In petroleum exploration, the geological model is developed and then proved, disproved, or modified by drilling. The subsequent commercial decisions are based on the existence and mobility of hydrocarbons within the pores of the reservoir rock under downhole conditions.
When a hydrocarbon-bearing formation is cored, the properties of the fluid contained in the pores changes as the core is brought to surface. Most of the more volatile components leak away during the decompression process and during surface handling.
Hence wireline logging, was developed. In order to understand the geological significance of the output from these logs, the science of petrophysics was developed. The aim of petrophysics has been formulated as follows:
"To determine, from laboratory measurements on rock samples, and from measurements with instruments in boreholes, the storage capacity of the rock, called porosity; the hydraulic conductivity called permeability; the fraction of the pore space filled with hydrocarbons, called saturation; and the acoustic velocity."
Mud logging was developed to sample the formation and fluids from the returning mud stream. However the uncertainty of the origin of the data has resulted in a depth uncertainty similar to that from seismic data.
Petrophysical data has been revolutionised by digitisation. This has bred a vast industry, within oil companies and their contractors, to handle, collate, merge, stretch, cross plot, compare and visualise this data, on both an inter- and intra-well basis. The very ease and cheapness with which these operations can now be done means that petrophysical databases have now become key assets for oil companies.