The earth is a spheroid (nearly a sphere) about 12700 km in diameter, depending on exactly where you measure it (there is an equatorial bulge and various bumps). Most of the planet is molten, although the central part is predicted to be Iron and Nickel under very high pressure. These are the stable elements on the binding curve.

The solid outer bit is called the crust and is only about 10 km thick under the oceans ( oceanic crust) and up to 70 km thick on land ( continental crust). The near surface liquid stuff is called magma and when it reaches the surface it is called lava. The liquid core, outer core and magma are in constant motion and it is this which creates the earth’s protective magnetic field.

Plate tectonics

The continents float around on this magma, like the hard skin on hot porridge. The Australian plate for example, is moving approximately northward at a rate of about 7 cm per year (adding complications to accurate GPS locations). As we are the surface of a spheroid, the crust breaks apart at weak spots ( mid ocean ridges) and is pushed under in other parts (the leading edge is subducted and the other side is thrust upwards).

It is this motion of the continental crust (provided by internal nuclear energy), together with the Earth’s water cycle (solar energy) which create our sedimentary processes. We call this soft rock geology.

The thrusted zone creates a mountain range.  Australia is moving northward, creating a mountain range in New Guinea (Nuigini).  Similarly the Indian plate has been moving northward at a much faster rate and has created the Himalayas about 55 million years ago.

Geological Age

The big thing with geology is the time it takes to happen. At 7 cm a year Australia would have moved about 3500 km in 50 million years.  As it is now understood, Australia, India and Antarctica were joined together in a landmass called Gondwanaland.  This breakup began about 180 million years ago in the Jurassic Period with most movement post Cretaceous (about 120 million years ago).  Geologists have divided geological time into various Periods, Epochs and Ages to keep track of what was happening at different times.  A 100 million years is a very long time.  If you had a millimetre of dust settle in your house every year then in 100 million years that would be a 100 km thick pile of dust!

Sediments

Mountains are thrust up and immediately start to erode, principally by the action of water. Rain and snow fall on mountain tops, creating great rivers which erode mega tonnes of sediments along the way (estimated 150 million tonnes of sediment from the Mississippi annually). These sediments are called fluvial sediments and are often coarse grained (sands).  When they flow into depressions, lakes are formed, where fine grained sediments settle out ( lacustrine).

As tectonic processes (plate movement) continue to occur, continents can be stretched with extensional depressions forming and growing into sedimentary basins over time. Depending on exactly where and how they form, many different basin styles form onshore and offshore.

The Cooper Basin in central Australia formed during Permian times (about 300 mya) as an extensional system. It overlies an older basin (the Cambrian to Ordovician  Warburton Basin) and is overlain by the Jurassic to Cretaceous Eromanga Basin and the Recent Eyre Basin.

During the Permo Carboniferous times the Earth came out of an ice age and life flourished in many areas.  The Cooper Basin developed into a very large coal swamp, where tonnes of organic matter were mixed with sediments being deposited.  These sediments became deeply buried over time and became compressed or consolidated into massive sandstones (coarse grains) and coals (another blog).

A massive extinction occurred to end the Permian Period. The Cooper Basin underwent tilting and erosion on its western flank and was covered by fine grained Triassic aged sediments.  These were in turn overlain by massive sands of the early Eromanga Basin deposition.  These massive sands were subsequently buried and compacted to become the Hutton sandstone, which is part of the massive Great Artesian Basin (GAB).

Coarse grained sediments form porous rocks while fine grained sediments form impermeable layers (mudstones, shales etc).  Over millions of years these form layered systems with various degrees of connectivity. The GAB is connected across to the mountains on the east coast of Australia and are filled with groundwater from rains and floods. In the west this water will flow to surface under its own pressure when a well is drilled into it.

Oil and Gas

When large amounts of organic matter are buried in swampy conditions (peat bogs), heat and pressure will cook this organic matter into coals.  Where organic materials are more dispersed in lacustrine systems organic shales will be deposited.

Biogenic gas (methane) will be released almost immediately the organic materials are deposited.  Over time and temperature all the other hydrocarbons will be cooked out.  Oil and gas will be formed in the system and expelled under pressure into any available pore space.  Oil and gas are less dense than water and will float to the top.  Where an impermeable layer exists the oil and gas will be trapped.  As it fills the trap it spills into the next available reservoir.

In the Cooper Basin massive amounts of gas were cooked from the coals and shales in the area and were trapped in sandstone reservoirs. Lesser amounts of gas liquids and oil products were also created.  Erosion on the western flank ( and in other areas) at the top of the Permian sediments has allowed substantial amounts of oil and gas to migrate up into the next sandstone, the Hutton.  Gas is slightly soluble and has become incorporated in the GAB waters, while oil has been trapped at the top Hutton.

Today we drill for gas in the Permian sediments and oil in the Jurassic sediments.  To find these deposits we need to map the subsurface using the seismic method (another blog).  Where water wells are drilled into the southern or western GAB, dissolved gas (methane) escapes with the water and can easily be set alight.  This is a common occurrence in aquifers that are exposed to coals or organic shales.

Marsh gases

When organic material decomposes in swamp conditions many different gases can form.  Methane ( CH4), Phosphine (PH3) and Hydrogen Sulphide (H2S) are three likely gases.  Hydrogen sulphide is also called rotten egg gas because of its atrocious smell.  When it is concentrated it has no smell but is very poisonous.  Many a life has been lost by sitting or sleeping next to a swamp on a still night due to Hydrogen sulphide poisoning.

Phosphine is a derivative of phosphorous and has the same property that it will self ignite.  When mixed with methane it creates its own fireball with no human intervention.  In olden days it was called Will o’ the Wisp or Jack of the Lantern and was associated with fairies and other supernatural beings.  In Australia we have the Min Min lights from this phenomena.

Scientists have only started making sense of this and other similar phenomena ( geological batteries, piezo electric lights etc) in recent years.  I think most people probably prefer the goblins, ghosts and fairies explanations!

 

Leave a Reply

You must be logged in to post a comment.