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Stratigraphy
Stratigraphic succession of Assam Shelf has been widely discussed by Evans (1932), Mathur and Evans (1964), Dasgupta (1977), andDeshpande et al. (1993). Regionally, Assam Shelf exhibits a “Weimerian layer cake” (Weimer et al., 1965) stratigraphy typical of
epicontinental basins overlying the Precambrian granite basement. Up to this time, more than 1200 wells have been drilled on UpperAssam Shelf, and subsurface sedimentary records have been confirmed from Paleocene to Recent in age with a major break in
sedimentation during Oligocene, late Miocene, and Pliocene times. The standardized lithostratigraphic succession (Deshpande et. al.,
1993) for Assam Shelf, shown in Figure 3, depicts the generalized stratigraphy of Upper Assam.
In Assam Shelf area, the Paleogene sediments are mostly deposited in shallow marine to transitional environments. Thickness of
these sediments increases from northwest to southeast, suggesting the basinal system towards the southeast (Bhandari et al., 1973;Murty, 1983). The end of Paleogene is marked by the widespread well recognized Oligocene unconformity.
Tertiary rocks rest directly over the granite gneiss and are divisible into two supergroups (Bhandari et al., 1973). The older Naga
Supergroup of Paleogene age is further divided into Jaintia and Barail groups.
Jaintia Group (Paleocene- Eocene) is composed of:
Therria/Tura/Basal Sandstone of Paleocene to early Eocene age consisting of calcareous sandstones with basal conglomeratedeposited in a fluvial to shallow-marine environment.
Sylhet Limestone of middle Eocene age includes fossiliferous limestones with shale and sandstone bands deposited in shallow-marine shelf. The clastic intercalations represent periods of regression.
Kopili alternations consisting of shale and fine-grained sandstones with marl streaks deposited in a shallow-marine to lagoonalenvironment during late Eocene period.
This Study
Objectives
The primary objectives of this study were:
to develop a depositional model for Kopili Formation;
to determine the extent and orientation of sand bodies within Kopili Formation,
to identify prospective areas
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Methodology
In the present study concerted efforts have been made, based on sequence stratigraphic approach, to collate well logs, seismic, andother G&G data. The stepwise approach was followed in the present study:
Well data study (to understand nature of contacts, thickness variations, sand / shale distribution, etc.) and analyze the logs byadopting sequence stratigraphic approach to develop a chronostratigraphic framework of Kopili Formation for the entire UpperAssam Shelf.
Identification of higher order sequence boundaries on the logs along dip, as well as strike, profile using biostratigraphic data inorder to understand the spatial and vertical distribution of different sequences and sand bodies within the identified sequences.
Calibration of sequence tops on seismic by using the available VSP data in the area.
Preparation of time contour map close to top of identified sequences.
Preparation of isochronopach maps to understand basinal slope.
Analysis of system tract and preparation of HST maps to bring out facies distribution map for each sequence.
Development of a comprehensive depositional model for Kopili reservoirs.
Data Used
Seismic as well as well data for this study was provided in 8 and 32 bit display format. Total of 18 PSDM volumes.
Geo-scientific data of 45 wells including core data, cutting sample descriptions, sidewall cores, well testing details, dipmeterdata, etc.
Inputs from linked laboratory projects.
G&G Analysis
Well Log Profiles
Forty-five deep wells in the area have been studied, and 6 regional electrolog profiles (Figure 4; showing 3 dip and 3 strike profiles)
were prepared for 22 wells; in most of them the entire Kopili sections are represented.
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Figure 1. Map showing study area.
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Figure 2. Map showing various structural elements of Assam and Assam-Arakan Basin.
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Figure 3. Generalized stratigraphy of Assam Shelf.
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Figure 4. Well location map showing various log correlation profiles.
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Figure 5. (a) Isochronopach map of Kopili Formation along with (b) 3D view.
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Figure 6. Conceptual contrast between lithostratigraphy and sequence stratigraphy. Sequence stratigraphic surfaces are event-significant and mark changes in depositional trends. In this case, their timing is controlled by the turnaround points between
transgressions and regressions.Lithostratigraphic surfaces are highly diachronous facies contacts. Note that the system tracts and sequence boundaries cross the
formation boundaries. Each system tract is composed of three depositional systems in this example and is defined by a particular
depositional trend; i.e., progradational or retrogradational (after Catuneanu et al., 2005).
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Figure 7. Biostratigraphic data of well #E.
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Figure 8. Type log, along with seismic response, showing sequence stratigraphic analysis in well G#250 depicting identified sequence
boundary, maxiumum flooding surfaces, and system tracts.
