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Study of the effect of Neotectonism in the Re-migration of Hydrocarbon in North Assam ShelfBlecy Tep, S.Mazumder and K.K.S.Pangtey

Remote Sensing & Geomatics Division, KDMIPE, [email protected]

Keywords

Morphotectonics, Re-migration, North Assam Shelf

Summary

The North Assam Shelf consists of a thicksedimentary sequence of Tertiary sediments whichhouses large oil fields and has been in production fora few decades. As an effort to identify additionalareas of hydrocarbon exploration, in this presentpaper a deductive approach in the form ofmorphotectonics analysis and tectonic stress basedreactivation analysis has been applied in a two-foldapproach. Firstly, to delineate structures that wouldact as locales for hydrocarbon accumulation.Secondly, to identify faults based on tectonic stressanalysis that would be instrumental in tertiary verticalremigration of hydrocarbons from their original areaof accumulation to the identified shallower andsmaller structures rendering them as newerexploration targets. This study uses seismic,electrolog, gravity, magnetic and geochemical data tovalidate the faults and structures identified and addsto the geological model.

Introduction

At present, Assam shelf is bounded by three principalthrusts, Mishmi Thrust in the Northeast, Naga Thrustin the Southeast and Main Boundary Thrust in theNorth, which are still active to accommodate theconvergence along the different plate boundaries andmakes the region neotectonically active. The NorthAssam Shelf (NAS) (Fig. 1) forms a part of theAssam shelf, north of the Jorhat Fault and west ofNaga Thrust and is covered by thick alluviumdeposits from the major rivers transversely drainingit. The neotectonically active status of NAS has twomajor implications in the hydrocarbon explorationscenario of the area. Firstly, that the existing faults offault bounded structural highs that act as the presentareas of hydrocarbon accumulations might have beenreactivated due to the ongoing active tectonics in thearea. As such the earlier structures that served as

hydrocarbon pools could be breached leading toremigration along the reactivated faults. Thesecontinuing phases of active tectonics might also leadto the formation of newer structural highs as well asnewer faults at a shallower level which might act asnewer pathways of migration and newer areas ofaccumulation. These newer formed satellitestructures to the already established ones might beareas of exploratory interest in the presentexploration scenario. And secondly to determinewhich set of faults will act as conduits for verticalmigration of hydrocarbons based on the direction ofprevailing tectonic stress.

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Fig. 1 a) Tectonic setup of Assam-Arakan Basin withLandsat image of Assam shelf; b) Landsat ETM+ FCC457 image of the study area.

11th Biennial International Conference & Exposition

Study of Neotectonism in Hydrocarbon Re-migration in North Assam Shelf

Methodology

a) MorphotectonicsIn a neotectonically active province, it is assumedthat older structural features are reactivated byyounger deformations and subsequently thesereactivated features are manifested on the surface asgeomorphic anomalies especially in case of drainageand topography (Shurr, 1982). Based on thesepremise and the principles dealt in detail inMazumder et al. 2011, a detailed morphotectonicanalysis based on drainage and other geomorphicdata had been carried out in the concerned area todelineate geomorphic linear and geomorphic highsthat are assumed to be surface signatures of subtleand shallow sub surface structures. Themorphotectonic analysis had been attempted mainlyon basis of prevalent drainage data of the area, whichwere extracted from pan-sharpened Landsat ETM+732 FCC image and SOI toposheets of 1:50000 scale.Anomalous features such as drainage offsets,rectangular drainages, rectilinear drainages,compressed meanders and abnormal sinuosities wereidentified. These had been translated into micro-faults, thus, acting as building blocks to regionalfaults.

Reactivated faults might interact with the surfacetopography causing a change in the surface slope andan anomalous linear slope direction i.e. Aspect. DEMdata are digital elevation data set recording thetopographic surface expression of any area and theterrain derivative maps (slope, aspect and curvatures)have largely demonstrated their usefulness forlineaments and fault extraction (Abdulla et al. 2010).30 m ASTER DEM of the area was processed usingSpatial Analyst module of ArcGIS 10.1 to derive theslope and aspect map of the terrain. Abrupt slopebreaks and lines of similar aspect were extracted asprobable faults. Lineaments were also extracted fromLandsat ETM+ FCC 432 image.

All these data sets (geomorphic linear from drainage,DEM and Image) are put on a GIS platformrepresenting as microfaults or building blocks oflarger regional faults (Fig.2a). This is then joined asper their trend and continuity to derive the probablefault network of the area (Fig.2b). To validate theseinterpretations, detailed fields check (Fig.2c) wascarried out along selected traverses to note the

signatures of neotectonic responses of theseinterpreted regional structures on the drainage andtopography of the terrain.

