural contour maps. Figure 4.6 shows the presence of faults in the study area. "The wells (Fateh-01 and Ichhri-01) drilled in the study area were found dried" (Zaigham et al., 2000). The reason might be the absence of suitable traps in case of Ichri-01 well as indicated by velocity models. In case of Fateh-01 Well, either the well was drilled below the oil-water contact (Zaigham et al., 2000).
(b) Massive Sand (Reservoir Rock)
The studied Horizon-2 of interest is identified as the Massive Sand in Chapter 3 which is considered as reservoir rock. Velocity contour maps were also generated using SURFER 10 software and available data. The velocity contour maps include 2D velocity contour map (Figure 4.7) and the 3D velocity contour map (Figure 4.8).
The overall model of P-wave velocity shows that there is lithological variation present in the area. The low velocity zones may be potential zone for Hydrocarbon. From the contour pattern shown in figures 4.7 and 4.8, the behavior of velocity interpreted to be different then velocity pattern in Upper Goru Formation. Moving towards the North-Eastern and South-Eastern parts of study area the velocity decreases gradually while in the North-Western and South-Western parts velocity is relatively high. In the study area low velocity zones predict the hydrocarbon reservoir which is also reported by Ahmad et al. (2004).
The Massive Sand of the Lower Goru Formation was the target reservoir in both wells Ichri-01 and Fateh-01in the study area but both the wells are abandoned because they fail to produce hydrocarbons. In the case of Ichri-01 the reason might be the lack of suitable traps and velocity of reservoir rock is relatively high which predicts that the zone does not have suitable condition for the accumulation of hydrocarbons as shown in figure 4.7. In Fateh-01 well the reason for abandoning might be the drilling below the oil-water contact or due to hydrocarbons needed more time for maturity (Zaigham et al., 2000).
(c) Chiltan Limestone (Source Rock)
The third studied horizon on seismic section is identified as the Chiltan limestone in Chapter 3which is considered as main source rock in the study area. The overall model P-wave velocity shows that there is less variation in lithology present in the Chiltan Formation. The low velocity zone may be potential zone for hydrocarbon exploration in Bitrism Block as described in the figure 4.9. From the figures 4.9 and 4.10 it is concluded that the velocity decreases in the East, South-East and Northeastern parts while increases in the North-Western and South-Western parts of the study area. The low velocity zones represent the zones of hydrocarbon potential in the study area. Figure 4.10 also shows that the area is dominated by normal faulting and the same structures are reported in Chapter 3 on the basis of time and depth contour maps.
4.3.1 3D Velocity Modeling
Three dimensional figures give clear identification of lithological and structural variation in the subsurface. In the present study 3D velocity structural map were prepared using the software SURFER 10, for the given data of Upper Goru Formation, Massive Sand of Lower Goru Formation and Chiltan Limestone (Appendix III). The model clearly shows the presence of horst and graben structures in the study area (Figure 4.11). Structural features that control accumulation of petroleum in the study area are fault- generated structures (horsts and grabens). It is interpreted that the structural feature encountered in Ichri-01 in Upper Goru Formation is a graben which changes with depth into a horst structure in Lower Goru Formation and Chiltan Limestone.
However, in Fateh-01 well horst feature is present in the Upper Goru Formation which changes with depth into a horst in Lower Goru Formation and Chiltan Limestone. Both the wells are found abandoned because hydrocarbons may be migrated vertically along reactivated basement-rooted faults or associated fracture zones along the flanks of these structures.
4.3.2 Rockworks Modeling
The velocity data of the three horizons i.e. H-1(Upper Goru Formation), H-2 (Massive Sand of Lower Goru Formation) and H-3 (Chiltan Limestone) have also been plotted in software Rockworks version 2006. "Rockworks software is a diverse software collection for applications in mineral exploration, oil and gas exploration, hydrology and environmental geology" (http://dx.doi.org/10.1016/0098-3004(95)00124-7). Rockworks 2006 provides useful ways to create grids that can be used for 3D modeling so that this software is used in the present study to construct 3D velocity models to confirm the results obtained by the 3D models of SURFER 10.
220.127.116.11 Interpretation of 3D velocity models prepared in Rockworks 2006
3D models of seismic velocities (P-wave and S-wave) are generated to examine the variation in velocity in the lithology of rocks and the shearing strength of the subsurface rocks in the study area. The results obtained are discussed as follows;
(a) P-Wave 3D Models velocity
"P-wavesÂ are a type ofÂ elastic waveÂ that can travel through any medium as it has the highest velocity and is therefore the first to be recorded".Â "P-wave velocity varies with the lithology, porosity and bulk density of the material, state of stress (such as lithostatic pressure) and fabric or degree of fracturing" (Lecture notes in Earth Sciences, 2009 and Gledart & Sheriff, 2004). The data set required for the 3D velocity modeling contains the longitudes/latitudes and the mean average velocity (P-wave and S-wave) of selected horizons (Appendix III, IV and V), the resulted model is shown in the figure 4.12. The exploration which is confirmed by 3D structural models prepared by the SURFER 10.
(b) Shear Wave Velocity
Shear waves bring additional knowledge to a seismic study because compressional and shear waves sample different rock properties. "Shear wave velocity does not pass through fluids and it moves through grain to grain because of this property it is very sensitive to lithology" (Ayres and Theilen, 2001). Compressional-wave velocity is a function of a medium's density, shear modulus and bulk modulus. Shear wave velocities has been estimated from Castagna's equation and it is further used in rock physics (Chapter 5).
(Castagna et.al. 1985) ...â€¦â€¦.. (4.6)
Where; Vs is the velocity of shear wave (km/sec) and Vp is the velocity of primary wave (km/sec).
The shear wave velocity is calculated on each CDP and given in Appendix III, IV and V for each studied horizon. The low values of shear wave velocity indicate the hydrocarbon zones.
3D models of S-wave velocity are also generated to study the variation in velocity which shows variation in elasticity of the rocks of study area. The overall S-wave velocity model shows that there is variation in elasticity figure 4.13. Western portion of the study area might be less elastic then Northeastern portion. The models clearly reflect that the studied formation are more or less elastic such that the shearing strength is less as compared to extensional forces and due to this no anticlines and synclines were noticed for the good hydrocarbon accumulation, and hence only the small scale horst and graben structures noted maximum in formations having sandy materials. Overall model of P-wave velocity shows that lithological variation present in the study area and zone where the velocity decreases may be the potential zone for hydrocarbon.
The 2-D velocity modeling of the horizons of interest indicate that North-Eastern and South-Eastern parts of study area comprises low velocity zones while North-Western and South-Western parts comprise relatively high velocity zones. This variation in velocity is due to lithological variation in formations. Lithologically in Upper Goru Formation the low velocity zones indicates the presence of shale and marl while the high velocity zones indicates the presence of sandstone. Shale, an abundant sedimentary rock of extremely low permeability, is considered a natural barrier to the migration of oil and gas. Therefore, the low velocity zones may be the zones of hydrocarbon potential in the study area. The Lower Goru Formation includes a massive sand bed and act as a reservoir rock, while the Chiltan Limestone contains thick massive bedded limestone and considered as main source rock of the studied area.
The interpretation results that the Southeast and East and in the Northwest and Northeast areas have the horst and graben geometries which are the appropriate entrapments of hydrocarbon. The horst and graben geometries confirm normal faulting in the study area.
In the present study 3D velocity contour maps with marked structures (fault) were prepared using the SURFER 10, for the given data of Upper Goru Formation, Massive sand of Lower Goru Formation and Chiltan Limestone. It was interpreted that Ichri-01 represents structurally in Graben features while with depth the features ch