C&N&DIA.N AOURNAL OF EXPLORAITION GEOPHYSICS ““L, 30. NO, 2 ,DECEMBER 19941, P, 103~114

PETROLEUM GEOLOGY OF NEPAL’

OR. FRIEDENREICH*, O.L. SLIN$, U.M.S. PRADHAN~ AND R.B. SHRESTHA~

ARSTRACT

Magnetic. gravity and seisn1ic surveys, exIcnSi”e geological field mapping programs. StrUCtUlal and straiigraphic studies and prtr”le”m grochrmicill in”esligati”ns have her” carried ““I in ,hr Trrai plains and the adjacent Siwalik Fold Belt of southern Nepal since the lxc 1970s. This work. which has hem recently incorp rated into a comprehensive evaluation. suggesrs that the country lhas mrac,iYe petroleum por,ihihties. Rocks with iignificam xn”rCe potential have been samplcd from oumops. and lhcmal modelling ruggcsts hat, ill the prDspecli”r pm 01 Ncpd ,hesc potential S,~“ICC rocks are in the oil and gas gmeration window. Effective resewoirr Em cxpeccml to be loud in IhC t,.owcr Siwalik. S”rkhCI amI Condrana sandrtoncs and in the carhonatci of the I.akharpata~Vindhyan G*O”p.

Enpl”rati”n “pp”nunities in the Trrai include Stl”Cl”lrS ilSS”Ci~ ated with blind thrusts. hascment~con,rollcd drape and bull ~b’uc- twes, stratigraphic pinch-outs and suhcrop trapr. Major folds and thrusts in the Siwalik Fold Belt are likely to provide str~ctwal traps. Ddling dcplh* to ohjeclivcs arc hm 1500 10 5000 mctrc* in the Terai and from 25011 fo 4000 rncms in the Siwalik Fold Bell.

1.0 INTRonUCTloN

This paper is based on the recently completed Nepal Source and Seal Study conducted by Alconsult International Ltd. (AIL) and the Nepal Petroleum Exploration Promotion Project/ Department of Mines and Geology (PEPP/DMG) HMG of Nepal (Slind, 1993). Geochemical sample collecting, labora- tory analysis, interpretation and modelling was conducted by the Geochemistry Section of the Institute of Sedimentary and Petroleum Geology (ISPG) of the Geological Survey of Canada. The study operated under the overall direction of Petro-Canada International Management Services (PCTMS) for the Canadian International Development Agency.

2.0 OtSJECTtVE STUDY

The main objective of the study was to better understand the petroleum geology of Nepal and to define its oil and gas potential by combining existing data with new information acquired during the course of the study.

3.0 PROCEDURF.

The project was a team effort consisting of Alconsult professionals in the fields of geology, geophysics (seismic, gravity), sedimentology, petrography and photogeology along with petroleum geochemists from the Geological Survey of Canada and geologists from the Petroleum Exploration Promotion Project of Nepal. The work done by the team consisted of: a) reviewing all petroleum-related information on Nepal and environs: b) collecting and analyr- ing oil and gas seep samples; c) conducting field traverses throughout southern Nepal to collect samples for geochemi- cal and petrographic analyses; d) field mapping (structure) in selected critical areas; e) studying the sedimentology 01 potential reservoir rocks; and f) reviewing and interpreting all the existing Nepal geophysical data.

4.0 DATA BASE

The data base included all previous work in which the PEPP/DMG was involved. This data included: a) more than IO years of geological field mapping (Schroeder, 19X I ; Shrestha and Shrestha. 1983); b) a regional aeromagnetic survey (CCC. 1980); c) a photogeological study (Hunting Geology and Geophysics Ltd., 1984); d) results of several geochemical investigations (Kay&ha, 1989): e) information from the Shell Biratnagar I well; and f) basic field data and interpretations from four geophysical surveys: 1983. I984 World BankKGG 1250 km reflection seismic program (Harris, 19X4), 1987.1988 PClAClAirborne 1651 km retlec- tion seismic and gravity program (Friedenreich and Slind. 1989), 1988-1989 Shell Nepal 1940 km (615 km used in this interpretation) reflection seismic program, 194 l-1993 French Government/CGG 422 km retlection seismic program.

