ABSTRACT OF THE THESIS OF
Tariq Majeed Jaswal for the degree of Master of Science in Geology presented on
March 14, 1990. Title: Structure and Evolution of the Dhurnal Oil Field, Northern Potwar Deformed
Zone, Pakistan.
Abstract approved: Robert J. Lillie
The North Potwar Deformed Zone (NPDZ) is part of the active foreland fold- and-thrust belt of the Salt Range and Potwar Plateau (SR/PP) in northern Pakistan.
About 500 km of seismic reflection profiles are integrated with surface geologic and drilling data to examine the structure of the NPDZ, in general, and the history of deformation of the Dhurnal oil field, in particular. The seismic lines suggest that the overall structure of the eastern NPDZ is a duplex structure developed beneath a passive roof thrust. The roof thrust is generated from a tipline in the Murree Formation of
Miocene age, while the sole thrust is initiated from the same Eocambrian evaporite zone that extends 80 km southward beneath the Soan syncline and Salt Range. The Dhurnal oil field structure is a pop-up at the southern margin of the NPDZ, developed beneath the passive roof thrust. The passive roof thrust crops out just north of Dhurnal on the steep, northern limb of the Soan syncline. An overstep passive roof thrust (Sakhwal fault) is interpreted west of Dhurnal, which developed due to southward progression of the deformation front beneath the earlier passive roof thrust.
Very gentle basement dip and almost zero topographic slope in the NPDZ suggest that the Eocambrian salt provides effective decoupling at the present position of the NPDZ. The strong deformation in the NPDZ appears to have developed farther north, in an area where the evaporites may be lacking. Since 2 Ma the NPDZ moved farther south over the evaporites without any further deformation, while erosion removed any former topographic slope. Restoring a balanced cross-section suggests the minimum shortening across the NPDZ is about 69 km. Assuming that this shortening occurred in the time interval from 5.1 to 2.0 Ma, the shortening rate is 22 mm/yr. This is about 50% of the 40-50 mm/yr convergence rate of the Eurasian and
Indian plates. Structure and Evolution of the Dhurnal Oil Field, Northern Potwar Deformed Zone, Pakistan
by
Tariq Majeed Jaswal
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Completed March 14, 1990 Commencement June, 1990 APPROVED:
Assistant Professor of Geosciences in charge of major
an of Department of Geosciences
Dean of Grad It- School
Date thesis is presented March 14, 1990 ACKNOWI FDGEMENTS
My first thanks and feelings of gratitude go to my advisor, Bob Lillie, whose patience, guidance and enthusiasm have made this thesis a reality.I am grateful to Dan
Davis for his reconnaissance visit to my field area and for many enlightening discussions on mechanics of foreland fold-and-thrust belts, and to Bob Lawrence and
Bob Yeats for their help and guidance, which significantly improved this work.
My thanks also go to Dan Baker, Mike Leathers, Yanick Duroy, Steve Jaume and Ned Pennock, whose investigations of the Salt Range and Potwar Plateau have yielded results of great value and provided a framework for this study.
I am grateful to every member of the staff and faculty of the Geology
Department, Oregon State University (OSU) for their help and cooperation during my stay in the department. I also owe thanks to my student colleagues who shared with me the dens of Wilkinson Hall basement, for their friendship and kindness, which made my stay at OSU a pleasant experience. I am grateful to the management of the Oil and Gas Development Corporation
(OGDC) for their approval of this project, release of data and providing assistance in the field.I also appreciate the coordination and assistance of Occidental of Pakistan Inc. (OXY) during this study. OXY and Pakistan Oil Fields Ltd. (POL) provided the recently acquired seismic and drilling data from their respective concessions in the
NPDZ. The Oregon State University project in northern Pakistan is supported by National Science Foundation (NSF) grants INT-8118403, INT-86009914, EAR-
8318194, and EAR-8608224. Support for work on the mechanics of thrusting was provided through the Petroleum Research Fund of the American Chemical Society, grant PRF 17932-G2. Additional financial support and technical discussions from Texaco Inc., Mobil Oil Corporation and AMOCO Production Company are gratefully acknowledged.
My special thanks go to Jeff L. Waldman and Shahid Khan for providing computer expertise on the Macintosh. I am also thankful to Linda Haygarth and Karen
French for the excellent drafting. TABLE OF CONTENTS
INTRODUCTION 1
REGIONAL SETTING 7
AVAILABLE DATA 11
STRATIGRAPHY 16
GENERAL STRUCTURE 20
BALANCED CROSS-SECTION 22
STRUCTURAL STYLE IN THE NORTHERN POTWAR
DEFORMED ZONE 32
Passive Roof Duplex 36
Overstep Backthrust 40
Basement warp 45
Thickness and Distribution of Salt 45
TECTONIC SHORTENING 48
Timing of Structural Events 48
Amount and Rate of Horizontal Shortening 49
CONCLUSIONS 50
REFERENCES CITED 52 LIST OF FIGURES
Figure Page
1.Sketch map of the Indian plate showing regional tectonic
features. 2
2.Generalized tectonic map of Pakistan. 5
3.Generalized map of the Salt Range and Potwar Plateau
showing prominent geologic and tectonic features 8
4. Map of project area showing shot point locations for
seismic reflection lines and positions of exploration/
development wells used in this study. 12
5.Composite geological map of the project area. 14
6.Generalized stratigraphic section of the Dhurnal oil field based
on horizons encountered in the Dhurnal well no. 3. 18
7.Composite seismic line (time section) across NPDZ, used for
balanced section along line A-A'. 24
8.Balanced and restored structural cross-section along line A-A',
showing structural details of the Dhurnal pop-up and overall
structure of the NPDZ. 27 9. Diagram showing the development of imbrications in the
hanging wall of thrust sheets. 30
10. Diagram showing different stages of erosion in the foreland of
fold-and-thrust belts. 33
11. Composite seismic line (time section) across eastern Dhurnal. 38
12. Cartoon showing the gradual development of overstep
backthrusts in the roof sequence of a passive roof thrust. 41
13. Portion of seismic line N-84-15, showing development of an
overstep backthrust. 43
14. Subsurface contour map at the top of Eocambrian basement. 46 STRUCTURE AND EVOLUTION OF THE DHURNAL OIL HELD,
NORTHERN POTWAR DEFORMED ZONE, PAKISTAN
INTRODUCTION
Crustal shortening due to northward underthrusting of the Indian Plate beneath
Eurasia continues to create active tectonic features on the northern fringes of the Indian craton since major collision began in Eocene time (Fig. 1). In the Himalayan foreland of Pakistan, thin-skinned tectonic features are developing in the Salt Range and Potwar
Plateau (SR/PP) as a wedge of sediments is being contracted and thrusted southward along a decollement in Eocambrian evaporite beds (Fig. 2). The Dhurnal oil field
structure is a product of this ongoing collision, located at the leading edge of an old deformation front in the Northern Potwar Deformed Zone (NPDZ; Fig. 3). For the last ten years, the Geology Department of Oregon State University
(OSU) has been involved in detailed studies of the foreland fold-and-thrust belts of
Pakistan. About 3000 km of older industry seismic reflection profiles of the SR/PP were provided to OSU by the Oil and Gas Development Corporation (OGDC) and the Ministry of Petroleum and Natural Resources of Pakistan. These data, along with
surface geologic, drilling and gravity data, were used to construct balanced cross- sections of the western, central and eastern SR/PP (Leathers, 1987; Baker, 1987; Pennock, 1988; Fig. 2). The same data set was used by Jaume (1986) to study the mechanics of thrusting in the SR/PP, and by Duroy (1986) to analyze lithospheric flexure in the Himalayas of Pakistan. Detailed reports based on integration of these surface and subsurface data (Khan et al., 1986; Lillie et al.,1987; Baker et al., 1988; Jaume and Lillie, 1988; Pennock et al., 1989; Duroy et al., 1989) have shed light on 2
Figure 1. Regional sketch map, showing geographical position of Pakistan and regional tectonic features of the Indian plate (compiled from Molnar & Tapponier, 1977, Valdiya, 1984, and Yeats and Lawrence, 1984). The large arrows show the direction and amount of convergence (cm.) of the Indian plate relative to the Asian plate, after Jacob and Quittmeyer (1979). AF= Altyn Tagh fault, BD= Bangladesh,
CF= Chaman fault, CLR= Chagos-Laccadive Ridge (Reunion Hotspot), HF= Herat fault, KF= Karakoram fault, MBT= Main Boundary thrust, MCT= Main Central thrust,
MKT= Main Karakoram thrust, MMT= Main Mantle thrust, MR= Murray Ridge,
NER= Ninetyeast Ridge (Kerguelen Hotspot), OFZ= Owen fracture zone, SL= Sri Lanka, SR/PP= Salt Range/Potwar Plateau, SRT= Salt Range thrust, TS= Tsangpo suture. 3
70° 80° 90°
TIBET BLOCK
30°
ARABIAN 20° SEA BAY OF BENGAL
10°
0 500 i Li L i 1 km I
Figure 1. 4 many long-standing structural and tectonic problems in the area, and they provide a framework for this thesis.