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Sequence III
This is the uppermost sequence of three identified sequences. The thickness of it is greater than that of the other two sequences.Gradually the thickness increases towards the southeast of the study area. Two types of system tracts (HST and TST) are present
within this sequence. A map has been prepared for the HST to show the distribution pattern of sands within this sequence (Figure 16).Based on the sand distribution pattern, it is interpreted that in Geleki, Amguri, Charali, Lakwa, Moran and Sonari areas sands have
been deposited as offshore bars. Towards the southwest and northwest, the depositional environment graded to lagoon. Farther
towards the west the depositional setting changed to a bayhead delta.
Seismic Response Close to Sequence III Top
The reflector ranges from 2600 msec in Disangmukh area to 5100 msec in Sonari–Nazira low area. It is discontinuous and patchy inthe whole area. TWT relief (structure) map with 3-D view has been prepared, after correlation of this event (Figure 17a and b).
Attribute Analysis
Kopili Formation is primarily an argillaceous section with some isolated, scattered, discrete sands distributed on the entire North
Assam Shelf. Sand thickness is found to be on the order of 8-30m. The depth of occurrence makes it difficult to resolve its
heterogeneity and spatial geometry with confidence. Delineation of these individual sands within the formation is difficult due to thefollowing reasons:
thin and discrete nature.
high degree of vertical and lateral variability in net sand thickness.
weak impedance contrasts at sandstone interfaces.
high impedance of underlying limestones.
limited bandwidth of seismic data.
To identify the prospective areas, locales of sand maxima coupled with structural advantageous positions have been our main interest.
To achieve this objective, the sand thickness trends were calculated from logs and a reasonable horizon window was calculated to
accommodate most of the sands from the Kopili package to avoid tuning constraints. Accordingly, a window of 100 msec above
Sequence II top was taken for generating attribute maps.
Instantaneous frequency and amplitude attributes were extracted, and spectral decomposition technique was also attempted. Several
discrete sand geometries are observed in the area. Kopili sands could be recognized by the moderate to high amplitude and low to
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Figure 9. Sequence stratigraphic analysis in profile through wells #E, #D, and #Y.
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Figure 10. Sequence stratigraphic analysis along profile III.
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Figure 11. Sequence stratigraphic analysis along profile IV.
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Figure 12, HST sand map of Sequence I, Kopili Formation, North Assam Shelf.
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Figure 13. a. TWT structure map close to Sequence I. b. 3-D view close to Sequence I.
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Figure 14. HST sand map of Sequence II, Kopili Formation, North Assam Shelf.
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Figure 15. a. TWT structure map close to top of Sequence II. b. 3-D view close to top of Sequence II.
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Figure 16. HST sand map of Sequence III, Kopili Formation, North Assam Shelf.
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Figure 17. a. TWT structure map close to top of Sequence III. b. 3-D view close to top of Sequence III.
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Figure 18. 3-D view of average instantaneous attributes (100 msec above Sequence II top).
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Figure 19. Composite depositional environment map.
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Figure 20. Tidal and wave ravinement surfaces in a wave-dominated estuarine setting (modified from Reinson, 1992; Dalrymple et al.,
1992; Zaitlin, 1994; Shanmugam et al., 2000).
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Figure 21. Map showing the prospective areas identified within Kopili Formation for the North Assam Shelf.
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Conclusions
With the established leads from the Kopili play in Geleki and Lakwa areas this study has incorporated enhanced quality 3D data
(PSTM/PSDM) inputs along with the G&G data within sequence stratigraphic framework to arrive at the following conclusions:
In this study stratigraphic analysis is based on the concepts of high-resolution sequence stratigraphy, which is essentially atime stratigraphic concept being used globally as a reliable tool for basin-scale correlation. Previously, stratigraphic correlationof different sand units within Kopili Formation was done based on classical concept of lithostratigraphy, which is found to be
ineffective in basin-scale correlation.
Kopili Formation is divided into three higher order sequences ( Sequence I, II, and III) primarily based on logs and available biostratigraphic and paleobathymetric data as the seismic imaging is found to be very poor within Kopili section
Sequence I is the lowermost sequence; Sequence III is the uppermost sequence, whereas Sequence II is the intermediatesequence identified within Kopili Formation.
Two types of system tracts (TST and HST) are found to be present in each sequence.
Integrated study has mapped facies distribution pattern and Barrier Bar- Lagoon-Tidal inlet-Tidal bar-Bay Head Deltadepositional model has been envisaged for Kopili Formation in the entire North Assam Shelf.
The study has shown Kopili to be a new play, rather than merely as a source rock with significant reserve potential. This islikely to open a very good exploration opportunity especially in the areas east and southeast of Geleki, Charaideo-Bihubar-
Laxmijan and Lakwa-Sonari– Nazira low and adjoining areas of North Assam Shelf.