5.0 GEoCnAPHICAL/GEoLoCICAI. SETTING

The Kingdom of Nepal, an independent country lying between India to the south and China to the north (Figure I), is 800 km long from east to west and ranges in width from

‘Presented at the CSEGKSPG Joint Convention, Calgary. Alhem, May I I. 1994. Manuscript received hy the Editor October 17. 1W4; wised manuscripr received January 23, 19% 2AI~~n~~It lntrmationai Ltd.. 450.5X1 6th Avenue SW.. Calgary. Alhem T2P US2 ?HMG Nepal Petroleum Exploration Promorion Project. Dcpartmcnt of Mines and Geology. Kathmandu. Nrpai Thanks are given fo Psrro-Canada lnterna&mal Management Services. prqject managers for Canadian International Developmcnl Agmcy. and IO Lhc Pclmlcum Exploration Promotion Project ofthe Departmcm of Mines and Geology of His Majesty’s G,ovcmmen~ d Nepal Cur lhcir a~sisuncc in prcpring rhis paper.

ClECi 103 Dccvlllbcr IV94

O.R. FKIEDENREICH. 0.L SLLND, U.M.S. PRADHAN and R.B. SHRESTHA

130 to 230 km. The country is naturally separated into four major geographical/geological zones that parallel its long dimension (Frank and Fuchs, 1970; Mitchell, 1979; StRcklin, 1980; Windley, 1983). Each zone has its own characteristic stratigraphy and structure and these zones are, from south to north, described below (Figures 2.3).

5.1 Terai

The Terai is the Nepal portion of the Indo-Gangetic Plain that extends from the Indian Shield in the south to the Siwalik Fold Belt to the north. The Plain is a few hundred metres above sea level and is underlain by a thick, relatively flat-lying sequence of Mid to Late Tertiary molasse (Siwalik Group) which unconformably overlies subbasins of early Tertiary to Proterozoic sediments (Surkhet, Gondwana and Vindhyan Groups) and igneous and metamorphic rocks of the Indian Shield (Agrawal, 1977; Acharyya and Ray, 1982; Raivemun et al., 1983). The only deep well in Nepal (Shell Nepal B.V. Biratnagar 1, TD 3530 m in 1989) was drilled in the far eastern part of the Terai.

5.2 Siwalik Fold Belt

The Siwalik Fold Belt is from 5 to 45 km wide and rises abruptly from the Terai along the main frontal thrust (MFT). It consists of a series of ridges and valleys composed of thick beds of folded and fault-repeated Tertiary molasse (Siwalik

-

30”

Group) thrust to the south (Parkash et al., 1980; Herail et al., 1986). Gravity measurements and detailed field mapping indi- cate that the cores of at least some of these structures contain pre-Siwalik rocks that are considered to be hydrocarbon explo- ration objectives (Friedenreich and Slind, 1989; Ebner, 1989).

The Siwalik Fold Belt and the Terai are a pat of the fore- land of the Himalayas (Parkash et al., 1980). They have some similarities with the foothills and western plains of Alberta and British Columbia and are the main areas of petroleum interest in Nepal.

5.3 Lesser Himal

The Lesser Himal is a wide, stratigraphically and strut- turally complex zone that lies immediately north of the Siwalik Fold Belt and is separated from it by the south-verging main boundaty fault (MBT). The majority of the Lesser Himal is composed of thrust sheets and nappes of metasediments and igneous rock of the Midland Super Group. The Group is of little hydrocarbon exploration interest, although the oil and gas seeps of the Dailekh area occur within the Midland Group (CPIT, !973). These seeps are interpreted to have been gener- ated in sediments below the nappes (Figure 2).

5.4 Higher Himal

The Higher Himal, which contains the spectacular peaks of the Great Himalayan Range, Everest, Annapuma, etc., is

80” 82” 84” 86”

200 km I I

S-N: Approx. Location of Schematic Cross-Section Fig. 2

!6’ 0 : Biralnagar - 1 Well

82’ 84” 86’ 88” I I I I

Fig. 1. Geological map of Nepal.