Newer seismic profiles in and around the Dhurnal oil field, drilling logs of
Dhurnal wells and surface geologic data were provided for this study by Occidental of Pakistan, Inc. (OXY). Pakistan Oil Fields, Ltd. (POL) also released two recently recorded seismic lines from their eastern Khushal Garh concession (Fig. 4). Surface geologic maps on 1: 50,(X)0 scale provided by OXY and OGDC, along with new measurements of structural attitudes as part of this thesis, provided important surface constraints on interpreting the subsurface structure.
The NPDZ can be divided into two parts. The western NPDZ is an emergent foreland fold-and-thrust front, while the eastern NPDZ is a buried one (Morley, 1986). This study emphasizes the geometry of a passive roof duplex zone developed in the eastern NPDZ as a key to understanding the subsurface structure and tectonic evolution of the Dhurnal oil field. The specific objectives of this study are to determine: 1) the geometry and deformation style in the Dhurnal area and the region farther north; 2) the distribution and thickness of salt in the NPDZ; 3) the shape and dip of the basement, and its involvement in deformation; 4) the amount, rate and timing of shortening of the sedimentary wedge and how the shortening relates to the regional tectonic scenario.
The timing of major deformational events around the NPDZ is well established (Raynolds, 1980; Burbank et. al., 1986; Johnson et. al., 1986; Burbank and Beck, 1989) and can be used to constrain structural events. A balanced cross-section constructed as part of this thesis reveals 69 km shortening from the Soan syncline to the
Main Boundary thrust (MBT) and suggests an average shortening rate of about 22.0 mm/yr based on the timing from 5.1 to 2.0 Ma. Factors which control the overall geometry of the thrust system in the NPDZ are evident in the cross-section, namely a gently-dipping basement, the presence of a northward-extending layer of Eocambrian evaporite beds, and a basement warp under the Dhurnal structure. 5
Figure 2. Generalized tectonic map of Pakistan, showing position of the Salt Range/Potwar Plateau (after Kazmi and Rana, 1982). The stippled area represents the foreland fold-and-thrust belts skirting the northwestern boundary of the Indian plate.
Note the trend of the Sargodha High (SH), which was modeled by Duroy (1986) and
Duroy et al. (1989) as the flexural bulge parallel to the main Himalayan trend (Yeats and Lawrence, 1984). L-L', B-B' and P-P' are cross-sections of Leathers (1987), Baker (1987) and Pennock (1988), respectively. The area boxed by the rectangle is shown in Figure 3. CMF=Chukhan Manda fault, IB= Islamabad, K= Karachi, KF= Kingri fault, KFTB= Kirthar foreland fold-and-thrust belt, KMF= Kurram fault, KRF= Kirthar fault, NR= Nagarparkar Ridge, ONF= Ornach Nal fault, P= Peshawar, PF= Pab fault, Q= Quetta, S= Sargodha, SFTB= Sulaiman foreland fold-and-thrust belt, SH=
Sargodha High, SR/PP= Salt Range/ Potwar Plateau, SRT= Salt Range thrust, ST=
Sibi trough. 6
70° 72° 74° 76° 38°
36°
34°
P.
C,(\''' ,- .),,..1...; UPPER . ...i INDUS if\-c E AND BASIN 61° 63° r/) -9 30° 65° 00tt. CHAGAI ARC CENTRAL INDUS BASIN.
28° / IRANi- dr.) \..
I if 26° LOWER MAKRAN RANGES INDUS BASIN
0
Figure 2. 7
The discovery of the Dhurnal oil field in May 1984 by OXY was the result of an extensive seismic-reflection survey, which confirmed the presence of a NE-SW trending, pop-up under the northern limb of the Soan Syncline. The aerial extent of the
Dhurnal anticline is about 6500 acres with 500 m vertical closure. The initial production from Dhurnal well #1 was 5900 BOPD and 17 MMCFGPD. Presently, the field is producing about 16,500 BOPD and 145 tons of LPG/D, which makes it the largest producing oil field of Pakistan (Brady, 1988). The discoveries at Dakhni in
1983 by OGDC and Dhurnal in 1984 by OXY in the strongly-deformed parts of the northern Potwar Plateau (Fig. 3) provided the necessary incentive for renewed exploration in the NPDZ. The foreland fold-and-thrust belt of the NPDZ is a comparatively young feature; critical features which are missing or eroded away in older foreland fold-and-thrust belts can be taken into consideration based on analogy with the NPDZ.
REGIONAL SETTING
Pakistan contains the northwestern boundary of the Indian lithospheric plate
(Fig. 1). The northern areas of the country represent features characteristic of continent
continent collision between the Indian and Eurasian plates, where full-thickness continental crust undergoes direct convergence. The separation of India from Africa and Madagascar probably occurred in the Late Cretaceous, and its northward drift from 80 to 53 Ma was rapid. India moved gradually toward Eurasia as the Tethys ocean closed at a rate of 150 mm/yr (Powell,
1979; Klootwijk and Peirce, 1979). The rate of Indian plate motion relative to Eurasia
calculated with the Reunion hot spot frame of reference is 130 mm/yr from 83 to 48 Ma 8
Figure 3. Generalized map of the Salt Range and Potwar Plateau (SR/PP), showing prominent geologic and tectonic features (see Fig. 2 for location). The area north of the
Soan syncline is the highly dissected part of the Potwar Plateau (NPDZ). Notice that the trend of the Dhurnal structure (DH) is comparable with the NE trending structures of the eastern SR/PP. Line A-A' is the balanced cross-section discussed in text (Fig. 8). The area bounded by the rectangle in the NPDZ is shown in Fig. 4, 5 and 14.
Structures are abbreviated as: A= Adhi oil field, AF= Ahmadal fault, BA= Butter anticline, CBK= Chak Be li Khan anticline, DF= Dhurnal fault, DJ= Dil Jabba fault,
DU= Dhulian anticline, DN= Dakhni anticline, KBF= Kalabagh fault, KF= Kanet fault, KH= Khaur anticline, KMF= Kheri Murat fault, KRF= Kharpa fault, LF= Langrial fault, M= Mahesian anticline, MA= Mianwala structure, MF= Mianwala fault, MY=
Meyal anticline, PH= Pabbi Hills anticline, PR= Pariwali structure, Q= Qazian anticline, RF= Riwat fault, SF= Sakhwal fault, T= Toot oil field, TB= Tanwin-Bains anticline. The DH, DN, MA, and PR structures are underthrusted anticlines of the
NPDZ, with synclinal surface expression. 9
71° 72° 73° 74° 34°
,V7TOCK-CHHARAT RANGE slamabad
KOHAT
PLATEAU \4(' SOAK 33° c, POTWAR Jhelum PH mlm...... ::::.. PLATEAU .ims:1177SW.41111111. --.1=
f::::..ii::::::.:".::.;:.:.:.;:;:::.:74'.:,.:. .. /mmimmmammmimmw.---...... mmis. Mianwali mmigaMITMMET''''' ...d7mmmileMqr ....
