Page 30 - Drilling Technology in Nontechnical Language

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OIL AND GAS RESERVOIR

FORMATION

Overview

Oil and gas (collectively called hydrocarbons) are rarely found in the
rocks where they were generated from the organic remains of plants and
animals. As hydrocarbons are generated, they migrate upwards within the
earth until they meet a barrier, where they may accumulate as an oil or
gas reservoir. If they do not meet a barrier, they will eventually reach the
surface, where they will be visible as an oil seep. The lighter hydrocarbons
will evaporate, possibly leaving behind the heaviest hydrocarbon elements
(e.g., the tar sands of Alberta, Canada).
There are seven recognized factors that all need to be present, in the
correct order, for a petroleum accumulation to occur. These are called the
Seven Pillars of Wisdom for oil accumulation and are given in the correct
sequence in the following:
1. Source rock
2. Hydrocarbon generation

3. Primary migration to a suitable structure (reservoir)
4. Structural trap
5. Reservoir rock
6. Seal rock
7. Secondary migration within the reservoir

This chapter will describe briefly the processes involved in petroleum
generation, migration, and accumulation into an exploitable reservoir. The
important rock properties for reservoir creation and the properties of the

fluids within the reservoir are described.

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