Naga Thrust

AAPG Bulletin, v. 86, no. 12 (December 2002), pp. 2023–2045 2023

Application of a ramp/flat-fault model to interpretation ofthe Naga thrust and possibleimplications for petroleumexploration along the Nagathrust frontW. Norman Kent, Robert G. Hickman,and Udayan Dasgupta

ABSTRACT

The Assam-Arakan thrust belt extends along the India-Myanmarborder, from the Chinese border on the north to the Bay of Bengalon the south. Tertiary nonmarine sediments dominate the stratig-raphy within the frontal zone of this fold and thrust belt. Thrust-fault flats occur in the upper Barail Group coaly interval, and rampsare localized by preexistent normal faults and stratigraphic discon-tinuities. As a result, the frontal zone of the thrust belt is charac-terized by multiscale imbricate structures. The Jaipur anticline oc-curs at the foreland edge of the Naga thrust imbricate zone, at thenortheastern end of the Assam valley. Application of flat/ramp geo-metric models, with a limited data set from the Jaipur anticline,allows creation of geometrically viable models for interpreting thegeneral structure of the Jaipur anticline and for developing hydro-carbon exploration leads. The results of this method indicate that(1) the proven productive foreland trend extends several kilometersbeneath the Naga thrust, and (2) zones of high dip in the thrustbelt mark the location of prospect leads probably related to localthrust imbrication. The method provides a testable process of iden-tifying prospective areas; therefore, it can minimize exploration ex-pense and optimize exploration planning.

INTRODUCTION

The Jaipur anticline is located on the southeast side of the upperAssam valley at the leading edge of the Naga thrust (Figure 1). Thisforelandmost surface expression of the Assam-Arakan fold and

Copyright �2002. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received March 21, 2001; revised manuscript received May 28, 2002; final acceptance June24, 2002.

AUTHORS

W. Norman Kent � Kent GeoScienceAssociates, P.O. Box 1082, Richmond, Texas,77406; wnkent@kent_geo.com

W. Norman Kent received a B.S. degree ingeology from the University of Arizona (1970)and an M.S. degree in geology from NorthernArizona University (1975). He spent 25 yearswith Unocal Corporation as an explorationgeologist (1975–2000). His backgroundincludes field mapping, well site geology,lease sale acreage evaluation, prospectgeneration, integrated regional studies, andproject management. He participated in andled exploration projects in North America,North Africa, the Middle East, India, andChina. He now operates a consulting firmspecializing in problems related to explorationin areas of limited data and complexstructure, with active projects in India, Alaska,and eastern North America.

Robert G. Hickman � Structural Solutions,1330 Sugar Creek Boulevard, Sugar Land,Texas, 77478; [email protected]

Robert Hickman received a B.S. degree ingeology from Stanford University and an M.S.degree and Ph.D. in geology from theUniversity of Wisconsin. He spent 25 yearswith Unocal Corporation as a structuralgeologist and coordinator of structuralgeology. His areas of expertise includeanalysis of compressional, extensional, andsalt regimes and integrated regional studies.He is currently a consultant specializing instructural analysis.

Udayan Dasgupta � Hindustan OilExploration Co., LTD, HOEC House, TandaljaRoad, Vadodara, India 390 020;[email protected]

Udayan Dasgupta is currently generalmanager for Hindustan Oil ExplorationCompany Limited (HOEC) in Baroda, India. Hereceived an M.Sc. degree in geology fromCalcutta University, India, in 1969.Subsequently, he migrated to Canada andobtained a Ph.D. from the University ofToronto in 1978 for his research on fracturingin Mississippian carbonate reservoirs ofAlberta. During this period, he also workedfor Cominco Limited in exploring for

2024 Application of a Ramp/Flat-Fault Model to the Naga Thrust (India)

thrust belt is within an area covered by a production-sharing con-tract for Block AAP-ON-94/1, currently held by Hindustan OilExploration Company of India. In the middle of 1998, Unocal Cor-poration and Hindustan Oil Exploration Company undertook ajoint assessment of the exploration potential of this acreage block.Block AAP-ON-94/1 enclosed 870 km2, and, although several sub-commercial discoveries were made in the block during the late1800s, there has been little modern exploration in the area.Modernsubsurface data in the block include one well with geophysical logsand 350 km of seismic data acquired in 1988.

The Jaipur anticline became the main focus of the investigationbecause Digboi oil field is located on Digboi anticline, which is thenortheastern extension of the regional Jaipur anticline. Addition-ally, several oil and gas fields are present along the northeasternflank of the Jaipur anticline, in the footwall of the Naga thrust.

The primary purpose of studying the Jaipur anticline was toevaluate the hydrocarbon exploration potential for the southernpart of the anticline. An important secondary goal was to providecompany management with a convincing indication that the area’spotential could be evaluated reliably with existing data or with newdata obtained within a reasonable time period.

Early in the joint Hindustan Oil Exploration Company/Unocalassessment of Block AAP-ON-94/1, attention focused on a crosssection by Oil India Limited through Digboi field (Figure 2A) thatis based on several of the Digboi field wells. Although the crosssection illustrates only a part of the structure, it raises several ques-tions about the structure’s geometry and the area’s exploration po-tential. These questions gain additional importance because a simi-lar cross section (Figure 2B), covering more of the structure(Mathur and Evans, 1964), had been discussed in several subse-quent publications (Berger et al., 1983; Mitra, 1990; Suppe andMedwedeff, 1990). Mathur and Evans’s cross section shows theDigboi anticline to be a fault-propagation fold in which the thrusthas apparently broken through the fold following the synclinal axialplane. This interpretation implies that, although there might besubthrust potential for the foreland trend, little potential exists inthe hanging-wall features, because the anticline is breached to thecore along most of its length south of the Digboi field.