PunjabPlains
Sargodha 0 50
m 32° NEOGENE MOLASSE
M PALEOCENE TO EOCENE
PERMIAN TO JURASSIC
PRECAMBRIAN TO CAMBRIAN SEDIMENTARY ROCKS 771 PRECAMBIAN INDIAN 1=1 SHELD
Figure 3. 10
(Duncan, 1990). At about 50 Ma (anomaly 22) India rotated counterclockwise relative
to Eurasia, around a close pole (Powell, 1979), and its northward velocity reduced to 40-60 mm/yr (Powell, 1979; Duncan and Hargraves, 1990). Finally, the northward convergence rate became stable at <50 mm/yr from 36 Ma to the present (Patriat and Achache, 1984). The erratic behavior of the Indian plate and plate reorganization in the
Indian ocean at about 44 Ma (anomaly 20) can be interpreted to be a result of collision between Indian and Eurasian continental crust (Patriat and Achache, 1984). The
continued crustal shortening since Late Cretaceous gave rise to the Himalaya as thick
layers of sediments thrust southward over the Indian craton (Powell and Conaghan, 1973; Molnar and Tapponnier, 1975; Quittmeyer et al., 1979; Valdiya, 1984). The
deformation front prograded southward gradually to the MBT from the Main Central Thrust (MCT) at about 15 Ma (Fig. 1; Klootwijk et al., 1985; Lefort, 1975). In Pakistan, the MBT is considered to be ramped up from a basement offset, along which
a Cretaceous to Tertiary section is thrust over the Neogene molasse of the NPDZ (Yeats and Hussain, 1987). The active foreland fold-and-thrust belt on the southern fringes of the Himalayan collision zone is broad in northern Pakistan ( 100 km), consisting of
platform sedimentary rocks and molasse thrust southward along a ductile, Eocambrian evaporite layer (Fig. 2: Seeber et al., 1979; Davis and Engelder, 1985). The overthrust
sheet of the Potwar Plateau is a large structural depression, the Soan syncline bounded on the south by the Salt Range, which is terminated against the undeformed Punjab Plains by the Salt Range thrust (SRT; Fig. 2). The area north of the Soan syncline is characterized by horizontal shortening and imbricate thrust faulting (NPDZ). Farther north, the Hill Ranges are uplifted along a separate series of thrusts. To the west, the SR/PP thrust sheet is bounded by the right-lateral Kalabagh Fault (McDougall, 1988), but in the east thrusting dies out more gradually (Pennock, 1989). Basement rocks are exposed about 70 km south of the Salt Range, along the Sargodha High (Fig. 2), which 11
has a regional trend parallel to the Himalaya of India and may represent the active flexural bulge (Yeats and Lawrence, 1984; Duroy et al., 1989).
The most recent deformation in the NPDZ occurred between 2.1-1.9 Ma
(Johnson et al., 1986). Yeats et al. (1984) mapped some deformational features of <0.4 Ma in the eastern Salt Range. Lillie et al. (1987) suggest that, as the deformation
in the NPDZ stopped about 2 Ma, the decollement propagated abruptly to the Salt
Range front. About 20 km of horizontal shortening has occurred along the SRT since that time, so that the overthrust rate for the SR/PP allochthon during the Quaternary is calculated to be about 10 mm/yr (Baker et al., 1988), or about 1/4 of the overall India- Eurasia convergence rate (37-45 mm/yr.; Chantelain et al., 1980). Part of this thesis is
an attempt to determine the amount of shortening that occurred in the northern portion
of the allochthon (NPDZ) prior to 2 Ma, and to estimate the corresponding shortening
rate.
AVAILABLE DATA
Seismic reflection lines from the joint OXY, OGDC, POL and Attock Oil Co.
(AOC) North Potwar concession Area, well logs from the Dhurnal oil field and old
surface geologic data were released by OXY and OGDC for this study (Fig. 4 and 5).
POL also released two recently recorded (1987) seismic lines from their eastern Khushal Garh concession in the NPDZ, which provide excellent constraints for the portion of the regional cross-section north of Dhurnal (Fig. 4). The vintage of most of the data is 1984 to 1987, and total seismic coverage is about 500 km. Some old
AMOCO seismic lines were also used to extend cross-section north and south of
Dhurnal (Fig. 4). Seismic data quality is very good on the Dhurnal structure and south of it, but it deteriorated north of Dhurnal where very complex deformation is present. 12
Figure 4. Map of the project area (see Fig. 3 for location), showing locations of seismic lines used in this study, wells drilled in the Dhurnal structure, and concession areas of OXY and POL. NP 84/86 lines are from the OXY/ OGDC/ POL/ AOC joint north Potwar concession. These lines are 48 fold, 8-60 Hz Vibroseis (Trademark CONOCO, Inc.), recorded in 1984 to 1986 by Seismograph Services Limited (SSL) and processed by Seiscom Delta United. Line numbers 86-KH-02 and 86-KH-05 are from the POL/ OGDC joint eastern Khushal Garh concession. The vintage of these lines is 1987 and they are 60 fold, 10-47 Hz Vibroseis, recorded by S.S.L. and processed by OGDC. The AW lines are from the old AMOCO/ OGDC joint north Potwar concession. These lines are 24 fold, 6-24 Hz Vibroseis, recorded in 1977 by
Western Geophysical Company and processed by Petty-Ray. Line A-A', near seismic lines 86-KH-05, NP84-12 and a segment of line AW-15AF, is the balanced structural cross-section (Figures 7 & 8). The bold lines along seismic lines NP84-03, AW-15AG (B) and NP84-15 (C) represent segments shown in Figs. 11 and 13. 13
72°30' 72°45' 72°50' 330 38'
::::::)NORTH OXY/OGDC/POL/AOC POTWAR CONCESSION nS POL/OGD EASTERN KHUSHALGAPH CONCESSION
NP84 /86- OXY SEISMIC LINES <723Fateh 86-K11- POL SEISMIC LINES Jang AW- AMOCO SEISMIC LINES
330 30'
\\ Sakhwal 0H-1
DH-3
33° 15'
Figure 4. 14
Figure 5. Composite geological map of the project area (see Fig. 3 for location). Compiled from geologic maps provided by OXY, OGDC and field mapping conducted during Fall of 1988. The Eocene rocks along KMF and Kamlial beds along DF rise above the ground level to develop the ridges in the area. Line A-A' indicates the position of the balanced cross-section. The bold seismic lines B and C are shown in Figs. 11 and 13, respectively. The wells drilled in the Dhurnal structure are also shown by the dots numbered 1 to 7 (DH1 to DH 7 in Fig. 4). Note all the Dhurnal wells are located on the south-dipping, northern limb of the Soan syncline. Locally, another syncline is developed in the surroundings of Dhurnal due to emergence of the Sakhwal fault (SF) and eastern plunge of the Khaur anticline. The Nagri Formation of the Siwalik Group is exposed at the surface in the vicinity of the Dhurnal well. BS= Bhal Saydian, DM= Dhok Maiki, Nt= Nautheh. For other abbreviations, see Fig. 3. 15
72°30 72°45' 72°50' 33°
H ig nlyfaulted zone. Eocene 5 olderrocks are exposed alongsome fault
Dault Para Syncline
330 30'
4:76 172
Sokhwal
33° -c DIP AND STRIKE 15' + VERTICAL DIP -ANTICLINE +SYNCLINE
(Dp Dhok Pathan Fm. fKIKamlial Fm. Al Alluvium M Murree Fm. No Nagri Fm. Sn Soon Fm Ch Chinji Fm. Eo Eocene and 0111111111101 older rocks
Figure 5. 16
A surface geologic map was prepared from the available data and from new field mapping on a 1:50,000 scale undertaken for this project (Fig. 5). The new mapping was conducted in critical locations such as traverses of seismic lines, formation contacts and surface outcrops of faults. The cross-section of the Dhurnal structure was designed to follow specific seismic lines, but all the lines were used to construct the subsurface contour map. Lateral velocity variations are anticipated due to facies changes in the molasse and thrusting of high velocity platform rocks from deeper to shallower positions.
Interval velocities used for depth conversions in the Dhurnal area and farther north were calculated from stacking velocities of seismic lines, sonic logs (where available), and possible ranges of velocities for different lithologies (Telford, 1976; Fig. 6).
STRATIGRAPHY
The stratigraphy of the SR/PP is well established from outcrops in the Salt
Range (Gee, 1980, 1989) and oil wells drilled in the Potwar Plateau (Khan et al.,
1986). The stratigraphic sequence in the NPDZ is not that well constrained due to lack of deep drilling. However, surface outcrops along the MBT and seismic profiles suggest that the stratigraphy of the NPDZ is similar to that of other parts of the SR/PP.
