Chapter 1 INTRODUCTION 1.1 INTRODUCTION OF THE STUDY AREA The study area is situated 105km southwest of Islamabad in Chakwal District. It is a small village covering an area 2550sq.km and its coordinates are Latitude 33°2'51"N, Longitude 72°51'16"E. It is 4km from the center of Chakwal City as shown in figure-1. The Minwal Oilfield lies in geologically situated in the south-southeast of the Salt Range-Potwar foreland basin. 1
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Regional and General geology and tectonics of Upper Indus basin
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Chapter 1
INTRODUCTION
1.1 INTRODUCTION OF THE STUDY AREA
The study area is situated 105km southwest of Islamabad in Chakwal District. It is a
small village covering an area 2550sq.km and its coordinates are Latitude 33°2'51"N, Longitude
72°51'16"E. It is 4km from the center of Chakwal City as shown in figure-1. The Minwal Oilfield
lies in geologically situated in the south-southeast of the Salt Range-Potwar foreland basin.
Figure-1.1:- Map showing Location of the study area (Mehmood, 2008).
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1.2 DATA OBTAINED FOR STUDY
The well data to be used is Minwal X-1 whereas the Seismic lines that were used in the
study are mentioned below (Figure-2) and been used with the permission of Directorate
general petroleum concession.
1. LINE: 93-MN-8 (Dip Line)
2. LINE: 93-MN-7 (Dip Line)
3. LINE: 782-CW-29 (Strike Line)
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Figure-1.2:- Shot point Base Map of the study area.
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Figure-1.3:- Satellite imagery showing shot point base map and block boundary of the study area.
1.3 OBJECTIVES OF THE STUDY
The purpose of this dissertation is to understand the various steps involved in seismic
reflection interpretation. This study is carried out to generate reasonable model and structure
of the subsurface of Minwal D & P lease area and to understand and enhance our knowledge on
different seismic interpretation techniques involved in 2-D seismic interpretation. Data
gathering on tectonics, description of structure, stratigraphy, and exploration history is an
integral part of this project.
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Chapter 2
GENERAL GEOLOGY AND TECTONICS OF UPPER INDUS BASIN
2.1 REGIONAL TECTONIC SETTING
The building of Himalayan mountain process in Eocene triggered compressional system.
Northward movement of Indian plate is about 40 mm/year (1.6 inches/yr) and is colliding with
Eurasian plate. 55 million years ago Indian plate collided with the Eurasian plate and building of
Himalayan mountain belt 30-40 million years was formed in the North Western Pakistan and
mountain ranges moved in the east west direction (Kazmi and Jan, 1997). Being one of the most
active collision zones in the world foreland thrusting is taking place on continental scale. It has
created variety of active folds and thrust wedges with in Pakistan passing from Kashmir fold and
thrust belt in North East, South West through the Salt range-Potwar plateau fold belt, the
Suleiman fold belt and the Makran accretionary wedge of Pakistan. As far as the Indian plate is
concerned which is subducting under the Eurasian plate at its Northern edge, a sequence of
north dipping south thrusts are being produced. The shortening of crust caused a large amount
of folds and thrust belt. The youngest basins in the Western Himalayan Foreland Thrust Belt are
Kohat Plateau, Bannu Basin and Potwar Plateau which have compressive stresses and
convergent tectonics. Pakistan is located at in the two domains Gondwanian and the Tethyan
Domains (Kazmi & Jan, 1997). The south eastern part of Pakistan belongs to Gondwanian
Domain and is supported by the Indo-Pakistan crustal plate whereas the northern-most and
western areas of Pakistan fall in Tethyan. Tectonically Pakistan is divided into (Qadri, 1995).
1. Northern Collision Belt.
2. Subduction Complex Association of Balochistan.
3. Chaman Transform Zone.
4. Ophiolites and Ophiolitic Melanges.
5. Platform Areas.
The Potwar Plateau is comprises of less internally deformed fold and thrust belt having a
width of approximately 150 km in N−S direction. The terrain in Potwar is undulated. Sakesar is
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the highest mountain of this region (1522 m). The Potwar is tectonically situated directly below
the western foothills of Himalayas and falls in Potwar Plateau. In north it extends about 130 km
from the Main Boundary Thrust (MBT) and is bounded in the east by Jhelum strike-slip fault, in
the west by Kalabagh strike-slip fault and in the south by the Salt Range Thrust (Aamir and
Siddiqui, 2006) see figure-2.1.
Figure-2.1:- Tectonic map of Northwest of Himalayas of Pakistan showing main tectonic divisions (modified from
Shami and Baig, 2002)
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2.2 GEOLOGICAL BOUNDARY OF THE POTWAR PLATEAU
The Potwar is bounded by the following two strike-slip and two thrust fault which are.
1. Kalabagh Fault.
2. Jhleum Fault.
3. Salt Range Thrust.
4. Main Boundary Thrust.
1. KALABAGH FAULT
It is right lateral strike-slip fault and its direction is from north to west 150 km which can
be seen as faulted block. It lies in the north of the Kalabagh City, Mianwali and is the Trans-
Indus extention of Western Salt Range (McDougal & Khan, 1990).
2. JHELUM FAULT
Extending from Kohala to Azad Pattan the Murree is hanging while Kamlial, Chingi and
Nagri formations are footwall. Starting from the Indus-Kohistan to Ravi it is the active aspect of
the Indian Shield. It is seen also in the map that MBT, Panjal Thrust and HFT cut shortened by
left-lateral reverse Jhelum Fault in west (Baig, Lawrence, 1987).
3. SALT RANGE THRUST
It is also known as Himalayan Frontal Thrust. Salt range and Trans-Indus Himalayan
ranges are the foothills.
4. MAIN BOUNDARY THRUST
The MBT which lies in the north of the Islamabad is called as Murree fault. The western
part of this fault is orienting to north east forming non-striking fault in its western part i.e.
Hazara Kashmir-Syntexis (Latif, 1970; Yeats and Lawrence, 1984: Greco, 1991) also this fault
strike the in the direction of east moving in the direction of Southern side of Kalachitta Range
and North of Kohat plateau (Meissner et at, 1974).
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In Potwar the structure trend is east to west or northeast to southwest and mostly large
surface anticlines are bounded by the thrust or reverse faults. The structure of Potwar basin is
affected by compressional forces, basement slope, and variable thickness of Pre-Cambrian salt
over the basement, and deposition of very thick molasse and tectonic events. In Potwar basin
some surface features mismatch subsurface structures due to decollements at different levels.