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References
Bhandari, L.L., Fuloria, R.C., and Sastry, V.V., 1973, Stratigraphy of Assam valley, India, AAPG Bulletin, v. 57, p. 642-656.
Catuneanu, O., 2005, Principles of Sequence Stratigraphy: Elsevier Science Ltd., Amsterdam
Curray, J.R., 1964, Transgressions and regressions, in Miller, R.L., ed., Papers in Marine Geology: New York, Macmillan, p. 175-203.
Dalrymple, R. W., B.A. Zaitlin, and R. Boyd, 1992, Estuarine facies models--Conceptual basis and stratigraphic implications: Journalof Sedimentary Petrology, v. 62/6, p. 1130-1146
Das Gupta, A.B., 1977, Geology of Assam– Arakan region, Oil Commentary, India: v. 15, p. 4 –34
Deshpande, S.V., et. al., 1993, Lithostratigraphy of Indian petroliferous basins, Assam-Arakan basin, v. I, II document X: KDMIPE,
ONGC.
Evans P., 1932, Tertiary succession in Assam: Trans. Min. Geol. Inst, India, v.27, p.155.
Goswami, P., and Dutta, A.J., 2000, Depositional environment, reservoir characteristics and source rock potential of Kopili Formation,Geleki Field: unpublished ONGC report.
Mathur, L.P., and and Evans P., 1964,Oil in India: 2nd International Geological Congress, New Delhi, India, p. 85.
Mohan, M., and Pandey, J.,1973, Early Paleogne eco-stratigraphy of Upper Assam: Bulletin Indian Geological Association, v.6, no.1,
p.47-62.
Murty, K.N., 1983, Geology and hydrocarbon prospects of Assam Shelf--Recent advances and present status: Petroleum Asia Journal,v. 1, p.1-14.
Naik, G.C., et al., 2001, Sequence stratigraphy and petroleum systems of Assam Shelf: unpublished report, BSD, KDMIPE, ONGC.
Posamentier, H.W., Allen, G.P., James, D.P., and Tesson, M., 1992, Forced regressions in a sequence stratigraphic framework:
Concepts, examples, and exploration significance: AAPG Bulletin, v. 76, p. 1687-1709.
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Rawat, R.S., Pundeer, B.S., and Phor, L., 1999, Sequence stratigraphy and petroleum systems in Assam Shelf (Palynological part):unpublished ONGC report.
Reinson, G.E., 1992, Transgressive barrier-island and estuarine systems, in: R.G. Walker and N.P. James (eds.). Facies Models:Response to Sea Level Change: Geological Association of Canada, p. 179-194.
Shanmugam, G., Poffenberger, M., and Alava, J.T., 2000, Tide-dominated estuarine facies in the Hollin and Napo (“T” and “U”)
formations (Cretaceous), Sacha Field, Oriente Basin, Ecuador: AAPG Bulletin, v. 84, no. 5, p. 652-682.
Van Wagoner, J.C., Mitchum, J.C., Campion, K.M., and Rahmanian, V.D., 1990, Siliciclastic sequence stratigraphy in well logs, core
and outcrop concepts for high-resolution correlation of time and facies: AAPG.Methods in Exploration Series no. 7, 55p.
Weimer, R.J., 1965, Late Cretaceous deltas, Rocky Mountain Region (abstract): AAPG Bulletin, v. 49, p. 363.
Zaitlin, B.A., 1994, The Stratigraphic Organi zation of Incised Valley Sy stems: Im plications to Hydrocarbon Exploration and
Production with examples from the Western Canada Sedimentary Basin (S39B) (Special Publications): Canadian Society of Petroleum
Geologists, 189 p.
Acknowledgements
The authors are highly indebted to ONGC Limited for giving an opportunity to work on this topic Authors express their deep gratitudeto D. K. Pande, Director (Exploration) for permission to publish this article. This work could not have been successfully completed
without the valuable support and guidance provided by P.K.Bhowmick, ED–Head KDMIPE. The authors also highly appreciate the
constructive suggestions of Shri. Jokhan Ram, the then - Executive Director and Head KDMIPE towards the progress of the project.Continuous inspiration and motivation from Dr. Manoj Asthana,GGM&Head BRG and Shri Sanjive Mayor,GM(Geology) are highly
acknowledged. Authors also acknowledge the encouragement given by peers and colleagues. System group KDMIPE is thankfullyacknowledged for their cooperation in carrying out the present work on IIWS. Support/inferences taken from the reports of various
authors is also gratefully acknowledged. Views expressed in this paper are those of the authors only and may not necessarily be those
of ONGC.