Additionally, geomorphic highs had been delineatedbased on anomalous drainage pattern such as radialand peripheral drainage, representing shallowstructural highs. Based on the association of theseinterpreted geomorphic highs with one or more offeatures such as pondings, rectangular drainage and

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Fig. 2 a) Geomorphic linears from drainage, DEM &Image, put together on GIS platform; b) Reconstructedfault network; c & d) Field checks



low drainage density, these geomorphic highs wereclassified as per their degree of confidence into high,medium and low to represent subsurface structuralhighs (Fig.3a). The geomorphic highs classified asper their degree of confidence had been overlain withthe drilled wells to show a correlation with theestablished hydrocarbon fields. Those geomorphichighs which do not correlate with the existing/established hydrocarbon fields but yet show a higherdegree of confidence merit an interest fromexploration point of view (Fig.3b).

b) Correlation with collateral dataIn order to establish the subsurface continuity of themorphotectonically interpreted faults and structuralhighs, they are correlated with collateral geophysicaldata-seismic, gravity and magnetic, where everavailable. Through this correlation it could beinferred that the interpreted surface faults arepenetrative, some of which reaches down to thebasement level and are basically products ofreactivations. Another significant inference is that inNAS area the NE-SW trending faults which areparallel to the Naga Thrust are the most reactivated,

whereas, in the Amguri-Teok area, the E-W faultscan be considered to be the most reactivated.

c) Tectonic Stress AnalysisStudy of tectonic stress analysis was carried out tounderstand the state of stress in the area. The analysiswas carried out using focal plane solution data andworld stress map for a regional analysis and FMI logsfor a more localized level. From the studies byAngeleir & Baruah, 2009 on the focal plane solutionof earthquake near NAS and Naga Thrust area, it isobserved that the earthquake is dominated by strikeslip component (Fig. 5a). From the focal pointsolutions and world stress map, the overall stressacting on the basin is envisaged to be directednorthwards or northeastwards (Fig. 5). TectonicStress analysis from FMI logs also was carried out bystudying Drilling Induced Fracture (DIFs) and bore-hole breakout in order to predict which set of faultswill act as conduits for hydrocarbon migration(Fig.6). In DIFs the directional maxima is thedirection of maximum horizontal stress (SHmax)whereas borehole breakouts occur perpendicular tothe SHmax.

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Fig. 4 Correlation of interpreted faults mapped at a) BCSand LCM levels; b) gravity data.

Fig. 3 a) Classified geomorphic highs map based onconfidence level; b) Categorized geomorphic highsoverlain with wells.

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Discussions

a) RemigrationEight wells were analyzed of which five wells (CD-4,PD-4, NRDS-1, Mekeypore-4 and DMLG-37)showed SHmax of NE-SW and three wells (BN-4,GLK-354 and LP-1) showed SHmax of NW-SE toNNW-SSE direction (Fig. 6). The Amguri-Teok areais dominated by an E-W trending Jorhat Fault withstrike slip activity, and it can be presumed that SHmax

might be E-W. Faults parallel to or at low angles toMax. Compressive Stress (SHmax) will help in verticalremigration of the accumulated hydrocarbon. Thus, itcan be concluded from stress analysis that NE-SWinterpreted faults will aid in hydrocarbon re-migration. While in Amguri-Teok area E-W orientedfaults are assumed to aid in hydrocarbon re-migration.

b) Evidences of Re-migrationRemigration along the delineated faults may bemanifested by a number of parameters such asshallow hydrocarbon shows, hydrocarbon seepages,geomicrobial anomalies and oil to oil correlation.Such features had been used as evidences to validatethe above interpretations. Hydrocarbon showsoccurring at shallow depth had been encountered in anumber of wells in the area and shows a positivecorrelation with the interpreted faults (Fig. 7a).Hydrocarbon seepages were observed mostly in theAmguri-Teok area and one in Charaideo area (Fig.7b). Microbial blooms concentration maps of Charali,Geleki, Panihiding and Disangmukh areas were

correlated with the interpreted faults depicting apositive correlation (Fig. 7c). Oil to oil correlationstudies carried out in Lakwa field (Goswami et al.1998) (Fig. 7d) indicating wells where oil of deeperhorizon correlates with that of Girujan were alsofound to show a positive relationship with theinterpreted faults that were interpreted to be conduitsof remigration.

c) New Shallower & Subtle Accumulations

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Fig. 7 Correlation of the delineated faults instrumental invertical remigration with a) shallow hydrocarbon shows; b)hydrocarbon seepages; c) geomicrobial anomalies and d) oil-to-oil correlation

Fig. 5 a) Focal plane solution of earthquakes in NAS andNaga Thrust area overlain on Landsat ETM+ 432 image;b) Stress direction from Bore-hole Breakout in the studyarea showing a northward stress direction (After WSM)

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Fig. 6 Rose diagrams of the DIFs and borehole breakout ofthe 8 wells showing approximate direction of σmax



As stated interpreted geomorphic highs are alsofound to match with structural highs in seismic andthus imply such structural highs might exist atshallower horizons. These structural highs might besmaller than the already established structures in thearea. A positive association between these interpretedfaults, acting as conduits of migration, andgeomorphic highs, manifesting shallower structure,implies vertical or secondary re-migration ofhydrocarbons from their original area of entrapment,which might have been breached due to reactivation(Fig. 8). And subsequent accumulation in shallowerstructural highs. This above premise also implieshydrocarbon accumulation at multiple levels andhence also might serve as targets of exploration.