Data from along the Naga thrust trend were proposed as coun-terpoints to this negative interpretation. In 1984, the Oil and Nat-ural Gas Commission, India, conducted two seismic surveys in thefrontal part of the thrust belt. The poor quality of the data that thecommission acquired prohibited meaningful interpretation andprompted acquisition and processing of an experimental line to de-fine better seismic methods. Major thrusts and events above andbelow the thrusts are discernible on the experimental line (Duttaand Ray, 1993). The Changki-1 well was drilled through the secondmajor thrust sheet, approximately 11 km toward the hinterlandfrom the surface trace of the Naga thrust. The well and the exper-imental seismic data confirm the continuity of the foreland stratabeneath the thrust belt, for a substantial distance (Figure 1). Ranga

ACKNOWLEDGEMENTS

Hindustan Oil Exploration Company and Uno-cal Corporation conducted a significant part ofthe data collection and analysis that led tothis article during a joint investigation of BlockAAP-ON-94/1. We wish to thank both compa-nies for permission and aid in publication ofthis article. We also thank our colleagues atUnocal and Hindustan Oil Exploration Com-pany for their help in preparing this article.We gratefully acknowledge the support of OilIndia Limited, who allowed us to use themaps and data without which the researchand documentation of this article would havebeen impossible. Geo-Logic Systems, LLC pro-vided valuable aid in allowing us to useLithoTech structural modeling software fortesting the viability of our interpretations. Forthis generosity, we are particularly apprecia-tive. We extend special thanks to CharlesStewart, David Courtis, Steven Boyer, ShankarMitra, Robert Milici, and W. A. Bally for theirhelpful critiques of this article.

carbonate-hosted base metals. He returned toIndia and joined Oil India Limited (OIL),where he worked in various capacities fromexploration and development to strategicplanning. During his early years with OIL, heworked in northeastern India, including thearea covered by this article. He joined HOECin 1994.

Kent et al. 2025

Figure 1. Location and major structural elements of the Assam valley (after Berger et al., 1983; used with permission of SchlumbergerTechnical Services). Rectangle in the main figure indicates the location of the Jaipur anticline.

Rao and Samanta (1987) published an interpretationof the structural style of the Naga thrust belt that theybased, in part, on the seismic data just described.

However, management and peers reviewing theinitial work discounted the applicability of the resultsbecause, the reviewers suggested, the two areas thatare separated by approximately 125 km could differ instructural style. To support this contention it wasnoted that, except for the Borholla-Champang field,which occurs immediately in front of and under theleading edge of the Naga thrust, all other fields provento extend beneath the Naga thrust are located wherethe mountain front is oriented east-west at nearly 90�to its normal northeast-southwest trend (Figure 1)(Krishna Rao and Parasad, 1982). The initial data setavailable to evaluate Block AAP-ON-94/1 was limitedbut was not atypical of data sets provided by govern-ments to evaluate areas covered by production-sharingcontracts. Data provided included a description of thearea, general geologic maps and cross sections, a seis-mic data set composed of every other line in the grid

within the block, and base map, well logs, and eleva-tions for stratigraphic markers for the Tarajan No. 1,Kusijan No. 1, and Namrup No. 1 wells. The geologicmaps were constructed by Oil India Limited at a scaleof 1 in. � 1 mi. The seismic grid was 24-fold 2-D data.Most of the lines were located immediately southeastof the thrust front, and only one dip line (D-58)crossed the Jaipur anticline (Figure 3). All of the seis-mic profiles except line D-58 terminated at the blockboundary. Two composite regional seismic lines pro-vided the only seismic data west of the Naga thrust.SPOT and Landsat images of the Jaipur anticline areaalso were available.

Given the initial inferences drawn from the twocross sections by Corps (1949) and Mathur and Evans(1964) and also given the limited data set, it mightseem reasonable to conclude that, even with the sub-stantial expenditure of money to obtain additionaldata, a reliable evaluation of the area could not beobtained. The intent of this article is to (1) describethe structural geology of Jaipur/Digboi anticline; (2)


Figure 2. (A) Cross section and map displayed at OIL facilities at Digboi. (Published with the consent of Oil India Limited.) Thecross section illustrates the structure of Digboi field and shows steep dip on the Naga thrust (�60�) (See Figure 3 for location). Thecross section is at true scale (1:1). (B) Cross section through Digboi field, from Mathur and Evans (1964). The cross section showstwo dip segments of the Naga thrust and illustrates that the Tipam Formation extends several kilometers beneath the Naga thrust(see Figure 3 for location).

illustrate procedures for deducing the quantitative ge-ometry of thrust-belt structures from a diverse butsparse data set; (3) indicate the relevance of the struc-tural development of the Jaipur anticline to hydrocar-bon exploration within the anticline and associatedforeland structures; and (4) illustrate that although theJaipur anticline and associated structures are complexstructural features, most of the data and analysis forscoping exploration can be accomplished with a rela-tively small exploration budget.

TECTONIC SETT ING

The greater Assam region consists of four tectonicprovinces: (1) the eastern Himalayan fold and thrustbelt, (2) the Mishmi Hills uplift, (3) the Assam-Ara-kan thrust belt, and (4) the upper Assam foreland ba-sin, which includes the Brahmaputra arch (Figure 1).The eastern Himalayan foothills belt consists of south-eastward-verging thrust sheets of Precambrian crys-

talline and metasedimentary rocks that are capped lo-cally by an Upper Carboniferous through Triassicsequence of sedimentary strata (Bhandari et al.,1973). The Himalayan foothills belt formed in the lateTertiary in response to the collision of the Indian platewith Eurasia, and extends for hundreds of kilometersto the west of the Assam region. Thrusts boundingthe eastern Himalayan foothills probably have beenactive since the Oligocene. The Mishmi Hills, at thenortheastern head of the Assam valley, consist of anupthrust block of Archean gneisses and granites.

The Assam-Arakan thrust belt bounds the Assamvalley on the southeast and extends southwestwardacross northern and western Myanmar (Burma). Thethrust belt developed as a result of differential move-ment between the Burma microplate and the Indianplate. Prior to collision of the Burma microplate, whatis now the western part of the thrust belt was theeastward-facing margin of the Indian plate. The north-western Assam part of the thrust belt is predomi-nantly thin skinned and mainly involves Tertiary sed-

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Figure 3. Index map showing geography of the Assam region and locations of subsurface data.

imentary strata. This part of the belt is 10 to 40 kmwide and extends 425 km from the Dauki fault southof the Shilong plateau to the Mishmi Hills uplift inthe north. This zone is strongly imbricated and con-sists of six to eight major overlapping thrust sheets.The northwestward margin of this belt is convention-ally called the Naga thrust, although it does not rep-resent a single continuous fault (Mathur and Evans,1964; Berger et al., 1983). Activity on the frontalfaults of this thrust belt probably started during themiddle Miocene.

The Assam valley extends through the upper As-sam basin, which is a composite foreland basin that islocated between the eastern Himalayan foothills andAssam-Arakan thrust belt. The basin is terminated tothe northeast by the Mishmi Hills block, and to thesouthwest it is partly disrupted by the Shilong plateaubasement uplift. In the subsurface, a ridge of Precam-brian rocks, here referred to as the Brahmaputra arch,roughly bisects the Assam basin. This arch plungesnortheastward, from outcrops in the Mikir Hills, tothe northeastern part of the basin, where the plunge

of the arch becomes northerly. On the northern flankof the arch, upper Paleozoic strata are interpreted tooverlie Precambrian rocks and to be truncated be-neath Paleogene strata. Elsewhere, Paleocene and Eo-cene units thin and lap out against the arch; overlyingOligocene and Miocene units thin over it. Sedimentson the south flank of the arch are cut by normal faultsthat roughly parallel and dip away from the axis ofthe arch (Bhandari et al., 1973; Das Gupta and Bis-was, 2000).

