Primary migration and compositional changes
| Exploring for Oil and Gas Traps | |
| |
| Series | Treatise in Petroleum Geology |
|---|---|
| Part | Critical elements of the petroleum system |
| Chapter | Migration of petroleum |
| Author | Martin D. Matthews |
| Link | Web page |
| Store | AAPG Store |
The composition of hydrocarbons expelled from a source rock is a primary control on the composition of reservoired hydrocarbons. In general, the larger-molecular-weight compounds are preferentially retained in the source rock while the smaller compounds are expelled.
Factors favoring oil expulsion
The following factors favor oil expulsion from a source rock:
- Type I or 2 kerogen
- Sufficient time in the oil window
- High levels of total organic carbon (TOC)
- Concentration of organic matter in laminae
- Low-capillary-pressure conduits
Factors favoring gas expulsion
Five factors favor gas expulsion from a source rock:
- Type 3 kerogen
- Rapid burial through the oil window
- Low Total organic carbon (TOC)
- Dissemination of organic matter
- High-capillary-pressure conduits
Composition of early vs. later generation
Early generation concentrates light products into large pores and fracture networks. Thus, the oil expelled is lighter in composition than the oil retained. However, as maturity continues, the difference between these two disappears and oil–source correlation improves.
Compositional changes and correlation
Expulsion favors light compounds over heavy compounds and saturated hydrocarbons over aromatics. This is due to molecular filtering and adsorption–desorption phenomena, particularly during the early stages. However, because significant quantities of hydrocarbons are retained in the large and medium pore systems within the source rock, the correlation of reservoired oil with its respective source rock is not significantly affected (see also Oil-oil and oil-source rock correlation. The effect of continued maturation of the source rock after expulsion is a more significant impediment to correlation.
See also
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