CIEC 104 December IV94

PETROLEUM CEOLOCY “t- PvEPAL

thrust southward over the Lesser Himal by the main central thrust (MCT; Figures I, 2). The zone is composed of a basal slab of metamorphic Proterozoic rocks overlain by a con- formable sequence of Cambrian to Eocene Tethyan sedi- ments (Bordet et al., 1981). Gas seeps occur in the upper Tethyan of northern central Nepal near the village of Muktinath (Figures 1.2).

6.0 STRATICRAPHY

The following discussion concentrates only on those rock units that have a direct bearing on hydrocarbon exploration (Figure 3).

Vindhyun Super Group ~ Vindhyan sediments, equiva- lent to those of the great Vindhyan Basin of Northern India, are interpreted to extend beneath the Ta% and Siwalik Fold Belt. The Vindhyan is reponed to have a maximum thickness of 5250 m (Agrawal, 1977; Srivastava et al., 1983) although at least 12 000 m of pre-Tertiary seismic reflections have been observed under the Terai. The Vindhyen, which is con- sidered to be a major hydrocarbon exploration objective, consists of an unmetamorphosed sequence of stromatolitic limestone, shale and sandstone. Although dating is difficult, the Vindhyan is considered to be equivalent to at least a part of the Lakharpata Group of the Lesser Himal.

Midland Super Group - The Midland Super Group, which encompasses most of the rocks of the Lesser Himal, is

not adequately dated, but is intcrprcted to be older than Carboniferous and to range well into the Precambrian (Frank and Fuchs, 1970; StGcklin, 1980).

Duilekh Group ~ The Dailekh Group, the lower part of the Midland Super Group, is made up of phyllites, garnetifer- ous schists, feldspathic greywacke, volcanics and stroma- tolitic dolomites (Figures 2, 7). These rocks have been raised to the low green schist facies in the centre of the Midlands and the metamorphic grade increases in successive thrust sheets to the north (Mitchell, 1979; Bordet et al., 1981). The rocks of the Dailekh Group are not considered to bc cxplo- ration objectives.

Lakharpata Group ~ The Lakharpata Group occupies the upper part of the Midland Super Group and, although not firmly dated, is interpreted to be of Late Precambrian to Late Paleozoic ages (Stiicklin, 1980: Shrestha and Shrestha, 1983). It is unconformably overlain by frequently organic dark shales and sandstones of the Gondwana and Surkhet Groups. This Group and its Vindhyan equivalent are an important exploration objective.

The Group contains the following formations and mem- bers:

Sangram Fm: Greenish grey to black splintery shale, con- taining zones of relatively rich organic intervals (up to 9% TOC); some shales being burned for fuel; major potential source rock:

tINDIA-+-NEPAL +.-----CHINA-

m ALLtJWUM SURKHET GP GRANITE

m SIWALIK FM LAKHARPATA GP OPHlOLlTE

m GONDWANA GP m b;$@&,C ROCKS m VOLCANICS

“INDHYAN GP TETHYAN SEDIMENTS * SEEPS

BASEMENT HIGH GRADE METAMORPHIC ROCKS

MFT MAN FRONTAL THRUST MET MAN BOUNDARY THRUST MCT MAIN CENTRAL THRUST MMT MA!N MANTLE THRUST

Fig. 2. Schematic cross-section through central Nepal

CIEG 105 Drcrmbr ,994

TERAI AND SIWALIK FOLD BELT

MAtNT;;;;y LESSER HIMAL

PLIOCENE ;$$gi.z; UPPER v

UPPER MtOCENE

CRETACEOUS UPPER

-GONDWANA

“INDHYAN

I SURKHET

LOWER

GROUP MIOCENE

(SW

-k

UPPER CRETACEOUS

LOWER GONDWANA CRETACEOUS

(GD)

-I-

UPPER PALEOZOIC

UPPER (7) PALEOZOIC

LAKHdRPATA TaOUP’

PALEOZOIC

w UNDIFFERENTIATED

- SOURCE ROCb . DIAMICTITE

UPPER

. PROTEROZOIC

Fig. 3. Stratigraphic zonations of central-southern Nepal.

