Chapter 1 INTRODUCTION 1.1 1.1
INTR INTROD ODUC UCTI TION ON OF THE THE STU STUDY DY AREA 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 st udy area (Mehmood, 2008).
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1.2 1.2
DATA DATA OB OBTA TAIN INED ED FO FOR R STUD STUDY Y
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: LINE: 93-MN 93-MN-8 -8 (Dip (Dip Line Line)) 2. LINE: LINE: 93-MN 93-MN-7 -7 (Dip (Dip Line Line)) 3. LINE: LINE: 782-CW782-CW-29 29 (Strike (Strike Line) 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 1.3
OBJE OB JECT CTIV IVES ES OF THE THE STUD STUDY Y
The purpose purpose of this dissertation is to understand understand the various steps invo involve lved d in seismi seismic c refle reflecti ction on interp interpre retat tatio ion. n. This This study study is carri carried ed out out to generate reasonable model and structure of the subsurface of Minwal D & P lease lease area area and and to unde underst rstan and d and and enhan enhance ce our our know knowled ledge ge on diffe differe rent nt 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 2.1
REGI RE GION ONAL AL TE TECT CTON ONIC IC SET SETTI TING NG
The building building of Himalayan Himalayan mountain mountain process in Eocene triggered triggered compres compression sional al system. system. Northwar Northward d movemen movementt of Indian Indian plate plate is about 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 and moun mountai tain n rang ranges es moved moved in the the east east west west direct direction ion (Kazm (Kazmii and and Jan, 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 fold belt, belt, the Sulei Suleiman man fold fold belt belt and and the the Makr Makran an accre accretio tiona nary ry wedge 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 folds and thru thrust st belt. belt. The The youn younges gestt basin basins s in the Wester Western n Himal Himalay ayan an Foreland Thrust Belt are Kohat Plateau, Bannu Basin and Potwar Plateau which which have have comp compres ressiv sive e stress stresses es and and conv converg ergen entt tecto tectoni nics. cs. Pakis Pakistan tan is located at in the two domains domains Gondwanian and the Tethyan Domains 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. North Northern ern Colli Collisio sion n Belt. 5
2. Subduct Subduction ion Complex Complex Associati Association on of Balochis Balochistan. tan. 3. Chama Chaman n Transf Transfor orm m Zone. Zone. 4. Ophiolit Ophiolites es and Ophio Ophioliti litic c Melanges. Melanges. 5. Plat Platfo form rm Area Areas. s. The Potwar Plateau is comprises comprises of less internally internally deformed fold and thrust belt having a width of approximately 150 km in N−S direction. The terrain in Potwar is undulated. undulated. Sakesar is the highest mountain mountain of this region (1522 (1522 m). The Potwar Potwar is tectonica tectonically lly situated directly directly below below the western 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 Kalabagh strike-slip fault and in the south by the Salt Range Thrust (Aamir and Siddiqui, 2006) see figure-2.1.
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Figure-2.1:- Tectonic map of Northwest of Himalayas of Pakistan showing main tectonic divisions (modified from Shami and Baig, 2002)
2.2
GEOLO GEOLOGIC GICAL AL BO BOUN UNDAR DARY Y OF OF THE THE POTWAR POTWAR PLATEA PLATEAU U
The Potwar is bounded bounded by the following following two strike-slip and two thrust fault which are. 1. Kala Kalaba bagh gh Faul Fault. t. 2. Jhle Jhleum um Faul Fault. t. 7
3. Salt Salt Ran Range ge Thru Thrust. st. 4. Main Main Boun Bounda dary ry Thru Thrust. st. 1. KALABA KALABAGH GH FAUL FAULT T
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 Kalabag Kalabagh h City, City, Mianwali Mianwali and is the Trans-Indus Trans-Indus extention extention of Western Western Salt Range (McDougal & Khan, 1990). 2. JHEL JHELUM UM FAUL FAULT T
Extending from Kohala to Azad Pattan the Murree is hanging while Kamlial, Chingi and Nagri formations are footwall. Starting from the IndusKohistan to Ravi it is the active aspect of the Indian Shield. It is seen also in the the map map that that MBT, MBT, Panja Panjall Thru Thrust st and and HFT cut cut shorte shortened ned by leftleft-lat latera erall reverse Jhelum Fault in west (Baig, Lawrence, 1987). 3. SALT SALT RANGE RANGE THRUS THRUST T
It is also known as Himalayan Frontal Thrust. Salt range and TransIndus Himalayan ranges are the foothills. 4. MAIN BOUNDAR BOUNDARY Y THRUST 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 nonstriking fault in its western part i.e. Hazara Kashmir-Syntexis (Latif, 1970; Yeats and Lawrence, 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). 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. faults. The structure structure of Potwar Potwar basin is affected affected by compres compression sional al forces, forces, basem basemen entt slope slope,, and and variab variable le thickn thickness ess of Pre-C Pre-Camb ambri rian an salt salt over over the 8
basem basemen ent, t, and depo deposit sition ion of very very thick thick molas molasse se and and tecto tectonic nic event events. s. In Potwar basin some surface features mismatch subsurface structures due to decollements at different levels. In such circumstances, it is necessary to integ integrat rate e seismi seismic c data data with with surfa surface ce geolog geologica icall infor informat matio ion n for for preci precise se 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 of the basement basement (steeper in western western Potwar Potwar Plateau). Plateau). 2. Thickness of of the Eocambria Eocambrian n evaporates evaporates beneath beneath the cover. cover. 3. Reactivation Reactivation of basement basement brittle brittle tectonics tectonics (more enhanced enhanced in the eastern eastern Potwar Plateau). In Potwar, the Eocambrian Eocambrian evaporite sequence is overlain by Cambrian 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 deposition or erosion and the strata from these periods are missing in Potwar sub-basin. The continental depositional envir environ onme menta ntall of Nilaw Nilawah ahan an grou group p of early early Permi Permian an is boun bounde ded d 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 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 Shinawari (limestone and shale s hale 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 present tectonic tectonic framewo framework rk and the position position of the Potwar Plateau have 9