Stratigraphic successions in the SR/PP can be broken into four major, unconformity- bounded sequences (Fig. 6):
1) a late Precambrian through early Cambrian platform sequence, including a
basal evaporite layer;
2) an early to late Permian platform sequence;
3) a Paleocene to early Eocene, marine carbonate and shale sequence; 17
4) a time-transgressive, Neogene molasse sequence. The Eocambrian evaporite beds of the Salt Range Formation were deposited unconformably on a Precambrian basement in a restricted, hypersaline basin of the
Gondwana interior. The overlying Cambrian rocks of the Jhelum Group are composed of sandstones, shale, dolomite and anhydrite. These rocks were deposited in shallow marine, restricted shallow marine, and lagoonal-nonmarine environments. The
Baghanwala Formation, which contains salt pseudomorphs, represents a regressive cycle of deposition in the late Cambrian (Khan et al., 1986). The overlying unconformity extends from the Ordovician to late Carboniferous. Erosion during this time resulted in the maximum preserved thickness of Cambrian rocks in the eastern
SR/PP, thinning gradually toward the west. Deposition again started in the early Permian with the glacial deposits of the Tobra Formation. In the Dhurnal area, Permian rocks directly overlie the Eocambrian Salt Range Formation and the whole Cambrian section is missing (Fig. 6). The rocks of Triassic, Jurassic and Cretaceous age were deposited on a west northwest-facing, passive margin after the breakup of Gondwana, such that the maximum development of Mesozoic rocks is in the western SR/PP, overlapped by
Paleocene strata towards the east (Yeats & Hussain, 1987). In the Dhurnal area the whole Mesozoic section is missing due to a combination of a thin original section and later erosion (Fig. 6).
During the Paleogene, shallow marine to lagoonal sediments were deposited on the earlier eroded surface. A thick sequence of carbonate and shale beds was deposited in the western and central SR/PP, thinning towards the east. The third period of uplift and erosion corresponds to major collision between the Indian and Eurasian plates in the late Eocene and continues today in the Himalayan foredeep, represented by
Pleistocene conglomerates which rest on the older rocks with an angular relationship 18
Figure 6. Generalized stratigraphic section based on horizons encountered in the Dhurnal #3 well (for location, see Figs. 4 and 5). The description is generalized from lithology of the rocks drilled in the well. The environments of deposition (Shah, 1980) may by seen in relation to tectonic setting. Interval velocities used for depth conversions from time sections are indicated. The Chorgali and Sakesar formations of
Eocene age, Patala Formation of Paleocene age and Wargal Formation of late Permian age produce hydrocarbons in the Dhurnal oil field. Hydrocarbon shows were recorded in the Amb Formation of Late Permian age in DH #3, and the Warcha Formation of early Permian age in DH #2. = hydrocarbon production; 14 = hydrocarbon shows. 19
z -[ , AGE ,.., z - r ' r . Z Z ....ZC Z I ' ' . ' . ' , , I, .11 I. :..) = '...,.,- = Cg..' ,-, ,,., FE >, c= - ,-.0 FORMATION DESCRIPTION i.... > C: L.) ..,..,"' :"- 4' = ,- i- F-. r-T.1 - C. ,...., Z '"' -, = < 7.: Sst. with subordinate sltst. & clay - NAGRI .-.,2.5 + intraformational conglomerates.Fluvial .- PLIOCENE .. 7 Interbedded red brown clay with Deltaic 75,,,, - CHINJI -=,,-,. 11 ,-.4 sst. and sltst. ";., = Interbedded greyish sst. & clyst. 115 ,6_, 7'..) c 393 1-71 -4-.... = KAMLIAL with minor sltst. Fluvial MIOCENE . > z Alternating light grey sst. with darkLacustrine < 1713 ==-. 'MURREE green heavy minerals, clyst. & sltst.Deltaic OcEL1GONE- Iv U dri r /....0". z /A ....firemr.....7. N <. Brick red clyst./sh. with sub. sltst.Red beds 0p W < NIAMIKHELand micritic Is. Coastal plain f4 Z Z (LI = L1.1 W < CHORGALI Grey shale with buff limestone. Marine U E--. U 0 ...."" .L.1 Light brown Is. with thin streaks ..) SAKESAR Marne E 87 :-F:c4 of dark grey shale. c LL1 Light grey Is. with thin streaks ofShallow marine rti c z PATALA to lagoonal a.. 168 q. 11.1 dark grey shale. Interbedded IL grey limestone and Shallow marine 11 8 LOCKHARTgrey shale. .-7 Light grey sandstone and dark greyMarine to HANGU 13 P. shale. Continental ir 7:1 ofer. CRETACEOUS / N 0JURASSIC TRIASSIC Buff Is. with thinstre of creamy c.) WARGAL Shallow marine 100 hsahksale. dolomite and greyis E LATE t Light grey sst. interbedded with AIV1B Shallow marine 83 ( i i '.1 N creamy Is. & greenish shale. Z Fred. bluish grey, purple, brn. sh.Lacustrine to =Q. 116 Z SARDHAI with minor white sst. & bin. clyst.Shallow marineEn c U < Mainly whitish sst. with reddish Fluvial to c g ,--- 0t4
CAMBRIAN Restricted Upper part is reddish clyst., olive 4600 marine 2+87 SALT RANGEgreen dolomite & grey sh., while m/s lower part is evaporites. Hypersaline -,Q PRECAMBRIAN Cont.Not 6000 BASEMENT Biotite schist, quartzite, rhyolite.Metamorphic INDIAN SHIELD Complex Shielddrilledm/s
Figure 6 20
(Seeber & others, 1981; Gee, 1983; Ni and Barazangi, 1984; Yeats and Hussain,
1987). The collision resulted in deposition of the fluvio-deltaic, time transgressive deposits of the Rawalpindi and Siwalik Groups during uplift of the Himalaya (Johnson et al., 1979). The presence of eroded and recycled crystalline and metamorphic material of the High Himalaya in the lower Siwalik rocks and recycled debris of the lower and middle Siwalik in the upper Siwalik rocks suggests continued uplift in the north (Keller et al., 1977). The southward creeping of the deformation front is the response to continued collision and crustal shortening farther north. The depocenter of the resulting molasse foredeep also migrated southward due to continued uplift in the north. Such southward migration of the molasse depositional axis is also reported in the foreland of
India (Acharyya and Ray, 1982). Detailed magnetostratigraphic studies in the SR/PP (Raynolds and Johnson, 1985; Johnson et al., 1982) provide important timing constraints for different deformational events in the area, and also suggest the south- southwestward, time-transgressive nature of the molasse. As a result of these studies, the migration rate of the molasse depocenter has been estimated as 20 mm/yr (Raynolds and Johnson, 1985).
GENERAL STRUCTURE
The NPDZ is a belt of Neogene deformation, extending southward from the MBT to the Soan syncline (Fig. 3). The project area is a part of the highly dissected NPDZ, where resistant rocks rise above the general ground level, along ridges such as
Khairi Murat and Kamlial (Fig. 5). Eocene rocks are exposed in the Khairi Murat uplift; otherwise, the molasse sequence constitutes the predominant exposure in the area. The general trend of formation outcrops and faults is ENE-WSW, approximately perpendicular to the tectonic transport direction (Fig. 5). The Soan River marks the 21 axis of the Soan syncline south of Dhurnal, and surface dips gradually increase toward the upturned Kamlial ridge, north of Dhurnal (Fig. 5). The Nagri Formation of
Pliocene age is exposed at the surface in the vicinity of the Dhurnal oil field, while the overlying Dhok Pathan Formation crops out south of the Dhurnal area on the northern
limb of the Soan syncline (Fig. 5). The Dhurnal structure underlies a local syncline developed due to surface expression of the Khaur anticline in the southwest and the Sakhwal fault (SF) west of Dhurnal. The SF is a backthrust developed in the roof sequence of the Dhurnal back- thrust, as the Khaur anticline developed southwest of Dhurnal. The SF also has some
right-lateral motion and slickensides can be seen in the fault zone north of Sakhwal
village. The SF trends NNE-SSW; it merges with the Ahmadal fault (AF) in the south and dies out in the Chinji Formation to the north (Fig. 5). In the southwest corner of
the project area, the Khaur anticline forms a smaller ridge rimmed by the Kamlial Formation (Elahi and Martin, 1961). The Kenet syncline developed on the back of the AF and is bounded by the SF and Kenet fault (KF) in the east and north, respectively
(Fig. 5). The trend of the Dhurnal structure is parallel to that of structures of the eastern
SR/PP (Fig. 3); the eastern plunge of the Khaur anticline also has the same trend (Fig.
