While rock mechanics can be a very complicated subject, there are a few basics that all petrophysicists will need in their day-to-day work, which will be covered here. In a normal reservoir, the formation rock is subject to greatest stress from the overburden. This stress arises from the weight of rock above and can be measured by integrating the density log to surface. Since density logs are not usually run to surface, a common working assumption is that the overburden stress is approximately 1psi/ft.

The vertical strain (i.e., compaction) caused by this stress is offset by the formation pressure, which helps "support" the rock. Because the structure is usually partially open-ended, the fluid will take only a proportion of the overburden stress. However, in overpressured reservoirs where the fluid is not free to escape, the formation pressure may become close to the overburden pressure. The net effective vertical stress seen by the formation is given by:

This is actually not the true effective vertical stress, which for given conditions of P _overburden and P _formation would result in the same strain in the sample if applied with zero pore pressure. This will now be demonstrated. Let K _m equal the bulk modulus of the matrix, when the pore pressure equals the vertical stress, defined by:

Let K _b equal the bulk modulus of the dry rock, as measured in a normal core measurement, defined by:

The strain is given by:

The true effective rock stress is...