Page 13 - Well Control for Completions and Interventions

8 Well Control for Completions and Interventions

Figure 1.3 Limestone outcrops in the Yorkshire Dales (United Kingdom).

Evaporites form when a body of saline water evaporates. As the salin-
ity increases, chemical precipitates build up in layers. Common evaporates
are gypsum, anhydrite, and halite (rock salt). Halite is of interest to oil-
field geologists, since some oilfields form around salt domes.
Most organic sediment is classified as “carbonate,” including limestone
and dolomite formations. Organic sediments are the skeletons of dead
marine creatures that sink to the bottom of the ocean. Over time, the
carbonate material builds into beds that can be many hundreds of feet
thick. Carbonate formations are an important reservoir rock, accounting
for approximately 60% of the world’s oil and gas production (Fig. 1.3).
A few hydrocarbon reservoirs are found in naturally fractured crystal-
line basement rocks, such as granite or basalt.

1.1.3.2 Hydrocarbon traps
Many theories concerning the origin of oil and gas have been advanced
over the years. Current thinking is that hydrocarbons are produced
through a complex chemical reaction involving the bacterial decay of
phytoplankton and algae. Once formed, hydrocarbons, being less dense
than surrounding formation water, migrate upwards from the source
rock. If there are no impermeable barriers in the overlying formations,