5). The boundary which marks the change in trend between the eastern and central
SR/PP appears to pass between the eastern plunge of the Khaur anticline and the
southern end of the SF.
The backthrust north of Dhurnal appears at first glance to be the eastward extension of the Kanet fault (KF). However, the new surface mapping and seismic data indicate that they are two different faults, with different senses of motion and origin. The Dhurnal fault (DF) is a south-dipping backthrust, initiated from a tip line in the upper beds of the Murree Formation, while the KF is a north-dipping emergent thrust developed in the direction of tectonic transport. The resistant Kam lial sandstone beds tilted upward along the DF, developing the northern limb of the Soan syncline. 22
The DF dies out on the northern limb of the Kenet syncline out of project area (Fig. 5),
as the deformation front exhibits emergent thrusts west of Dhurnal (Fig. 3).
The dips in the area between the north-verging DF and the south-verging
Mianwala fault (MF) are steep to vertical, in places overturned. Highly deformed Murree Formation is exposed in this zone and shows very erratic trends. The MF is a high angle, intraformational thrust which can be traced at the surface in streams where
good rock exposures are present, on the basis of shear zones, fault breccia and
secondary calcite. The rocks exposed between the MF and Khaki Murat faults (KMF) have high
dips and represent the northernmost exposure of Siwalik rocks. The surface exposure
of the KMF is very prominent as the Eocene beds are thrusted up against the Siwalik rocks. Slickensides can be observed on large blocks of massive limestone near Dhok
Maiki, west of Gali Jagir (Fig. 5). The Chorgali Limestone of Eocene age is exposed along the KMF, and in thick Chorgali Limestone beds are exposed with steep dips east of Gali Jagir (Fig. 5). The area north of the KMF is mostly covered by alluvium and only middle to
lower Murree beds are exposed in patches. The exposed Murree beds in this area are
highly faulted, and dips are moderate to high (Fig. 5). Farther north, the MBT is represented by a fault zone consisting of many high angle thrusts, along which Eocene
and older rocks are exposed at the surface.
BALANCED CROSS-SECTION
A balanced section has been constructed (Fig. 8) which provides the simplest possible interpretation based on available surface and subsurface constraints. It is quite possible that subsurface structure is more complex, particularly north of the KMF. 23
Two different techniques were used to construct the balanced cross-section of the NPDZ. The mechanically competent platform rocks of Permian to Eocene age were balanced by using bed lengths (Dahlstrom, 1969; Dixon, 1982). In contrast, the
Eocambrian evaporite section (Salt Range Formation) and molasse sediments were balanced by applying the area method (Dahlstrom, 1969), due to ductile behavior of the evaporites and presence of bedding-plane slip in molasse during deformation. One of the problems in balancing the sedimentary section in the NPDZ is that the basement shape and dip are not known at the original place of deformation, north of the current position of the NPDZ. The sedimentary wedge, which was telescoped and thrust southward along a low angle decollement in the Salt Range Formation, is about 8 km thick near the MBT.
About 4000 m of molasse and 1050 m of Permian to Eocene platform rocks were drilled in the Dhurnal well # 3. The thickness of Eocene to Permian rocks indicates a hinterlandward, stratagraphic thickening of the platform sequence,which is only about
600 m thick in the Salt Range. Thick beds of the Salt Range formation are present under Dhurnal; Latif (1973) suggested that they are correlative with gypsiferous deposits farther north, in Hazara. Depths to basement north of Dhurnal are not well constrained because overthrusting of high velocity rocks results in velocity pull-up on seismic time sections, inducing errors in depth conversion.
Siwalik rocks generally pinch out toward the north, and their previous northern limits are not known due to deformation and erosion north of the passive roof thrust. The only surface exposure of Siwaliks in that area is just south of the KMF (Fig. 5), where they crop out with almost vertical dips, riding on the back of the Mianwala fault (MF). The balanced section (Fig. 8B) assumes that faults ramp against sandstone units in the Siwaliks, and form flats in shale or clay beds. North of the KMF, only lower Murree beds of the Rawalpindi Group are exposed at the surface. Either rocks of the
Siwalik Group were not deposited that far north or eroded after deformation and uplift 24
Figure 7. (A) Composite seismic line (time section) across NPDZ, used for balanced section A-A' (see Figs.3, 4 and 5 for location). Line AW-15-AF is migrated, 24 fold, 6-24 Hz, Vibroseis source, recorded in 1978 by Western Geophysical Company and processed by Petty-Ray. Line NP-84-12 is migrated, 48 fold, 8-47 Hz Vibroseis source, recorded in 1985 by Seismograph Services Limited (SSL) and processed by Seiscom Delta United. Lines 86-KH-02 and 86-KH-05 are migrated, 60 fold, 10-47 Hz Vibroseis source, recorded in 1987 by S.S.L. and processed by OGDC.
(B) Generalized interpretation of (A). Note that mapped surface dips (Fig. 5) are projected and plotted above the section. 25
A. Uninterpreted 0 2km SE SPLICED 0 2km N SPLICED 0 2km i I I 0 0 0
1
2
.,,,>_:--',,_,'-_-7. -----..-__,-,-,.. _...e.,,.,,--- --+ ,--
7.7,---.. ,..".'7..-a-, ------.:, --<''''..M----:-..------=-'-"--7,--:-.".-----.--..-- _--Tr....---- 4
-,--- ,-_-_-..---.-_ Offset 4 km -__....- _ --,---_,----....---:-.,-----_-_ -_,------17,-- .---- -....,-,- -,-..r..-. 7--..-Z--- 7------_ -- -- 7 -7----- ',--- -= ' -r ------.------1--.,---.77-; 1 --" along strike NP-84-12 (Migrated) AW-15-AF 86-KH-02 (Migrated) 86-KH-05 (Migrated) Offseti 5 km along strike (Migrated)
B. Interpreted DHURNAL EASTERN PLUNGE SE N POP-UP KHAUR ANTICLINE SOAN RIVER SPLICED SPLICED 442 ac40 .20 MBT 80? KmF eofNAF 0 76? DF 0 0
1 cn z 2-- 0 SR S R F 3 :---_f=LLIT
""------4 _ 4 _ PRE -E BASEMENT - £ BASEMENT £ BASEMENT C. 7-7 86-KH-05 (Migrated) NP-84-12 (Migrated) AW-15-AF 86-KH-02 (Migrated) (Migrated)
Figure 7. 26 of the NPDZ. The thickness of Murree beds increases toward the north; about 3800 m
(tectonic thickness) of Murree beds were encountered by OGDC in the Bhal Saydian well, near Bhal Saydian village west of Fateh Jang (Fig. 5).
The Eocene to Permian platform rocks have a very strong and characteristic seismic signature, which is traceable from the Soan syncline to Dhurnal. Farther north, the data quality deteriorates, but these reflectors are traceable along imbricate faults. From the KMF to the MBT, the data quality is very poor, and these reflectors can only be seen in patches. The only good reflection in that area is associated with the top of the basement. The balanced cross-section A-A' of the foreland fold-and-thrust belt of the
NPDZ has many of the characteristic features found in foreland fold-and-thrust belts (Fig.7, 8, 9 and 10). The presence of a basal decollement and termination of faults into the decollement are important features in thin-skinned deformation of foreland fold-and- thrust belts (Jones,1987). The NPDZ developed by south-verging, piggyback thrusting which initiated from the master decollement in the Salt Range Formation; section balancing reveals that out of sequence thrusting is also present. The present-day frontal culmination wall at the southern edge of the NPDZ overrides the back of the passive roof thrust (DF), developing the northern limb of the Soan syncline (Fig. 10).
