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Nair, K. M.
- Stratigraphy and Sedimentation of Bombay Offshore Basin
Authors
1 ONGC, Priyadarshini, Eastern Express Highway, Bombay 400022, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 40, No 5 (1992), Pagination: 415-442Abstract
Correlation of seven chronostratigraphic sequences in over 300 wells drilled in the six structural blocks constituting the Bombay Offshore Basin has helped in defining the stratigraphy, paleogeography and facies distribution and suggesting a model for shelf carbonate deposition. The sequences correlated are: Paleocene-Early Eocene, Middle-Upper Eocene, Lower Oligocene, Upper Oligocene, Lower Miocene, Middle Miocene and Middle Miocene-Holocene.
The most significant observations arc 1) Surat depression and its southward extension through other grabens up to Ratnagiri block contain a restricted marine, paralic and continental shale, sand and coal facies of 500-3000 m. thickness, deposited during syn-rift. Paleocene-Early Eocene phase. This is the principal source rock in the basin. 2) The main reservoirs are the Miocene limestones in Bombay High and Middle Eocene and Lower Oligocene limestones in other areas. 3) The main sandstone reservoirs are in the Oligocene sequences in Daman-Tapti area of Surat depression and in the Paleocene-Lower Eocene sequence in Heera-Bassein and Ratnagiri blocks. 4) By tracing the clastic dispersal.pattern, it is shown that Saurashtra basin and Shelf Margin basin with their excessive Upper Oligocene-Holoccne fill are less favourable than the rest of the basin in hydrocarbon potential.
A homoclinal ramp model is suggested for the shallow shelf carbonate deposition. The suggested carbonate depositional model indicates that the platfonn areas are susceptible to shale incursions from the east which could affect the reservoir properties of limestone in many areas. Almost all the major carbonate pay zones are associated with unconformities and diastems and the principal porosity is telogenetic.
Keywords
Stratigraphy, Sedimentation, Carbonates, Bombay High.- Quaternary Geology of South Kerala Sedimentary Basin - An Outline
Authors
1 LBS Centre for Science and Technology, Thiruvananthapuram - 695 033, IN
2 Centre for Earth Science Studies, Thuruvikkal PO, Thiruvananthapuram - 695 03 1, IN
3 Agarkar Research Institute, G G Agarkar Road, Pune - 41 1004, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 67, No 2 (2006), Pagination: 165-179Abstract
The South Kerala Sedimentary Basin(SKSB) extending along the coast between Kollam and Kodungallur, is the landward extension of the offshore Kerala-Konkan Basin (KKB). The SKSB has a fill of about 700 m of sediments, of which > 80 m are made up of Quaternary sediments, the rest being Miocene formations. Data from over 120 bore holes drilled for groundwater extraction from Tertiary aquifers, geotechnical investigations and stratigraphic studies form the basis of this paper. Based on sediment characteristics and tectonic setting, the SKSB can be divided into three separate blocks (a) Southern Block (uplifted during Pleistocene end), (b) Central Depression (undergoing subsidence from Early Miocene to Present) and (c) Northern Block (uplifted during Pleistocene-Holocene transition but undergoing subsidence during Holocene).
The Late Pleistocene sedimentary sections indicate continental environments at the bottom. The marine influence during Pleistocene predates 42 ky BP. Ecology and environment deduced from sedimentology, palynology micropalaentology and geochemistry suggest climatic variations from higher-Than-Present to low rainfall rates during deposition of Pleistocene sediments.
Holocene sedimentation also started in continental environments. The marine transgression giving rise to littoral-Lagoonal environments dates back to 7-6 ky BP. The high precipitation in Holocene gradually got reduced and by ca 3 5 ky BP the area witnessed prolonged exposure and development of ferruginous palaeosols.
Within the constraints imposed by insufficient samples and inadequate geochronological data, the paper gives a broad picture of the structural framework, stratigraphy, palynology and depositional environments of SKSB. From these, inferences on sea level oscillations, palaeoclimatic variations and depositional environments are made.