TERT IARY STRATIGRAPHY OF THE ASSAMBASIN AND THE NAGA HILLS

The upper Assam basin and Naga Hills, which makeup the frontal part of the northern Assam-Arakanthrust belt, share a common, but subtly different,Tertiary stratigraphy (Figure 4). The Tertiary se-quence is divided into Paleogene and Neogene se-quences that are separated by a major Oligocene un-conformity (Raju and Mathur, 1995). In the Assam


Figure 4. Generalized strati-graphic column and petroleumsystem information for the up-per Assam basin (modifiedfrom Raju and Mathur, 1995;reprinted with permission fromElsevier Science).

basin, Paleogene strata are divided into three majorunits. The Paleocene Langpar Formation is composedof marine quartzitic sandstone and minor claystone.The overlying Jaintia Group consists of the lower Eo-

cene Sylhet Formation, which is composed of siliceousshale with limestone and sandstone interbeds, and themiddle to upper Eocene Kopili Formation, which con-sists of shale and fine-grained sandstone. All of these

Kent et al. 2029

units were deposited in a shelfal area along the south-east-facing continental margin that existed prior to thecollision of the Burma microplate with India. Basin-ward, within what is now the interior of the Assam-Arakan thrust belt, deep-water strata that are partlyage-equivalent to these latter units are referred to asthe Disang Group.

The third major Paleogene unit is the upper Eo-cene to lower Oligocene Barail Group, which is com-posed of sandstones of the Tinali Formation and over-lying mudstones and coals of the Moran Formationthat were deposited in a marginal-marine to fluvialsetting along the southeast-facing continental margin.The close of Barail deposition was followed by basinuplift and erosion that probably was related to earlystages of Himalayan uplift, but that also coincideswith the middle Oligocene global-eustatic lowstand(Vail et al., 1977).

Unconformably overlying the Barail Group is theNahorkatiya Group, which comprises the lower Ti-pam Formation and the upper Girujan Formation.The upper Miocene Tipam Formation sandstone islargely of fluvial origin, and the heavy mineral contentof the unit (Bhandari et al., 1973) indicates derivationfrom the rising Himalayas. Depositional transport ofthe Tipam Formation sand was toward the south,which in the early Miocene was not blocked by theAssam-Arakan thrust belt and the Shilong plateau up-lift. The overlying lower and middle Miocene GirujanFormation consists of lacustrine shales and fluvial de-posits. It may reflect changes in drainage patterns re-lated to initial development of the thrust belt to thesouth, or lacustrine flooding related to foreland basinsubsidence. This unit is thickest in the eastern part ofthe Assam valley and thins and interfingers with flu-vial sandstone to the west. The Girujan Formation isunconformably overlain by the upper Miocene Nam-sang Formation, which consists of poorly consolidatedfluvial sandstone with interbedded clay and lignite.Both the Girujan and the Namsang formations aretruncated by a regional unconformity that cuts down-section to the west. West of the Jaipur anticline, thePliocene Dhekiajuli Formation unconformably over-lies the Girujan and Namsang formations. The Dhek-iajuli Formation consists of unconsolidated fluvial andalluvial fan deposits and appears to grade verticallyinto the overlying alluvium. East of the Jaipur anti-cline, the Dhekiajuli Formation correlates with thebasal part of the Dihing Formation, which also ap-pears to grade vertically into the overlying alluvium(Das Gupta and Biswas, 2000).

PETROLEUM SYSTEMS

There are two petroleum systems in the upper Assambasin (Figure 5). As a result of tectonism, hydrocarbonaccumulations related to each system occur along twodistinct trends. The oldest petroleum system is thePaleocene–Eocene system. Source rocks of this systemare organic-rich carbonaceous shales, coals, and a fewthin carbonate units of the upper Paleocene–lowerEocene Sylhet Formation and the upper PaleoceneLangpar Formation (Handique and Bharali, 1981).The producing clastic units of the lower Eocene andupper Paleocene Lakadong and Langpar formationsare interbedded with the source rock intervals. Thecarbonate units reflect marine incursions. Organicmatter is predominantly type II, with subordinatetype I macerals. Although conclusive evidence ofmatching source rock to oil has not been developed,proximity indicates a link between oil in the Eocene–Paleocene reservoirs and the interbedded shale units.

The second petroleum system is the Oligocene–Miocene petroleum system. In this system, the sourcerocks and reservoir units also are intercalated. Sourcerock intervals occur within the Oligocene BarailGroup. Biomarker ratios in carbonaceous shale unitsof the Moran Formation of the Barail Group areclosely correlated with the moderate-gravity, waxy oilproduced from the Barail and Tipam reservoirs (Rajuand Mathur, 1995). The Barail source rocks were de-posited in fresh to brackish water in turbid conditions,as indicated by the presence of interbedded coals,spherulitic siderite concretions, and abundant arena-ceous foraminifera (Saikia and Dutta, 1980). The res-ervoir units of this petroleum system are the Oligo-cene fluvial sandstone units of the Barail Group andthe fluvial-lacustrine Miocene Tipam Sandstone andGirujan Clay.

The fluvial-paralic character of reservoir units inboth petroleum systems results in many local-intra-formation seals. The Miocene Girujan Clay providesa regional top seal. Burial and loading by the devel-oping Assam-Arakan fold and thrust belt provided theoverburden necessary for generation of the hydrocar-bons within both systems. Although long-distance mi-gration is not required for either petroleum system,the volume of trapped hydrocarbons suggests that mi-gration over moderate distance probably occurred.Nahorkatiya, Jorajan, and associated satellite fields arealong the crest of a structural nose that plunges be-neath the Naga thrust and forms a focus for migration(Figure 6). The area of mature source rocks illustrated

Figure 5. Petroleum systemelements and associated depo-sitional and tectonicenvironments.

Figure 6. Hydrocarbonkitchen and migration pathwaysfor petroleum systems of theupper Assam basin. The map il-lustrates a dynamic systemrather than an event; thus, theboundary of the active sourceareas migrated northwestwardin time. The base map for thisfigure is modified from Rath etal. (1994).

