Wednesday, February 7, 1-00pm

Mr. Pradipta Kumar Das

Ph.D. Candidate, NCPGG, The University of Adelaide

Seismic expression of layer-bound polygonal fault systems in the Late Tertiary of the Bass Basin: indications of syneresis of colloidal sediments ?


This paper documents a new type of fault structure in the Bass Basin: the
polygonal fault system. This complex fault system is composed of almost
randomly oriented minor extensional faults of extremely high density and
organised in a polygonal network. The component faults are typically
500m-1000m long and have throws ranging from 10m to 40m. The average fault
spacing ranges from 60m to 500m. Seismically, there are at least three
units or stratigraphic tiers observed within the deformed interval showing
a range of variation in the reflection characteristics and fault pattern
development. The polygonal network of fault system shows a near equal
distribution of fault strike orientations suggesting an isotropic stress
regime during deformation of each unit within the deformed interval.

The polygonal fault system deforms the very fine-grained Late Tertiary
calcareous clay and marl-dominated succession of the basal Torquay Group in
almost the entire offshore Bass Basin. The seismic expression of this
pervasively deformed unit suggests that there is no displacement transfer
to the basement structures and the stratigraphy overlying and underlying
this sequence is undisturbed and is characterised by continuous reflection

The development of polygonal fault systems by three-dimensional volumetric
contraction of muddy sediments during early burial was first reported in
literature from central North Sea Basin. This layer-parallel volumetric
contraction measured in seismic sections from the North Sea has recently
been attributed to a process called syneresis of colloidal smectitic gels
during early compaction history of sediments. Syneresis results from the
spontaneous contraction of a sedimentary gel without evaporation of
constituent pore fluid. This process occurs by the domination of
inter-particle attractive forces in marine clays and is governed by the
change of gel permeability and viscosity with progressive compaction. A
similar process is attributed to the seismically observed complex polygonal
fault system in the Bass Basin.

Biography of the Speaker

Pradipta Kumar Das did Masters in Exploration Geophysics from Indian
Institute of Technology, Kharagpur, India in 1984 and since 1985, has been
working as a geophysicist at various capacities ranging from geophysical
data acquisition duties in the field to basin analysis work in the inhouse
Research Institute for the Oil and Natural Gas Corporation Limited, based
in Dehradun, India.In 1989-90, he was sponsored by ONGC to undertake a
Masters in Technology degree in Petroleum Exploration from Indian School of
Mines, Dhanbad.Since 1997, he has taken study leave from his parent
organisation to carry out a PhD study at the University of Adelaide.