This topographically high wall has been eroded since the time of its formation, so that the preserved bed length in the roof sequence is considerably less than the restored bed length of rocks under it. The imbricate slices developed into a hinterland dipping duplex that was emplaced under a passive roof thrust, resulting in a "triangle zone" geometry. The presence of an upper detachment means that each thrust fault beneath it generates a ramp anticline where it flattens along the upper detachment surface (Jones,
1987). The passive roof duplex geometry in the Dhurnal area is further distorted by the development of backthrusts in the northern limbs of the Dhurnal and Khaur anticlines, 27
Figure 8. Balanced and restored cross-section along line A-A' (for location see Figures
3, 4 and 5). The labels Fl to F10 represent the chronological development of major faults, from oldest to youngest. A) Post-Siwalik restoration. The Siwalik rocks are present south of KMF (F6) but their northward extension is not well constrained. B) Balanced cross-section showing present structural geometry of the NPDZ. Positions of nearby seismic lines (see Fig. 4 and 7) are projected and shown below the section. Intrafommtional faults that occur at the surface in the Murree Formation north of the KMF, east of line A-A' (Fig. 5), are not depicted on this section, although we expect that they continue westward beneath the alluvium. 28
NORTHERN POTWAR DEFORMED ZONE
F3 F4 F6
. .. .
. 141:1 . F6A moimm==mmrimmommowommommoliiiiiimmiiimrZiesnom N" 7 , \-77 , 7, 7 7 _ 2 .. 7 N .7 \ " \-7 \ N N N Vr\ r 60 50 40 30 20 10 0 118 110 100 90 80 70 KILOMETERS
DHURNAL POP-UP Eastern Plunge Soan KMF MF DF Khaur Anticline Syncline MBT A' A 68 76 42 46 20 17 18 6. 411.. PERMIAN TO EOCENE 5? 1; 64; er° SIWALIK } TERTIARY -4. -0. -4. MOLASSE RAWALPINDI SALT RANGE FORMATION AMP A/r 4117 it1 4111111/Ain` ANS' AWAl911A' AIM," w PRECAMBRIAN BASEMENT All 47AV-A=1/ ,AIN=MINIMINIP/MMEr410, AI ...or> -4 .ANNIK" K111'- a/NW 4=P` -rep e 0 ANI1111111111/..A=P" don_ws. -6 72km SHORTENING MBT TO SOAN SYNCLINE (69km AFTER CORRECTION FOR BEND IN A-A') 1,1/ /NMI .AIMMIINIVAIM111 AM ADMII/IMMIs- 8
10 20 10 118 110 100 90 80 70 60 50 40 30 NP -84 -12 KILOMETERS I- 86-KH-05
86-KH-02
1
Figure 8. 29
and tectonic thickening of salt in the core of the Dhurnal structure.
Overstep faults are well recognized throughout the SR/PP (Burbank and Beck,
1989; Leathers, 1987; Johnson et. al., 1986) and other foreland thrust belts. They are mechanically explained as needed to maintain critical taper (Davis et al., 1983). The
palinspastic restoration of the balanced section revealed major overstep (out of sequence) thrusting in the NPDZ. The KMF (F6 in Fig. 8) is an overstep fault that probably developed after the emplacement of the MF (F5 in Fig. 8). With the
piggyback development of the KMF, the confirmed bed length (about 6 km) of Kamlial
Formation, just south of the KMF, cannot be achieved. By emplacing the MF first, this problem was resolved, suggesting that the KMF is an out-of-sequence thrust, younger
than the MF. The development of a pair of thrusts, where one thrust projects horizontally in the transport direction from another, higher-angle thrust, is referred to as a system of
"divergent thrust faults" (Jones, 1987). Jones (1987) modeled different stages in the piggy-back development of such faults, and discussed their origin. The piggy-back development of such faults must be modeled very carefully, since slip on the horizontal
thrust should be comparable with the ramp height of the vertical fault (Jones, 1987).
Two such sets of divergent thrust faults are present in the balanced section A-A' (Fig. 8). One set is present just north of Dhurnal (F7 and F9 in Fig. 8), while the other pair is interpreted along the KMF (F6 and 6A in Fig. 8). The restoration of the balanced
section suggests that faults F6A and F9 developed due to overstep faulting.
Imbrication observed in the area north of the KMF (F6A and F6B in Fig. 8) is commonly considered to be developed from a lower to a higher sequence, after emplacement of the basal thrust. Dahlstrom (1970) suggests that if imbricates develop near the surface, they will break back (Fig. 9A); if they develop at depth they should break forward, in the footwall (Fig. 9B). Butler (1982) termed the imbricates as hangingwall imbricates and footwall imbricates, depending upon their position. Jones 30
Figure 9. The development of imbrications progressing from fault 1 to fault 4 (from Dahlstrom, 1970). A. Overstep development of imbrications in the hanging wall.
B. Piggy-back development of imbrications in the foot wall. 31
A. Imbrication in the hanging wall.
B. Imbrication in the footwall.
Figure 9. 32
(1987) discussed the matter with computer-generated models, suggesting that imbricates can be formed in any sequence, backward or forward, depending on when the major thrust of the sequence developed.
Highly faulted beds of Murree formation are present north of the KMF, where most of the area is covered by alluvium. The subsurface structure suggests the development of a hinterland-dipping duplex structure in the platform strata (Fig. 7 and
8). However, it does not appear that the overlying molasse strata are deformed through imbricate thrusting. Two explanations can be given for this disharmony in structural style: 1) there is a large amount of slip of molasse sediments along a weak zone at the top of the platform strata (Fig. 8A); 2) a roof thrust has developed at the base of the molasse sequence. According to the classification of foreland fold-and-thrust belts on the basis of erosion level (Jones, 1987), the foreland fold-and-thrust belt of the NPDZ lies somewhere near stage 2, depicted in Fig. 10. At that stage, most of the roof sequence is eroded away and only some remnants are present as large synclines bounded by narrow, faulted anticlines. This surface expression of the exposed geology has long been reported in the NPDZ, particularly in the western part (Gill, 1951).
STRUCTURAL STYLE IN THE NORTHERN POTWAR DEFORMED ZONE
The NPDZ is complexly folded and faulted as compared to other parts of the
Potwar Plateau. Baker (1987) interpreted older seismic lines which revealed that the gross structure of the NPDZ changes abruptly from south to north. In the southern
NPDZ, the northern limb of the Soan syncline dips steeply southward above an upper level detachment or "passive roof thrust" (Fig. 7 and 8). In the north, the competent rocks are stacked along imbricate thrusts in a hinterland-dipping duplex (Fig. 7 and 8). 33
Figure 10. (A) Foreland fold-and-thrust belt without any erosion, showing foreland syncline, sole thrust, roof thrust and hinterland-dipping duplex.
(B) Same as (A), but with different stages of erosion. Stage 1: Thrust zone is not exposed, while foreland syncline and other folds constitute the surface expression (British Columbia foothills, Canada; southern Oman fold belt).
Stage 2: In this stage, the upper part of the folds have been eroded away, exposing narrow faulted anticlines bounded by broad synclines. The foreland syncline is still present, and the roof thrust exposed (Folded Alpine Molasse Basin; Central Alberta foothills; NPDZ and Soan syncline in this study, Fig. 8). Stage 3: Thrust zone is completely exposed and only the lower remnants of the foreland syncline are still present (Southern Alberta foothills). stage 4: Advanced stage of erosion where only remnants of thrust zone are present. Overthrusted foreland margin and foreland syncline are eroded away. (Utah-Wyoming overthrust belt, USA).
Figure modified after Jones, 1987. 34
A. Hinterland-dipping Duplex
Roof Sequence
Sole Thrust Buried Frontal Tipline
Foreland B. Syncline
16,V...,"el,, Imo1 Roof rert-- Sequence
Roof Thrust
4. ff]
Figure 10. 35
Passive roof duplexes or "triangle zones") have been reported in the frontal portions of
the Sulaiman and Kirther Ranges of Pakistan (Banks and Warburton, 1986), the eastern
Rocky Mountain Foothills of Canada (Price, 1981; Jones, 1982; Price, 1986), the
Adavale basin of Queensland, Australia (Remus et al., 1988), and the Southern Norwegian Caledonides (Morley, 1986). Many styles of deformation have been
recognized in duplexes and other imbricate thrust systems, depending upon changes in
geometric relationships between folds and thrusts, and their internal structural parameters (Boyer and Elliot, 1982; Suppe, 1983; Mitra, 1986).
