The recent origin of Bass Strait oil and gas
by A.A. Snelling
This article reports recent research which indicates that the oil and gas deposits
in Bass Strait were formed from deep rapid burial of the same coal measures as those
in the Latrobe Valley Brown Coal Deposits. The evidence indicates that the oil and
gas are still being formed, a factor which strongly supports the conclusion that
Bass Strait oil and gas are of recent origin.
The Gippsland Basin sediments
Fig. 1: Location of the Gippsland Basin
The Gippsland sedimentary basin is located in south-eastern Victoria, and about
80% of its total area is offshore in Bass Strait (Fig. 1). The sediments which fill
the offshore part of the basin are shown in the idealised time cross-section of
Fig. 2.
Fig. 2: Sediments of the offshore Gippsland Basin, under Bass Strait—a
time cross-section. The oil and gas are supposed to have formed 20–40 million
years ago.
The first sediments deposited on the downwarped metamorphic basement rocks were
the sediments of the Strzelecki Group—various greywackes (sandstones) with
minor shales (sometimes slightly carbonaceous) up to 3,500m thick. James and Evans1 describe these sediments as
‘rapidly dumped’!
Overlying the Strzelecki Group, in places unconformably, are the quartz sandstones,
coals, mudstones, siltstones and shales of the Latrobe Group. In the Latrobe Valley
these coal measures are about 400m thick (stacked coal seams representing up to
250m of this), but offshore while the sequence thickens to almost 5,000m, the coal
seams gradually diminish in thickness and number.2
Complex channel systems were carved into the top of the Latrobe Group sediments
soon after deposition (while still soft) and were then infilled with shale and coarse
elastics (see Fig. 2). The top of the Latrobe Group is bounded by a regional unconformity
or unconformities (representing, in this case, a brief erosional period between
‘waves’ of tidal flood sedimentation). Overlying formations are the
Lakes Entrance mudstone and the Gippsland limestone.
The oil and gas deposits
Exploration since 1964 has resulted in the discovery of at least four commercial
oil fields (Halibut, Kingfish, Mackerel and Tuna) and three gas fields (Barracouta,
Marlin and Snapper) (Fig. 3). At present between 60% and 70% of Australia's oil
requirements are met from this basin; further exploration is continuing.
Fig. 3: Location of exploration wells in the Gippsland Basin. Note
the increasing depth to the top of the Latrobe Group southeast from the coastline.
Most of the hydrocarbons are trapped at the top of the Latrobe Group in highly porous
and permeable sandstones forming highs on the Latrobe Group/Lakes Entrance Formation
unconformity surface (Fig. 4). The cap rock and lateral seal in such cases are the
Lakes Entrance Formation mudstones. The Bass Strait oil and gas fields are primarily
structural traps and occupy culminations along major anticlinal axes. The Marlin
reservoir, for example, has 300m of vertical closure covering an area of 137 sq
km while to the southeast the smaller Halibut reservoir has a vertical closure of
approximately 160m and an areal extent of 27 sq km.
The source and generation of the hydrocarbons
Brooks and Smith3 and Brooks4 have established that the
Bass Strait hydrocarbons were generated as a result of the burial and subsequent
diagenesis (heat and pressure processes due to deep burial) of land plant material
such as waxy leaf cuticles, pollen, and spore coatings associated with the carbonaceous
muds and coals of the Latrobe Group. Hocking2 has noted that there is
a parallel basin increase, determined by the depth of burial of the Latrobe Group
coals and depths to the hydrocarbon reservoirs, in the ‘maturity’ of
the resultant carbonaceous sediments and in the heaviness of the liquid hydrocarbons
(Fig. 5). He further concluded that the hydrocarbons experienced only short-distance
migration from source rocks to reservoir rocks, since the two are intimately associated,
as indicated by the almost identical molecular distribution of paraffins from Marlin
coal and oil.2,4
Fig. 4: Representative Bass Strait hydrocarbon reservoirs at the
Latrobe Group/Lakes Entrance Formation unconformity.
Fig. 5: hydrocarbon trends, Gippsland Basin from the coast southeast
into Bass Strait corresponding to increasing depth of burial of coal.
|
Fig. 6: Reflectance of vitrinite in offshore Gippsland Basin wells
plotted against depth.
Fig. 7: Simplified cross-section of the offshore Gippsland Basin
showing vitrinite reflectance profiles.
Examination of drill core coal samples obtained from depths between 1050 m and 2300
m in different wells shows a gradual increase in diagenesis with increasing depth,
the carbon content of the dry, ash-free coal increasing from 74% to 81% and the
coal rank from sub-bituminous to high-volatile bituminous.3 By comparison,
the offshore coals are of distinctly higher rank than the main onshore seams of
brown coal at Yallourn (66.6% carbon and Morwell (70.2%), but those seams are only
under between 9 m and 45 m of overburden.
Another method of measuring coal rank is by microscopic examination of the coal
constituents (or macerals), and in particular, the capacity of one of these constituents,
namely vitrinite, to reflect light, quoted as % reflectance. As the rank of coal
increases, so does the % reflectance. Most research workers agree that the main
zone of oil generation from carbonaceous material in sediments and coal is only
reached after the reflectance of contained vitrinite has risen above at least 0.7%.5
Reflectance measurements of vitrinite in Bass Strait drill core samples5,6 indicate a 0.3% to 0.7% range
for most samples (Fig.6). Hence, samples of mature or actively generating organic
matter are not readily available, except perhaps from a few of the deepest wells.