resulted from the northward under-thrusting by the Indian plate under its own sedimentary cover (Khan, 1986). Salt horizon of Eocene in Kohat area is separated due to structural difference. Data being gathered through (Butler and others, 1987; Leathers, 1987 1987;; Baker Baker and and other others, s, 1988; 1988; Jaumé Jaumé and Lillie Lillie,, 1988 1988;; Penno Pennock, ck, 1988 1988;; Pennock and others, 1989; Raza and others, 1989; Hylland, 1990; Jaswal, 1990 1990;; McDou McDougal galll and Husai Husain, n, 1991 1991)) seismi seismic c prof profile iles, s, well well logs, logs, Boug Bougue uerr 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 According to the interpretaion interpretaion of seismic in structures in Potwar region may be divided into. 1. Pop-u Pop-up p antic anticlin lines es 2. Sanke Sanke head head anti anticli clines nes 3. Salt Salt cored cored ant anticl icline ines s 10
4. Tria Triang ngle le zone 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 structure. The location of the well was at SP 232 on Seismic Line No: 93-MN-08. The Eocene Bhadrar and Sakesar form format atio ions ns were were the the prim primar ary y obje object ctiv ive. e. The The well well is loca locate ted d in the the high 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 anticline are in the SW. The Northern Northern limb showing dips which which are are steepe steeperr as compa compared red to the Sout Souther hern n limb limb,, which which are sligh slightly tly 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 2.3
TECT ECTONIC STRUCTU UCTUR RES
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 Potw Potwar ar Plat Platea eau u (Yea (Yeats ts and and Lawr Lawren ence ce,, 1984 1984). ). In Potw Potwar ar larg large e wedg wedge e of Phan Phaner eroz ozoi oic c rock rocks s are are thru thrust sted ed over over the the Punj Punjab ab plai plains ns alon along g basa basall deco decoll llem emen entt in the the Eoca Eocamb mbri rian an evap evapor orit ite e sequ sequen ence ce of the the Salt Salt Rang Range e Formation. Basement in Sargodha is gently dipping northwards which does not not cau cause stru struct ctur ural al defo deform rmat atio ion. n. Sout South h of the the Soan Soan Rive Riverr is near nearly ly unde undefor formed med but but is defor deformed med on its north norther ern n and easte eastern rn marg margin ins. s. The The potwar is divided into the following structural zones see figure-2.3. 1. Northern Northern Potwar Potwar Deform Deformed ed Zone (NPDZ (NPDZ). ). 2. Soan Soan Syn Syncl clin ine. e. 3. Easter Eastern n Potwar Potwar Plat Platea eau. u. 4. Souther Southern n Potw Potwar ar Plateau Plateau.. 5. Wester Western n Potwar Potwar Plate Plateau. au.
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Figure-2.3:- Geology and new trends for petroleum exploration in Pakistan (modified from Kamal, 19 91)
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2.4 2.4
SOUT SO UTHE HERN RN POTW POTWAR AR PLAT PLATEA EAU U
The study area lies in the southern part of the Potwar Plateau which is chara characte cteri rized zed by north northwa wardrd-di dipp ppin ing g strat strata a and and local local open open folds folds of low 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 plateau and is divided divided along left lateral lateral Vairo and Dhab Kalan Kalan faults. faults. The hanging wall anticline is represented by the triangular zone orienting from southeast to northwest northwest flanks. flanks. The triangle triangle zone is the result of two phases phases of Himalayan thrusting (Shami and Baig, 2002). 1. The thrust thrust and back-thr back-thrust ust phases phases are the result of northwest northwest southea southeast st successive Himalayan compression. 2. The thrusts thrusts initiated as southeast southeast and northwest northwest vergent vergent fault fault propagated propagated folds. The fault propagated folds were later on displaced by these thrusts.
2.4 2.4
SOUT SO UTHE HERN RN POTW POTWAR AR PLAT PLATEA EAU U
The study area lies in the southern part of the Potwar Plateau which is chara characte cteri rized zed by north northwa wardrd-di dipp ppin ing g strat strata a and and local local open open folds folds of low 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 plateau and is divided divided along left lateral lateral Vairo and Dhab Kalan Kalan faults. faults. The hanging wall anticline is represented by the triangular zone orienting from southeast to northwest northwest flanks. flanks. The triangle triangle zone is the result of two phases phases of Himalayan thrusting (Shami and Baig, 2002). 1. The thrust thrust and back-thr back-thrust ust phases phases are the result of northwest northwest southea southeast st successive Himalayan compression. 2. The thrusts thrusts initiated as southeast southeast and northwest northwest vergent vergent fault fault propagated propagated folds. The fault propagated folds were later on displaced by these thrusts. Fina Finall lly y thes these e oppo opposi site te dire direct cted ed thru thrust sts s form formed ed the the tria triang ngle le zone 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 trian triangle gle zone zone has has been been drill drilled ed for for oil oil and gas wherea whereas s the hangin hanging g wall wall antic anticli line ne along along nort northw hwest estern ern flank flank of the trian triangle gle zone zone is untap untapped ped.. 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 STRATIGRAPHY OF THE AREA The stratigraphic stratigraphic column is divided into three unconformity-boun unconformity-bounded ded sequ sequen ence ces. s. Thes These e unco unconf nfor ormi miti ties es in the the stud study y area area are are Ordo Ordovi vici cian an to Carboniferous, Mesozoic to Late Permian, and Oligocene in age (Figure-3.1) . These unconformities unconformities are difficult to identify in the seismic profiles due to comp compli lica cate ted d thru thrust stin ing. g. The The Potw Potwar ar subsub-ba basi sin n is fill filled ed with with thic thick k infr infraaCambrian 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, dolomi dolomite, te, and shales shales (Muhamma (Muhammad d Aamir Aamir and Muhamma Muhammad d Maas Siddiqu Siddiqui, i, 2006). The Salt Range Formation is best developed in the Eastern Salt Range. The salt lies unconformably unconformably on the Precambrian Precambrian basement. basement. The overlying 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 this time time inte interv rval al were were depo deposi site ted d in the the basi basin. n. The The seco second nd sudd sudden en 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 and overl overlyin ying g molas molasse se sectio section n where where the entir entire e Olig Oligoce ocene ne sedime sedimenta ntary ry record is missing. The molasse deposits include the Murree, Kamlial, Chinji, 14