In such circumstances, it is necessary to integrate seismic data with surface geological
information for precise delineation of sub-surface configuration of various structures (Moghal
et al, 2007). Tectonic of the Potwar Plateau is controlled mainly by the following factors:
1. Slope of the basement (steeper in western Potwar Plateau).
2. Thickness of the Eocambrian evaporates beneath the cover.
3. Reactivation of basement brittle tectonics (more enhanced in the eastern Potwar Plateau).
In Potwar, the Eocambrian evaporite sequence is overlain by Cambrian rocks of Jhelum
Group which comprises Khewra Sandstone, Kussak, Jutana, and Bhaganwala formations. From
middle Cambrian to Early Permian the Jhelum group consist of limited deposition or erosion
and the strata from these periods are missing in Potwar sub-basin. The continental depositional
environmental of Nilawahan group of early Permian is bounded to the eastern part of
Potwar/Salt Range. The late Permian Zaluch group extends over western and northern/central
part of Potwar/Salt Range. Mianwali and Tredian formation of Triassic age deposited in deep to
shallow marine environment and Kingriali formation consists of shallow water dolomite. The
Jurassic formations include Datta Sandstone, Shinawari (limestone and shale sequence) and the
Samana Suk (Limestone) formations (Moghal et al, 2007).
The Kohat basin comprises of salt in sufficient enough to form the allocation within the
sedimentary basin gliding far in southward direction and has suffered relatively less northward
movement. It is heterogeneous in style of tectonic intensity, direction and extension. An
evidence for this ongoing deformation and uplifting is shown by the meandering course of the
Soan River which straightens near the younger structure of Khur and Dhulian. The present
tectonic framework and the position of the Potwar Plateau have resulted from the northward
under-thrusting by the Indian plate under its own sedimentary cover (Khan, 1986).
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Salt horizon of Eocene in Kohat area is separated due to structural difference. Data
being gathered through (Butler and others, 1987; Leathers, 1987; Baker and others, 1988;
Jaumé and Lillie, 1988; Pennock, 1988; Pennock and others, 1989; Raza and others, 1989;
Hylland, 1990; Jaswal, 1990; McDougall and Husain, 1991) seismic profiles, well logs, Bouguer
gravity anomaly, and surface geology to construct regional structural cross sections map that
detail the thrust-related tectonics of the area. The Salt in the basement has created different
structural pattern in Potwar and the cross-sectional figure 2.2.
Figure-2.2:- Generalized cross section showing structure through the Potwar Plateau (modified from Malik et al.,
1988).
According to the interpretaion of seismic in structures in Potwar region may be divided
into.
1. Pop-up anticlines
2. Sanke head anticlines
3. Salt cored anticlines
4. Triangle zone
Minwal X-1 lies in near Joya Mir. This region is active area for oil and gas exploration and
production. This Well is drilled by POL drill on the Joya Mair in North Eastern limits of the
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structure. The location of the well was at SP 232 on Seismic Line No: 93-MN-08. The Eocene
Bhadrar and Sakesar formations were the primary objective. The well is located in the high
fractures which could contribute in an excellent well productivity.
Structurally it is a broad anticline with its axis running SW-NE direction. The limbs of the
anticline are in the SW. The Northern limb showing dips which are steeper as compared to the
Southern limb, which are slightly gentler. The dips of the Northern limb are in between 50° - 60°
while that of Southern limb shows 55° - 75°dips. On the NE side, the anticline is separated by
Chak Naurang-Wari fault which is a major fault in the area.
2.3 TECTONIC STRUCTURES
Tectonic features in Potwar are divided from South to North into three major tectonic
elements (1) the Jhelum Plain, (2) the Salt Range and (3) the Potwar Plateau (Yeats and
Lawrence, 1984). In Potwar large wedge of Phanerozoic rocks are thrusted over the Punjab
plains along basal decollement in the Eocambrian evaporite sequence of the Salt Range
Formation. Basement in Sargodha is gently dipping northwards which does not cause structural
deformation. South of the Soan River is nearly undeformed but is deformed on its northern and
eastern margins. The potwar is divided into the following structural zones see figure-2.3.
1. Northern Potwar Deformed Zone (NPDZ).
2. Soan Syncline.
3. Eastern Potwar Plateau.
4. Southern Potwar Plateau.
5. Western Potwar Plateau.
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Figure-2.3:- Geology and new trends for petroleum exploration in Pakistan (modified from Kamal, 1991)
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2.4 SOUTHERN POTWAR PLATEAU
The study area lies in the southern part of the Potwar Plateau which is characterized by
northward-dipping strata and local open folds of low structural relief and axes that is generally
parallel to the trend of the Salt Range. Minwal triangular zone is segmented and lies in the
southern potwar plateau and is divided along left lateral Vairo and Dhab Kalan faults. The
hanging wall anticline is represented by the triangular zone orienting from southeast to
northwest flanks. The triangle zone is the result of two phases of Himalayan thrusting (Shami
and Baig, 2002).
1. The thrust and back-thrust phases are the result of northwest southeast successive
Himalayan compression.
2. The thrusts initiated as southeast and northwest vergent fault propagated folds. The fault
propagated folds were later on displaced by these thrusts.
Finally these opposite directed thrusts formed the triangle zone geometry. The drag
along the thrust and back-thrust formed the hanging wall anticlines. The hanging wall anticline
along the southeastern flank of the triangle zone has been drilled for oil and gas whereas the
hanging wall anticline along northwestern flank of the triangle zone is untapped. The structure
geometry, source and cap rock of the northwestern flank indicates that there is potential for
hydrocarbon exploration (Shami and Baig, 2002).
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Chapter 3
STRATIGRAPHY OF THE AREA
The stratigraphic column is divided into three unconformity-bounded sequences. These
unconformities in the study area are Ordovician to Carboniferous, Mesozoic to Late Permian,
and Oligocene in age (Figure-3.1). These unconformities are difficult to identify in the seismic
profiles due to complicated thrusting. The Potwar sub-basin is filled with thick infra-Cambrian
evaporite deposits overlain by relatively thin Cambrian to Eocene age platform deposits
followed by thick Miocene-Pliocene molasse deposits. This whole section has been severely
deformed by intense tectonic activity during the Himalayan orogeny in Pliocene to middle
Pleistocene time. The oldest formation penetrated in this area is the Infra- Cambrian Salt Range
Formation, which is dominantly composed of halite with subordinate marl, dolomite, and shales
(Muhammad Aamir and Muhammad Maas Siddiqui, 2006).
The Salt Range Formation is best developed in the Eastern Salt Range. The salt lies
unconformably on the Precambrian basement. The overlying platform sequence consists of
Cambrian to Eocene shallow water sediments with major unconformities at the base of
Permian and Paleocene. The Potwar basin was raised during Ordovician to Carboniferous;
therefore no sediments of this time interval were deposited in the basin. The second sudden
alteration to the sedimentary system is represented by the complete lack of the Mesozoic
sedimentary sequence, including late Permian to Cretaceous, throughout the eastern Potwar
area. In Mesozoic time the depocenter was located in central Potwar, where a thick Mesozoic
sedimentary section is present. A major unconformity is also found between the platform
sequence and overlying molasse section where the entire Oligocene sedimentary record is
missing. The molasse deposits include the Murree, Kamlial, Chinji, Nagri, and Dhok Pathan
Formations (Muhammad Aamir and Muhammad Maas Siddiqui, 2006).
