Publication Name: PESA Journal
Authors: Dianne Edwards, David M. McKirdy and Roger E. Summons
Publication Issue: 26
Date Published: December 1998
Number of Pages: 25
Reference Type: Journal Article
Abstract:
Jet black asphaltite ( 4-18 o API) recovered from the shoresof Western Australia, South Australia, Victoria and
Tasmania is the least common but most spectacular variety
of Australian coastal bitumen. It typically strands in the
form of large (up to 7 kg), flattened, ovoid lumps which
exhibit deep shrinkage cracks on their upper surface. This
study documents the chemistry of some 40 asphaltite
strandings collected over the last 97 years along the
southern Australian mainland coast from Cape Chatham
(Western Australia) in the west to Cape Otway (Victoria) in
the east; and from Kangaroo Island (South Australia) and
King Island (Tasmania) to as far south as Macquarie Island
in the Antarctic Circle. Despite their diverse stranding
localities, these asphaltites exhibit a striking uniformity in
chemical and isotopic composition. They are aromaticintermediate
to aromatic-asphaltic oils, rich in sulfur (S = 3-
4% ), and in this regard quite unlike any other known
Australian crude oil. The mean bulk carbon isotopic
compositions of their saturated and aromatic hydrocarbons
(813C = -30.1%o and -29.5%o PDB, respectively) are 2-5%o
lighter than those of the corresponding fractions in the more
prolific and widespread waxy coastal bitumens. Their
individual n-alkanes are also light (813C = -28 to -34%o) over
the range C12. 29 ? The asphaltites have maturities that fall
within the conventional early oil window (MPI-1 = 0.44-
0.64, calculated Ro = 0.66-0.79%). Source- and age-specific
aspects of their molecular geochemistry indicate that they
originated from a Mesozoic (most likely Late JurassicCretaceous)
marine shale containing sulphur-rich Type II
kerogen, deposited under anoxic/sulphidic conditions.
These asphaltites are considered to be either products of an
as-yet-unidentified southern margin petroleum system, or
historical artefacts of the early Australian whale and seal
harvesting industry.