Nagri Nagri,, and and Dhok Dhok Patha Pathan n Forma Formatio tions ns (Muha (Muhamm mmad ad Aamir Aamir and and Muha Muhamm mmad ad Maas Siddiqui, 2006). Rock Rock unit units s rang rangin ing g in age age from from Infr Infraa-Ca Camb mbri rian an to Camb Cambri rian an are are exposed in the Potwar Province of the Indus basin where the Salt Range Formation with salt, marl salt seams and dolomite is the oldest recognized unit through surface and subsurface geological information and forms the basement for the fossiliferous Cambrian sequence (Shah, 1977). Since Since the comp complet lete e sectio section n of Salt Salt Range Range Form Formati ation on has has not not been 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 PreCambrian basement. Howev However, er, the the wells wells drill drilled ed up to the basem basemen entt on Punj Punjab ab Platf Platfom om,, Pakistan and Bikaner-Nagaur basin of India situated south of Potwar reveal that the Salt Range Formation is underlain by Infra-Cambrian sediments of Bila Bilara ra Form Format atio ion n foll follow owed ed by Jodh Jodhpu purr Form Format atio ion. n. Exte Extent nt of thes these e two 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
LITHO LITHOLO LOGIC GICAL AL DESCRI DESCRIPTI PTION ON OF FOR FORMAT MATION IONS S
Follo Followin wing g are the litho litholo logic gical al descri descripti ption on of the sectio section n drill drilled ed 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 electric logs. A brief, brief, general generalized ized description description of the formatio formations ns drilled drilled in Balkassar Oxy #1 is given below. 3.1.1 INFRA-CAMBRIAN INFRA-CAMBRIAN THE SALT RANGE FORMATION
The oldest formation of the cover sequence known to lie at top of the base baseme ment nt is the the Eoca Eocamb mbri rian an Salt Salt Rang Range e Form Format atio ion. n. The The Form Format atio ion n 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 Formation exhibits varied lithology, lithology, dominantly 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 inter intercal calati ation ons s of kero keroge gen n shale shale or oil oil shale shale have have been been found found in the Salt Salt Range Formation. Formation. A trachy basalt trap, called the Khewra Trap or Khewrite 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
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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 irregular gypsum, dolomite beds and the “Khewrite Trap” (3-100 meters). 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 3.1.2 CAMBRIAN CAMBRIAN KHEWRA FORMATION
The Khewra Formation Formation overlies the Late Proterozoic Proterozoic Salt Range Formation without any apparent disconformity (Shah, 1977). Type locality is the Khewra Gorge in the Eastern Salt Range. The Khewra Formation 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. interclations. The sandstone is characteristically characteristically cross-bedded, has abundant abundant ripple marks and mud cracks, and, in places, exhibits convolute bedding. Thickness of the Khewra Formation Formation is 150m at the type locality in the Eastern Salt Range. Apart from rare trace fossils, the formation is devoid of fossil fossils. s. Becau Because se of its posit positio ion n betwee between n the the late late Prot Protero erozo zoic ic Salt Salt Range Range Forma Formatio tion n and and the fossil fossilife ifero rous us early early Camb Cambria rian n Kussa Kussak k Form Formati ation on,, the the Khew Khewra ra Form Format atio ion n is thou though ghtt to repr repres esen entt the the basa basall part part of the the Lowe Lowerr Cambrian. 3.1.3 3.1.3 PERMIAN PERMIAN 19
TOBRA FORMATION
The Tobra Formation Formation rests unconformably unconformably upon different Cambrian 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 Formation is exposed throughout throughout the Salt Range. It was also encou encount ntere ered d by the the wells wells in the Koha Kohat-P t-Potw otwar ar area. area. In the the Easte Eastern rn Salt Salt Range, the Tobra Formation consists mainly of polymict conglomerates with pebbles and boulders of igneous, metamorphic and sedimentary rocks. The thic thickn knes ess s of the the form format atio ion n is 20m 20m at the the type type loca locali lity ty.. Its Its age age is earl early y Permian. DANDOT FORMATION
The Tobra Formation Formation is overlain conformably conformably by the Dandot Formation (Shah, 1977). Type locality is the village of Dandot, northeast of Khewra, in the Eastern Salt Range. The formation formation is well represented in the Eastern and Central Central Salt Salt Range. Range. The formatio formation n mainly mainly consists consists of dark dark greenis greenish-gr h-grey, ey, splintery shale and siltstone with intercalated intercalated sandstone, whereas in the Salt Rang Range e gree greeni nish sh grey grey to blac black, k, carb carbon onac aceo eous us shal shales es with with sand sand flas flaser ers s alternate with cross-bedded 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 Formation rests rests conformably conformably upon the Dandot Dandot Formation. Formation. Type locality is the Warchha Nala in west-Central 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 coarsegrained and arkosic. Intercalated purple to dark grey shale layers reach a thickness of several meters each. The Warchha Formation is unfossiliferous. It is cons consid ider ered ed Earl Early y Perm Permia ian n beca becaus use e of its its posi positi tion on betw betwee een n the the 20
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 transitional contact with the overlying 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. age. The The paleo paleo-en -envir viron onme ment nt is interp interpre reted ted as main mainly ly terres terrestri trial, al, partl partly y 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 Formation unconformably unconformably overlies overlies various formations 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 upward into fossilifer fossiliferous ous shale shale and calcareo calcareous us sandston sandstone. e. At places, 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 Formation is early Paleocene Paleocene in age. age. LOCKHART FORMATION
The Lockhart Lockhart Limestone Limestone conformably conformably overlies the Hangu Formation (Dav (Davie ies, s, 1930 1930 and and Fatm Fatmi, i, 1973 1973). ). Its Its type type sect sectio ion n is expo expose sed d near near Fort Fort Lock Lockha hart rt.. It cons consis ists ts of grey grey,, medi medium um to thic thickk-be bedd dded ed and and massi assive ve 21
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 Formation overlies the Lockhart Formation Formation conformably 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. sandstone. Marcasite nodules nodules are found found in the shale. The The sandstone 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. 3.1.5 3.1.5 EOCENE EOCENE SAKESAR FORMATION
With increase in limestone beds, the Nammal Formation transitionally passes into the overlying Sakesar Formation, 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 Sakes Sakesar ar Format Formatio ion n later laterall ally y grad grades es into into massi massive ve gypsum gypsum.. Its thickn thickness ess ranges from 70m to about 450m. Its age is early Eocene. CHORGALI FORMATION