Kent et al. 2031

in Figure 6 is diagrammatic. The actual western limitis controlled both by depositional and tectonic load-ing, and it migrated westward over time. The easternlimit of the source area is a function of both wherethe source rocks become supermature and where theBarail thrust carries them.

PRODUCING TRENDS

Assam Shelf Producing Trend

Along the Brahmaputra arch, traps for both petro-leum systems form a single producing trend. Normalfaults associated with early Tertiary extension formthe major trap type. The similarity of trap type andthe superposition of the two petroleum systems resultin two parallel subtrends. The two trends are parallelwith the axis of the arch, with the older petroleumsystem forming a producing subtrend that is closer tothe crest of the arch. However, several fields havestacked pay and produce from reservoirs of both pe-troleum systems. Additionally, some of the Paleogenefields are formed by stratigraphic pinch-outs onto thebasement.

Exploration in the Assam shelf trend began withseismic surveys in the 1950s and resulted in the dis-covery of several large fields, including Nahorkatiyafield (Carmalt and St. John, 1984), which has an es-timated ultimate recovery of 707 MMBOE. The As-sam shelf trend produces from sandstone reservoirs ofPaleocene to Miocene age. Initially, the main reser-voirs discovered were the Oligocene fluvial sandstoneunits of the Barail Group, but production was estab-lished soon after in the Miocene Tipam Sandstoneand, recently, in Paleocene and lower Eocene sand-stone units. Currently, nearly half of Oil India Lim-ited’s production in Assam comes from these olderunits (Rath and Devi, 1999).

Naga Foothills Trend

The Naga foothills trend produces from compressionalfolds and fault traps along the leading edge of the As-sam-Arakan fold and thrust belt. In the Naga foothillstrend, the primary reservoirs are the lower units ofthe Tipam Formation, with secondary reservoirs in theGirujan and Namsang formations. The high wax con-tent of the oil from Digboi field suggests an upperBarail source and supports Corps’s (1949) suggestion

that the oil in the Naga foothills may come from ear-lier accumulations in the Barail or Tipam that weredisrupted by the compressional tectonic events. How-ever, two distinct oil types are produced from inter-bedded reservoirs at Kharsang field (Figure 3). One ofthe two hydrocarbon types is high-wax (8–10% wax)34� API oil, whereas the second is low-wax (0.5%wax) 17� API oil. The proximity of two apparentlydifferent oil types suggests that tectonic activity hasmixed oils from two petroleum systems, possibly thesame two systems seen in the Assam Shelf producingtrend.

Exploration in Assam began in the Naga foothillstrend with the drilling of wells near oil seeps in theMakum-Namdang-Ledo mining area in the 1860s(Figure 3). Although the first commercial oil discoveryin Assam was made in this trend at Digboi in 1889(Visvanath, 1997), the trend is relatively immature.Fewer than a half dozen significant exploration wellshave been drilled in this trend, and only three fieldscurrently produce from it.

STRUCTURE OF THE JAIPUR ANTICL INE

The Jaipur anticline is an asymmetrical, arcuate, dou-bly plunging fold bounded by the surface trace of theNaga thrust along its northwest side (Figure 7). Bedsalong the narrow northwest limb dip from 50 to 70�

into the Naga thrust. The outcrop width of the south-east flank of the fold is more than 10 times the widthof the opposite flank. Dip on the beds is steep at thecrest (70 to 50�) and decreases toward the southeast-ern flank to as low as 10�.

At the location where the Assam Railway crossesthe Jaipur anticline, the structure is offset approxi-mately 1.5 km by a right-lateral fault, the Assam Rail-road tear fault. Northeast of the railway, in the Digboiarea, the crest of the anticline is eroded to just belowthe top of the Tipam Sandstone. The fold appears lessasymmetrical, and both limbs are well developed.Southeast of the railway, in the Kusijan area, twosmall anticlinal closures occur at the crest of the majorfold and are separated by a small syncline cored byGirujan Clay. South of the Kusijan area, the structureis eroded through the Tipam Sandstone and is ex-pressed as only a monoclinal dip that decreases from60� at the Naga thrust, to 20� to the southeast wherethe beds are covered by alluvium (refer to Figure 7for locations).


STRUCTURAL INTERPRETATION

Methods

Building on concepts first developed by J. L. Rich(1934), Suppe (1983) published a landmark article inwhich he outlined geometric and kinematic propertiesof folds developed above thrusts with ramp-flat ge-ometry. After publication of this article, it was dem-onstrated that, in the brittle frontal region of fold andthrust belts where no thick ductile units occur in thestratigraphic column, most folds can be modeled aseither fault-bend folds or fault-propagation folds. Ele-ments in both of these classes of folds have beenshown to have well-defined geometric relationships tothe underlying fault (Suppe, 1983, 1985; Mitra,1990).

The most important geometric relationship forthese structures is the relationship between fault shapeand fold shape. An observation in Suppe’s 1983 article

that is significant for interpretation of the Jaipur anti-cline structure is that most fault-bend folds involvemultiple imbrications. As a consequence, each succes-sive imbrication produces a quantum increase in theforward and back dip in overlying imbricates. The dippanels in structurally higher thrust sheets have a dipspectrum (0, I, II, etc.) related to the number of faultramps underlying the structural panel. If the funda-mental (undeformed) cutoff angle can be determined,then the geometry of deeper imbricates can be pre-dicted from the pattern of shallow-dip domains. Con-versely, the dip spectra are a function of the numberof fault ramps at a given location and of the funda-mental cutoff angle.

Given the preceding assumptions, Mathur andEvans’s (Figure 2B) cross section provides a reasonablerepresentation for the general geometry of the Jaipuranticline (see Figure 3 for location). Surface geologicdata, together with well data from the Kusijan and Ta-rajan areas, provide insights into geometric changes

Figure 7. Generalized geo-logic map of the Jaipur anti-cline, with location of oil andgas fields (Oil India Limited,1952; reproduced with the con-sent of OIL). The location of theKusijan structural model (Figure9) is noted. The rectangle en-closes the area in which down-plunge viewing was used to de-termine the anticipatedgeometry of the structural mod-els. The dashed line within thisarea illustrates the down-plungeprojection of two dip segmentsof the Naga thrust, as seen inFigure 2B. The change in dip ofthe Naga thrust occurs near thepoint at which the fault inter-sects the anticlinal axial plane.It is equivalent to the point atwhich the Naga thrust breaksthrough the fault-propagationfold, from the initial thrustramp to the trend of thesynclinal axis.

