Reassessing Prospectivity of Tura Sandstone in Rudrasagar ... · Reassessing Prospectivity of Tura...
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N. P. Singh , Room No. 522, 5 th Floor GEOPIC, KDMIPE Campus, ONGC Dehradun, Email: [email protected] P-360 Reassessing Prospectivity of Tura Sandstone in Rudrasagar area: Application of 3-D Seismic Attributes and Impedance N. P. Singh*, Harilal and S. R. Gupta, ONGC GEOPIC Dehradun Summary 3-D seismic and well data of Rudrasagar area in the North Assam Shelf sub-basin has been interpreted for assessing the prospectivity of Paleocene to Early Eocene Tura Sandstones which are hydrocarbon bearing in the northern part of the Basin. In the area, Tura represents basal clastic facies sandwiched between underlying granitic basement and overlying Eocene Sylhet Formation. The rock properties and resultant log and seismic signatures of Tura Formation are affected by enclosing layers. Tura sandstones are of relatively low impedance than overlying carbonates and underlying basement. The acoustic impedance contrasts at respective interfaces are significant for generating good reflections. Structure maps show major NE-SW trends separated by faults of varying orientations, NNE-SSW, NE-SW and E-W. Seismic attributes, generally, follow the structural trends. Low to medium impedance is distributed in selected structural trends. Relatively high amplitude and medium impedance depict good sandstone reservoir facies. Integration of seismic attributes, impedance and structure show favourable locales for hydrocarbon entrapment. Introduction The Rudrasagar area is located in the western central part of Uppet Assam Shelf of Assam and Assam Arakan Basin, India (Figure 1). In this area Oligocene to Miocene Barail and Tipam formations contain main sandstone reservoir rocks and are producer of oil and gas. Late Paleocene – Early Eocene Tura sandstone, is yet to be discovered as hydrocarbon bearing in the area, though it is producer in nearby Disangmukh, Panidihing and Lakwa areas. In the area, four wells were targeted to Tura formation and all are dry. For reassessing the potential, in light of dry wells, newly processed 3-D seismic data have been interpreted over an area of 75 km 2 in integration of logs of 3 dry wells within 3-D coverage. Basic framework for attribute and impedance analyses was generated by establishing well–to seismic-tie and correlating horizons and faults. Seismic attributes were extracted along horizons and in windows with reference to horizons and model based poststack inversion was done. Relatively low impedance and high seismic amplitudes are considered as favourable for good sand facies and thickness. Structures with medium to low impedance and high amplitude may be favourable locales for hydrocarbon. Stratigraphy and General Geology Evolution of Basin and depositional environments and processes are deliberated by number of authors from time to time (Bhandari et al., 1973; Kuldeep Chandra et al., 2001; Mathur and Evans, 1964; Raju, 1968). The stratigraphic succession ranges from Pre-Cambrian Granitic basement to the Plio-Pleistocene Namsang Formation, with intervening Jaintias of Palaeocene and Barails of Oligocene and Tipams of MioPliocene age (Figure 2). In the area, Tura Formation unconformably overlies the Precambrian basement and ranges in age from Paleocene to Early Eocene. It is product of fluvial to marginal marine depositional systems during the initial phase of shelf development by rifting/drifting and block faulting. It is underlain by Eocene Sylhet Formation which has range of depositional environments and is divided into number of members. In the study area, Sylhet is represented by shelf carbonate with interbedded siltstones and clay. The top of the Sylhet is marked by a regional unconformity on which shallow marine to lagoonal shales and interbedded limestone of the Eocene Kopili Formation are accumulated. In cores, Tura comprises cyclic alternations of thin sands and shales. Sandstones consist of quartz arenites and quartz wacke with bimodal grain distribution at places (N. K. Verma et. al, 2005). The formation is well correlatable
Transcript of Reassessing Prospectivity of Tura Sandstone in Rudrasagar ... · Reassessing Prospectivity of Tura...
N. P. Singh , Room No. 522, 5th
Floor GEOPIC, KDMIPE Campus, ONGC Dehradun, Email: [email protected]
P-360
Reassessing Prospectivity of Tura Sandstone in Rudrasagar area:
Application of 3-D Seismic Attributes and Impedance
N. P. Singh*, Harilal and S. R. Gupta, ONGC GEOPIC Dehradun
Summary
3-D seismic and well data of Rudrasagar area in the North Assam Shelf sub-basin has been interpreted for assessing the prospectivity of Paleocene to Early Eocene Tura Sandstones which are hydrocarbon bearing in the northern part of the Basin. In the area, Tura represents basal clastic facies sandwiched between underlying granitic basement and overlying Eocene Sylhet Formation. The rock properties and resultant log and seismic signatures of Tura Formation are affected by enclosing layers. Tura sandstones are of relatively low impedance than overlying carbonates and underlying basement. The acoustic impedance contrasts at respective interfaces are significant for generating good reflections. Structure maps show major NE-SW trends separated by faults of varying orientations, NNE-SSW, NE-SW and E-W. Seismic attributes, generally, follow the structural trends. Low to medium impedance is distributed in selected structural trends. Relatively high amplitude and medium impedance
depict good sandstone reservoir facies. Integration of seismic attributes, impedance and structure show favourable locales for hydrocarbon entrapment.