The Chorgali Chorgali Formation Formation rests conformably over the Sakesar Formation Formation (type (type local locality ity Chor Chorga gali li Pass) Pass) (Pasc (Pascoe oe,, 1920 1920 and and Fatmi, Fatmi, 1973 1973). ). It consi consists sts large largely, ly, in the the lower lower part, part, of thin-b thin-bedd edded ed grey grey,, partl partly y dolo dolomit mitiz ized ed and argil argillac laceou eous s limes limeston tone e with with bitum bitumino inous us odou odour, r, and and in the uppe upperr part, part, of greenis greenish, h, soft calcareo calcareous us shale shale with interbeds interbeds of limeston limestone. e. Its thicknes thickness s ranges from 30m to 140m. It contains molluscs, ostracods and foraminifera . 22
The age of the Chorgali Chorgali Formation 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 Surg Surgha harr Rang Range, e, the the lowe lowerr part part comp compos osed ed of blui bluish sh grey grey marl marl with 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 type local locality ity is Namma Nammall Gorg Gorge e Salt Salt Range Range,, Punja Punjab b and and thickn thickness ess of this this formation is 100m at type locality. Its age is early Eocene. 3.1.6 3.1.6 MIOCENE 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-be inter-bedded dded greenish greenish sandston sandstone e with sub-ord sub-ordinat inate e intra-for intra-formati mationa onall conglomerate (Wynne, 1873). The thickness of the 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 Formation is in the southwest of Kamlial, the formation overlies the Murree Formation conformably and transitionally; thou though gh at some some local localiti ities es it lies lies unco unconfo nform rmabl ably y on the the Eocen Eocene e Sakes Sakesar ar Forma Formatio tion n (Pinfo (Pinfold ld,, 1918 1918,, Lewis, Lewis, 1937 1937,, Fatmi Fatmi,, 1973 1973 and and Cheem Cheema a et al., al., 1977). The formation consists mainly of grey to brick red, medium to coarsegrain grained ed sandst sandston one e interb interbed edded ded with with purp purple le shale shale and and intra intrafor format mation ional al conglomerate. A number of mammalian fossils have been found (Pascoe, 1963). The age of the Kamlial Formation is middle to late Miocene. 23
3.1.6 3.1.6 PLIOCENE PLIOCENE SIWALIK GROUP 1. Chingi Chingi form formati ation on
The type locality of Chingi formation formation is South of Chinji, Campbellpur, 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 NAGRI FORM FORMATI ATION ON
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 PATHAN FORM FORMATIO ATION N
Its Type locality is Dhok village Campbellpur Campbellpur District, Punjab is the type section of this formation formation of this formation. Lithology Lithology comprises of sandstone, sandstone, clay and conglomerate. Its thickness at type section ranges from 1330m2000m and its age is Middle Pliocene. 4. SOAN SOAN FORM FORMATI ATION ON
Its type locality locality is Gaji Gaji Jagir, Jagir, Sahil Sahil Road Road near Mujahid Mujahid village village north.of north.of Soan Soan Rive River, r, Camp Campbe bell llpu purr Dist Distri rict ct,, Punj Punjab ab and and Lith Lithol olog ogy y comp compri rise ses s of Conglomerate, siltstone and thickness of this formation ranges from 300m3000m. The age of this formation is late Pliocene.
24
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 sync syncli line nes. s.
Tanw Tanwin in-B -Bai ains ns-B -But utta tarr
and and
Joya Joya
Mair Mair-C -Cha hak k
Naura aurang ng-A -Adh dhii-
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 the frac fractu ture red d carb carbon onat ates es of Pale Paleoc ocen ene e and and Eoce Eocene ne age age but but Meso Mesozo zoic ic sand sandst ston ones es and and Pale Paleoz ozoi oic c carb carbon onat ates es and and sand sandst ston ones es has has prod produc uced ed additional oil (Ahmed, 1995) in the area.
25
Oil and gas exploration in Eastern Potwar area mainly in south of Soan Syncl Synclin ine e are are enlon enlonga gated ted syncli syncline nes s which which are trend trending ing from from NE-SW NE-SW have have stee steep p dipp dippin ing g flan flanks ks beca becaus use e of the the salt salt poppop-u up. The The thru thrust sts, s, faul faultt 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 cont contin inen enta tall marg margin in,, thic thick k mari marine ne sedi sedime ment ntar ary y sequ sequen ence ce,, sour source ce and and reservoir and cap rocks (Riva, 1983). The optimization temperature and the thick overburden of 3047m of molasse provides burial depth (Pressure) for the the achi achiev evin ing g the the oil oil form format atio ion. n. Beca Becaus use e of this this in Salt Salt Rang Range e Potw Potwar ar 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 and Dhulian Dhulian Oil fields fields (Kozary, 1968). 1968). Appro Approxim ximate ately ly 135,0 135,000 00 barr barrels els of oil oil is bein being g gener generate ated d from from the Karsal field of Central Potwar. Seismic data of 2002 by PPL did not revealed any any stru struct ctur ural al clos closur ure e whic which h indi indica cate tes s that that prod produc ucin ing g well wells s are 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 cor core
which are
som sometim times are are
asymm symme etri tric
to overtur turned. ed.
The The
hydr hydroc ocar arbo bon n in most most of the the area areas s of Potw Potwar ar may may be attr attrib ibut uted ed to the the structural styles. The structural style framework is the result of the intensive struc structur ture e forma formatio tion n in easte eastern rn part part which which conta contain ins s netwo network rk divid divides es and and altered geological sections and collaborate other data forms. In Potwar subbasin the structural development is due to the faults and decollement levels. In the Potwar Potwar sub-basi sub-basin, n, there there are local local decollem decollement ent levels levels recogni recognizabl zable e besi beside des s two two main main at the the inte interf rfac aces es of Eoce Eocene ne-m -mol olas asse se sequ sequen ence ce and and platform-evaporite sequence (Salt Range Formation). Based on the structural styles Potwar sub-basin is divided into various zones. Structures have been 26
in diff differ eren entt orie orient ntat atio ion n and styl styles es and and have have been been obse observ rved ed thro throug ugh h 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
Producti
Field
on
Dhurnal Eocene/Paleo
Lockhart
Limestone
Dakhni
cene
Sakesar
Balkassar
Chorgali
Chalk-
Oil
Naurang Jurassic
Permian
Cambrian
Datta
Sandstone &
Minwal Dhulian,
Samana Suk
Limestone
Toot
Nilawahan
Conglomerate &
Meyal Adhi
Zaulch Group
Limestone
Dhurnal
Oil
Khewra
Sandstone
Adhi
Gas
Sandstone
Oil
Missa Keswal
Table-1:- Hydrocarbon significance of different rock units in t he study area (modified after Kadri, 1995)
27
Figure-4.1:- Subsurface geometry of area in relation to structure and entrapment of oil and gas (modified from Moghal, 2003).
