The reservoir can be divided into two sections (1) Rock (2) Fluid, whole reservoir engineering revolves around these two elements and their interaction at different pressure and temperature conditions. These interactions are further complicated by injecting different fluids to pump out residual hydrocarbons from the reservoir (EOR techniques).
Rock properties mainly consist of fundamental questions of porosity, permeability and saturation determination. While fluid properties are the determination of fluid thermodynamic behavior. First by measuring the composition and then using EOS (equation of state) models to predict behavior at different pressure/temperature conditions. Finally, rocks and fluids are combined to determine their interactions, as properties of both of them can change when they are combined and the world becomes more complex/interesting once rock & fluid are combined. For example: when flow occurs in nanopores (due to variation in density near the walls) critical points of fluid changes. Similarly, relative permeability changes due to slippage of molecules on the pore walls. In some unconsolidated sands rock compressibility changes drastically (Ekofisk, Norway). Along with the change in individual characteristics of the elements (rock/fluid), the combination results in the definition of newer properties like surface tension, capillary pressure etc. Finally, Once enough data is available about the system reservoir modeling is done using geostatistical techniques and mathematical simulations are used to predict future behavior of reservoir.
Engineers have three different methods to “MEASURE” these properties at different scales (a) Core analysis (b) Well-logs (c) Well-Tests (pressure/rate transient). But, countless empirical correlations & methods (generally, adopted from chemical engineering) exists to “CALCULATE” combined properties of rocks, hydrocarbons & their interactions.
This site deals with newer methods and models which can be envisioned so measure/calculate these properties using laboratory experiments/theoretical laws.