Rock units ranging in age from Infra-Cambrian to Cambrian are exposed in the Potwar
Province of the Indus basin where the Salt Range Formation with salt, marl salt seams and
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dolomite is the oldest recognized unit through surface and subsurface geological information
and forms the basement for the fossiliferous Cambrian sequence (Shah, 1977).
Since the complete section of Salt Range Formation has not been observed in any of the
wells of Potwar sub-basin and the formation is not completely exposed along the Salt Range, it
was therefore, assumed in the past that the Salt Range Formation is the oldest rock unit
overlying the Pre-Cambrian basement.
However, the wells drilled up to the basement on Punjab Platfom, Pakistan and Bikaner-
Nagaur basin of India situated south of Potwar reveal that the Salt Range Formation is underlain
by Infra-Cambrian sediments of Bilara Formation followed by Jodhpur Formation. Extent of
these two formations toward north and examination of seismic data indicate that the
mentioned formations may also be present in the eastern Potwar region.
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Figure-3.1:- Schematic stratigraphic column of the study area. (S. Grelaud et al, 2002)
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3.1 LITHOLOGICAL DESCRIPTION OF FORMATIONS
Following are the lithological description of the section drilled at Balkassar Oxy#1 which
was drilled down to a depth of 3131 meter into Salt Range Formation of Infra Cambrian age.
The Formation tops were initially picked at the well site, which were further refined and
confirmed by the electric logs. A brief, generalized description of the formations drilled in
Balkassar Oxy #1 is given below.
3.1.1 INFRA-CAMBRIAN
THE SALT RANGE FORMATION
The oldest formation of the cover sequence known to lie at top of the basement is the
Eocambrian Salt Range Formation. The Formation is exposed along the outer edge of the Salt
Range from Kalabagh in the west to the Eastern Salt Range. The age assigned to the Salt Range
Formation is Infra Cambrian.
In the Punjab Plains the Salt Range Formation extends to at least 29° N-Latitude, south
of the Sargodha High, as confirmed by its thin occurrence in some exploratory wells. More likely
evaporates were deposited in smaller intra-cratonic basins.
The Salt Range Formation exhibits varied lithology, dominantly composed of reddish
brown to maroon gypseous marl interbedded with thin layers of gypsum, dolomite, clay, salt
marl and thick seams of rock salt. Thin intercalations of kerogen shale or oil shale have been
found in the Salt Range Formation. A trachy basalt trap, called the Khewra Trap or Khewrite is
present in some localities, consisting of decomposed radiating needles of a light colored
mineral, probably pyroxene. Stratigraphic division of Salt Range Formation in Khewra Gorge is
as follows:
SAHWAL MARL MEMBER
It is composed of two units, dull red marl beds with some salt seams and 10 meters thick
gypsum bed on top (more than 40 meters) and bright red marl beds with irregular gypsum,
dolomite beds and the “Khewrite Trap” (3-100 meters).
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BANDAR KAS GYPSUM MEMBER
Massive gypsum with minor beds of dolomite and clay (more than 80 meters).
BILLANWALA SALT MEMBER
It is composed of ferrigenous red marl, with thick seams of salt (more than 650 meters).
One of the most important features of the Salt Range Formation is its behavior as a zone of a
decollement between underlying rigid basement and overlying platform sequence.
3.1.2 CAMBRIAN
KHEWRA FORMATION
The Khewra Formation overlies the Late Proterozoic Salt Range Formation without any
apparent disconformity (Shah, 1977). Type locality is the Khewra Gorge in the Eastern Salt
Range. The Khewra Formation is widely exposed in the Salt Range. The Khewra Formation
consists mainly of reddish brown to purple, thick-bedded to massive sandstone with few brown
shale interclations. The sandstone is characteristically cross-bedded, has abundant ripple marks
and mud cracks, and, in places, exhibits convolute bedding. Thickness of the Khewra Formation
is 150m at the type locality in the Eastern Salt Range. Apart from rare trace fossils, the
formation is devoid of fossils. Because of its position between the late Proterozoic Salt Range
Formation and the fossiliferous early Cambrian Kussak Formation, the Khewra Formation is
thought to represent the basal part of the Lower Cambrian.
3.1.3 PERMIAN
TOBRA FORMATION
The Tobra Formation rests unconformably upon different Cambrian Formations and the
Salt Range Formation respectively (Shah, 1977). Type locality is the village of Tobra, north of
Khewra, in the Eastern Salt Range. The Formation is exposed throughout the Salt Range. It was
also encountered by the wells in the Kohat-Potwar area. In the Eastern Salt Range, the Tobra
Formation consists mainly of polymict conglomerates with pebbles and boulders of igneous,
17
metamorphic and sedimentary rocks. The thickness of the formation is 20m at the type locality.
Its age is early Permian.
DANDOT FORMATION
The Tobra Formation is overlain conformably by the Dandot Formation (Shah, 1977).
Type locality is the village of Dandot, northeast of Khewra, in the Eastern Salt Range. The
formation is well represented in the Eastern and Central Salt Range. The formation mainly
consists of dark greenish-grey, splintery shale and siltstone with intercalated sandstone,
whereas in the Salt Range greenish grey to black, carbonaceous shales with sand flasers
alternate with cross-bedded sandstones. The formation consists of rich fauna as well as spores.
On the basis of its faunal content and its gradational contact with the underlying Tobra
Formation, the Dandot Formation has been dated as Early Permian (Teichert, 1967).
WARCHHA FORMATION
The Warchha Formation rests conformably upon the Dandot Formation. Type locality is
the Warchha Nala in west-Central Salt Range. The Warchha Formation is widely exposed in the
Salt Range. The formation is generally thick-bedded to massive, reddish-brown, cross-bedded,
medium to coarse-grained and arkosic. Intercalated purple to dark grey shale layers reach a
thickness of several meters each. The Warchha Formation is unfossiliferous. It is considered
Early Permian because of its position between the fossiliferous Early Permian Dandot and
Sardhai Formations. The thickness of the Warchha Formation reaches 150m to 165m in the Salt
Range (Kadri, 1995).
SARDHAI FORMATION
The Warchha Formation has a transitional contact with the overlying Sardhai Formation
(Shah, 1977). Type locality is the Sardhai Nala in the Eastern Salt Range. The formation has an
areal distribution similar to the Warchha Formation. The prevailing lithology in the Eastern and
Central Salt Range is bluish-grey, purple or reddish claystone. Plant remains and fish scales have
occasionally been found. The fossils indicate the early Permian age. The paleo-environment is
18
interpreted as mainly terrestrial, partly lagoonal, with marine incursions, which become more
frequent towards the west. The thickness of the Sardhai Formation is 40m at the type section.