Introduction
The Rudrasagar area is located in the western central part of Uppet Assam Shelf of Assam and Assam Arakan Basin, India (Figure 1). In this area Oligocene to Miocene Barail and Tipam formations contain main sandstone reservoir
rocks and are producer of oil and gas. Late Paleocene –Early Eocene Tura sandstone, is yet to be discovered as hydrocarbon bearing in the area, though it is producer in nearby Disangmukh, Panidihing and Lakwa areas. In the area, four wells were targeted to Tura formation and all are dry. For reassessing the potential, in light of dry wells, newly processed 3-D seismic data have been interpreted over an area of 75 km2 in integration of logs of 3 dry wells
within 3-D coverage. Basic framework for attribute and impedance analyses was generated by establishing well–to seismic-tie and correlating horizons and faults. Seismic attributes were extracted along horizons and in windows with reference to horizons and model based poststack inversion was done. Relatively low impedance and high seismic amplitudes are considered as favourable for good sand facies and thickness. Structures with medium to low
impedance and high amplitude may be favourable locales for hydrocarbon.
Stratigraphy and General Geology
Evolution of Basin and depositional environments and processes are deliberated by number of authors from time to time (Bhandari et al., 1973; Kuldeep Chandra et al., 2001; Mathur and Evans, 1964; Raju, 1968). The
stratigraphic succession ranges from Pre-Cambrian Granitic basement to the Plio-Pleistocene Namsang Formation, with intervening Jaintias of Palaeocene and Barails of Oligocene and Tipams of MioPliocene age (Figure 2). In the area, Tura Formation unconformably overlies the Precambrian basement and ranges in age from Paleocene to Early Eocene. It is product of fluvial to marginal marine depositional systems during the initial phase of shelf
development by rifting/drifting and block faulting. It is underlain by Eocene Sylhet Formation which has range of depositional environments and is divided into number of members. In the study area, Sylhet is represented by shelf carbonate with interbedded siltstones and clay. The top of the Sylhet is marked by a regional unconformity on which shallow marine to lagoonal shales and interbedded limestone of the Eocene Kopili Formation are accumulated.
In cores, Tura comprises cyclic alternations of thin sands and shales. Sandstones consist of quartz arenites and quartz wacke with bimodal grain distribution at places (N. K. Verma et. al, 2005). The formation is well correlatable
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through out Upper Assam Shelf. It can be divided into two parts based on litho and bio facies characterizations. The Upper part is hydrocarbon bearing in the nearby areas.
Fig.1 Location of area in Upper Assam shelf
Fig. 2 General stratigraphy of the area
Well-to-Seismic matching and structural framework
building
Basement, Tura Top and Sylhet Top markers are identified on logs in Figure 3. Tura sandstone layer is interpreted by low impedance as compared to overlaying Sylhet limestone and underlying basement. Due to these characteristics it
generates high amplitude peak from top interface and high
amplitude trough from bottom interface (Figure 4). Seismic section in Figure 5 shows typical seismic signatures of basement, Tura and Sylhet. Character based synthetic to seismic matching is good with 80% correlation coefficient. Reflector corresponding to Sylhet Top, Tura Top and
basement top were correlated on the seismic and time structure and depth maps were prepared. Depth map at top of Tura Top is shown in the Figure 6. The map shows general dips towards southeast and NNE-SSW, NW-SE and E-W trending faults forming few fault closures.
Seismic attributes
Seismic attributes were computed in volume, constant time windows with reference to correlated horizons and along horizons. RMS amplitude along the Tura top is shown in Figure 7. Since Tura Top is generally carbonate-clastic interface, amplitude variations may be directly related to variation in underlying rock properties. The high amplitudes may be related to richness of clastic facies. The structure map of Tura is superimposed on the attribute map.
Amplitude and structural feature (nose and fault closures) are in good corroboration.
