So far we have looked at how a Sedimentary Basin forms in a mid continent setting (intra cratonic). These basins have continental sediments, from a nearby provenance (mountain range) and are often coal and gas prone. Australia typically has plenty of coal and gas but little oil. Continental areas may also have some marine sediments if the area was once inundated by sea, or if the area has been uplifted by plate tectonic forces. Many other deposition situations can occur elsewhere, including deltas and marine.
Rivers transfer massive amounts of sediments and organic matter and when they reach the sea (or large lakes) form deltas. This is where a lot of the sediment load is dumped if conditions are right. If not, the sediments are further transported out to sea. Deltas dump sediments in piles and as these build up they start to block the flow of water and switch to a new exit path.
These sediments are unconsolidated, essentially like a beach sand, with layers of organic material and fine silts. Water is trapped in the sands and as the organic material starts to decompose, biogenic gas (methane) forms. As more sediments are piled on top, the original sediments are compacted, either releasing the water and gas if seals are inadequate or burying them as gas fields. With more sediments piled on, pressure and temperature increase and more oil and gas are cooked out.
During the compaction phase gas and water are likely to be expelled causing the sediments to subside (ie take up less space). Additionally these sediments are likely to have dissolved salts in high concentrations. Arsenic, for example attaches to iron oxides and is commonly found in deltas.
Modern cities and subsidence
You might think that the last place to build a modern city would be on a major river delta. We like to build our cities where fresh water and harbours are present and so deltas are common place – eg New York Hudson River), New Orleans (Mississippi), Dhaka (Bangladesh)(Ganges), Venice (Po)etc. We also love to build them next to flooding rivers and active mountain building locations!
So a city built on a delta will suffer from flooding (delta switching), subsidence and poisoned ground water. About 150 million people world wide are affected by arsenic poisoning in ground water (naturally occurring), with a particular problem in the Ganges delta in Bangladesh. In Venice for example, ground level is subsiding by 1-5 mm a year. Groundwater withdrawal (from bores) assists this process.
In Australia we don’t have much in the way of modern accreting deltas but we do have subsidence in some local areas. Notably both Port Adelaide and Fremantle suffer subsidence at a rate of about 2-3 mm /year.
Isostasy and Eustacy
When anything heavy is put on a floating continental plate it pushes that part of the plate down. When lifted the plate rises up again. This process is called isostasy. When a large dam is built, the weight of the water causes the land mass to sink. A large mine site/ quarry will create a hole and the land mass will rise around that area. These processes cause earthquakes while the land mass equilibrates. Most of these are very small (there are thousands of small earthquakes around the world daily), but some are significant. A Chinese study after the Three Gorges Dam was built found that there were about 3 times the number of seismic events compared to before the dam, with one reaching a magnitude of 4.1 (Richter).
During the last Ice Age a huge weight was placed on the north of the planet with the kilometres thick ice caps extending into central Europe, Asia and North America. Once all this ice melted these land masses started to rebound. The weight was so significant that the earth is still rebounding today. In the UK for example Scotland is rebounding at about 10 cm / 100 years while southern England is experiencing a corresponding downward movement of about 5 cm / 100 years, exacerbating modern day flooding there.
Eustacy is sea level movement in response to the volume of water available and the size of the hole that it sits in ( ie the size of the oceans). During the Ice Ages, so much water was locked up as ice that sea level was about 120m lower than it is today. In geological time, sea level is continuously rising and falling in response to plate tectonics and global temperatures. These sea level changes have been studied very thoroughly by sedimentary geologists as they are the boundaries that geophysicists regularly interpret on seismic data. The location of a delta will be in an entirely different place when sea level is 120 m lower!
So between plate tectonics, isostasy, subsidence and global temperature changes, Mean Sea Level (msl) can be hard to determine accurately. The large scale stuff of hundreds of metres is reasonably well documented. The small scale stuff is much harder. Even with satellites, the shape of the earth, influence of the moon (tides) and weather (pressure systems) will influence MSL.
In New York for example, ground level is subsiding due to sediment compaction, the weight of the city and ground water extraction. Ground level is also rising due to new sediments coming in and glacial rebound (New York was under a massive glacier about 150000 years ago). Sea level is also rising due to recent melting glaciers and warming oceans (hot water has more volume than cold). Storms and low pressure systems just add to the problem (water level will rise under lower pressure).
Once sediments have reached the edge of the land mass, a significant part of the sediment load will continue offshore. Great canyons are carved into continental shelfs where rivers continue past the delta system. and sediments are then deposited in the ocean. Where these rivers continue into the ocean they are called turbidity currents and they deposit large amounts of coarse grained sediments. Fine grained sediments and organic matter form a cloud initially and when conditions are right, they settle on the ocean floor. In this way they create an organic mud. When buried and compacted these form organic shales which are rich source material for oil formation. Gas will also form along with the oil and where these are trapped in the coarse grained sandstones, they form oil and gas reservoirs.
Reefs will also grow in shallow oceans and when buried become carbonate reservoirs. Reefs and carbonate platforms are ephemeral (they do not survive for long). At the same time, they are plentiful during geological history, particularly during warm periods. The Great Barrier Reef for example can only be about 10000 years old. Before that, during the last Ice Age, sea level was 120 m lower, meaning that the land that the reef now sits on was about 100 metres above sea level!
Water is known as the Universal Solvent due to the fact that pretty much everything dissolves in it to some degree. In the Himalayas there are vast amounts of rock salt. This was deposited in early seas and then uplifted many kilometres above sea level. Today it is commercially mined and sold as the pink rock salt many of us use. It tastes great. Salt deposits left after the sea dried contain pretty much every element that occurs naturally and this is why Himalayan Rock salt suppliers advertise that it contains 84 trace elements (out of a naturally occurring 92). Whether it does or not is arguable, but if it does it means it contains highly poisonous substances such as polonium, radium, arsenic, thallium etc. Enjoy it but don’t believe the nonsense.
What it does mean though, is that marine sediments, in particular fine grained sediments will trap many dissolved elements and molecules. Coarse grained sediments are more likely to be flushed (in either continental or marine situations) and are less likely to contain contaminants.
Marine rocks can be uplifted and now be found in continental settings. Where they are drilled into or mined, they are more likely to contain a variety of dissolved salts and poisonous elements.
In South Australia, our gas fields are generally within relatively clean, coarse grained continental sandstones. In Central and Eastern US, much of the oil and gas is sourced from a black, marine, Devonian aged shale called the Marcellus Formation. The shales also contain iron, pyrite and uranium in significant quantities. Drilling and extracting hydrocarbons from this will be much more problematic.
Gas can be extracted from continental sandstones (eg Cooper Basin and Otway Basin in SA), marine sandstones (WA Browse Basin etc), from Coal beds (in Australia from Eastern states Permian aged coal beds) or continental or marine shales. It can also be extracted from carbonate reservoirs (marine reefs), frozen methane called clathrates (generally in deep sea environments) and land fill.
Each of these deposits require different techniques to extract the gas and each may use either vertical or horizontal drilling together with some form of stimulation (fraccing). The next few blogs will go through some of these processes together with similar processes in water drilling (groundwater) and coal extraction.
Should be fun!!