Nannofossils for Refined Stratigraphic Time Resolution in Mumbai Offshore Basin of India

Abstract

Abstract

Nannofossils are used for biochronology of the sedimentary sequences in the exploratory wells drilled in Western Offshore of India. The nannofossil events subdivide sedimentary column into various bio-chronostratigraphic units vertically and laterally to correlate with the adjoining area. The wells from Tapti-Daman, Heera-Panna-Bassein, DCS and Ratnagiri Blocks (C-37-D, B-28A-E, WO-16-F, D-33-B and DLS-A and Ratnagiri-A) from Mumbai Offshore Basin were selected for the nannofossil biochronostratigraphic studies and to record the sequence stratigraphic boundaries in the Cenozoic succession. The area has excellent sedimentary succession after cessation of Deccan Trap volcanism, which forms the basement. The standard nannofossil bioevents/zones, applicable worldwide are utilized to establish Cenozoic biochronostratigraphic framework in the Mumbai Offshore Basin of India.

Exceptionally well-preserved and diverse Middle Paleocene to Miocene nannofossil assemblages are described and provide a zonation scheme, which is mostly applied in the oil industry while dealing with the cutting samples. The Nannofossil biochronostratigraphic data is utilized for the sequence stratigraphic aspects of the Cenozoic sediments of Mumbai Offshore in providing regional sequence stratigraphic boundaries. The CI10 sequence boundary represents unconformity between the trap and overlying sediments. The oldest sediments of rift sequence of Mumbai Offshore overlies on Deccan traps, are dated as Selandian (Middle Paleocene) age in the Ratnagiri area. First regional marine transgressive phase with in rift-fill sediments of basin is witnessed during Thanetian (Late Paleocene) times between CI10 and CI20 sequence boundaries. A third order unconformity spanning 2.0-2.8Ma at CI20, demarcating the Passive margin sequence with rift sequence of Selandian – Thanetian age. The overlying first order passive margin sequence are punctuated by regional hiatuses/unconformities of varying magnitudes have been identified at sequence stratigraphic boundaries CII30, CII40 and CIII 30. The sequence stratigraphic boundary CII30 with the span of unconformity 4.4-6.2Ma of second order is easily correlatable in the HPB, Tapti-Daman and Bombay High-DCS area in the Mumbai Offshore. The CII40 sequence stratigraphic boundary spanning 2.1 -11.1Ma during Late Bartonian to Priabonian times is identified and correlatable in the HPB and Bombay High-DCS area. The CIII30 sequence boundary is widely recognized as Early/ Late Oligocene unconformity with the estimated time span of 2.5-3.0Ma and suggests an increase in duration of hiatus from west to east. The seismic marker H3CGG corroborates with the 3^rd order Late Oligocene / Miocene unconformity with low magnitude in the eastern side. The youngest CII100 sequence stratigraphic boundary is represented by the correlative conformity between Miocene-Pliocene sediments.

The sequence stratigraphic boundaries are identified in the Cenozoic sections of Mumbai Offshore using nannofossil bioevents. The nannofossil biochronostratigraphy have brought out value added geological information in refining the subsurface sequence stratigraphic boundaries as applicable to sequence stratigraphy. The study would help in interbasinal correlation and in refining the petroleum system modelling for new discoveries in Western Offshore of India.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016