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Figure 8. Dip-domain mapgenerated by contouring sur-face dips taken from 1:63,360scale geologic maps made byOil India Limited personnel.Heavy dashed line shows the–I/0 dip-domain boundary. Thisis the surface projection of thepoint where the Naga thrustchanges from flat to ramp ge-ometry. Areas of high dip at theleading edge of the Nagathrust, indicated by the arrowslabeled 1, are interpreted to beplaces where one or moresmall-scale imbricates occurnear the front of the thrust.Larger areas of high dip, indi-cated by the arrows labeled 2,are interpreted to representconcealed medium-scale imbri-cates of the Naga thrust.

along the trend of the Jaipur anticline and Naga thrustand suggest potentially prospective areas.

Naga Thrust

Kusijan Model (Surface and Well Data)For folds that develop above faults, and where foldedstrata have relatively uniform thickness, the cutoff an-gle can be estimated from the dip of the beds withinthe back-dip panel, because these beds parallel the un-derlying fault ramp (Tearpock and Bischke, 1991). Thelowest dip observed on the back-dip panel of the Jaipuranticline is 20�, suggesting a similar value for the cutoffangle. However, in the case of the Jaipur anticline, thedips in the back-dip panel are not constant and mayindicate a concealed imbricate. A contour map wasconstructed on dip measurements extracted from thegeologic map. The purpose of this map was to visualizethe rate of change in dip and the areas of anomalousdip and to determine where the Naga thrust steps from

its decollement to its ramp (the boundary between the�I/0 dip-domain) (Figure 8). The stratigraphic posi-tion of the decollement was estimated by examiningthe geologic map in areas where the fault-propagationfold has been removed or nearly removed by erosion.In such an area south of the Burai Dihing River, coalseams (Figure 3) in the Moran Formation of the BarailGroup are the oldest known strata in the hanging wall.This relationship suggests that the decollement iswithin this unit. Well data from the Kusijan No. 1 wellprovide the remaining data necessary tomake a prelim-inary structural model. This well penetrated the Nagathrust plane at �1233 m and the base of the Morancoal unit in the footwall at approximately �3800 m.Accounting for regional dip can further refine thestructural model. The regional dip at the top of theBarail Group was estimated to be 3.5� (Mallik andRaju, 1995).

The location and estimated magnitude of the foot-wall cutoff can be determined by making a cross


Figure 9. Kusijan structural model constructed using kink-fold geometry (Suppe, 1983) and data from Kusijan No. 1 well and thesurface geology map. The model depicts the large-scale geometry of the Jaipur anticline and the Naga thrust, but does not resolvethe smaller scale features.

section that connects the surface trace of the fault, thefault cut in the Kusijan No. 1 well, and the subsurfaceprojection of the �I/0 dip-domain boundary (Figure9; see Figure 7 for location). Using the footwall cutoffangle and the internal angular relationships describedby Suppe (1985), in conjunction with down-plungeviewing, a representation of the near-surface structurecan be constructed. However, no attempt was made inthis study to model the near-surface small-scale imbri-cate structures.

Seismic InterpretationAs noted in a preceding section, the data set includeda grid of seismic data (Figure 3). Line D-58, the onlyline in the data set that crosses the Jaipur anticline (Fig-ure 10), is coincident with the previously describedcross section (Figure 9). Although this line clearlyshows the location of the –I/0 dip-domain boundary(Figure 10B), the location and geometry of the Nagathrust are less obvious.

The initial interpretation followed the cross sec-tion model and interpreted the Naga thrust to be anearly planar surface connecting the surface trace ofthe fault with the base of the ramp (�I/0 domainboundary). In this interpretation, the strong reflectivezone east of the �I/0 dip-domain boundary at ap-proximately 2.5 s is interpreted to be a reflector asso-ciated with the upper Barail coals (Figure 10A). Themap-view thickness of the Barail outcrop is less than600mwhere it lies parallel to the fault and is thickenedby imbrication. If the Barail units above the fault arebetween 300 and 500m thick, and the interval velocityof the upper Barail is between 3000 and 3200 m/s(Prasad andMani, 1983), the Naga thrust should occurbetween 0.100 and 0.156 s below the top of the Barail.At 0.120 s below the near-top Barail reflector in theseismic data, there is a continuous reflection thatmarksthe termination of other reflectors and that suggests apossible position for the fault. This horizon wasmapped as the Naga thrust.

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To further investigate the fault geometry, thetime-structure map of the Naga thrust was combinedwith a top Tipam Formation time-structure map fromthe information docket (Ministry of Petroleum andNatural Gas, 1994) and overlain on a regional mapshowing fault trends (after Mallick and Raju, 1995)(Figure 11). This map confirms the location of the flat-to-ramp transition, as determined from the surfacegeologic map, but does not explain the width of theGirujan Formation outcrop or the steep dips that havebeen mapped near the surface projection of the �I/0dip-domain boundary (Figure 10D).

Seismic line D-58 demonstrates problems com-mon to seismic data in fold and thrust belts. In areaswhere beds have high dips, seismic energy is reflectedaway from the geophones. Data are either not recordedor the data recovery is poor, as was the case in theregion between the surface trace of theNaga thrust andshotpoint 1245 on line D-58. Beneath thrust planes,seismic data are degraded by the velocity pull-upcaused by the superimposed higher-velocity materialof the thrust sheet and by refraction of seismic wavesaway from the surface of the dipping thrust. Ray trac-ing of the seismic waves shows that, of the two effects,the dispersal of energy by refraction is the most diffi-cult to deal with because it results in a data shadow-zone that is hard to image. The combination of highdips and high contrasts in velocity can produce seismicimages in the time domain that are quite different fromthe same data displayed in depth (Figure 10C). Giventhe quality of the seismic data, the inherent weaknessof seismic techniques noted in the preceding section,and concerns about unexplained structural elements,the initial interpretation was reexamined.

All available geologic data were posted on the pro-file before the data were interpreted. Posting of thesedata on the seismic line shows that beds dipping at 40�occur within 2 km of the surface projection of the baseof the ramp (�I/0 domain boundary). Erosion of theNamsang Formation before deposition of the DihingFormation removed beds with high dips even closer tothe (�I/0) domain boundary. Beds dipping at nearlytwice the value of the cutoff angle estimated in Figure10, in close proximity to the (�I/0) domain boundary,suggest that two fault imbricates may be present. Onepossible interpretation that honors the geology andseismic data and includes two faults is shown in Figure10C. This diagrammatic interpretation shows deposi-tional thickening of the Girujan Formation, but re-quires less thickening than would be required in a one-fault solution. In this interpretation, the higher than

expected dips are the result of imbrication caused bya fold-accommodation fault within the Girujan For-mation at the flank of the Jaipur anticline.