The present structural setting of the NPDZ, namely, low surface topographic
slope, low basement dip and the presence of Eocambrian evaporite beds above the
basement, is not compatible with the intense deformation of this zone. The strong
deformation and multiple imbrication in the NPDZ must have produced a broad taper
(basement dip + topographic slope) which implies high friction at the basal decollement (Davis et al., 1983). The NPDZ appears to have formed prior to 2 Ma, as the foreland
strata were progressively accreted onto the overriding thrust sheet by duplex formation north of the northern margin of the evaporites of the SRF. The passive roof duplex
(triangle zone) also appears to have developed along a high friction decollement north of
the weak salt horizon. As the deformation front reached the northern edge of the salt
basin at about 2 Ma, the Dhurnal structure developed as a pop-up and the thrust front jumped from the NPDZ to the SRT (Jaume and Lillie, 1988). By the present, the NPDZ has been translated about 20 km southward on the SRT. Note that the Dhurnal
pop-up is expressed as a symmetrical anticline bounded by forward and backthrusts (Fig. 11). The pop-up reveals no preferred direction of vergence, a geometry
consistent with compressional structures developed over salt (Davis and Engelder,
1985; Pennock et al., 1989). The salt may have thickened under the Dhurnal structure
in response to a basement warp (Fig. 8); as the salt thickened sufficiently, the Dhurnal pop-up slid over the warp. Subsequent erosion of the NPDZ then led to a very low 36 topographic slope (Fig. 8), in accordance with the required narrow cross-sectional taper
(Jaume & Lillie, 1988). The later development of another backthrust in the northern limb of the Khaur anticline distorted the earlier structure (Fig. 8).
Though structural features show considerable parallelism along strike, the thrust front exhibits variable geometry from east to west. The eastern segment of the
NPDZ exhibits stacking of blind thrusts due to sticking of the tipline (buried thrust front), which gave rise to the passive roof thrust and development of the foreland syncline (Morley, 1986; Banks and Warburton, 1986; see Fig. 3, 7 and 8). In contrast, the western part of the NPDZ exhibits an emergent thrust front (Morley,1986), where compressed, faulted folds are separated by wide synclines (Fig. 3).
Passive Roof Duplex A passive roof duplex can be developed when the sole thrust ramps to an upper detachment level. A tipline develops due to sticking, which generates a back thrust. Continued convergence from the hinterland stacks thrust sheets under the backthrust (or "passive roof thrust"). The passive roof thrust exhibits an apparent backthrust sense of motion, even though there may not be any actual motion toward the hinterland (Banks and Warburton, 1986; Remas et al., 1988). The beds above the backthrust develop a syncline on its foreland side, due to upward tilting of the passive roof thrust (Fig. 10,
11 and 12A). In the case of the Dhurnal passive roof duplex, the sole thrust ramped upward
(F7) and flattened gradually along bedding at the top of Murree Formation. At this point the passive roof thrust developed from a tipline along some weak zone at the top of Murree Formation, to facilitate some of the south-directed thrusting (Fig. 8). The development of the Dhurnal pop-up at the northern edge of the salt basin (F8 in Fig. 8) disturbed the passive roof duplex geometry, but elevated the passive roof thrust. As a wedge of thrust slices (F9 in Fig. 8) moved under the backthrust, the passive roof 37
thrust was further tilted southward, resulting in the steeply-dipping north limb of the
Soan Syncline. The later development of another backthrust (F10 in Fig. 8) in the
northern limb of the Khaur anticline distorted the overall triangle zone geometry (Fig.
8). The ductile behavior (if the Salt Range Formation also contributed to this
disharmonic structure, because evaporites flowed into the core of the anticline.
As the effect of the eastern plunge of the Khaur anticline diminishes gradually
eastward, a clear and simple structural picture can be seen (Fig. 11). The southern limb of eastern Dhurnal develops into a fault propagation fold (Suppe, 1983). The strong
reflections from the platform sequence are offset above thick Salt Range Formation on
line NP-84-03, and the displacement along the fault gradually decreases to zero in the
thick molasse sequence. It appears that the development of the fault propagation fold
ceased, due to gradual development of the passive roof thrust at the top of Murree
Formation and the northern limb of the Soan Syncline tilted upward due to underthrusting of thrust sheets. The backthrust in the northern limb of Dhurnal is an
earlier thrust emplaced during the development of the Dhurnal pop-up, but renewed displacement can occur along this fault through later compression or continued salt
flow in to the core of the structure.
Because the eastern NPDZ is a tip-stick type of thrust front, at the time of deformation the sole thrust (lied out in the subsurface and developed the passive-roof
duplex geometry. The later development of the Khaur anticline southwest of Dhurnal may have also provided a buttress, because seismic sections show that the Dhurnal structure is locked against the northeastern limb of the Khaur anticline (Fig. 13). In a regime where the thrust front is locked and fading stress from the hinterland is unable to move the thrust, enough strain may develop in the thrust sheet to produce complicated structures (Cello, 1988). Tip-stick thrust fronts commonly exhibit high strain accumulation; this high strain may be manifested in the Dhumal structure in the form of fracturing and the development of small faults. These fractures and faults provide the 38
Figure 11. (A) Un interpreted time section along lines NP84-03 and AW-15AG, across eastern part of the Dhurnal structure (for location, see bold line B of Figs. 4 and 5).
(B) Generalized interpretation of (A), showing the development of a fault propagation fold (Suppe, 1983) on the southern limb of eastern Dhurnal, and the development of the passive roof duplex due to southward underthrusting of thrust sheets north of Dhurnal. UDM= undifferentiated molasse, for other abbreviations see Fig. 5. 3 9
A.UNINTERPRETED SOAN SYNCLINE S N SPLICED
._,,--,. -'-.-- __:".,..'C., <-:-. .4'...`"'.....-4.-.f y!,..n. _--4__ ' .....,,,4ou w 44..4 . , 4
taA, NP- 84-03 (Migrated) 01 2 km AW -15 -AG (Migrated)
B. INTERPRETED EASTERN SOAN SYNCLINE N DHURNAL SPLICED DF f(1 Ch Na Dp 30
"_-
2 Zo w Cn P-E
-.=1074..- BASEMENT PRE . BASEMENT
NP -84 -03 (Migrated) 0tstsI1 2 km AW-I5 AG ( Migrated)
Figure 11. 40 excellent porosity and permeability for hydrocarbon accumulation. The massive competent units can develop fractures and relatively small faults at the leading edge of the hanging wall, as they cannot form concentric folds without internal deformation (Jones, 1987). The deformation mechanisms and their timing can reduce or enhance the porosity of reservoirs (Mitra, 1988). The fractures formed during the last deformational phase can remain open while earlier developed fractures can be sealed by pressure solution ( Mitra, 1988).
The area north of Dhurnal exhibits vertical stacking of hinterland-dipping duplexes. Thrusts ramped steeply through competent platform rocks, while in molasse they formed flats. The poor seismic data quality in this area may be indicative of high internal deformation of the incompetent rocks.
Overstep Backthrust The Sakhwal backthrust, which dies out north of Sakhwal and merges into the south-verging Ahmadal fault south of Nauthian (Fig. 5), is an overstep, passive roof thrust. The development of a foreland-dipping, overstep backthrust within a roof sequence over an earlier passive roof thrust is discussed by Banks and Warburton (1986) and reported in Ziegler (1969) and Jones (1982). It is more likely that the roof sequence became imbricated with the inclusion of new duplex horses, instead of a single continuous passive-roof sequence over many duplexes (Fig.12). When duplexes form only by a passive-roof mechanism, the development of overstep backthrusts is obvious (Butler,1982). The overstep, passive backthrust geometry is clear on line NP- 84-15, on the western plunge of the Dhurnal structure (Fig. 13). The Sakhwal overstep backthrust (SF) cuts the surface just north of Dhurnal, while the Dhurnal passive roof thrust (DF) and the Kanet fault (KF) cut the surface farther north. The SF developed within the roof sequence as the deformation front migrated southward, with the successive addition of the duplex in the Khaur area, west of Dhurnal (Fig. 13). 41
Figure 12. Development of an overstep backthrust in a roof sequence, as a new thrust sheet is added to the deformation front (A), then the tipline migrates farther toward the foreland (B). The thrust numbering indicates their order of development (after Banks and Warburton, 1986). 42
A Passive Backthrust Foreland Syncline FORELAND
Roof Sequence
Buried Tip line
B Passive Overstep Backthrust Backthrust Foreland Syncline
1
Figure 12. 43
Figure 13. (A) Uninterpreted seismic time section NP-84-15 across the Dhurnal pop- up and eastern plunge of the Khaur anticline (for location, see boldline C of Figs. 4 and 5). (B) Generalized interpretation of (A), showing the development of an overstep backthrust within the roof sequence above the Dhurnal passive roof thrust (DF). This geometry developed due to inclusion of the Khaur structure to the west of Dhurnal. Line NP-84-15 is migrated, 48 fold Vibroseis, sweep frequency 10-60 Hz, recorded in 1984 by S.S.L. and processed by Seiscom Delta United. Dhurnal #3 is the deepest well in the Dhurnal pop-up, which penetrated the whole platform sequence and about
87 m of Salt Range Formation (Fig. 6). DF= Dhurnal fault, 1U= Kanet fault, P-E=
Permian to Eocene, SF= Sakhwal fault, SRF= Salt Range Formation.