d) Regional ModelThrough regional lineament analysis of LandsatETM+ 432 image of the area, it is observed that twomajor regional lineament acts as two major boundingstrike-slip faults in Assam Shelf creating a wrenchmechanism with the sigmoidal geometry ofBrahmaputra (Fig. 9a). The one on the southcoincides with the Jorhat fault and the one on thenorth occurs probably north of Tinsukia. The faultpattern of the interpreted morphotectonic faults,faults mapped on top of Girujan in the South and inthe North bank of Brahmaputra, faults mapped on topof Barail all appears to be in concordance with themodeled wrench fault system defined by Harding(1964) (Fig. 9b). Based on the model, it is proposedthat the bounding strike slip fault might have led todevelopment of three main fault systems. First, NE-SW faults with an appreciable amount of strike slipcomponent, second, E-W trending faults parallel tothe bounding faults and third, NW-SE trending faults.Also because of the compressive effect of the Naga

Thrust (in the study area) it is possible that the NE-SW faults are more re-activated leading to a clockwise rotation of them. This might cause some amountof compression or thrust component in these faultsalong with consequent development of en-echelonNE-SW oriented structures compartmentalized byNW-SE transverse faults.

Interpreted seismic profiles suggest the developmentof flower structures in the NAS area suggesting astrike slip component, which corroborates to theassumed regional model. As some of the interpretedfaults are deep-seated, based on the correlation withgravity data, it is possible that they connect deeplyseated source rocks with reservoirs in shallow partsof the sedimentary section to form newer petroleumpools (Fig.10). Such type of structure is analogous toBohai Sea basin, South China Sea where the foldsassociated with wrench offsets formed favorabletraps and many oil and gas fields along the wrenchzones have been found (Zhang et al. 2011).Similarly, active tectonics might have led to theformation of shallow reactivated structures in NAS.

Based on the above models and the premise statedabove, geomorphic highs of medium and high

Fig. 8 Geomorphic highs denoting shallow structuralhighs associated with delineated faults for remigration

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Fig. 9 a) Bounding strike slip faults resulting in aprobable top to the right shear system with acompressional component due to Naga Thrust and MBT;b) Wrench fault model defined by Harding, 1974.

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confidence categories when associated withreactivated faults, delineated to be acting as conduits,might serve as areas of exploratory interest in thearea (Fig. 11).

Conclusions

The overall study thus gives a new approach toexploration based on remote sensing inducedgeomorphic studies and present day stress analysis toidentify shallow and subtle structure in addition tothe already established ones. Additionally, the studyhelps to delineate newer areas of exploration in theNorth Assam Shelf which might be of muchsignificance in the present day exploration scenario.Lastly, the study proposes a newer alternativegeological model to explain the formation of

structures in the North Assam shelf which might beof relevance.

Acknowledgements

The authors express their gratitude to GGM-HOI, KDMIPEand GM-Head Geology Group for their permission inpublishing and presenting this paper in SPG. The viewsexpressed here are solely those of the authors and notnecessarily of the organization where they are working.

References

Abdullah Anwar, Akhir J.M and Abdullah Ibrahim. 2010,the extraction of lineaments using slope image derivedfrom digital elevation model: case study of SungaiLembing- Maran area, Malaysia, Journal of AppliedSciences Research, 6,11, 1745-1751.

Angelier Jacques and Baruah Saurabh, 2009,Seismotectonics in North-East India: a stress analysis offocal mechanism solutions of earthquakes and its kinematicimplications, Geophisics Journal International, 178, 303-326.

Goswami B.G et al. 1998, Oil/Gas source correlationstudies of oils and gases discovered within the Girujan clayformation of Lakwa-Lakhmani field, Upper Assam, ONGCInternal Publication.

Shurr G.W., 1982, Geological significance of lineamentsinterpreted from LANDSAT images near the NorthernBlack Hills, Proceedings of Fourth International WillistonBasin Symposium, 198, 313 – 320.

Mazumder S, Pangtey K.K.S and Mitra D.S, 2011,Geomorphic analysis in Cachhar area and its tectonicimplications based on Remote Sensing and integratedstudies, ONGC Unpublished Report.

Singh H.J, Siawal A, Akhtar MD.S, Chakravorty D andKumar N, 2011, Play and prospectivity analysis of AssamArakan Basin, 2011, KDMIPE, E& D Directorate,GEOPIC and Basin, ONGC Internal Publication.

Harding T.P., 1974, Petroleum Traps associated withWrench Faults, AAPG Bulletin, 58, 7, 1290-1304.

Zhang L, Luo X, Vasseur G, Yu C, Yang W, Lei Y, SongC, Yu L and Yan J, 2011, Evaluation of geological factorsin characterizing fault connectivity during hydrocarbonmigration: Application to Bohai Bay basin, Marine andpetroleum geology, 28, 9, 1634-1647.

Fig. 11 Map showing areas of exploratory interest.

Fig. 10 Interpreted Seismic sections along Line A and Bcorrelated with surface morphotectonic elements i.e. faultsand geomorphic highs.


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