Ramkot Fm. Pink to grey sandstone and purple to grey shale 850 m thick;

Gawar Fm. Limestone and dolomite with abundant stro- matolites and algal mats and becomes more shaly and sandy upwards; reaches a maximum thickness of 1700 m and is considered an exploration objective;

Khara Fm. Dark grey, microcrystalline limestone and shale;

Katwa Fm. Dark grey to black shales with minor stroma- tolitic dolomite; a potential source rock;

Am Fm. Dense dolomite with lesser amounts of sandstone and shale; restricted to south-central part of Nepal; a poten- tial subcrop reservoir.

Gondwana Group (Late Carboniferous to Early Cretaceous) ~ The Gondwana is a group of rocks occurring in basins on the lndian Craton (unconformably below the base of the Siwalik Group of the Terai) and in the southern thrust sheets of the Lesser Himal. The thickness of the Group in eastern and central Nepal, which is quite variable, is approximately 1000 m in the ridges immediately above the MBT in central Nepal and is interpreted from seismic analy- sis to be at least that thick in the basins of the Terai (Bashyal, 1980; Sakai, 1985; Datta et al., 1983). The Group is sepa- rated into two formations. The Sisne Formation uncon- formably overlies the L&harp& Group and is composed of diamictite and shale and contains late Paleozoic marine

fauna. The formation has a few intervals that contain up to 10% TOC and is considered an important potential source rock. The Taltung Formation disconformably overlies this Sinse and is comprised of conglomerate sandstone shale.

Gondwana sediments are considered an exploration objec- tive in Nepal. They probably occur in small pockets beneath the base Siwaliks (Figures 8, 9) in the eastern Nepal Terai, possibly in the Gandak depression in the Birganj and Lumbini areas and could be caught up in smaller thrust slabs beneath the Siwalik Fold Belt and main boundary thrusts. Good reservoir rocks were not seen in outcrop hut several organically rich intervals have been mapped. The lower coal- hearing Gondwana could be both a source and reservoir for gas and the marine Upper Gondwana may be a potential oil source rock.

Surkhet Group (Late Cretaceous to Early Miocene) - The Surkhet Group occurs in the southern, central and western part of the Lesser Himal where it unconformably overlies Lakharpata or Gondwana rocks. The Group attains a maximum thickness of 1200 m and is separated into the Melpani, Swat and Suntar Formations (Sakai, 1983). The Group is correlated with the oil and gas producing formations of the Assam and Potwar basins (Rangarao, 1983) and with the “Unnamed Formation” of Northern India (Schroeder, 1981) sediments.

The Swat Formation is composed mainly of shale that contains intervals with greater than 2% TOC and some of the

CEG 106 DIsElnkr lYY4

F ig. 4. Migrated Line 59 in eastern Terai. showing two stages of thrusting (see Figure 8 for location)

Mrlpani organic shnlcs have 20%, TOC. In the Danp Valley, the porosity of a distinct Melpani sandslonc is filled with solid hydrocarbon Wind, IYY~J.

Siwdik GIW[I (Late Miocene to Pliocene) ~ The Siwalik Group is a thick section of fresh-water molasse. sxposcd in a ssties of fault slices that make up the Siwalik Fold Belt and lit beneath the alluvial deposits of the Tcrai (Delcaillau, 19x6). The Group is up to 4500 m thick and is scpxated into three f& mntions: Lower Siwalik (LS), Middle Siwalik (MS1 and MS2) and Upper Siwalik (US). The lower part of the Group i\ con- posed of fine-grained sandstones containing shale intcl-vals (IS and MS I ) and becomes cot~rsfr @nrd upwards (MS2 and US). Petroleum ohjcctives are expected to he restricted Iu the lower part of the Group where thcrc is an effective combination of potential sandstone rcscr~oirs and shale se&s.