3.1.4 PALEOCENE
HANGU FORMATION
The Hangu Formation unconformably overlies various formations of Paleozoic to
Mesozoic age (Davies, 1930 & Fatmi, 1973). The type locality is south of Fort Lockhart in the
Samana Range. It consists largely of grey to brown, fine to coarse-grained, silty and ferruginous
sandstone which grades upward into fossiliferous shale and calcareous sandstone. At places,
the formation is intercalated with grey argillaceous limestone and carbonaceous shale. In the
Makarwal and Hangu areas, it contains coal beds in the lower part. Its thickness ranges from
about 15m in Hazara to 150m at Kohat Pass. The Hangu Formation is early Paleocene in age.
LOCKHART FORMATION
The Lockhart Limestone conformably overlies the Hangu Formation (Davies, 1930 and
Fatmi, 1973). Its type section is exposed near Fort Lockhart. It consists of grey, medium to thick-
bedded and massive limestone, which is rubbly and brecciated at places. Its thickness ranges
from about 30m to 240m. It contains foraminifera, molluscs, echinoids and algae (Cox, 1931;
Davies & Pinfold, 1937; Eames, 1952 and Latif, 1970). The age of the Lockhart Formation is
Paleocene.
PATALA FORMATION
The Patala Formation overlies the Lockhart Formation conformably and its type section
is in the Patala Nala in the Western Salt Range (Davies and Pinfold, 1937). It consists largely of
shale with sub-ordinate marl, limestone and sandstone. Marcasite nodules are found in the
shale. The sandstone is in the upper part. The formation also contains coal, and its thickness
ranges from 27m to over 200m (Warwick, 1990). It contains abundant foraminifera, molluscs
and ostracods (Davies & Pinfold, 1937, Eames, 1952, and Latif, 1970). The age of the Patala
Formation is Late Paleocene.
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3.1.5 EOCENE
SAKESAR FORMATION
With increase in limestone beds, the Nammal Formation transitionally passes into the
overlying Sakesar Formation, the type locality of which is the Sakesar Peak (Gee, 1935 and
Fatmi, 1973). It consists of grey, nodular to massive limestone, which is cherty in the upper
part. Near Daudkhel, the Sakesar Formation laterally grades into massive gypsum. Its thickness
ranges from 70m to about 450m. Its age is early Eocene.
CHORGALI FORMATION
The Chorgali Formation rests conformably over the Sakesar Formation (type locality
Chorgali Pass) (Pascoe, 1920 and Fatmi, 1973). It consists largely, in the lower part, of thin-
bedded grey, partly dolomitized and argillaceous limestone with bituminous odour, and in the
upper part, of greenish, soft calcareous shale with interbeds of limestone. Its thickness ranges
from 30m to 140m. It contains molluscs, ostracods and foraminifera . The age of the Chorgali
Formation is Early Eocene. It is overlain unconformably by the Neogene sequence.
Namal Formation
It comprises grey to olive green shale, light grey to bluish grey marl and argillaceous
limestone. In Salt Range, these rocks occur as alternations. In Surghar Range, the lower part
composed of bluish grey marl with interbedded calcareous shale and minor limestone while
upper part consists of bluish grey to dark grey limestone with intercalation of marl and shale. Its
type locality is Nammal Gorge Salt Range, Punjab and thickness of this formation is 100m at
type locality. Its age is early Eocene.
3.1.6 MIOCENE
Murree Formation
The type section of Murree Formation is in north of Dhol Maiki. Murree Formation is composed
of thick monotonous sequence of red and purple clay and inter-bedded greenish sandstone
with sub-ordinate intra-formational conglomerate (Wynne, 1873). The thickness of the
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formation increases from 180m to 600m in the Salt Range to 3,030m in the northern Potwar
area. It is poorly fossiliferous though plant remains and some vertebrate bones have been
found. This fauna indicates early Miocene age of the Murree Formation.
KAMLIAL FORMATION
The type section of Kamlial Formation is in the southwest of Kamlial, the formation
overlies the Murree Formation conformably and transitionally; though at some localities it lies
unconformably on the Eocene Sakesar Formation (Pinfold, 1918, Lewis, 1937, Fatmi, 1973 and
Cheema et al., 1977). The formation consists mainly of grey to brick red, medium to coarse-
grained sandstone interbedded with purple shale and intraformational conglomerate. A
number of mammalian fossils have been found (Pascoe, 1963). The age of the Kamlial
Formation is middle to late Miocene.
3.1.6 PLIOCENE
SIWALIK GROUP
1. Chingi formation
The type locality of Chingi formation is South of Chinji, Campbellpur, Punjab. And its
lithology comprises of Clay, sandstone with minor siltstone. According to Shami and Baig
thickness of this formation is 750m at type locality. The age of Chingi formation is Late Miocene
to early Pliocene.
2. NAGRI FORMATION
Nagri village, Campbellpur District, Punjab is the type section of the nagri formation. Its
lithology comprises of salt, conglomerate, clay. Thickness of this formation ranges from 200m-
3000m. Its age is early Pliocene.
3. DHOK PATHAN FORMATION
Its Type locality is Dhok village Campbellpur District, Punjab is the type section of this
formation of this formation. Lithology comprises of sandstone, clay and conglomerate. Its
thickness at type section ranges from 1330m-2000m and its age is Middle Pliocene.
21
4. SOAN FORMATION
Its type locality is Gaji Jagir, Sahil Road near Mujahid village north.of Soan River,
Campbellpur District, Punjab and Lithology comprises of Conglomerate, siltstone and thickness
of this formation ranges from 300m-3000m. The age of this formation is late Pliocene.
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Chapter 4
PETROLEUM GEOLOGY OF AREA
The geological history of this basin begins from Precambrian age. East of Potwar Plateau
is salt-cored which anticlines are separated by the wide synclines. Tanwin-Bains-Buttar and Joya
Mair-Chak Naurang-Adhi-GungrillaKallar are such main trends. The cores of these salt anticlines
are thrusted and originated due to the compression of Himalayan orogeny in Miocene-Pliocene
age. The oil and gas in the area has been produced from the fractured carbonates of Paleocene
and Eocene age but Mesozoic sandstones and Paleozoic carbonates and sandstones has
produced additional oil (Ahmed, 1995) in the area.
Oil and gas exploration in Eastern Potwar area mainly in south of Soan Syncline are
enlongated synclines which are trending from NE-SW have steep dipping flanks because of the
salt pop-up. The thrusts, fault propagation folds and triangle and pop-up zones which are
double edged and are believed to be formed by the strike- slip movement along the decollment
surface. The Western Potwar lacks the evaporite sequence as compare to the Eastern Potwar
and Central Potwar (Moghal et al, 2007).
