To get to Lakes Entrance from Melbourne follow the Princes
Highway to Bairnsdale and continue East via the coast road. To find
the cache you will need to drive a few hundred metres along the
unsealed Old Lake Tyres Road to a roadside picnic area.
Lakes Entrance is well known for its endless beaches, beautiful
coastal lakes and great fishing but it is less well known as the
historic home of Australian oil. The first real Australian oil
field was found at this site, near Lake Bunga, in 1924. Not only
was the first oil field found here but eventually, after much
exploration, very large commercial quantities of oil and gas were
discovered in associated off shore deposits. They are further
described in Earthcache Well well well two.
Several more wells on-shore lead to the discovery of the Lakes
Entrance oil pool. Sixty four wells, drilled between 1930 and 1941
produced over 3000 barrels of oil and a further 4935 barrels of
very heavy crude were extracted during WWII. While this field is
estimated to contain over 50 million barrels the viscosity of the
oil and the geological conditions limit probable recovery to about
50,000 barrels making it uneconomic to develop further.
Oil and gas are hydrocarbons that form inside some rocks
because the sediments that formed the rocks originally contained a
variety of organic debris from plants and animals. Deeply buried
sediments experience increased pressure and temperature and the
organics in them undergo a series of changes and the complex
molecules in them 'crack' or break down into smaller molecules that
eventually form oils and gases. This takes a long time and source
rocks for most oils are millions of years old.
Not all oil and gas gets trapped in the rocks they form in and some
does not get trapped at all. Oil fields are not gigantic holes in
the rocks but are in fact vast volumes of rock containing huge
amounts of very tiny spaces in between the grains of sand that make
up the sandstones. This is called the
porosity of the rock. The spaces are all joined up, allowing
water, oil and gas to move from one place in the rock to another.
This is called the
permeability. The more permeable the rocks, the more easily
fluids move through the rocks.
If the rocks where the hydrocarbons form are permeable it is
possible for the hydrocarbons to migrate from where they form to
either the surface or to another place in the rocks where they may
get trapped. Surface leaks, called seeps, allow the hydrocarbons to
escape and get dispersed by surface processes. This happens in many
places around the world and is a tell tale sign of hydrocarbon
formation in the rocks below.
Hydrocarbon traps form in rocks in many ways but usually form
because oil and gas floats on water and rises to the top and
encounters another rock which is not permeable. Oil and gas
reservoirs are chance combinations of rocks that:
- have lots of organics buried in them
- have been buried deep enough and long enough to form oil and
gas
- are both porous and permeable
- have been faulted, folded or tilted in such a way that traps
keep vast volumes of oil and gas inside them.
Here, off-shore at Lakes Entrance, these rocks have that chance
combination in abundance.
To log your visit to this site you need to send photos of your
visit and answer the following questions to GeoGeckoEd (through
profile above):
a) At what depth was oil encountered in Bunga-1?
b) What is the tall metal structure at this site intended to
represent?
c) What type of rock was Bunga-1 oil found in?
d) What was oil found in association with at Bunga-1?
Once verified, you can log your visit, but you must include the
number of people in your group who visited this Earthcache.