Publication Name: PESA Journal No. 28
Authors: A. Crostella and C. J. Boreham
Publication Volume: 28
Date Published: December 2000
Number of Pages: 15
Reference Type: Journal Article
Abstract:
Widespread gas within Cretaceous rocks in the OnslowTerrace, Peedamullah Shelf and inner Exmouth Sub-basin
is dry and considered to have a major biogenic input as
indicated by its low vitrinite reflectance equivalent, light
carbon isotope values and high C,/C1 5 ratios. This gas may
have been generated in association with liquid
hydrocarbons and then progressively altered by
biodegradation. Alternatively, it may be primary biogenic
in origin and sourced by immature strata. Such an origin
requires Cretaceous or younger source rocks, anoxic
depositional environments and moderate temperatures.
The non-hydrocarbon gases carbon dioxide (C02) and
nitrogen have a primary abiogenic source although
biodegradation can lower the C02 content.
On the Peedamullah Shelf, background gas, as indicated
by well-site recording instruments, is anomalously high,
reaching eight per cent by volume, and is almost pure
methane. In places stringers of gas-bearing sandstone are
scattered throughout the Cretaceous section and in some
wells, such as Mangrove-1 and Mardie-2, produced gas
when tested. In the Investigator Sub-basin the giant dry
gas fields have undoubtedly undergone methane
enrichment through biogenic processes. In that region,
however, there is also a significant input of methane from
deep, overmature source rocks.
Numerous indications of biodegraded residual oil have
been found in the study area. Residual oils reservoired in
the Roller and Skate oilfields in the innermost Barrow
Sub-basin probably were biodegraded by the same
bacterial processes that produced the dry gas. Within the
basin, heavy oil and biogenic gas are present only within
Cretaceous sandstone units but show little correlation
with depth, whereas the stratigraphic distribution of light
oil and associated thermogenic gas is more widespread.
As the majority of hydrocarbon traps are mid-Miocene in
age, the hydrocarbon charge was during the Late Tertiary. In the Investigator Sub-basin, the generation of large
amounts of thermogenic hydrocarbons pre-date effective
seals and were subsequently lost, whereas in the Barrow
Sub-basin and on the Peedamullah Shelf oil from prejurassic
sources is preserved in restricted areas, or became
biodegraded and locally mixed with a younger light oil
charge. In the Late Tertiary, local biogenic gas generation
accompanied generation of the major light oil and
associated thermogenic gas charge sourced from the main
jurassic source rocks. Undersaturated oil columns suggest
migration of gas independent of oil migration, the former
extending out onto the Onslow Terrace and the
Peedamullah Shelf.