First, what it isn't; it isn't oil and it isn't shale. The rock is a marlstone rich in calcium carbonate that is intermediate between mudstone and limestone. The black stuff is kerogen, a precursor to oil.
Because kerogen is a mixture of organic material, rather than a specific chemical; it cannot be given a chemical formula. Indeed its chemical composition can vary distinctively from sample to sample. Kerogen from the Green River Formation
oil shale deposit of Colorado contains elements in the proportions Carbon
215 : Hydrogen
330 : Oxygen
12 : Nitrogen
5 : Sulfur
In order to convert kerogen to oil it must be heated to high temperatures. Nature converts kerogen into oil by burying the rock to the point where the temperature and pressure is sufficient to do the job. Colorado's oil shale was never buried deeply enough to reach the conditions of converting the kerogen to oil. Historically, humans have tried to do this artificially by bringing oil shale to the surface and roasting it in kilns. Today, instead of bringing the shale to heat, companies are using different techniques to take heat down to the oil shale.