The seismic interpretation was converted fromtime domain to depth domain (Figure 12A) andrestored using LithoTech cross section balancing soft-ware (Figure 12B). The restoration implies that pre-Dhekiajuli/Dihing normal faults with down-to-the-east displacement have been omitted from the footwallinterpretation, as have the small-scale imbrications inthe Girujan Formation at the �I/0 domain boundary.These additional faults are necessary to explain the ad-ditional Girujan/Namsang thickness in the subthrust.The abrupt thickening of the Girujan Formation in thehanging wall may or may not actually occur. The thin-ning coincides with the �I/0 dip-domain boundaryand may be the result of post-Girujan erosion. Becausethere is no velocity control for the shallow units eastof the anticline, the thickening may also be an artifactof velocity or it may be due to structural featureswithin the upper 0.5 s of data mute of the seismicsection.

Summary of Large-Scale Structural Interpretation

The surface geology (Figure 7), combined with the twocross sections (Figures 2, 9) and the seismic section(Figure 10), indicates that the Jaipur anticline is a fault-propagation fold with nearly constant overall geometryalong its trend. However, these data also suggest thatvariation along the length of the structure is the resultof modification of the fault-propagation fold by sec-ondary structures.

Lateral projection of the line defining the flat-to-ramp boundary of the Naga thrust into the forelandillustrates that in many places the boundary is parallelwith the trend of foreland normal faults (Figure 11).This observation suggests that the preexisting exten-sional fault pattern influenced the location of the com-pressional structures. Such an interpretation, as illus-trated by Mathur and Evans in 1964 (Figure 13A),implies discontinuous eastward thickening of both theCenozoic and Mesozoic stratigraphic units across theextensional faults, rather than the continuous eastwardthickening of units shown in our models.

Cutoff-angle data also suggest that normal faultsinfluenced the initiation of ramp development. Themechanism for development of “large-scale tip linefolds,” as described by Saint Bezar et al. (1998) andillustrated in Figure 13B, is similar to that envisionedfor the Jaipur anticline. Dahlen et al. (1984) show that


in western Taiwan, reactivated normal faults producethe highest range (28–31�) of basal cutoff angles of theobserved fault types. The relatively high estimated cut-off angle for the Naga thrust may also be a result ofthis mechanism. Although the large-scale structure ofthe Jaipur anticline may be relatively straightforward,the modification of this structure by smaller scale fea-tures creates the points of economic interest for petro-leum exploration.

TEAR FAULTS AND LATERAL RAMPS

Although most folds associated with frontal ramps canbe modeled as either fault-bend folds or fault-propa-gation folds, structures associated with lateral rampscan be more diverse. A block diagram for a footwall

frontal ramp and associated lateral ramp is shown inFigure 14A. Figure 14B1 illustrates the predeformationgeometry through an incipient ramp along profile 1 inFigure 14A. Figure 14B2 illustrates the geometry alongprofile 2 in Figure 14A, after thrust displacement ofthe hanging wall. Figure 14C1–C2 illustrates the trans-port of a more complex hanging wall ramp, from po-sition 1 to 2 in Figure 14A.

Using these concepts, we can more easily demon-strate the relationship between preexisting normalfaults and lateral ramps of the Naga thrust. A normalfault intersects the west flank of the Jaipur anticline atan oblique angle, at approximately the point where theAssam Railway crosses the structure (Figure 7). Theseismic data are equivocal, but they suggest, alongwiththe surface data, that the normal fault displaced thedecollement horizon prior to thrusting. This had the

Figure 10. Seismic line D-58. (A) Seismic data without interpretation. (B) Seismic data illustration areas where data are questionableor absent. Continued.

Kent et al. 2037

effect of creating a lateral ramp that causes the differ-ence in surface expression of the Jaipur anticline in theDigboi and Kusijan areas. Thus the Assam Railwaynormal fault is an important structure in providing lat-eral closure for Digboi field.

FAULT IMBRICATION

Possibly the most striking feature of the frontal part ofthe Assam-Arakan thrust belt is the degree of struc-tural imbrication. This characteristic prompted earlyworkers to refer to the area as the “Belt of Schuppen”(imbricates). The overlapping arcuate map pattern

that results from imbrication is apparent on both small-scale and large-scale maps.

Three types of imbrication are noted in the Belt ofSchuppen: (1) large-scale imbricate structures origi-nating from a similar or common decollement, (2)medium-scale imbricate structures originating fromsimilar or common ramps, and (3) small-scale imbri-cate structures that occur at the leading edge of theNaga thrust.

Leading-Edge Imbrication

Leading-edge imbrications are small-scale featuresthat occur at the locus of maximum displacement

Figure 10. Continued. (C) Seismic data with interpretation. The interpretation shows the position of the transition of the Nagathrust from the decollement to the ramp (�I/0 dip-domain boundary). This point coincides with the point independently determinedfrom the geologic map. Beds dipping at 40� within 2 km of the base of the ramp indicate that two faults are present in the section.However, the dip data do not reveal whether the additional faulting or imbrication is above or below the Naga thrust. (D) Geologicmap showing the location of seismic line D-58 and surface data used in interpretation.


Figure 11. Regional map showing fault trends (after Mallick and Raju, 1995; used with permission from the Society of ProfessionalWell Log Analysts) with time-structure map overlays. The rectangle encloses a time-structure map of the Naga thrust plane, based onseismic data. East of this map are dashed contours extracted from a top Tipam Formation time-structure map from the informationdocket for Assam and Arunachal Pradesh blocks (Ministry of Petroleum and Natural Gas, 1994). This map compares the transitionfrom flat to ramp of the Naga thrust (�I/0 dip-domain boundary), as derived from seismic data and from the geologic map. Thestrike of this boundary is also compared with the trend of the basinal normal faults by projecting it laterally into the basin (heavydashed lines northwest of Naga thrust).

Figure 12. The seismic inter-pretation of Figure 10, restoredusing LithoTech cross sectionbalancing software. Panel A isthe seismic interpretation con-verted from time domain todepth domain, onto which sur-face and well data have beenprojected. Panel B is the depthinterpretation, restored using aflexural slip algorithm. Interpre-tation of the hanging-wall geol-ogy is simplified, because in thehanging wall, there is a com-plex interrelationship betweenthe regional unconformitiesnoted at arrows 1 through 2and fold-accommodation faultssuch as the one noted atpoint 3.

