Page 32 - Hydrocarbon

CHA P T E R 3

Exploration

Introduction and Commercial Application: This section will firstly examine the
conditions necessary for the existence of a hydrocarbon accumulation. Secondly,
we will see which techniques are employed by the industry to locate oil and gas
deposits.
Exploration activities are aimed at finding new volumes of hydrocarbons, thus
replacing the volumes being produced. The success of a company’s exploration
efforts determines its prospects of remaining in business in the long term.

3.1. Hydrocarbon Accumulations

3.1.1. Overview
Several conditions need to be satisfied for the existence of a hydrocarbon
accumulation, as indicated in Figure 3.1. The first of these is an area in which a
suitable sequence of rocks has accumulated over geologic time, the sedimentary basin.
Within that sequence there needs to be a high content of organic matter, the source
rock. Through elevated temperatures and pressures these rocks must have reached
maturation, the condition at which hydrocarbons are expelled from the source rock.
Migration describes the process which has transported the generated hydro-
carbons into a porous type of sediment, the reservoir rock. Only if the reservoir is
deformed in a favourable shape or if it is laterally grading into an impermeable
formation does a trap for the migrating hydrocarbons exist.

3.1.2. Sedimentary basins
One of the geo-scientific breakthroughs of the last century was the acceptance of
the concept of plate tectonics. It is beyond the scope of this book to explore the
underlying theories in any detail. In summary, the plate tectonic model postulates
that the positions of the oceans and continents are gradually changing through
geologic times. Like giant rafts, the continents drift over the underlying mantle.
Figure 3.2 shows the global configuration of major plate boundaries.
The features created by crustal movements may be mountain chains, like the
Himalayas, where collision of continents causes compression. Conversely, the
depressions of the Red Sea and East African Rift Basin are formed by extensional
plate movements. Both type of movements form large-scale depressions into which
sediments from the surrounding elevated areas (highs) are transported. These
depressions are termed sedimentary basins (Figure 3.3). The basin fill can attain a
thickness of several kilometres.

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