Page 35 - Hydrocarbon

22 Hydrocarbon Accumulations

Max. paleo Hydrocarbon
Hydrocarbon maturity -temp. (°C) product
0
biogenic
1 immature (early)
methane
60
2
Depth (km) 3 initial maturity 80 oil
(zone of oil
generation)
115
4 130 Heavy
condensate/ hydrocarbons
mature & 165 wet gas Light
5 post mature high hydrocarbons
(high temparature 180 temperature
methane) methane Methane
6
Figure 3.4 Hydrocarbon maturation.

The most important factor for maturation and hydrocarbon type is therefore
heat. The increase of temperature with depth is dependent on the geothermal gradient
which varies from basin to basin. An average value is about 31C per 100 m of depth.

3.1.5. Migration

The maturation of source rocks is followed by the migration of the produced
hydrocarbons from the deeper, hotter parts of the basin into suitable structures.
Hydrocarbons are lighter than water and will therefore tend to move upwards
through permeable strata.
Two stages have been recognised in the migration process. During primary
migration the very process of kerogen transformation causes micro-fracturing of the
impermeable and low porosity source rock which allows hydrocarbons to move into
more permeable strata. In the second stage of migration the generated fluids move
more freely along bedding planes and faults into a suitable reservoir structure.
Migration can occur over considerable lateral distances of several tens of kilometres.

3.1.6. Reservoir rock
Reservoir rocks are either of clastic or carbonate composition. The former are
composed of silicates, usually sandstone, the latter of biogenetically derived detritus,
such as coral or shell fragments. There are some important differences between the
two rock types which affect the quality of the reservoir and its interaction with
fluids which flow through them.
The main component of sandstone reservoirs (siliciclastic reservoirs) is quartz
(SiO 2 ). Chemically it is a fairly stable mineral which is not easily altered by changes