28
4.1
RESERVOIR
The main oil producing producing reserviors reserviors in Minwal are the Cambrian, 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 yell yellow ow gray gray,, silt silty, y, part partly ly dolo dolomi miti tic c and and thin thin bedd bedded ed lime limest ston one. e. It was was deposited in intratidal conditions where sebkha conditions dominated (Shami and Baig, 2002). The calcite calcite cement has occupied occupied the pore spaces and compaction 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
4.1
RESERVOIR
The main oil producing producing reserviors reserviors in Minwal are the Cambrian, 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 yell yellow ow gray gray,, silt silty, y, part partly ly dolo dolomi miti tic c and and thin thin bedd bedded ed lime limest ston one. e. It was was deposited in intratidal conditions where sebkha conditions dominated (Shami and Baig, 2002). The calcite calcite cement has occupied occupied the pore spaces and compaction 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 Some samp sample les s have have also also show showed ed that that prim primar ary y poro porosi sity ty has has comp comple lete tely ly destroyed due to the over burden pressure of the rock especially compaction and and ceme cement ntat atio ion n and and the the logs logs like like Bore Bore Hole Hole Comp Compen ensat satee-Ga Gama ma Ray, Ray, Compensate Neutron Log-Lithodensity Logging has not indicated the primary porosity and permeability. In the North Western Potwar the fractured porosity is compar comparati ativel vely y very very high high becau because se these these rocks rocks have have defor deformed med into into the process process of Himalayan Himalayan orogeny. orogeny. The rock fractures develop parallel, oblique and perpendicular to the fold axes of anticlines (Shami and Baig, 2002). 4.2
SOURCE RO ROCK
The potential source rock in Minwal is the grey shales of Mianwali formation, Datta formation and Patala formation. The Eocambrian Salt Range Formation 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 29
rock rock of oil oil prod product uctio ion n in Potw Potwar ar sub-ba sub-basin sin accor accordi ding ng to the the oil oil to source source correlation.
4.3
CAP ROCK
The thin-skinned thin-skinned tectonics has developed developed the traps creating the faulted anticlines, pop-up and positive flower structures above Pre-Cambrian 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).
30
Chapter 5 2D SEISMIC DATA ACQUISITION AND PROCESSING 5.1 5.1
SEIS SEISMI MIC C DAT DATA A ACQ ACQUI UISI SITI TION ON
Geophysics is technique used to probe the internal structure of the earth (shallow and deep) and also to understand the extent of the different forma formatio tion n on map and and to conclu conclude de the the inter internal nal physi physica call prop proper ertie ties. s. By analyzing analyzing the geophysical geophysical data it is observed that how physical properties properties of the earth earth can vary vary vertical vertically ly and horizontal horizontally. ly. Differen Differentt scales scales are being 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. Reflecti Reflection on Seismi Seismic c acquisi acquisitio tion. n. 2. Refracti Refraction on Seismi Seismic c acquisi acquisition tion.. In our our diss disser erta tati tion on refl reflec ecti tion on seis seismi mic c surv survey ey has has been een used. sed. In reflection reflection seismic survey elastic waves between different geological geological layers in the subsurface is used to produce the geological model of the subsurface for hydr hydroc ocar arbo bon n expl explor orat atio ion. n. This This meth method od prov provid ides es us the the pict pictur ure e of the 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 SO SOURCE
31
The seismic source releases energy with in the localized region causes stre stress ss in a surr surrou ound ndin ing g medi medium um.. The The exam exampl ple e of seis seismi mic c sour source ce is an explosion. The main criteria of seismic sources are. 1. The seismic energy energy must must be satisfactory satisfactory provide provide sufficient sufficient energy enough enough to be recordable. 2. Seismic energy energy is recorded recorded in the form form of wave energy energy either either P-wave or Swave. wave. Other Other unwan unwanted ted energ energy y signa signals ls woul would d creat create e disto distorti rtion on in the recorded data and this is called as coherent noise. 3. Seismic energy energy which which is converted converted into into waveform waveform must repeat repeat itself. itself. 4. The seismic seismic energy energy source must must be non-hazar non-hazardou dous/saf s/safe e which is safe and efficient and must be environmental protected (Philip Kearey et al, 2002). The land seismic sources falls under two categories categories which are mentioned below. 1. Expl Explos osiv ive. e. 2. NonNon-ex expl plos osiv ive. e. 5.2.1 EXPLOSIVE SOURCE DYNAMITE
In seismic line no. 782-CW-29, dynamite 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 NUMBER/ POINT SHORT
Array SHORT
9
POINT
656
INTERVAL 32
MEASURED SYSTEM
ft
Table-5.1:- Source Parameter for line NO. 782-CW-29
5.2.2 NON-EXPLOSIVE NON-EXPLOSIVE SOURCE VIBROSEIS
In seismic line no. 93-MN-07 and 93-MN-08, vibroseis has been used as a source source for acqu acquisi isitio tion. n. It prov provid ides es a preci precisio sion n and repea repeated ted signa signal. l. A vibr vibrato atorr is load loaded ed in a truck truck which which passes passes the the vibr vibrati ation on into into the the grou ground nd 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 parameter used for the seismic lines lines are mentioned mentioned in table table 1 and 2. ENERGY SOURCE SWEEP FREQUENCY SWEEP LENGTH GROUP INTERVAL
Vibroseies 9-72HZ 14sec 40M
Table-5.2:- Source parameter used for the seismic lines 93-MN-08 and 93-MN-07
5.3
RECEIVERS
The geophones geophones or seismogram 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 geological information visible in recording 33
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 Array Length Geophone Group Array Type
Group Interval 40M 40M 36M Inline
Table-5.3:- Shows the geophone parameter used for the seismic lines.
