Chapter 4: Geology

This chapter presents those geological considerations, from microscopic to macroscopic, that affect waterflood performance. The discussion is limited to the major internal geological characteristics and pore systems of the oil-bearing rocks.¹ Other details of rock characteristics and peripheral geological factors such as plate-tectonic processes and hydrocarbon-generation, -migration, and -trapping mechanisms are considered beyond the scope of the chapter.

Reservoir rocks form through a sequence of geologic events and processes that occur over geologic time spans, typically tens of millions of years. Fig. 4.1 illustrates a simple schematic of the key geologic processes: deposition, burial, and diagenesis (Slatt 2006). During deposition, the sediment’s grain size and sorting, which are initially and usually the primary controls on rock quality, are established. Also, the areal size, thickness, and internal relationships of the various sedimentary beds are fixed during deposition and subsequent burial. Initially, the sediments are deposited as discrete beds of unconsolidated carbonate or terrigenous clastic grains. Several changes occur to a sedimentary bed as sedimentation continues, namely (1) the weight of younger sediments compact and reduce the pore space (porosity), (2) pressure and temperature increase, (3) lithification typically occurs as unconsolidated material is solidified, and (4) a variety of diagenetic processes such as cementation and leaching can occur. Over millions of years, there can be several cycles of chemical cementation and leaching, but typically rock porosity decreases as burial depth increases. The result of all these geologic processes is an oil reservoir that is discovered and produced, typically through a combination of primary production, waterflooding, and possibly enhanced-oil-recovery processes.