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Figure 13. (A) Mathur and Evans (1964) diagrammatic section across upper Assam showing normal down-to-basin faults later inverted during the compression related to thedevelopment of the thrust belt. (B) The Jaipur anticline appears to have a kinematic evolution similar to that of the Jebel Ta’bast and Taddighoust anticlines of Morocco, asinterpreted by Saint Bezar et al. (1998; reprinted with permission from Elsevier Science), if differences in system scale are noted. In this interpretation, preexistent normal faultsare lines of weakness that initiate transition from flat to ramp on later thrust faults.


along the strike of a fault-propagation fold (Figure 8).The leading-edge imbricate structures are present onthe Digboi and Kusijan cross sections (Figures 2A, 9)and appear to occur in areas where neither flexuralnor nonbedding parallel slip alone can accommodatestrain. These features explain areas of high dip nearthe Naga thrust, because each imbricate fault in-creases the order of the overlying dip domain (areasindicated by the number 1 in Figure 8). Althoughthese leading-edge imbricate structures are small rela-tive to the size of potential hydrocarbon traps andto the Jaipur anticline, features of this magnitudecan become economically significant during fielddevelopment.

Ramp Imbrication

The map pattern of the overlapping imbricates alongthe trace of the Naga thrust, the position of the areasof higher-order dip near the intersection adjacent arcsof the fault trace (Figures 3, 8), and the poor-qualityseismic data of line D-58 together suggested theworking model illustrated in Figure 15. Based on theempirical bow-and-arrow rule (Elliott, 1976), thelength of each arc along the trace of the fault isroughly 15 times the displacement of the related im-bricate. Thus, the multiscale arcuate pattern suggestsa composite of overlapping imbricate faults on a va-riety of scales.

Figure 14. (A) Block diagram of footwall frontal and lateral ramps (from Woodward et al., 1989; reproduced by permission of theAmerican Geophysical Union). (B1) Cross section along line 1 in Figure 13A prior to deformation; (B2) Cross section along line 2 inFigure 13A after movement of the hanging wall up the footwall ramp. These two cross sections illustrate the deformation related todipping lateral ramps and the effect of inverting the volume above the thrust fault. (C1) Cross section along line 1 in Figure 13A priorto deformation. Cross section C1 is a before-deformation cross section at line 1 in Figure 13A as the section is modified to includethe second dipping lateral ramp and a vertical lateral ramp at the arrow. (C2) Cross section along line 2 in Figure 13A, after movementof the hanging wall up the ramp. These two cross sections show the development of a tear fault as the result of a lateral ramp. Thisset of illustrations, developed by Boyer (in Woodward et al., 1989), is for fault-bend folding. However, similar effects are seen forfault-propagation folds and breakthrough fault-propagation folds.

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Figure 15. Map and cross sections of accommodation zone for breakthrough fault-propagation fold. The map shows three imbricatefaults that transfer thrust displacement along strike. This map is proposed as a model for the Naga thrust, in which the Naga thrustis envisioned as a composite structure. Following the bow-and-arrow rule (Elliott, 1976) the length of each arc along the trace of thefault is predicted to be roughly 15 times the displacement of the related imbricate. Cross section AA� illustrates a cross section locatedwhere only one fault is encountered. The dashed line is the location in which a second imbricate could have formed. Cross sectionBB� shows the development of a second imbricate as displacement is transferred to the next segment of the thrust fault. The existenceof the imbricate is reflected at the surface by the second-order negative (hinterland-facing) dip domain (�II). Cross section CC� islocated at a point where the effect of the upper imbricate is diminishing. Where the depth to the decollement is small, a positive(basinward) dip domain as well as a second-order negative dip domain will develop. As the depth to decollement increases, thepositive dip domain may be concealed as a result of upward domain convergence.

Although no hard data demonstrate the presenceof ramp imbrication, the areas of anomalously high dipthat occur north of the Dilli River on the east flank ofthe Jaipur anticline (areas indicated by the numeral 2in Figure 8) suggest the possibility. We propose thatthe zones of high dip represent domains of second-or-der (�II) (and locally third-order [�III]) dip. Theseareas of higher dip result from the projection of oneimbricate beneath an adjacent imbricate (Figure 15).Such hidden imbricate faults may be prospects, if atransverse structure such as a lateral ramp or tear faultprovides lateral closure. Note that unrecognized shal-low imbricates could also create areas of nonprospec-tive higher dip (as seen in Figures 10, 12).

POSSIBLE EXPLANATIONS FOR BELT-OF-SCHUPPEN IMBRICATE STRUCTURALSTYLE

The prominence of the imbricate structural style in theAssam-Arakan thrust belt is likely to be related to theheterogeneity and low strength of the stratigraphic sec-tion that makes up the frontal zones of the belt. Thestratigraphic succession in this region consists pre-dominantly of nonmarine Tertiary strata that arerelatively weak and are characterized by numerous lat-eral facies changes. Abundant coal horizons in the Bar-ail Formation likely serve as multiple detachment sur-faces. In contrast, the more continuous lateral


geometry of many thrust belts that involve Paleozoicor Mesozoic strata is strongly influenced by strong,thick, regional carbonate units or well-indurated sand-stone beds.

IMPLICATIONS FOR HYDROCARBONEXPLORATION

With the exception of Digboi field, anticlinal pros-pects along the Naga thrust west of that field are lim-ited because the relatively narrow fault-propagationfold has been eroded away, leaving only east-dippingbeds along the southeastern part of the Naga thrust.However, the structural analysis described in this ar-ticle provides additional information about the extentof the upper Assam petroleum systems and their as-sociated producing trends. In the Jaipur anticline area,the intersection of the Tipam and upper Barail reser-voirs by the Naga thrust marks the eastward limit ofthe Assam shelf trend, and the intersection of the Bar-ail Formation with the Margherita thrust defines theeastward extent of proven effective source rocks. Theknown existence of active petroleum systems in theregion is encouraging and suggests the possibility thatsubtle or hidden traps occur elsewhere in the struc-turally complex area.

Naga Foothills Trend

The dip-domain analysis suggests that a hidden faultimbricate exists at the southern plunge of the Jaipuranticline (Figure 8). If seismic or drilling data confirmthis structure, it could have reserves equal to orgreater than those of Digboi field. The probability ishigh that both reservoir and top seal exist, and thereis little doubt regarding the presence of an active hy-drocarbon source. Determination of the imbricate ge-ometry and the existence of a southern lateral sealwould be the main points needed to reduce risk.