5.4 5.4
THE THE ARR ARRAY AY SY SYST STEM EM PROF PROFIL ILIN ING G
The single shot and single geophone 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 geop geopho hone nes s must must cancel cancel the grou ground nd roll rolls s and noise noises s which which are trave travelin ling g 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). 1988). There are different types of spread used in the field to acquire seismic data. 1. End End Spr Sprea ead. d. 2. Inlin Inline e Offse Offsett Spr Spread ead 3. Spli Splitt Spr Sprea ead. d. 4. Cros Cross s Spr Sprea ead. d. 5. L Sha Shaped ped Spre Spread. ad. The seismic lines have used inline spread geometry. geometry. It is a spread shot from from a shot shot-p -poi oint nt whic which h is sepa separa rate ted d from from the the spre spread ad an appr apprec ecia iabl ble e distance but along the line of the spread (Sah, 2003). 5.5 GEOPHO GEOPHONE NE INTER INTERVAL VAL
The distance between two sets of geophones geophones either next to or adjoining geophones is called a geophone interval. The seismic line no. 782-
34
CW-29 has geophone interval of 20 m. The seismic line no. 93-MN-08 and 93MN-07 have geophone interval of 36 m. 5.6 GROUP GROUP INTER INTERVAL VAL
It is the horizontal horizontal distance between two sets of geophones geophones either next to or adjo adjoin inin ing g geop geopho hone nes. s. The The seis seismi mic c line line no. no. 782782-CW CW-2 -29 9 has has grou group p interval of 328 and for the seismic line no. 93-MN-08 and 93-MN-07 have group interval of 40 m.
5.7 5.7
SEIS SEISMI MIC C DAT DATA A PROC PROCES ESSI SING NG
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 anal analog og form form and and are are transf transfor ormed med in the proc processi essing ng center center.. The prim primary ary objective is to remove or suppress all Noises and to increase the signal to noise ratio. The type of surface condition condition have tells that how much energy penetrates into the ground. The environmental condition, surface condition and and demog demograp raphy hy play play an impor importan tantt role role in quali quality ty of field field data. data. Besid Besides es processing also depend upon the technique used in processing (Dobrin and Savi Savit, t, 1988 1988). ). The The main main obje object ctiv ives es of the the seis seismi mic c data data proc proces essi sing ng are are summarized as below. 1. Improvin Improving g Signal Signal to to Noise Noise ratio ratio.. 2. Representation Representation of geology geology in seismic seismic cross-section. cross-section. 3. To acquire acquire the the target target provide provided d by client. client. The seismic data processing chart is shown in figure-5.2. To increase the the Sign Signal al to Nois Noise e whic which h cons consti titu tute tes s the the foll follo owing wing corr correc ecti tion ons s and and adjustments are applied during the seismic data processing. 35
1. Time. 2. Ampl Amplit itud ude. e. 3. Frequen Frequency-Ph cy-Phase ase content content.. 4. Decon Deconvo volu lutio tion. n. 5. Corr Correl elat atio ion. n. 6. Stacking Stacking/Dat /Data a Compressi Compressing. ng. 7. Veloc Velocity ity Analy Analysis sis.. 8. Prep Prepro roce cesso ssors rs.. 9. Fil Filter ters. 10. 10.
Migr Migrat atio ion/ n/Im Imag agin ing/ g/Da Data ta Posi Positi tion onin ing. g.
36
Figure-5.1:- Detailed Processing Sequence Flow Chart (Modified from Rehman, 1989)
37
5.7.1 5.7.1 TIME
The time adjustment adjustment falls under under two categories. categories. a. Dyna ynamic. b. Static. a. DYNAMIC TIME CORRECTION (NORMAL (NORMAL MOVE-OUT) 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 STATIC TIME TIME CORR CORRECTIO ECTION N
If the the ray path is vertic vertical al bene beneath ath any shot shot or detec detecto torr the static 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 datum.. The adjus adjustme tment nt of travel travel time time to datum datum can be achiev achieved ed 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
38
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)
The two kinds of static corrections corrections which are applied are mentioned mentioned below. a. Elevat Elevatio ion n Correc Correctio tion. n. b. Weathe Weatherin ring g Corr Correct ectio ion. n. 5.7.2 AMPLITUDE CORRECTION 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 Control (AGC) (AGC) or Structural Structural Gain Control. b. Relative Relative True Amplitud Amplitude e Gain. Gain. a. AUTOMATIC
GAIN
CONTROL
(AGC)
OR
STRUCTURAL
GAIN
CONTROL
To improve the the quality of the low low amplitudes amplitudes at later stages the AGC AGC or the automatic gain control is applied (Dobrin and Savit, 1988). b. RELATIVE RELATIVE TRUE TRUE AMPLITUD AMPLITUDE E GAIN
Amplitud Amplitude e informat information ion concern concerned ed with the facies facies changes, changes, porosit porosity y variatio variations, ns, and gaseous gaseous hydroc hydrocarbo arbons ns are maintain maintained ed (Dobri (Dobrin n and Savit, Savit, 1988). 5.7.3 FREQUENCY FREQUENCY PHASE CONTENT
To enhance the signal and to reduce noise the frequency-phase 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).