7.0 GEoPHYStCs

Geophysical exploration methods, rcflcction seismic. jiravitv and magnctics, combined with surface mapping and hasin analysis. have cstahlished the subsurlhcc fmmewo~-k uf

southern Nepal. Seismic control in eastern Nepal is quits dense (appnximatcly 2 km x 4 km). This xca is character-

ired by many bassment~contn,llcd structures offsettin@ srdi-

ment-filled suhbasins and erabcns beneath the Siwalik

molasse. The grid (IO km x IS km) of rcconnaissancc s&s-

mic lines covering the halance of the Terai has identified

se~cral gculogical settines which hnvc the potential for

hydnxarhon pnxpccts. Thcsc include structured traps rclatcd

to normal faulting involving prc-Siwalik formations and

thrusting involving the Siwaliks (Fi@w 4): subcrop traps

involving pre-Siwalik sequcnccs (Figure 51: and structural

trap\ associated with blind ti~lds in front of the Siwalik Fold

Belt (Figures 6. 7). The location\ of seismic lines show11 in

Fipures 4. 5 and 6 are shown on Figure 8.

The hasc Siwdik seismic marker (Figures 4, 5, 6) is an

important reflection in the Tel-ai region. It rcprcsents an

oncunlbunity that separates the Siwaliks (mid-Miwxnc and

younger) fri~nt older strata (L~wel- Miucenc to Precambrian).

Figum 8 and Y reprcxnt structural maps of this marker in

(‘1 i, i 107 ,kr,l,,rr, ,,,I,

N

“,R. FRIEDBNREICH. 0.L SI.,ND. I,.M.S. PKADHAN and R.B. SHKCSTHA

TERAI SIWALIK FOLD BELT ” LESSER HIMAL

SCALE: H = V 10km

Fig. 7. Structure cross-section along tine 31.

the eastern and western Terai, respectively. The unconfor- mity lies between 3500 and 4600 m below the Terai plain which is approximately 100 m above sea level (al.). There is no dominant east-west gradient to the base Siwalik marker; however, the dipping of the marker toward the Siwalik Fold Belt is very apparent.

Nine seismic and gravity traverses cross the Siwalik Fold Belt and identify major folds and thrust faults that may con- tain substantial thicknesses of potentially prospective pre- Siwalik rocks. Line 89 (Figures 9, IO) cr”sses a large Siwalik fold and demonstrates good reflections on the flanks of the structure but there is no coherent date in the core. Gravity measurements along this line show a substantial 25 mgal anomaly above regional f”r the traverse. Model studies show (using densities derived by methods described by Gardner et al., 1974) that the anomaly is caused by highier) density rocks being involved in the core of the structure. These model den- sities are comparable with those of the prospective pre- Siwalik (Lakharpata) dolomite (Figures I I, 12).

From the above, it is apparent that the gravity method may be very beneficial and cost-effective in c”ncert with seismic surveys in delineating hydrocarbon prospects in the southern put of Nepal.

8.0 GEocHEMtsTtw

A considerable amount of geochemical work has now been completed on rocks from Nepal. The oil from the Dailekh

seeps is interpreted to c”me from a conventional s”urce rock beneath the nappes and rocks with source potential have been sampled from the Lakhqaw, Gondwana and Surkhrt Groups. Thermal modelling suggests that potential source rocks in the prospective part of Nepal are in the marginally mature-f”l--oil t” late-in-the-gas generation window.

9.0 R~sliwom

Studies of outcrop samples and analysts of Shell Biratnagar I indicate that effective reservoirs and seals are likely to be found in the Lower Siwalik, Surkhet and Gondwana sandstones and shales. Outcrop studies have not identified large areas of carbonate reservoir development in the Lakharpata/Vindhyan Group, although it is expected that the unconformity at the base of the Siwalik molarsc will pro- vide a large number of attractive carbonate as well as cl&c exploration objectives.