45
Basement Warp A NE-SW trending basement warp is interpreted below the Dhurnal structure
(Fig. 14). This basement anomaly marks a change in basement dip, from 2° to 3° in the south, to < 2° in the north (Fig. 7 and 8). Basement warps are not as strong stress concentrators as basement faults, but they can accumulate enough stress to affect deformation (Wiltschko and Eastman, 1983). The increased salt thickness under
Dhurnal and just north of it may be due to the presence of this basement warp. The basement anomaly can also be interpreted as a small, down-to-north, normal fault.
However, this interpretation of the seismic time sections is speculative because; 1) the overlying structure is complex: 2) the effects of velocity pullup as the salt thickens distorts the basement reflection; and 3) the depth of basement is about 7 krn under
Dhurnal.
Thickness and Distribution of Salt
Loading by imbricate thrusts in the NPDZ is one of the factors resulting in salt flow towards the south (Gee, 1983, 1989). The basement warp under the Dhurnal structure provided the perturbation that initiated salt build up. As the salt grew thick enough, the thrust sheet pushed across the warp (Fig. 13). The salt has about 2700 m
(1.3 sec.) thickness under the Dhurnal structure, and about 2400 m (1.1 sec.) under the eastern plunge of the Khalir anticline on line NP84-12, south of Dhurnal. Farther south, the salt thickness gradually decreases towards the axis of the Soan Syncline. North of the Dhurnal pop-up structure, the salt beds show a gradual decrease in thickness; the appearance of some patchy, strong reflectors within the salt beds north of
Dhurnal may indicate a northward facies change to less evaporitic material. 46
Figure 14. Subsurface contour map at the top of Precambrian basement. Contour interval is 100 in, with depths relative to a sea-level datum. Note the warp under the Dhurnal structure. Two-way travel times were converted to depth with interval velocities given in Fig. 6. 47
72°30' 72°45' 72°50' 33°
OXY/OGDC/POL/AOC NORTH POTWAR CONCESSION (4) POL/OGD EASTERN KHUSHALGARH CONCESSION
Fateh Jang
33° 30'
IHURNAL/
Sakhwal 7400 m 7300 7200 7100 11111.1)111.'7000 m 6900
33° 67 00 15' ppco
Figure 14. 48
TECTONIC SHORTENING
Timing of Structural Events The tectonic convergence responsible for deformation of the northwestern
Himalaya, Hill Ranges and the SR/PP is directed approximately north-south ( S15°E; Yeats et al., 1984). The intensity and age of this deformation generally decrease southward, so that the less complicated and younger structures are at the southern edge of the deformation, in the Salt Range and Soan syncline.
The chronology of different structural events in the NPDZ must be inferred from studies of surrounding areas, as the NPDZ lacks such studies. The Peshawar basin area north of the M13T was strongly active until about 3 Ma, while the 1.8 Ma date from Dheri Choan, district Attock suggests a phase of uplift and deformation along the MBT (Johnson et. al., 1986). Opdyke et. al. (1982) suggested that approximately
10° of counterclockwise rotation of the northern limb of the Soan Syncline accompanied southward thrusting. Tauxe and Opdyke (1982) reported about 8° counterclockwise rotation in the Khaur area; they also concluded that the youngest Siwalik beds present in the Khaur area are 6.5 Ma old. Uplift and deformation along the Riwat fault, south of the Soan River in the eastern Potwar Plateau, occurred between 3.4 and 3 Ma (Johnson et. al., 1986). The top of the Dhok Pathan Formation is dated as 5.1 Ma in the central Potwar Plateau (Johnson et al ., 1982), and it is folded with the northern limb of the Soan syncline, right above the southern edge of the NPDZ (Fig. 5). These relationships suggest that the earliest time for initiation of deformation in the southern
NPDZ is 5.1 Ma. The structures north of Dhurnal show strong southward vergence, but in contrast the Dhurnal area exhibits both south and north vergent thrusts. The Dhurnal pop-up developed as a symmetrical anticline (Fig. 13) at the northern edge of the salt basin at about 2 Ma. The most recent deformation in the northeastern Potwar Plateau 49 occurred between 2.1 and 1.9 Ma (Raynolds, 1980; Johnson et al.,1986). Lillie et al.
(1987) suggested that, as the deformation in the NPDZ stopped about 2 Ma, the deformation front propagated abruptly to the Salt Range along the decollement in the
Eocambrian salt beds. However, age-dating just north of the Salt Range by Burbank and Beck (1989) suggested that some uplift of the Salt Range may have occurred much earlier, about 4.5 Ma. Thus, in the last 5 Ma the deformation may have been distributed throughout the SR/PP, with much "out of sequence thrusting" (Burbank and Beck,
1989).
Amount and Rate of I lorizontal Shortening
About 30 km of horizontal shortening in the competent platform strata has been calculated by restoring the balanced cross-section from the Soan syncline to the KMF
(Fig. 8). Shortening for the area north of the KMF is less constrained, as the seismic data quality is poor in this part of the NPDZ. The interpreted structure from the KMF to the MBT shows about 39 km horizontal shortening. The total minimum shortening from the Soan syncline to the MBT is estimated about 69 km (after correcting for bends in the section A-A'), a shortening of about 60% (Fig. 8). This percentage is comparable to the amount of shortening in other foreland fold-and-thrust belts. Considering the time span of structural events in the NPDZ ( 5.1-2.0 Ma; Johnson et al., 1986; Lillie et al., 1987), the minimum rate of shortening in the NPDZ is 22 mm/yr
(69 km in 3.1 Ma). 50
CONCLUSIONS
The Dhurnal oil field is a pop-up structure developed at the southern edge of the
NPDZ, in a passive roof duplex, under the northern limb of the Soan syncline.
The backthrust in the Dhurnal area had been considered as the eastward extension of the Kenet fault. However, the Dhurnal backthrust is altogether a different fault, with a different sense of motion; it joins the Kenet fault west of Dhurnal and then it diverts toward the southwest, gradually dying out at the surface. In this paper, this backthrust is called the "Dhurnal Fault" (DF). The DF is a passive roof thrust which has a backthrust sense of motion; as the thrust sheets form duplexes, they wedge under it, lifting and tilting the roof thrust to form the northern limb of the Soan syncline. A passive-roof duplex geometry can be predicted in foreland fold-and-thrust belts, where the deformation front is marked by a foreland dipping nionocline, in spite of foreland verging emergent thrusts (Banks and
Warburton, 1986). An overstep passive-roof thrust (Sakhwal backthrust) is interpreted from the seismic and surface geologic data, northwest of Dhurnal. This thrust developed in the
Siwalik sedimentary sequence above the earlier passive roof thrust, due to inclusion of another thrust sheet, as the deformation front moved farther south to the Khaur structure. The NPDZ has almost flat topography and gentle basement dip north of
Dhurnal, because Eocambrian evaporite beds are present under the platform strata of the
NPDZ. This study supports the earlier interpretation (Jaume and Lillie, 1988) that the northern Potwar Plateau existed as a strongly deformed and tapered fold-and thrust belt prior to 2 Ma; it has since overridden the north edge of the salt basin and erosion has removed its former topographic slope. About 69 km of horizontal shortening has been calculated for the zone between 51 the Soan syncline and the MBT by cross-section balancing and restoration. Taking into account the age of upper beds of the Dhok Pathan Formation (5.1 Ma), which are involved in deformation on the northern limb of the Soan syncline, and the termination of deformation in the NPDZ at about 2.0 Ma, the minimum rate of shortening in the
NPDZ is estimated as 22.0 mm/yr. 52
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