The Salt Range Fore Land Basin falls under the class of extra continental down wrap
basin. It has plenty of tectonic structures and hosts continental margin, thick marine
sedimentary sequence, source and reservoir and cap rocks (Riva, 1983). The optimization
temperature and the thick overburden of 3047m of molasse provides burial depth (Pressure)
for the achieving the oil formation. Because of this in Salt Range Potwar Foreland Basin is
producing oil from the depth of 2750-5200 m. This resulted in the formation of source,
reservoir and seal in the areas of Minwal, Joyamir, Toot, Meyal and Dhulian Oil fields (Kozary,
1968).
Approximately 135,000 barrels of oil is being generated from the Karsal field of Central
Potwar. Seismic data of 2002 by PPL did not revealed any structural closure which indicates that
producing wells are on a monocline/flexure nose and permeability from the local field in
surrounding areas of faults. Basins are faulted and anticlinal in nature and contain salt in its
23
core which are sometimes are asymmetric to overturned. The hydrocarbon in most of the areas
of Potwar may be attributed to the structural styles. The structural style framework is the result
of the intensive structure formation in eastern part which contains network divides and altered
geological sections and collaborate other data forms. In Potwar sub-basin the structural
development is due to the faults and decollement levels. In the Potwar sub-basin, there are
local decollement levels recognizable besides two main at the interfaces of Eocene-molasse
sequence and platform-evaporite sequence (Salt Range Formation). Based on the structural
styles Potwar sub-basin is divided into various zones. Structures have been in different
orientation and styles and have been observed through interpretation of maps. The table-4.1
shows the oil and gas fields in Upper Indus Basin. Figure-4.1 shows the structural evolution of
the triangular zone in the area.
Age Formations Lithology Oil & Gas Field Production
Eocene/
Paleocene
Lockhart
Sakesar
Chorgali
Limestone
Dhurnal
Dakhni
Balkassar
Chalk-Naurang
Minwal
Oil
Jurassic Datta
Samana Suk
Sandstone &
Limestone
Dhulian, Toot
Meyal Oil
Permian Nilawahan
Zaulch Group
Conglomerate &
Limestone
Adhi
Dhurnal Oil
Cambrian Khewra Sandstone Sandstone Adhi
Missa Keswal
Gas
Table-1:- Hydrocarbon significance of different rock units in the study area (modified after Kadri, 1995)
24
Figure-4.1:- Subsurface geometry of area in relation to structure and entrapment of oil and gas (modified from Moghal, 2003).
25
4.1 RESERVOIR
The main oil producing reserviors in Minwal are the Cambrian, Permian, Jurassic,
Paleocene and Eocene. Primary Porosity is lower in these reservoirs as compare to the
secondary porosity. The main oil producing reservoirs in Minwal area are fractured carbonates
which are of Sakesar and Chorgali Formations. The massive light yellow gray and partly
dolomitized of Sakesar limestone contain chert. The Chorgali Formation is creamy yellow to
yellow gray, silty, partly dolomitic and thin bedded limestone. It was deposited in intratidal
conditions where sebkha conditions dominated (Shami and Baig, 2002).
The calcite cement has occupied the pore spaces and compaction and its cementation
helped it to destroy its porosity also primary porosity than <1% in the Chorgali and Sakesar
Limestone during the core analysis in the Meyal, Dhulian and Minwal oilfields have been
observed due to dolomitization. Some samples have also showed that primary porosity has
completely destroyed due to the over burden pressure of the rock especially compaction and
cementation and the logs like Bore Hole Compensate-Gama Ray, Compensate Neutron Log-
Lithodensity Logging has not indicated the primary porosity and permeability. In the North
Western Potwar the fractured porosity is comparatively very high because these rocks have
deformed into the process of Himalayan orogeny. The rock fractures develop parallel, oblique
and perpendicular to the fold axes of anticlines (Shami and Baig, 2002).
4.2 SOURCE ROCK
The potential source rock in Minwal is the grey shales of Mianwali formation, Datta
formation and Patala formation. The Eocambrian Salt Range Formation contains oil shales with
27%-36% TOC in isolated pocket of shales are the source rock in the Salt Range Potwar Foreland
Basin (Shami and Baig, 2002).
In Potwar, the TOC 1.57 and hydrogen Index of 2.68 in shales have been observed
(Porth and Raza, 1990). Patala formation is the key source rock of oil production in Potwar sub-
basin according to the oil to source correlation.
26
4.3 CAP ROCK
The thin-skinned tectonics has developed the traps creating the faulted anticlines, pop-
up and positive flower structures above Pre-Cambrian salt. The lateral and vertical seal to
Eocene reservoir is provided by the Murree Formation’s clays and shales (Shami and Baig,
2002).
27
Chapter 5
2D SEISMIC DATA ACQUISITION AND PROCESSING
5.1 SEISMIC DATA ACQUISITION
Geophysics is technique used to probe the internal structure of the earth (shallow and
deep) and also to understand the extent of the different formation on map and to conclude the
internal physical properties. By analyzing the geophysical data it is observed that how physical
properties of the earth can vary vertically and horizontally. Different scales are being
investigated for entire surface of the earth (global geophysics; e.g. Kearey & Vine, 1996) and for
engineering purpose (Vogelsang, 1995 & McCann et al, 1997). The seismic data acquisition can
be done with two methods.
1. Reflection Seismic acquisition.
2. Refraction Seismic acquisition.
In our dissertation reflection seismic survey has been used. In reflection seismic survey
elastic waves between different geological layers in the subsurface is used to produce the
geological model of the subsurface for hydrocarbon exploration. This method provides us the
picture of the subsurface. For 2D seismic survey the source and the receiver are placed inline.
The reliable interpretation of and processing of the 2D seismic data depends upon its field
parameters. The poor parameter and design of the survey can generate distort the subsurface
picture in the seismic section.
5.2 SEISMIC SOURCE
The seismic source releases energy with in the localized region causes stress in a
surrounding medium. The example of seismic source is an explosion. The main criteria of
seismic sources are.
1. The seismic energy must be satisfactory provide sufficient energy enough to be recordable.
28
2. Seismic energy is recorded in the form of wave energy either P-wave or S-wave. Other
unwanted energy signals would create distortion in the recorded data and this is called as
coherent noise.
3. Seismic energy which is converted into waveform must repeat itself.
4. The seismic energy source must be non-hazardous/safe which is safe and efficient and must
be environmental protected (Philip Kearey et al, 2002).
The land seismic sources falls under two categories which are mentioned below.
1. Explosive.
2. Non-explosive.
5.2.1 EXPLOSIVE SOURCE
DYNAMITE
In seismic line no. 782-CW-29, dynamite has been used as a source for acquisition and it
is blasted in shallow shots and it meant for improving the coupling of the energy source and to
minimize surface damage. It provides the higher resolution of a data. Because they are drilled in
short holes they are slow to use on land. In modern processing, repeated precise source
signature is required which is the major drawback of dynamite (Philip Kearey et al, 2002).
