To clear up a few possible misconceptions about oil reservoirs, if you think of a pan full of marbles, the marbles would be the grains in the rock, the spaces between them is porosity.
Depending on the composition and type of rock, where/how it was laid down, and the alterations which happen to it with burial and the pressure from the weight of the rock above it, that porosity will either become better developed or degrade or be filled in.
That porosity is almost always filled with fluid at depth, most commonly salt water (most of that rock was laid down in an ocean), and the water is squeezed out of other rocks as they are compacted (or was caught up in the rock to begin with).
If there was a significant amount of organic matter (plankton, really or plants--dinosaurs are only a very tiny fraction), the hydrogen and carbon are present to act as building blocks to make oil and gas.
With pressure and the increased temperature of burial, oil and gas can form, and if the pore spaces are connected, migrate to accumulate in areas which are higher (like the tops of folds in the rock), or even to the surface in areas where there is no cap rock to contain them. Oil can also go lower, into rock with pores from the source layer (like in the Three Forks from the Lower Bakken Shale), but that is less common.
A cap rock is simply rock that the oil cannot migrate through, it has no effective connections between any pores present (no permeability).
We refer to those accumulations of oil as "pools" or "pockets" but for the most part, they are still spread through the pores in the rock, like water in a sponge. Almost every oil producing rock formation has both water and oil, but the concentration of oil, the shape of the pores, and the permeability of the rock all work to make it possible to extract the oil and make money doing so.
Fraccing (hydraulic fracturing) is a production technique which allows us to enhance the natural spaces in the rock the oil moves through on its way to an already drilled hole by cracking the rock the oil is in, providing an easy way for the oil to get to the wellbore and ultimately the surface. It does increase reservoir pressure briefly, as the pressure of the fluid pumped in has to be enough to break the rock. That excess pressure diminishes quickly during the initial production phase, and ultimately, the rock supports the rock above it, as it always did, just with some new cracks propped open with sand grains pumped in to keep those cracks open.
Because the oil is disseminated in the rock, there is no cavity to collapse, so that won't cause an earthquake.
The only possible link to oil wells that may increase seismic activity comes with the injection of salt water produced with the oil back into the rocks in injection wells. If there is a fault in the area, it is possible that the increase in fluid pressure, if it occurs along the fault, can cause that fault to be lubricated and release seismic stresses in the form of minor earthquakes.
The anti-oil lobby has been trying to make this out as if it will cause huge earthquakes on the order of the Alaska Earthquake of the early '60s or 'the big one', but if the faults aren't already there, there won't be an earthquake at all.
Look at North Dakota, where saltwater injection from oil wells has been going on over 50 years.
Not on the hit parade of seismic hotspots.
So, the real problem here, is that in the rush to judgement of the oil industry, other seismic hazards may be ignored which may just be coming to light after we have been keeping records for a mere couple hundred years (a mere eyeblink in geologic time), and ignoring those hazards might prevent understanding potential seismic threats (even relatively minor ones) along previously unknown faults which have always existed but are just becoming manifest.
When relatively recent earthquakes in Southern California reveal faults which had been hidden at the surface, despite the intensity of seismic study in that area, it would come as no surprise that new faults would be revealed in areas of the continent where previously they had not been documented.
Bump.
Thank you for taking the time to explain the geology of oil deposits.
If I may ask, what “chemicals” are injected in fracking oil and gas? Do lefties’ complaints about underground water “poisoning” and smell justified?
NY outlawed fracking without any supporting studies that shows harm to the environment. Exactly the opposite in case of the XL Pipeline where “environmental and State Department studies” CLEARED it but no, it won’t be built until the bastard leaves office.
Asto your comment:
“When relatively recent earthquakes in Southern California reveal faults which had been hidden at the surface, despite the intensity of seismic study in that area, it would come as no surprise that new faults would be revealed in areas of the continent where previously they had not been documented.”
We have undocumented faults and it doesn’t mean that when discovered, it must be our fault, no pun intended.
There is always unforeseen and unintended consequences when we fool around with Mother Nature, that includes extracting oil and gas and changing some physical balances and conditions below surface near an undiscovered fault. However, we do have to live, don’t we?
Again, thanks for taking the time.