Abstract:
Four exploration wells (Dorado North, Dorado, Barracuda, and Wallago) were drilled in
the deep-water Mannar Basin during the second phase of hydrocarbon exploration. The
Barracuda well was confirmed as gas potential due to presence of 85 wt% methane in the
Upper Cretaceous sediments. Therefore, the current study focused to evaluate
geochemistry of core sediments (depth from 2139–4741 m) in Barracuda well using XRay Diffraction (XRD) and stable isotopes analyses. The upstream petroleum industry
prefers such supportive geochemical data at the development stage. Stable carbon (δ
13 C)
isotopic values (–33.86 ‰ to – 24.88 ‰) and C/N ratios (17.3–158.8) suggest a
considerable supply of terrestrial plants organic matter (gas prone Type III kerogen) to
sediment. XRD results identified several paleoclimatic chronozones based on dominant
clay mineral assemblages. The Early Campanian age consists of montmorillonite
dominant sediments suggesting that arid climate. Kaolinite dominant Late Campanian to
Late Maastrichtian sediments indicates the warmer/wetter paleoclimatic condition.
However, montmorillonite dominant sediments in the Late Maastrichtian can be
controlled by short-term global cooling events and/or volcanogenic sediments followed
by Deccan-Reunion basalt volcanism. The Early-Late Paleocene sediments in the
Barracuda well suggest arid (mock aridity) climate in the Indian Ocean. The Late
Paleocene to Middle-Late Eocene age is composed of kaolinite dominant sediments that
correlate warmer and wetter greenhouse paleoclimate in the Indian Ocean. The EarlyMiddle Oligocene to Miocene sedimentary succession of the Barracuda well also contains
kaolinite dominant clay mineral assemblage. This wet climate characteristic is linked to
the sharp decline of atmospheric CO2 concentration followed by the growth of Antarctica
ice sheets after the Eocene-Oligocene climate transition (ca. 34 Ma) and development of
present-day South Asian monsoon system.
Keywords: Paleoclimate, Sediments, Clay mineralogy