SOURCETYPE Dynamite
PATTERN Array
NUMBER/ SHORT POINT 9
SHORT POINT INTERVAL 656
MEASURED SYSTEM ft
Table-5.1:- Source Parameter for line NO. 782-CW-29
5.2.2 NON-EXPLOSIVE SOURCE
VIBROSEIS
In seismic line no. 93-MN-07 and 93-MN-08, vibroseis has been used as a source for
acquisition. It provides a precision and repeated signal. A vibrator is loaded in a truck which
29
passes the vibration into the ground through its pads. This causes vibration in the ground which
is called sweeps (Philip Kearey et al, 2002).
Vibrator in vibroseis requires a firm ground or base to operate causes no damage to the
town or significant disturbance to the environment. One the major disadvantages of the
vibroseis accounts that costs a half a million US dollar (Philip Kearey et al, 2002).
The source parameter used for the seismic lines are mentioned in table 1 and 2.
ENERGY SOURCE Vibroseies
SWEEP FREQUENCY 9-72HZ
SWEEP LENGTH 14sec
GROUP INTERVAL 40M
Table-5.2:- Source parameter used for the seismic lines 93-MN-08 and 93-MN-07
5.3 RECEIVERS
The geophones or seismogram are the receivers in seismic data acquisition which
converts the ground signal into electrical signal caused by the shooting of the seismic energy.
The sismic industry uses two types of geophones electromagnetic geophone (for land Survey)
and Hydrophones (for marine survey). This signal comprises of instrument system and the
aftermath of this is the subsurface geological information visible in recording section (Dobrin
and Savit, 1988). Figure-5.1 shows common cross-sectional view of a geophone and the table-3
shows the geophone parameter.
Line Group Interval
Group Interval 40M
Array Length 40M
Geophone Group 36M
Array Type Inline
Table-5.3:- Shows the geophone parameter used for the seismic lines.
30
5.4 THE ARRAY SYSTEM PROFILING
The single shot and single geophone was used in early days for each traces for seismic
data acquisition. The concept of spreading of geophones over 10-100s of feet connected in the
series or parallel arrangement was introduced in 1930s. The purpose of this geometry was for
that the first six geophones must cancel the ground rolls and noises which are traveling
horizontally. Vibroseis is capable of shooting many shots as seen in dynamite in which one has
to drill many shot holes (Dobrin and Savit, 1988). There are different types of spread used in the
field to acquire seismic data.
1. End Spread.
2. Inline Offset Spread
3. Split Spread.
4. Cross Spread.
5. L Shaped Spread.
The seismic lines have used inline spread geometry. It is a spread shot from a shot-point
which is separated from the spread an appreciable distance but along the line of the spread
(Sah, 2003).
5.5 GEOPHONE INTERVAL
The distance between two sets of geophones either next to or adjoining geophones is
called a geophone interval. The seismic line no. 782-CW-29 has geophone interval of 20 m. The
seismic line no. 93-MN-08 and 93-MN-07 have geophone interval of 36 m.
5.6 GROUP INTERVAL
It is the horizontal distance between two sets of geophones either next to or adjoining
geophones. The seismic line no. 782-CW-29 has group interval of 328 and for the seismic line
no. 93-MN-08 and 93-MN-07 have group interval of 40 m.
31
5.7 SEISMIC DATA PROCESSING
The raw data recorded in the field is processed to construct a useful geological model so
that its interpretation possible. The step called seismic data processing is applied. Its results and
output depends upon the field acquisition parameters. The data in is field is recorded either in
digital or analog form and are transformed in the processing center. The primary objective is to
remove or suppress all Noises and to increase the signal to noise ratio.
The type of surface condition have tells that how much energy penetrates into the
ground. The environmental condition, surface condition and demography play an important
role in quality of field data. Besides processing also depend upon the technique used in
processing (Dobrin and Savit, 1988). The main objectives of the seismic data processing are
summarized as below.
1. Improving Signal to Noise ratio.
2. Representation of geology in seismic cross-section.
3. To acquire the target provided by client.
The seismic data processing chart is shown in figure-5.2. To increase the Signal to Noise
which constitutes the following corrections and adjustments are applied during the seismic data
processing.
1. Time.
2. Amplitude.
3. Frequency-Phase content.
4. Deconvolution.
5. Correlation.
6. Stacking/Data Compressing.
7. Velocity Analysis.
8. Preprocessors.
9. Filters.
10. Migration/Imaging/Data Positioning.
32
Figure-5.1:- Detailed Processing Sequence Flow Chart (Modified from Rehman, 1989)
33
5.7.1 TIME
The time adjustment falls under two categories.
a. Dynamic.
b. Static.
a. DYNAMIC TIME CORRECTION (NORMAL MOVE-OUT)
If the source and receiver are located at the same point in zero offset than the
difference between the travel time ΔT and the reflected arrivals at x is the NMO (Philip Kearey
et al, 2002).
b. STATIC TIME CORRECTION
If the ray path is vertical beneath any shot or detector the static correction is applied.
The time taken by the signal from the source to the receiver which is called a travel time is
corrected for the time taken to travel the vertical distance between the shot or detector
elevation and the survey datum. The adjustment of travel time to datum can be achieved if the
correction of the height interval between the base of the weathered layer and datum is
substituted by the material which contains the velocity of the top layer (Philip Kearey et al,
2002). Figure-5.3 illustrates the how the datum elevation is being corrected.
Figure-5.2:- Static corrections (a) Seismograms showing time differences between reflection events on adjacent
seismograms due to the different elevations of shots and detectors and the presence of a weathered layer. (b)
The same seismograms after the application of elevation and weathering corrections, showing good alignment
of the reflection events (After O’Brien, 1974)
34
The two kinds of static corrections which are applied are mentioned below.
a. Elevation Correction.
b. Weathering Correction.
5.7.2 AMPLITUDE CORRECTION
Because of spherical divergence and energy dispersion in the earth the Amplitude
Adjustment is applied. The types of Amplitude Adjustment which are applied are mentioned
below.
a. Automatic Gain Control (AGC) or Structural Gain Control.
b. Relative True Amplitude Gain.
a. AUTOMATIC GAIN CONTROL (AGC) OR STRUCTURAL GAIN CONTROL
To improve the quality of the low amplitudes at later stages the AGC or the automatic
gain control is applied (Dobrin and Savit, 1988).
b. RELATIVE TRUE AMPLITUDE GAIN
Amplitude information concerned with the facies changes, porosity variations, and
gaseous hydrocarbons are maintained (Dobrin and Savit, 1988).
5.7.3 FREQUENCY PHASE CONTENT
To enhance the signal and to reduce noise the frequency-phase content of the data is
handled. The suitable bandpass filter can be selected by referring to frequency scan of the data
which helps in calculating the frequency content of the signals (Dobrin and Savit, 1988).
5.7.4 DECONVOLUTION (INVERSE FILTERING)
A process designed to restore a wave shape to the form it had before it underwent a
linear filtering action (convolution) (Sheriff, 1989). The examples to remove the effects caused
by the filtering include.
a. Deterministic Deconvolution.