39
5.7.4 DECONVOLUTION DECONVOLUTION (INVERSE FILTERING)
A process designed to restore a wave shape to the form it had before it unde underw rwen entt a line linear ar filt filter erin ing g acti action on (con (convo volu luti tion on)) (She (Sheri riff ff,, 1989 1989). ). The The examples to remove the effects caused by the filtering include. a. Determi Deterministi nistic c Decon Deconvol volutio ution. n. b. Spiking Spiking Deconvo Deconvoluti lution. on. c. Predi Predicti ctive ve Decon Deconvo volu lutio tion. n. d. Sparse-sp Sparse-spike ike Deconvo Deconvoluti lution. on. a. DETERMIN DETERMINISTIC ISTIC DECONV DECONVOLUT OLUTION ION
If the characteristics of a system are known than it can be used to remove the effects of the recording recording instrument. If the reflection reflection from the sea floo floorr and and the the trav travel el time time in wate waterr is know known n than than this this dete determ rmin inis isti tic c 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 SPIKING DECON DECONVOL VOLUTIO UTION N
To make the embedded 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. PREDICTIV PREDICTIVE E DECONVOL DECONVOLUTIO UTION N
In the the proc proces ess s of pred predic icti tive ve deco deconv nvol olut utio ion n the the effe effect cts s of some some multip tiples les
are bein eing removed whi which uses ses
the
later ter portio tion of
the the
autocorrelation (Sheriff, 2004). d. SPARSE-SP SPARSE-SPIKE IKE DECONVOL DECONVOLUTIO UTION N
The sparse-spike sparse-spike deconvolution deconvolution is being applied to reduce the reflections and to emphasize more on large amplitudes (Sheriff, 2004). 40
5.7.5 CORRELATION CORRELATION
It is measu measurem remen entt of chara characte cterr and and time time align alignmen mentt of two traces traces.. Beca Becaus use e corr correl elat atio ion n is conv convol olut utio ion n so an iden identi tica call freq freque uenc ncy y doma domain in operation also applies to correlation (Yilmaz, 2001). There are two types of correlation technique which are applied. a. Cross Cross Corre Correlat latio ion. n. b. Auto Auto Corre Correlat latio ion. n. a. CROSS CROSS CORREL CORRELAT ATION ION
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 cross-correlation (Telford et al, 1990). b. AUTOCO AUTOCORR RRELA ELATIO TION N
Cros Cross s corr correl elat atio ion n of a time time seri series es with with itse itself lf is know known n as auto 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) COMPRESSION)
The stacking is the process of combination combination of traces which is a composite record from different records (Sheriff, 2001). The technique uses the phenomena phenomena of common midpoint midpoint (CMP) stack in which one trace is being achi achiev eved ed by summ summin ing g up all all the the offse offsets ts of comm common on midp midpoi oint nt gath gather er.. Generally 48-96 fold stacks are commonly being used. 5.7.7 VELOCITY ANALYSIS
It is being carried out on a suitable s uitable 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). 41
a. Inter Interval val veloci velocity. ty. b. Root Root Mean Mean Squa Square re Veloc Velocity. ity. c. Norma Normall Move Move Out Out Velo Velocit city. y. d. Stacki Stacking ng Veloc Velocity ity.. 5.7.8 PREPROCESSORS PREPROCESSORS
The preprocessor preprocessor has three three components. components. a. Muting. b. Edit c. Deml Demlti tipl plex exin ing. g. a. MUT UTIN ING G
This process is useful in removing removing ground roll, air waves, or noise bursts out of the stack. In this process the relative stacking components must must be chang changed ed with with recor recorded ded time time and and befor before e the the begin beginni ning ng of this 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 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 the gain gains s in recor recordi ding ng instru instrunm nmen entt and repla replacin cing g a corr correct ect valu value e for spherical divergence nevertheless it also useful for static-shift and normalmoveout corrections (Sheriff, 2001). c. DEMULT DEMULTIPL IPLEXI EXING NG
It is used to separate the individual component channels that have been multiplexed (Sheriff, 2001). 42
5.7.9 5.7.9 FILTERS 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, wavelength, moveout, coherence, or amplitude. amplitude. Linear filtering in geophysical data processing is called Convolution. 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.
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 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 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 positions (Philip Kearey et al, 2002). The time migration migration (Post-Migration) involves involves the change of velocity in vertical direction whereas the horizontal change in the velocity is called the Depth Migration (Prestack Migration) (Sheriff, 2001).
43
Chapter 6 SEISMIC DATA INTERPRETATION 6.1 6.1
SEIS SEISMI MIC C DATA DATA INTE INTERP RPRE RETA TATI TION ON
The final step in seismic study of an area is to interpret interpret the processed seismic section so that a geological model of sub-surface can be developed. Here He re the the obje object ctiv ive e of seis seismi mic c refl reflec ecti tion on inte interp rpre reta tati tion on is to stud study y the the subsurface structures that help in discovering the hydrocarbon 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 hydr hydroc ocarb arbon ons s in the subsur subsurfac face, e, so the seismi seismic c refle reflecti ction on metho method d only only indicates the geological situations where the hydrocarbons can accumulate. Seismic can be interpreted in two modes: 1. The first first mode mode is in areas areas of substan substantia tiall well well contr control ol,, in which which the well 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 second mode mode is in areas areas of no well well contr control ol (fronti (frontier er areas) areas) in which which the seismic data provide both definition of structure and estimates of depositi depositional onal environ environment ments. s. Seismic Seismic velociti velocities es and seismic seismic stratigr stratigraphi aphic c concepts are used to define the lithology. Seismic reflection amplitudes help to detail velocities and serve as a guide to pore constituents. 44
Seismic interpretation is the transformation of seismic reflection data into a structural picture, contouring of subsurface horizons and further depth conv convers ersio ion n by appl applyi ying ng some some suita suitabl ble e velo velocit cities ies.. The seismi seismic c refle reflecti ction on 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 methods for the interpretation interpretation of the seismic seismic section are. are. 1.
Structural Analysis
2.
Stratigraphic An Analysis
1.
STRUCTURAL ANALYSIS
It is the study of reflector geometry on the basis of reflection 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 twotwo-wa way y refl reflec ecti tion on time times s rath rather er dept depth. h. And And time time stru struct ctur ural al maps maps are are constru constructed cted to display display the geometry geometry of selected selected reflection reflection events. events. Some Some seismic sections contain images that can be interpreted without difficulty. Discont Discontinue inues s reflecti reflections ons clearly clearly indicat indicate e faults faults and undulat undulating ing reflecti reflections ons reveal folded beds. 2.
STRATIGRAPHIC ANALYSIS
Stratigraphic analysis involves the subdivision of seismic sections into seque sequence nce of refle reflecti ction ons s that that are are inter interpr prete eted d as a seismi seismic c expres expressio sion n of genet genetica icall lly y relat related ed sedim sediment entary ary seque sequence nces. s. The prin princip ciples les behin behind d this this seismic sequence analysis are of two types. Firstly, reflections are taken as chronostratigraphical units, since the type type of rock rock inter interfac face e that that prod produce uce refle reflecti ction ons s are are strata strata surfac surfaces es and and unconformities, by contrast the boundary of diachronous lithological units tend to be transitional and not to produce reflections.
