Jody,

I thought I'd provide you with some basic understanding of petroleum geology prior to getting into fracking.

There are four major components to the classic oil reservoir:

Source Rock
Fluid pathway
Resevoir rock
Trap

First of all, we need a source rock. Despite common knowledge, it is not dinosaurs remains, but all the microscopic goo that is created in a marine setting - plankton, algae, and fish shit. Back in the day, many millions of years ago, there were fairly shallow seas that were spread out over the continents. they were teaming with life, whose remains and what not settled to the seafloor creating an organic rich shale, a few centimeters a year, for millions of years, creating deposits thousands of feet thick. How is it possible that a shallow sea creates deposits thousands of feet thick? Well, the sediments, as the they thicken, push the crust downward, allowing the deposits to grow thicker and thicker. Eventually, as the the seas receeded and the land lifted due to plate tectonics (called a regression), the mud was buried under new sediments shed by the erosion of the new mountains. Mud was followed by silt, silt by sand, limestone coral reefs, then sand, and coarser gravels as the seas gave way to more terrestiral sediments. As the mud was buried deeper and deeper, the pressures and temperatures increased, and all that organic material began to change to new things, like kerogen, a waxy material that is the precursor to petroleum. Eventually, the kerogen was distilled into all kinds of hydrocarbons - tar, oil, and natural gas.

But shales (what becomes of mud after this process) in not very porous (the spaces between the tiny little mud grains is very small) nor permiable (few pathways between those grains for fluids to move). However, given the high pressure, and that these hydrocarbons get squeezed out of the source rock, and a whole lotta time, they start to move.

These fluids will find pathways within the strata to escape - faults and joints and cracks that are created when the rock is buried and when the rock is pushed by plate tectonics into folds - like pushing a carpet from one end.

As the fluid move upward, they'll find another rock unit, like a sandstone, that has lots of space between the grains for fluids to inhabit. Picture a sponge full of small holes that can fill with all of this hydrocarbon fluid. This is the reservoir rock. A rock with lots of interconnected holes that contain the oil, water, and natural gas.

Now unless there is another impermemiable layer above that reservoir rock, or that layer is cracked, the fluids will just keep on travelling upward until it reaches the surface; where it will pool on the ground as an oil seep.

However, it there is a shale unit above the reservoir, that upward migration is kept in check, creating a trap. These are often in folds of rock known as anticlines. Picture that folded carpet again and see how half of the folds create a ridge. If the hydrocarbons can collect in one of those ridges, and there is a good seal above them, then that pool of hydrocarbons just sits there, seperating out into layers of water, natural gas, and oil, getting more and mor mature and pressurized as time goes by. The great Texas oil fields are a good example. they were under so much pressure they'd blow out the rig when drilled into - a gusher!

So when you picture an oil field, don't think of a big open cavern full of oil in the ground, picture a rock that't like a sponge under pressure.

...more later...