35
b. Spiking Deconvolution.
c. Predictive Deconvolution.
d. Sparse-spike Deconvolution.
a. DETERMINISTIC DECONVOLUTION
If the characteristics of a system are known than it can be used to remove the effects of
the recording instrument. If the reflection from the sea floor and the travel time in water is
known than this deterministic deconvolution helps to remove the ringing that results from
those waves which have undergone more than one bounce in water layer (Sheriff, 2004).
b. SPIKING DECONVOLUTION
To make the embedded wavelet short close to a spike a special a type of deconvolution
method is applied which is called spiking deconvolution in which the frequency bandwidth of
the data is limited to some extent (Sheriff, 2004).
c. PREDICTIVE DECONVOLUTION
In the process of predictive deconvolution the effects of some multiples are being
removed which uses the later portion of the autocorrelation (Sheriff, 2004).
d. SPARSE-SPIKE DECONVOLUTION
The sparse-spike deconvolution is being applied to reduce the reflections and to
emphasize more on large amplitudes (Sheriff, 2004).
5.7.5 CORRELATION
It is measurement of character and time alignment of two traces. Because correlation is
convolution so an identical frequency domain operation also applies to correlation (Yilmaz,
2001). There are two types of correlation technique which are applied.
a. Cross Correlation.
b. Auto Correlation.
36
a. CROSS CORRELATION
It measures the similarity between the two time series. One data set value is moved
with respect to the other and values which are infront of each other are multiplied and their
products are than summed to give the value of the cross-correlation (Telford et al, 1990).
b. AUTOCORRELATION
Cross correlation of a time series with itself is known as auto correlation. In this case the
correlation of the data is being done with itself (Telford et al, 1990).
5.7.6 STACKING (DATA COMPRESSION)
The stacking is the process of combination of traces which is a composite record from
different records (Sheriff, 2001). The technique uses the phenomena of common midpoint
(CMP) stack in which one trace is being achieved by summing up all the offsets of common
midpoint gather. Generally 48-96 fold stacks are commonly being used.
5.7.7 VELOCITY ANALYSIS
It is being carried out on a suitable CMP or CDP gather. Its output is the velocity
spectrum which is the table of numbers as a function of velocity vs. two way zero off set time.
There are several types of velocities in reflection seismic data analysis (Telford et al,
1990).
a. Interval velocity.
b. Root Mean Square Velocity.
c. Normal Move Out Velocity.
d. Stacking Velocity.
5.7.8 PREPROCESSORS
The preprocessor has three components.
a. Muting.
b. Edit
37
c. Demltiplexing.
a. MUTING
This process is useful in removing ground roll, air waves, or noise bursts out of the stack.
In this process the relative stacking components must be changed with recorded time and
before the beginning of this process the record, long-offset traces must be muted and removed
from the stack. The deconvolution and other operators may be changed in muting. It can occur
gradually or abruptly (Sheriff, 2001).
b. EDIT
The raw data seismic data obtain from the field acquisition is in multiplexed form and
contains some unwanted signal such as ground rolls, air waves or noise and dead traces.
Demultiplexing which is done through some calculations corrects the information such as the
removal of the effect of the gains in recording instrunment and replacing a correct value for
spherical divergence nevertheless it also useful for static-shift and normal-moveout corrections
(Sheriff, 2001).
c. DEMULTIPLEXING
It is used to separate the individual component channels that have been multiplexed
(Sheriff, 2001).
5.7.9 FILTERS
It is a system which recognizes the distinction against some of the data entering in it.
This disction is normally based on the frequency but the other are based on wavelength,
moveout, coherence, or amplitude. Linear filtering in geophysical data processing is called
Convolution. The system is generally being convolved either in time domain or spectral shaping
in the frequency domain. The types used in filters are (Sheriff, 2001) as in table-4.
1. Low pass frequency filters.
2. High pass frequency filters.
38
3. Band pass frequency filters.
4. Notch filters.
5. Inverse filters/Deconvolution.
6. Velocity filters.
7. F-K filters.
5.7.10 MIGRATION/IMAGING
The seismic section is reconstructed in such a manner that events caused by the
reflection are moved to their actual position according to their correct surface location and
correct vertical time. It mainly concerns with the energy which is extends over the Fresnel zone
and reducing the diffraction patterns which are the results of point reflectors and faulted beds.
Migration is required because in dipping horizons and variable velocities recorded at the
surface position differs from the subsurface positions (Philip Kearey et al, 2002). The time
migration (Post-Migration) involves the change of velocity in vertical direction whereas the
horizontal change in the velocity is called the Depth Migration (Prestack Migration) (Sheriff,
2001).
39
Chapter 6
SEISMIC DATA INTERPRETATION
6.1 SEISMIC DATA INTERPRETATION
The final step in seismic study of an area is to interpret the processed seismic section so
that a geological model of sub-surface can be developed. Here the objective of seismic
reflection interpretation is to study the subsurface structures that help in discovering the
hydrocarbon accumulation in the subsurface sedimentary rocks. As science has not yet
discovered the direct method of finding the oil and gas, or of assessing the quantities of
hydrocarbons in the subsurface, so the seismic reflection method only indicates the geological
situations where the hydrocarbons can accumulate.
Seismic can be interpreted in two modes:
1. The first mode is in areas of substantial well control, in which the well information is first
tied to the seismic information, and the seismic then supplies the continuity between the
well for the zone of interest.
2. The second mode is in areas of no well control (frontier areas) in which the seismic data
provide both definition of structure and estimates of depositional environments. Seismic
velocities and seismic stratigraphic concepts are used to define the lithology. Seismic
reflection amplitudes help to detail velocities and serve as a guide to pore constituents.
Seismic interpretation is the transformation of seismic reflection data into a structural
picture, contouring of subsurface horizons and further depth conversion by applying some
suitable velocities. The seismic reflection interpretation usually consists of calculating the
positions, and recognizing the geologically, covered interfaces or sharp transition zones from
seismic pulses which is reflected from the ground surface.
The main methods for the interpretation of the seismic section are.
1. Structural Analysis
2. Stratigraphic Analysis
40
1. STRUCTURAL ANALYSIS
It is the study of reflector geometry on the basis of reflection time. The key use of the
structural analysis of seismic section is in the search for structural traps containing
hydrocarbons. Most structural interpretation uses two-way reflection times rather depth. And
time structural maps are constructed to display the geometry of selected reflection events.
Some seismic sections contain images that can be interpreted without difficulty. Discontinues
reflections clearly indicate faults and undulating reflections reveal folded beds.
2. STRATIGRAPHIC ANALYSIS
Stratigraphic analysis involves the subdivision of seismic sections into sequence of
reflections that are interpreted as a seismic expression of genetically related sedimentary
sequences. The principles behind this seismic sequence analysis are of two types.
