Petroleum geology is largely concerned with the study of fluids, not just the oil and gas discussed in Chapter 2, but the waters with which they are associated and through which they move. Before examining the generation and migration of oil and gas in Chapter 5, the subsurface environment in which these processes operate should be considered.

This chapter begins with an account of subsurface waters, then considers pressure and temperature and their effects on the condensation and evaporation of gas and oil. The chapter concludes by putting these ingredients together and discussing the dynamics of fluids in basins.

4.1 SUBSURFACE WATERS

Two types of water can be recognized in the subsurface by their mode of occurrence:

1. Free water

2. Interstitial, or irreducible, water

Free water is free to move in and out of pores in response to a pressure differential. Interstitial water, on the other hand, is bonded to mineral grains, both by attachment to atomic lattices as hydroxyl radicals, and as a discrete film of water. Interstitial water is often referred to as irreducible water because it cannot be removed during the production of oil or gas from a reservoir.

Figure 4.1: Eh-pH graph showing the approximate distribution of the various types of subsurface fluid.

4.1.1 Analysis

Subsurface waters are analyzed for several specific reasons, apart from general curiosity. As discussed in Chapter 3, the measurement of the resistivity of formation water ( R _w) is essential for the accurate assessment of S _w,...