The Kusijan area is the only part of the Jaipuranticline where west-dipping beds are preserved, andthree wells have been drilled straddling the crest ofthe anticline. Complications present in Digboi fieldsuggest that the Kusijan wells might have missed orbypassed an accumulation. Two lines of evidence sug-gest this possibility. Seismic line D-58 and surfacemaps indicate that reservoir rocks of the upper Barailsection are present at the core of the Jaipur anticline.Strata in this interval include parts of the upper BarailJaipur oil sands, which seep oil in outcrops at the

southern end of the anticline. Although the KusijanNo. 1 well is east of the surface trace of the anticlinalaxis, the well did not test this area of possible struc-tural/fault closure. Dead oil in outcrop at the surfacetrace of the Naga thrust, between the Kusijan No. 2and Jorajan No. 10 wells, indicates that any trapdowndip was near a migration pathway and has thepotential to be charged (Figure 16).

Review of Digboi field as an analog indicates thefollowing. Although the producing area of Digboi fieldis more than 17.6 km (11 mi) long, it is only about

Figure 16. Kusijan-Jorajan area lineament map, as inter-preted from drainage and alluvium outcrop patterns. The line-aments may result from differential compaction in the footwallof the thrust and may indicate the location of subthrust normalfaults. This possibility is demonstrated by the correlation of thelineament trend with a normal fault, mapped on seismic data,that separates Kusijan wells No. 2 and No. 4. If additional cor-relation could be demonstrated, the procedure could provide amethod for extrapolating the extent of Jorajan field beneath theNaga thrust. This map also shows the location of oil-stainedsandstone in outcrop, along the surface trace of the Naga thrust.Oil staining at this location is an indication that if a hanging wallstructure exists southeast of Kusijan No. 1 well location, thatstructure has a high probability of being along a migration path-way and of being charged.

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400 m (0.25 mi) wide. The upper pay zones are dis-continuous, and barren or water-bearing zones sepa-rate producing areas in these reservoirs. Determina-tion of productive zones by use of geophysical welllogs is difficult, because some reservoir zones containfresh to low-salinity interstitial water that makes themdifficult to distinguish from oil-bearing zones. Watersalinity increases eastward with depth, with the high-est concentration of dissolved salt in the shallow payzones reaching only 7500 ppm (Corps, 1949). Thesedata suggest that structural modeling to accurately lo-cate the crest of the structure at reservoir depth, andcareful reexamination of existing well logs and fluiddata to distinguish between hydrocarbon and fresh-water zones, might generate a second shallow (low-cost), relatively low-risk prospect on the Jaipurstructure.

Assam Shelf Trend

The Assam shelf trend is proven by more than 50 yrof production. The part of the basin beneath the Nagathrust represents an unexplored part of the trend. Theeastern prospective limit for each producing intervalof the Assam shelf trend is the intersection of the baseof that unit with either the Naga or the Margheritathrust, an area of several hundred square kilometers(Figure 17).

The structure of beds above the bedding-planepart of the Naga thrust fault is slightly modified bytranslation, but the beds remain relatively undefor-med by the fault. The limited seismic and well data,in conjunction with the structural modeling, wereused to make a form-line structure map of the upperBarail Group (Figure 17). This map indicates that thestructural nose on which Nahorkatiya, Jorajan, andother foreland fields are located extends under andsoutheast of the surface trace of the Naga thrust.

At prospect scale, the Jorajan No. 10 well, whichwas drilled east of the surface trace of the Naga thrustinto the subthrust extension of Jorajan field, illustratesthe potential for the extension of the Assam shelftrend beneath the frontal part of the thrust belt (Fig-ure 16). Jorajan field is a complex of at least five sepa-rate normal fault blocks that had initial reserves ofmore than 350 MMBOE.

If the assertion is correct that the trapping normalfaults of the Assam shelf trend predated and influ-enced the thrust belt development, then it may bepossible to deduce structure beneath a thrust sheet bycombining subsurface interpretations with surface ge-

ology. Patil (1993) has shown that there is a relation-ship between subthrust normal faults and suprathrustnormal faults within the Belt of Schuppen. He rea-soned that this relationship resulted from the relaxa-tion of strain at the end of the compressional episode,and continued differential compaction within the sub-thrust horst-and-graben system.

A similar relationship between surface lineamentsand subthrust faulting can be seen in the area of theKusijan wells. This correlation between surface line-aments and subsurface faults may provide a methodfor extrapolating the Assam shelf structural trends be-neath the Naga thrust. Some alluvial outcrops in theJorajan area have strikingly linear contacts (Figure 16).The strike of lineaments associated with the contactsis similar to two of the orientations of faults in theAssam shelf trend. The northernmost lineamentshown passes between the Kusijan No. 2 and No. 4wells, in which the elevation of the top of the sub-thrust Tipam Formation differs by about 80 m. Thisrelationship suggests that the seismically mapped nor-mal fault in the foreland passes between the twowells, along the trend defined by the lineament.

Figure 17. Form-line structure map of structure near the topof the upper Barail Group. This map is based on the limitedseismic and well control, as interpreted by concepts derivedfrom the structural models. Line A is the map trace of point Ain Figure 9 and represents the intersection of the Naga thrustwith the top of the Barail. Line B is the map trace of point B inFigure 9 and represents the intersection of the �I/0 dip-domainboundary with the top of the Barail. Both seismic lines D-3 andD-23 show regional dip of reflectors to the northeast. The Nagathrust occurs as a bedding-plane fault in seismic line D-23.


CONCLUSIONS AND EXPLORATIONECONOMICS

The principal part of this study was based on a datapackage available from the government of India for acost of a few thousand dollars. An integrated data setwas created by combining subsurface data from thedata package; surface data from geologicmapping donein the 1940s, 1950s, and 1960s; satellite images; andpublished articles. From this diverse data set it was pos-sible to derive the elements of quantitative structuralmodels that are testable. The structural models, inturn, when combined with other quantitative data,such as fluid properties, or more qualitative data, suchas hydrocarbon shows, can be expanded into concep-tual exploration models.

The strength of conceptual models derived by thismethod is twofold. First, one can follow the methodfor linking data and assumptions, which allows one totest critical points in the data or logic string. One canthen develop an exploration plan that follows a courseof project management in which critical points areidentified and the associated risk mitigated (Archibald,1992). Second, because key risk elements can be de-fined early in the project and at a relatively low cost,exploration dollars can be better focused to mitigatethese risks. In this case, additional seismic data mightbe acquired only over areas suggested by the integra-tion of existing data to be prospective, thereby reduc-ing seismic acquisition costs. An additional benefit isthat this conceptual approach links well with project-management techniques that track costs and accom-plishments as the exploration plan proceeds. Many ar-eas exist, particularly in thrust belts, where surfacegeologic data and key subsurface data can be assembledrelatively inexpensively, and where this integratedstructural interpretation approach can be applied.

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