Thus the main source rocks appear to be the lower part of the Latrobe Group at depths
greater than those reached by any of the exploration wells. A simplified cross-section,
Fig. 7, showing vitrinite reflectance profiles based on the gradient shown in Fig.
6, supports the conclusion that organic matter in the lower part of the Latrobe
Group is the source material for the hydrocarbons.
Fig. 8: Section through the Kingfish 1 well showing the depth to
the oil and gas generating zones below the reservoir in the lower Latrobe Group.
Microscopic characterization of Latrobe Group carbonaceous material in core samples
reveals high maceral (or coal constituent) contents of vitrinite (from lignin-rich
plant material) and exinite (from algal and lipid-rich materials): a typical maceral
composition is vitrinite 84%, exinite 12%, and inertinite 4%. The exinite content—waxes
and leaf, pollen, and spore cuticles—is unusually high compared with other
Australian coals. The presence of exinite with a strong greenish yellow to yellow
fluorescence in virtually all the cores examined by Shibaoka et al.5
is further evidence that the upper part of the Latrobe Group is above the maturation
(or oil generation) zone. Hence microscopic work also shows that the upper part
of the Latrobe Group, although rich in exinite, has not acted as the source of the
oil and gas deposits. A few samples from near the bottom of the Barracouta No. 3
well, however, suggests a similar distribution and high content of exinite in the
lower part of the Latrobe Group at depths greater than 4,000m. Shibaoka et al.5
therefore conclude that at temperatures greater than 130°C at these depths it
is reasonable to relate most of the oil to thermal breakdown or cracking of the
lipid-rich exinite. Comparable thermal cracking of vitrinite (and some exinite)
at the same and higher temperatures (at even greater depths) would yield mainly
methane (gas) and a solid non-volatile residue. Thus, on average, the gas has migrated
further than the oil before both have been trapped in reservoirs under the Latrobe
Group unconformity5,7 (see
Fig. 8).
The rate and date of oil and gas formation
Brooks and Smith3 conducted laboratory experiments heating Yallourn and
Morwell brown coals under conditions which would be expected to simulate accelerated
diagenetic (sedimentary burial) conditions. Such thermal treatment in the presence
of water brought about an increase in carbon content corresponding to a conversion
to a high-volatile bituminous (black) coal accompanied by the formation of liquid
and gaseous hydrocarbons from the contained waxes and leaf, pollen, and spore cuticles,
all in a matter of 2–5 days!3 This process is, of course, nearly
identical to the even more rapid coal-to-oil conversion (or liquefacation) of the
same brown coal now under commercial feasibility study.
Using conditions which would duplicate rapid burial of brown coal, thermal treatment
of the Yallourn and Morewell brown coals in the presence of water, brought about
an increase in carbon content corresponding to a conversion to a high-volatile bituminous
(black) coal, accompanied by the formation of oil (liquid and gaseous hydrocarbons)
from the contained waxes and leaf, pollen, and spore cuticles, all in a matter of
2–5 days!— Brooks and Smith, 1969
When endeavouring to correlate their vitrinite reflectance and geothermal gradient
measurements from the various exploration wells, Shibaoka et al.5
found that the best explanation of differences was disequilibrium caused by recent
rapid subsidence of the sediments. However, this disequilibrium not only indicates
a recent rapid event, but an incomplete one. Thus, since the sediments are still
subsiding, new exinite and vitrinite in the sediments are constantly being carried
down into the oil and gas generating zones. As a result Shibaoka et al.5
concluded that ‘hydrocarbon generation must be occurring strongly at the present
time with the products migrating relatively rapidly either into traps or to the
surface.’ This conclusion is consistent with the facts that (a) the hydrocarbon
traps under Bass Strait were full when discovered, and (b) most of the oil in the
reservoirs was low in sulphur5 indicating it has not been extensively
altered by bacterial or other processes.
The above evidence alone indicates recent rapid burial of the coal-bearing sediments
followed by rapid generation of hydrocarbons and very rapid migration of oil and
gas into traps. This is clearly contrary to the popular concepts of slowly forming
coal bearing sediments and oil formations.
Related articles
Further reading
Recommended Resources
References
- James, E.A. and Evans, P.R., APEA Journal
11(1):71–74, 1971. Return to text.
- Hocking, J.B., APEA Journal 12(1):132–137,
1972. Return to text.
- Brooks, J.D. and Smith, J.W., Geochim. Cosmochim. Acta
33: 1183–1194, 1969. Return to text.
- Brooks, J.D., APEA Journal 10(2):35–40,
1970. Return to text.
- Shibaoka, M., Saxby, J.D. and Taylor, G.H., AAPG Bulletin
62(7):1151–1158, 1978. Return to text.
- Kantsler, A.J., Smith, G.C. and Cook, A.C. APEA Journal
18(1): 143–156, 1978. Return to text.
- Saxby, J.D., APEA Journal 18:(1):137–142,
1978. Return to text.
| They say time is money. Well, this site provides over 30 years of information. That’s a lot of money and time. Would you support our efforts to keep this information coming for 30 more years?  | | |
|