Firstly, reflections are taken as chronostratigraphical units, since the type of rock
interface that produce reflections are strata surfaces and unconformities, by contrast the
boundary of diachronous lithological units tend to be transitional and not to produce
reflections.
Secondly, genetically related sedimentary sequences normally comprise the set of
concordant strata that exhibit discordance with underlying and overlying strata.
According to Dobrin and Savit, 1988 throughout the history of the reflection method, its
performance in locating hydrocarbons in stratigraphic traps has been much less favorable than
in finding structurally entrapped oil and gas.
Stratigraphic oil traps can result from reefs, pinchouts, or other features associated
erosional truncation, facies, transition and sand lenses associated with buried channels, lakes,
or similar sources.
6.2 INTERPRETATION OF SIESMIC LINES OF THE STUDY AREA
The seismic data interpretation has been carried out on 93-MN-8, 93-MN-7(Dip Lines)
and 782-CW-29(Strike Line). Pre-Stack Time Migration version of 2-D seismic lines has been
interpreted. The seismic data interpretation revealed that the structure of the area is a fault
41
bounded anticline trending SW-NE direction. In the north the anticline is bounded by south east
dipping back thrust, whereas the southern flank of anticline is gentle. After interpretation of
seismic lines two way time and Depth contour maps were generated on Chorgali level. The
study area has shown two types of fault.
1. Reverse faults.
2. Thrust faults.
Thrust faults were observed in seismic lines 93-MN-08, 93-MN-07 and 782-CW-29.
These faults have created the pop up structure which can generate hydrocarbon in the area.
The faults are identified on the seismic section by sudden change in the position of the
reflectors and distortion or disappearance of the reflection.
6.3 SEISMIC SECTION
Seismic Section is the outcome of the seismic reflection survey. The seismic section
shows the high values of traces in vertical line which are called recorded peaks in the cross
section. Most importantly it points out some the features of a geologic cross-section. These
high value traces in seismic section is filled in with black shows the wiggle-variable area. The
seismic section display or plot the data of the seismic line. The vertical scale in the seismic
section displays the arrival time (Two way Travel Time). Seismic section plots or displays seismic
data along a line. The vertical scale is usually arrival time but sometimes depth and the
Horizontal axis shows the short points and CDP.
6.4 SEISMIC HORIZONS
The reflection that can be traced across a seismic section is called a seismic horizon.
Since Chorgali formation is producing reservoir in the area so Chorgali horizon is marked in the
seismic sections. In seismic sections the basement shows no good continuous reflection and has
very short, disordered and discontinuous reflection. The geology of the area it reveals that it
has Salt in its basement. The horizon named Chorgali is marked on the basis that it is producing
reservoir in the area and has excellent visibility and good continuity of reflection so we can
trace well over the whole seismic lines. So this horizon can be easily recognized in the seismic
42
section. Hence two way travel time for the Chorgali formation was taken from the seismic
sections. The red line marks the Chorgali reflector. Whereas the black lines marks the faults
observed in the seismic lines. The distorted very short and disordered reflection pattern in the
bottom of the seismic sections is basement in the seismic section. The Minwal X-1 is drilled in
the seismic line 93-MN-08 at short point 232. The seismic sections are given in figure 6.1, 6.2
and 6.3.
43
Figure-6.1:- Shows the seismic section 93-MN-08
44
Figure-6.2:- shows the seismic line no. 93-MN-07
45
Figure-6.3:- showing the line no. 782-CW-29
46
6.5 CONTOUR MAPS
A line that connects the line of equal values is called a contour line. Such maps show us
steepness of slopes, elevation top of the subsurface of the sedimentary rock layer and also the
two way travel time of the horizon in milliseconds (Norman, 2001).
6.5.1 TIME CONTOUR MAP
Time was taken from the seismic section. The next step was the generating the time
contour map. As the time was in milliseconds so it was converted into seconds and then plotted
into the base map. The Time contour map of top of Chorgali is given below.
47
Figue-6.1:- Time contour map of top of Chorgali Formation.
48
6.3.2 DEPTH CONTOUR MAP
The depth contour map marks the depth of structure. The depth contour map in the
subsurface mainly shows the faults, anticlines and folds. So after marking the time contour map
the depth contour map is being generated in the area of the Eocene top by using the following
formula.
S=V*T/2
Where,
T = Two way reflection time (sec)
V =Average velocity (meter/second)
49
Figure-6.2:- Depth map contour map of the top of Chorgali Formation.
50
6.3.3 TIME VS DEPTH GRAPH
The time and velocity in seismic line no. 93-MN-08 at short point 232 data is given
below and depth was calculated through this data and after that time Vs depth curve was
generated.
Short Point 232
CDP 475
Time Velocity S=V*T Depth=V*T/2000
8 3135 25080 12.54
316 3222 1018152 509.076
729 3389 2470581 1235.2905
861 3442 2963562 1481.781
1458 3614 5269212 2634.606
2240 3870 8668800 4334.4
2451 3953 9688803 4844.4015
5000 4559 22795000 11397.5
51
Figure-6.3:- Shows the time VS depth graph.
52
CONCLUSIONS1. According to the picture reflected by Time Structure and Depth Contour Maps, Minwal
structure is an a fault bounded anticline and has a combination of North and South verging
thrust with downthrown block in the middle.
2. In the north the anticline is bounded by south east dipping back thrust, whereas the
southern flank of anticline is gentle.
3. South verging thrust does have a sub-thrust resulting in small up thrown block to the
north. The thrust fault in the area indicates the compressional tectonic movement.
4. These opposite directed thrust formed the triangle zone geometry.
5. The carbonates of Chorgali and Sakesar (Eocene) formation are reservoir rocks in this
area.
53
REFERENCES1. Aamir, Muhammad., and Siddiqui, Muhammad Mass., 2006, Interpretation and visualization
of thrust sheets in a triangle zone in eastern Potwar, Pakistan, Society of Exploration
Geophysicists, v. 25; no. 1; p. 25, 26, 28, 29.
2. Ahmed, Shahid., 1995, Production Of Crude Oils In Pakistan: Outlook For The Future,
Ministry of Petroleum and Natural Resources, JOJ9-A, Pak Plaza, Fazal-e-Haq Road, Blue
Area, Islamabad, Pakistan, PP-1.
3. Baig, M. S. and Lawrence, R. D., 1987. Kashmir J. Geol., 5, PP 1-22.
4. Baker, D.M., 1988, Balanced structural cross section of the central Salt Range and Potwar
Plateau of Pakistan-Shortening and overthrust deformation: Corvallis, Oregon State
University, PP-120.
5. Butler, W.H., Harwood, G.M., and Knipe, R.J., 1987, Salt control on thrust geometry,
structural style and gravitational collapse along the Himalayan Mountain front in the Salt
Range of northern Pakistan, in Leche, I., and O’Brien, J.J., eds., Dynamic geology of salt and
related structures: San Diego, Calif., Academic Press, PP-339–418.