How is porosity defined in a geological context?

Porosity in a geological context is defined as the percentage of space between the grains of rock. It indicates how much void space is available within a rock formation, which is critical for understanding fluid storage and movement within reservoirs. High porosity means that the rock can hold a significant amount of fluids, such as water, oil, or gas, making it an essential parameter in fields like petroleum geology and hydrogeology. This definition is central to evaluating the potential of rock formations for resource extraction and groundwater storage.

In contrast, the other definitions provided do not accurately encompass the concept of porosity. The volume of water contained in rock pertains more specifically to saturation rather than porosity itself. Similarly, density of the rock material and weight per unit volume relate to the physical characteristics of the rock without reflecting the significant aspect of void space that defines porosity.