2) Fracking might have led to fundamental changes in the sector in the sense that the investments undertaken are very different from offshore drilling, for example. Fracking companies can very quickly increase production in response to higher prices, or decrease production if the price of oil falls beyond a certain threshold value, which would make fracking unprofitable (I have read a few years ago that this could be somewhere below 30 dollars per barrel, but nowadays it might be even lower still).
3) Never reason from a price change ! (This is one of the fundamental laws of economics). Prices change either as a result of a shift in demand, or a shift in supply (or both). In order to figure out what's going on, you need to understand what exactly has changed.
So why has the price of oil plummeted last week?
I can think of two things. First, it looks like global growth is slowing down, maybe even quite substantially. Eurozone GDP growth looks already looked weaker last quarter and probably will go down even further this quarter. At the same time, the ECB seems to be determined to end Quantitative Easing and move towards a tighter monetary policy, which will slow down Eurozone activity. At the same time, it also looks like the Chinese economy is slowing down too, even though we can't exactly be sure of that yet as of now because the Chinese GDP figures are inaccurate. Finally, while the US economy is on par to grow at about 3% this year, this is mostly due to the expansionary tax cut. The Fed is tightening as well and US growth will slow down in 2019. It thus looks like the global expansion is loosing some speed, and maybe unexpectedly so. The decrease in demand will lead to a lower price of oil, all else equal.
The second factor seems to be supply related. The OPEC cartel is much less powerful than it once was. Russia is now the second largest oil produces in the world, before Saudi Arabia. Even more surprising, thanks to the fracking revolution in the US, the US is now the largest oil producer in the world. Additionally, it looks like the US is becoming self-sufficient and thus might become a net exporter of oil any time soon. The increase in supply has thus created a general glut, which might last for a few years. The average oil price since 2013 has been relatively low precisely because of all the additional supply that has been put on the market by American oil producers (see long-term chart below). Don't expect this to change any time soon.
PS: Of course, the low oil price is bad news for global warming since it leads to more consumption and it also lowers the incentives to come up with an alternative energy sources.
Oil production by country, Top 10
PPS: Just looking at the production figures, it seems unlikely that a major Iran crisis would have large effects on the oil price. There definitely won't be a repeat of the 1970s. Also, only 5 out of the top 10 oil producing countries are OPEC countries and they are not even in the top half of the table.