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Characteristics of Tidal Sand Bars in the Gulf of Khambhat using Satellite Images and Field Mapping, Western India*

Sourav Saha¹, Anupam Ghosh¹, Stuart Burley², Santanu Banerjee¹, and Pratul Kumar Saraswati¹

Search and Discovery Article #50050 (2007)

Posted August 17, 2007

*Adapted from oral presentation at AAPG Convention, with SEPM, Long Beach, California, April 1-4, 2007

¹Indian Institute of Technology, Bombay, Mumbai, India ([email protected])

²BG Exploration and Production India Ltd, BG House, Hiranandani Business Park,Mumbai, India, Mumbai, India

The modern day Gulf of Khambhat is identical in depositional setting to the underlying hydrocarbon-bearing Oligo-Miocene sedimentary succession of the Tapti Fields. Although many wells have been drilled on these fields, only limited core is available through the reservoir interval, and seismic imaging of reservoir bodies is poor. The modern-day tidal sand bodies have been mapped and their length, height and width measured from satellite images as well as on outcrop.

Tidal bars in the outer gulf are linear with curved crests and are spaced 3-5 km apart, forming narrow, high relief (~20 m) ridges 60 to 100 km long and 3-5 km wide, oriented 170-290 degrees to the main tidal current (N-S). In contrast, estuarine tidal sandbars are more equant shaped, with lower relief, spaced 50m to 1 km apart and oriented at small, oblique angles of 50-80 degrees counterclockwise to the main flow (E-W). In these shallow-water settings, sandbars spread laterally, amalgamate, and are dissected by flood- and ebb-tide channels. As a result bars in the inner estuary are typically 5 to10 km long and 4 to 6 km wide, flat-topped with large widths, and are oriented parallel to one another and the estuarine valley walls.

Cumulative-probability curves (CPC) of tidal-bar dimensional data from the Gulf of Khambhat are used to assess chance of success (COS) of sand bodies of specific reservoir area, thickness, and spacing are being developed. A CPC plot of tidal bar dimensional data indicates 50% probability (P50) of finding sand bars of 1229 m length, 470 m width, 321 m spacing and 1.1 m thick. This can be used to predict probability of tidal sand occurrence in analogous Oligo-Miocene reservoirs.

uAbstract

uOuter Gulf

uStudy Methods

uSelected Figures

uEstuaries

uStudy Methods

uSelected Figures

uGulf as Model

uConclusions

uReferences

uAcknowledgments

uAbstract

uOuter Gulf

uStudy Methods

uSelected Figures

uEstuaries

uStudy Methods

uSelected Figures

uGulf as Model

uConclusions

uReferences

uAcknowledgments

uAbstract

uOuter Gulf

uStudy Methods

uSelected Figures

uEstuaries

uStudy Methods

uSelected Figures

uGulf as Model

uConclusions

uReferences

uAcknowledgments

uAbstract

uOuter Gulf

uStudy Methods

uSelected Figures

uEstuaries

uStudy Methods

uSelected Figures

uGulf as Model

uConclusions

uReferences

uAcknowledgments

uAbstract

uOuter Gulf

uStudy Methods

uSelected Figures

uEstuaries

uStudy Methods

uSelected Figures

uGulf as Model

uConclusions

uReferences

uAcknowledgments

Outer Gulf

Study Methods

Locate the tidal bars with satellite images
Study older as well as new bathymetric maps
Geometry of the bars by side scan sonar
Simple logs of the shallow core from the bars
Shallow seismic of the bars

Analysis of dimensional data and generation of modern of tidal bar and use of it as reservoir model

Selected Figures

	Shoreline shifts on western Indian shelf. Pandey (1986) shows the same concept applied through Neogene.
	Present-day depositional depositional system. Large macro-tidal estuary with very low depositional slope.
	Geometry and extent of giant tidal bars (50-100 km long, 2-8 km wide, and 20 m high. These were first described by Off (1963). What is their geometry and do they migrate with time?
	Persistence of tidal bars. Significant question is whether they migrate daily, annually or in response to sea level.
	Geometry and architecture of tidal bars, which are asymmetric, but is the steep slope depositional?
	Vertical sequences through tidal bars, as shown by log of offshore ridge (left) and example from Oligo-Miocene (log of reservoir-bearing section and amplitude seismic map).
	Implication of depositional model to geometry (in cross-sectional view). Tidal-bar evolution influences geometries and internal structure.
	Implication of depositional model to geometry (in cross-sectional view as revealed by shallow seismic). Recent tidal-bar evolution shows preservation of both ridge and sheet geometry.

Estuaries

Study Methods

Study satellite imagery to select field locations
Field work at selected locations
Documentation of the structure and collection of samples with emphasis on tidal bars

Selected Figures

	Location map of Narmada and Mahi estuaries, with the second highest tidal range in the world.
	Typical tidal sand features of the dominantly muddy Narmada Estuary.
	Relevance of modern tidal sand to subsurface reservoir. Seismic amplitude map of the Oligo-Miocene (left) with modern setting of estuarine tidal sand.
	Interpreted cross-section across estuary, illustrating sediment distribution in a ‘valley fill.’ Deposition of this sequence is measured in thousands of years.

Gulf of Khambhat as Model

Geo-cellular model of the modern tidal bar (left) and geo-cellular model of the Oligo-Miocene hydrocarbon reservoirs.

Conclusions

Combination of satellite and sonar remote mapping techniques with ground truth from outcrop and core data enables tidal sand deposits to be characterized.
- Outer Gulf giant tidal bars - elongate and asymmetric in nature with curved crest. 50 km length, 3-5 km width, up to 20m high, and N-S oriented.
- Estuarine tidal bars- lozenge shaped with lower relief. 1-7 km length, width 300m to 1 km, 1-3 m high, and E-W oriented.
- 50 % probability of finding the tidal bar of length-1229 m, width- 470 m, spacing 321 m and height 1.2 m.
- Surfaces overlap in this type of basin characterized by incision and flooding.

The present-day sediments are an excellent analogue for depositional processes for the equivalent Oligo-Miocene incision and flooding cycles in hydrocarbon reservoirs.

Study of present-day and Holocene processes enables interpretive model of channel evolution to be developed that can be applied to better understand and characterize subsurface reservoir geometry and extent in areas where seismic imaging is sub-optimal.

References

Chappell, J., and Shackleton, N.J., 1986, Oxygen isotopes and sea level: Nature, v. 324, p. 137–140.

Hashimi, N.H., Nigam, R., Nair, R.R., and Rajagopalan, G., 1995, Holocene sea level curve and related climatic fluctuations for western Indian continental margin. An update: J. Geol. Soc. India, v. 46, p. 157–162.

Juyal, N., Kar, A., Rajaguru, S.N., and Singhvi, A.K., 2003, Luminescence chronology of aeolian deposition during the Late Quaternary on the southern margin of Thar Desert, India: Quaternary International, v. 104, p. 87-98.

Khadkikar, A.S., and Rajshekhar, C., 2005, Holocene valley incision during sea level transgression under a monsoonal climate: Sedimentary Geology, v. 179, p. 295-303.

Off, T., 1963, Rhythmic linear sand units caused by tidal currents: AAPG Bulletin, 47, 324–341.

Pandey, J., 1986. Some recent palaeontological studies and their implications on the Cenozoic stratigraphy of the Indian subcontinent: Bulletin of Oil and Natural Gas Corporation, v. 23, p. 1-24.

Wood, L. J., 2004, Predicting tidal sand reservoir architecture using data from modern and ancient depositional systems, in Integration of outcrop and modern analogs in reservoir modeling: AAPG Memoir 80, p. 45– 66.