10.0 DRILIJNG

Nepal is virtually unexplored. The country’s only well was drilled on a seismically defined, basement-controlled anomaly in the far eastern part of the country near Biratnagar. The well was abandoned at 3530 m after pene- trating 3143 m of typical Siwalik molnsse and 387 m of arkose and shale that was interpreted to be of Eocene or younger age. No hydrocarbons were reawered, although

C)H(; 110 Derrnlh.,~ ,w*

PCTROLF,UM clxxocY or NEPA,

a LESSER HIMAL SEaJENCES 1

m SWAM FOLD BELT ii 28N m SiWALlK MOLASSE

Con,o”is in m bsi

!7N !7N

0 0 50 km 50 km

Line 59: See Figure 4 Line 52: See Figure 5 Line 31: See Figures 6 & 7

83E 84E 85E 86E 87E

Fig. 8. Eastern Terai. structure map 01 Base Siwalik unconformity and location of Lines 31, 52 and 59,

small amounts of background (mud) gas were encountered in the lower part of the Middle Siwalik.

11.0 OPPOHTUNITIW

Exploration opportunities in the Terai consist of structures associutcd with blind thrusts, bascmcnt-controlled structures, stratigraphic pinch-outs and subcrop traps. Major folds and thrusts in the Siwalik Fold Belt are likely to provide strut- turally controlled traps containing Lakharpata to Siwalik reservoirs. Drilling depths to objectives are from 3500 to 5000 mctrcs in the Tcrai and from 2500 to 4000 metres in the Siwelik Fold Belt.

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(IK

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111 I>rwlilhrr i’l’,.l

81E 82E 83E 84E

m LESSER HIMAL SEQUENCES 1

29N.

2222x,, m SiWALlK FOLO BELT

j: SWAM MCILASSE

Contours I” ml t’s,

Lx 896s Figures 10, 11 & 12

8iE 82E 83E 84E

Fig. 9. Western Terai, structure map of Base Siwalik unconformity and location of Line 89 (Siwalik Fold Belt traverse)

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oppurtuniries: Bmchurc. ~, 1990. Exploration opportunih in Nepal: Bmchure. ~( 19w. The King&,“7 <>l Nepal. expl”rati”” opponunities. Brochure. Ganrser. .!.. 1%9. Geology o/the Himulayas: Wiley Inlcrscicncc. Lundon.

New Ywk. Karunilkaran. C. ;and Rm A.R.. IWh. Slatu, UI cxpluraiion hi hydmcarbon

in lhc Himalayx region contributions lo rlratigraphy imd slru~tw~: Himalayan Gculugy Seminar. Sec. 111, New Delhi. Mis Puhl. Gcol. Sure. Ilidiil, IW’). 41 (5) Mh.

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Shxma. KC.. 1990. Ccohfy of Ncpnl Hmolsya and adjaccni cuwmics: Sangeel;, Shwm;,. Hishnl Nqa’. Ka,h,llan<i”. Nepal. Jan. ,690.

c,,:ci 112 “rcc>l>hcr Wil

O.K. FKIEIxaKEICII. O,L, SLlNLl LI.M.S. WADHAN :ind K.B. SHKESTHA

SW NE LVMBlNl PLAiN SOMESHWAR RAPTI “ALLEY MAHABHARAT

RANGE (ES,) LEKH

LEGEND

A THRUST FAULTS

G G SEISMIC MARKERS X x SEiSMlC REFLECT

PLEiSTOCENE RECENT

MIOCENE PLIOCENE su “PER SIWN.lK SM MlDDLE SIWRUK SL LOWER SIWMK

CRETACtOUS MlOCENE SURKHET GROUP SN SUNTAR SW SWAT

ChRBONlFEROUS TO

LOWER CAETACEO”S

GD GONDWANA GP

PRECAMBRIAN 70 PAEOLOIC

“IN “INOHYAN GP

LK LAKHARPATA GP

MB1 MAIN BOUNDARY THRUST

MFT, MAIN FHON~reL lHHUST

SCALE: H =” 0 5 k”,

Fig. 11. Structure cross-section along Line 89.

OBSERVED GRAVITY c 720 CALCULATED GRAVITY 3 g .- E 710 x > 9 a 700

690 0 5 10 15 20 25

km

0

F r 2 f $ 4 0

6

0 5 10 15

km

20 25 30 SCALE H=V

Fig. 12. Model study for Line 89 (Siwalik Fold Belt traverse).

(lMi 114 “UCC,l’k,~ ,‘,‘W