To get to Lakes Entrance from Melbourne follow the Princes
Highway to Bairnsdale and continue east via the coast road. The
cache is at a roadside look out on a high point just to the west of
Lakes Entrance. On a clear day you can see oil rigs, even without
the pay-as-you-go binoculars provided.
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 near Lakes Entrance in 1924 [see Earthcache Well
well well one]. 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 that this cache points out.
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 gas
and liquids 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, that are both porous and permeable and
rocks that 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. Most of the hydrocarbons here are reservoired in
coarse-grained, higly permeable sandstones in the suite of
sedimentary rocks called the Latrobe Group.
In 1965 the Glomar-III drilling vessel discovered gas at a depth
of 1060m. This site is now known as Barracouta-1. The large Marlin
gas field was discovered near by in 1966. Shortly after, in 1967,
Australia's largest oil field was discovered at Kingfish-1. This
field had an estimated 1.2 billion barrels of recoverable oil. An
amazing 15 of the first 16 wells drilled found hydrocarbons and
included the major gas fields of Barracouta, Marlin and Snapper and
Australia's two largest oil fields, Kingfish and Halibut. By 1970
five of the eleven fields discovered were in production. It may
have all began in 1924 but the 1960's saw the rocks off shore of
Lakes Entrances become the most important oil and gas field in
Australia.
Discoveries of commercial and non-commercial deposits continued
in the 1970's with Cobia in '72, Sunfish in '74, Hapuku in '75 and
Fortescue, Seahorse and West Halibut in '1978. In the 1980,s came
discoveries at Basker and Manta, West Seahorse, Baleen and Sperm
Whale and the Blackback oil and gas field.
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) How many gas and oil rigs are pointed out by this cache?
b) How far is it, in kilometres, to the first gas site at
Barracouta?
c) How far away, in nautical miles, is Tasmania?
d) Is the Marlin field SSW, SSE, NNW or NNE of this cache?
Once verified, you can log your visit, but you must include the
number of people in your group who visited this Earthcache.