Page 109 - Hydrocarbon Exploration and Production Second Edition

96 Reservoir Geology

Most reservoirs in the GoM and the North Sea are contained in a clastic
depositional environment; many of the giant fields of the Middle East are contained
in carbonate rocks. Before looking at the significance of depositional environments
for the production process, let us investigate some of the main characteristics of both
the categories.

6.1.1.1. Clastics
The deposition of a clastic rock is preceded by the weathering and transport of material.
Mechanical weathering will be induced if a rock is exposed to severe temperature
changes or freezing of water in pores and cracks (e.g. in some desert environments).
The action of plant roots forcing their way into bedrock is another example of
mechanical weathering. Substances (e.g. acid waters) contained in surface waters can
cause chemical weathering. During this process minerals are dissolved and the less stable
ones, like feldspars, are leached. Chemical weathering is particularly severe in tropical
climates.
Weathering results in the breaking up of rock into smaller components which can
then be transported by agents such as water (rivers, sea currents), wind (deserts) and
ice (glaciers). There is an important relationship between the mode of transport and
the energy available for the movement of components. Transport energy determines
the size, shape and degree of sorting of sediment grains. Sorting is an important
parameter controlling properties such as porosity. Figure 6.1 shows the impact of
sorting on reservoir quality.
Poorly sorted sediments comprise very different particle sizes, resulting in a
dense rock fabric with low porosity. As a result, the connate water saturation is high,
leaving little space for the storage of hydrocarbons. Conversely, a very well sorted
sediment will have a large volume of ‘space’ between the evenly sized components, a
lower connate water saturation and hence a larger capacity to store hydrocarbons.

Sorting

poorly moderately well very well

5% Porosity % 35%
Saturation %
Content of clay / fines

Figure 6.1 Impact of sorting on reservoir quality.