Poststack acoustic impedance inversion
Model based inversion was carried out by incorporating 3 wells of the area and acoustic impedance volume was generated. A section from impedance volume in Figure 8
shows low impedance for Tura Formation. Minor variations within Tura can also be seen. Impedance maps were generated for thin layers representing sand intervals on the logs. Impedance map within a interval of 26-30 ms window with reference to Tura Top reflector and overlay of structure map is shown in Figure 9. Low to medium impedance values are in corroboration of structural features. Very low and high impedances are influenced by shale and basement rocks, respectively.
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Sylhet
Tura
Fig. 3. Log signature
Fig. 4 Well to seismic ties and seismic signatures Tura
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Fig. 5 Seismic signature of Basement, Tura and Sylhet .
Fig. 6 Structure map of Tura Top.
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Fig. 7 Amplitude map with overlay of structure of Tura top.
Fig. 8 Impedance section showing relatively low impedance in
Tura Formation. Section is flattened at Tura Top.
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Fig. 9 Impedance map with overlay of structure Discussion and results
Seismic reflections at an interface can reveal about hard versus soft layers provided the phase and polarity of the
seismic data are known (Brown, 2004). In this case, polarity of seismic data is “low to high impedance- positive” and phase is near zero. The peak is representing hard-to-soft-interface (carbonate over clastic) and trough is representing soft-to-hard-interface (clastics over basement). The thin internal layers within Tura are not resolved and we are getting only composite reflections. The inferences based on the attribute analysis are confirmed by impedance
analysis. Grossly, Tura sandstones are low impedance layer enclosed between high impedances. The integrated approach has depicted the distribution of Basal clastic facies within area. Tura, Sylhet and Kopili lithological tripelet is a complete source-reservoir-seal assemblage. Kopili shales are major source rock. Post depositional faulting may cause
juxtaposition of source sequences against reservoir facies of Tura and Sylhet. Occurrences of hydrocarbon in nearby areas show the evidence of generation and migration. The present structural model suggests unfavorable position for the existing drilled dry wells (Figure 10). However, earlier authors have suggested the limited generation and selective
migration (J. Sahoo et al, 2005). The entrapment conditions are depicted by structural maps as the vertical and lateral seals are provided by shales of Sylhet and Kopili formations. The integrated map of amplitude, impedance and structure (Figure 10) indicate possible locales for structural/stratigraphic entrapments.
Conclusions
The integrated study has brought out that Tura sandstones have lower impedance than overlaying and underlying layers and are recognized by high amplitude reflection. Seismic attribute and impedance have depicted good reservoir facies in Tura Formation. Amplitude, impedance and structure show good locales for exploration.
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Fig. 10 Integrated map showing structure (black contours), Impedance (red contours) and amplitude (colour fill).
Acknowledgements
We express our sincere gratitude towards Director (E), ONGC, India, for according permission for submission and publication of this paper. We are grateful to Shri S. K. Das, ED-HOI GEOPIC, GEOPIC for assigning and facilitating
the project. We are grateful to Shri Shyam Mohan GM-Head INTEG, GEOPIC for guidance and supervision during this study. The views expressed in this paper are exclusively of the authors and need not necessarily match with official views of ONGC.
References
Bhandari, L.L, Fuloria, R. and Sastry, V.V. (1973) Stratigraphy of Assam Valley, India. Amer. Assoc. Petrol. Geol. Bull., v.57(4), pp.642-652.
Brown A. R., 2004, Interpretation of three-dimensional seismic data, AAPG Memoir 42, SEG investigation in Geophysics No. 9, sixth ed. AAPG SEG publication. Jayaram Sahoo, S. Sinha and G. S. Rana, 2005, Assessment of oil migration in North Rudrasagar area, Upper Assam: a case study, Petrotech 2005.
Kuldeep Chandra, D.S.N. Raju,Anil Bhandari, and C.S. Mishra, 2001: Petroleum systems in Indian Sedimentary Basins: Stratigraphic and Geochemical Perspective, ONGC Bulletin Vol. 38 Nov. 1, pp.1-45.
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Mathur, L.P. and evans, P. (1964) Oil in India, Internat. Geol. Congress, 22nd Session, pp.1-85. Narendra K. Verma, P.V.S.J. Sharma, G.S. Chaturvedi, V.
Rangachari and S.K. Majumdar, 2005, Eocene Tura-Sylhet-Kopili petroleum system: a promising play with “yet to find hydrocarbon potential” in matured exploration area of Upper Assam Shelf, India, petrotech 2005. Raju, A.T.R. (1968) Geological evolution of Assam and Cambay Tertiary basins of India. Amer. Assoc. Pet. Geol. Bull., v.51 (12), pp.2422-2437.
