Good question, but I don't know the answer. Maybe a petrophysicist will know, but I suspect the answer depends on the type of source material, pressures, depths, etc.
And how did it get so deep?
Primarily plate tectonics. The crust is moving all the time, even today. Mountains are forming, mountains are eroding, and plates are sliding over one another (resulting in volcanic activity). A plate is forced up (to form mountains) and the opposing plate is driven deep into the crust.
I have seen the theories about plate tectonics being the cause of organic matter being drawn into the depths, but does it really compute?
Yes, it does compute. There is often fossil evidence retrieved in core samples. For example, you might find brachiopod shells at a depth of 12,000 ft. These are fossilized remains of shellfish formerly on the sea floor.
You can find brachiopods here in Ohio. I found a Neospirifer Rocky Montanus brachiopod completely intact from a gravel pit here in Fairborn, Ohio. The clam was shut, and you could see mud jammed along the crack. Perfectly consistent with sudden death by mud. Based upon my estimate from holding the specimen in my hand, it was more like 3000 years old, not the 300 million you find in the literature. So yes, the ocean was here, but it was only for a visit. If you accept the theory that every process must happen very slowly, then you have to concoct a wildly unbelievable theory about how the ocean, over zillions of years occupied the North American continent, then retreated, then came back again, etc....You should reread the silliness that is used to explain the Grand Canyon. On the other hand, even tectonic plates can be subjected to unspeakable violence. I can see how Mt. Sill could rise suddenly 10,000 feet above the desert plains of Nevada, by the way it does have the best view of the Sierras. Having seen the Sierra crest, but not the provenience that claims a clam was found 12,000 feet under, what should one believe?