45
Seco Second ndly ly,,
gene geneti tica call lly y
rela relate ted d
sedi sedime ment ntar ary y
sequ sequen ence ces s
norm normal ally ly
comp compri rise se the the set set of conc concor orda dant nt stra strata ta that that exhi exhibi bitt disc discor orda danc nce e with with underlying and overlying strata. Accordin According g to Dobrin and Savit, Savit, 1988 1988 through throughout out the history 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. Stra Strati tigr grap aphi hic c oil oil trap traps s can can resu result lt from from reef reefs, s, pinc pincho hout uts, s, or othe otherr features associated erosional truncation, facies, transition and sand lenses associated with buried channels, lakes, or similar sources. 6.2
INTER INTERPRE PRETAT TATION ION OF SIES SIESMIC MIC LIN LINES ES OF THE THE STUDY STUDY AREA AREA
The seismic data interpretation interpretation has been carried out on 93-MN-8, 93MN-7(D MN-7(Dip ip Lines) Lines) and and 782-C 782-CW-2 W-29( 9(Str Strike ike Line) Line).. Pre-S Pre-Stac tack k Time Time Migrat Migration ion vers versio ion n of 2-D 2-D seis seismi mic c line lines s has has been been inte interp rpre rete ted. d. The The seis seismi mic c data data interpretation revealed that the structure of the area is a fault 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 fa faults. Thrust faults were observed in seismic lines 93-MN-08, 93-MN-07 and
782-C 782-CW-2 W-29. 9. These These faults faults have have create created d the the pop pop up struct structur ure e which 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
46
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 seismi seismic c sectio section n displ displays ays the arriv arrival al time time (Two (Two way way Travel Travel Time). 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 HO HORIZONS
The reflection 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 base baseme ment nt show shows s no goo good cont contin inu uous ous refl reflec ecti tion on and and has very very shor short, t, 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 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 diso disord rder ered ed refl reflec ecti tion on patt patter ern n in the the bott bottom om of the the seis seismi mic c sect sectio ions ns 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.
47
Figure-6.1:- Shows the seismic section 93-MN-08
48
Figure-6.2:- shows the seismic line no. 93-MN-07
49
Figure-6.3:- showing the line no. 782-CW-29
Figure-6.3:- showing the line no. 782-CW-29
50
6.5
CONTOUR MA 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 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 conv convert erted ed into into secon seconds ds and and then then plot plotted ted into into the base base map. map. The Time Time contour map of top of Chorgali is given below.
6.5
CONTOUR MA 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 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 conv convert erted ed into into secon seconds ds and and then then plot plotted ted into into the base base map. map. The Time Time contour map of top of Chorgali is given below.
51
Figue-6.1:- Time contour map of top of Chorgali Formation.
52
6.3.2 DEPTH CONTOUR CONTOUR MAP 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) (sec) V =Average velocity (meter/second)
6.3.2 DEPTH CONTOUR CONTOUR MAP 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) (sec) V =Average velocity (meter/second)
Figure-6.2:- Depth map contour map of the top of Chorgali Formation.
54
6.3.3 TIME VS DEPTH 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.
Time 8 316 729 861 1458 2240 2451 5000
Short Point 232 CDP 475 Veloci S=V*T Depth=V*T/2 ty 3135 3222 3389 3442 3614 3870 3953 4559
25080 1018152 2470581 2963562 5269212 8668800 9688803 2279500
000 12.54 509.076 1235.2905 1481.781 2634.606 4334.4 4844.4015 11397.5
6.3.3 TIME VS DEPTH 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.
Time 8 316 729 861 1458 2240 2451 5000
Short Point 232 CDP 475 Veloci S=V*T Depth=V*T/2 ty 3135 3222 3389 3442 3614 3870 3953 4559
25080 1018152 2470581 2963562 5269212 8668800 9688803 2279500
000 12.54 509.076 1235.2905 1481.781 2634.606 4334.4 4844.4015 11397.5
0
Time Time Vs Depth 0
1000
2000
3000
4000
5000
6000
0 2000 4000 Depth=V*T/ V*T/20 2000 00 6000 8000 10000 12000
Figure-6.3:- Shows the time VS depth graph.
55
CONCLUSIONS 1.
According to the picture reflected by Time Structure
and Depth
Contour Maps, Maps, Minwal structure structure is an a fault bounded anticline anticline and has a combination of North and South verging thrust with downthrown block in the middle. 2.
In the north north the antic anticlin line e is boun bounded ded by south south east east dipp dippin ing g bac back k thr thrust ust,, whereas the southern flank of anticline is gentle.
3.
South ver verging ing thr thrust doe does have a sub sub-th -thrust resu result ltin ing g in smal smalll up throw thrown n block block to the the north north.. The The thrust thrust fault fault in the area indica indicate tes s the compressional tectonic movement.
4. 5.
Thes These e oppo opposi site te dir direc ecte ted d thru thrust st form formed ed the the tri trian angl gle e zone zone geo geome metr try. y. The The carbonates ates of Cho Chorgali and and Sakesar sar (Eo (Eocene ene) form forma atio tion are are reservoir rocks in this area.
56
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64
65
Short Points 103
Two Way Travel Time Time TWT (sec) (millisec) 1.61 1610 66
105 108 110 113 115 118 120 123 125 128 130 133 135 138 140 143 145 148 150 153 155 158 160 163 165 168 170 173 175 178 180 183 185 188 190 193 195 198 200 203 205 208 210
1.63 1.65 1.64 1.65 1.65 1.66 1.66 1.65 1.66 1.66 1.64 1.63 1.64 1.64 1.62 1.62 1.61 1.6 1.59 1.58 1.57 1.56 1.54 1.52 1.52 1.51 1.49 1.48 1.47 1.46 1.45 1.44 1.43 1.42 1.41 1.4 1.4 1.38 1.37 1.36 1.35 1.34 1.33
1630 1650 1640 1650 1650 1660 1660 1650 1660 1660 1640 1630 1640 1640 1620 1620 1610 1600 1590 1580 1570 1560 1540 1520 1520 1510 1490 1480 1470 1460 1450 1440 1430 1420 1410 1400 1400 1380 1370 1360 1350 1340 1330 67
213 215 218 220 223 225 228 223 210 235 238 240 243 245 248 250 253 255 258 260 263 265 268 270 273 275 278 280 283 285 288 290 293 295 298 300 303 305 308 310 313 315 318
Fault Fault
Fault Fault
1.32 1.31 1.3 1.27 1.26 1.25 1.26 1.15 2.3 2.34 2.32 2.3 2.28 2.26 2.24 2.22 2.2 2.1 2 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88
1320 1310 1300 1270 1260 1250 1260 1150 2300 2340 2320 2300 2280 2260 2240 2220 2200 2100 2000 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 68
320 323 325 328 330 333 335 338 340 343 345 348 350 353 355 358 360 363 365 368 370 373 375
1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9
1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1880 1900 1900 1900 1900 1900 1900 1900 1900
Table-1:- showing the time noted for the seismic line no. 93-MN-08
69
