TABLE OF CONTENTS Chapter 1 showed that one of the five essential prerequisites for a commercial accumulation of hydrocarbons is the existence of a reservoir. Theoretically, any rock may act as a reservoir for oil or gas. In practice, the sandstones and carbonates contain the major known reserves, although fields do occur in shales and diverse igneous and metamorphic rocks. For a rock to act as a reservoir it must possess two essential properties: It must have pores to contain the oil or gas, and the pores must be connected to allow the movement of fluids; in other words, the rock must have permeability.
This chapter is concerned with the nature of hydrocarbon reservoirs, that is, with the internal properties of a trap. It begins by describing porosity and permeability and discussing their relationship with sediment texture. This discussion is followed by an account of how postdepositional (diagenetic) changes may diminish or enhance reservoir performance. Following this discussion is a section on the vertical and lateral continuity of reservoirs and the calculation of oil and gas reserves. The chapter concludes with an account of the various ways of producing hydrocarbons from a reservoir.
Porosity is the first of the two essential attributes of a reservoir. The pore spaces, or voids, within a rock are generally filled with connate water, but contain oil or gas within a field. Porosity is either expressed as the void ratio, which is the ratio of voids to solid rock, or,...
