A porous medium, e.g. an oil or gas reservoir is normally composed of rock and fluid. However, its pressure profile is calculated as a function of fluid type only. For the case of water occupying porous media, the gradient is taken as 0.45 psi/ft (or something similar), which becomes less for oil and gas reservoirs. The problem here is why the pressure of rock is ignored.
Reza,
During deposition there is a full continuous column of fluid (water) and to calculate the pressure of fluid at any point like in cylinder or barrel you take its gradient. Now the oil and gas also are continuous columns of fluids that happened during migration from source rock beneath the reservoir mostly. As for your question regarding the rock, it is taken into account as second factor in the process which as was mentioned to you the stress (overburden stress). So, think of it this way: column of water and each droplets of water at certain depth carries the pressure of the fluid it is part of which is water in this case. Any additional pressure due to rock is carried by the matrix (rock grains). During injection and increasing the pressure of the fluid as you can imagine there is less pressure now need to be carried by the matrix and vice versa during depletion there is more pressure to be carried by the matrix which results in subsidence.
I hope this explains
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