Source Rocks: The Origin of Petroleum

Petroleum Generation and Migration

Petroleum generation and migration are essential processes in the formation and accumulation of hydrocarbons. Understanding how these processes occur is crucial for evaluating the potential of petroleum reservoirs. Let’s explore the key aspects of petroleum generation and migration.

Thermal Cracking

Thermal cracking is a process in which organic matter within source rocks undergoes chemical decomposition due to increased temperature and pressure. As the temperature rises with increasing burial depth, the organic molecules in the source rock begin to break down, transforming into hydrocarbons. This thermal cracking process is responsible for the conversion of kerogen, the complex mixture of organic compounds, into crude oil and natural gas.

Expulsion of Hydrocarbons

Once hydrocarbons are generated through thermal cracking, they need to be expelled from the source rock and migrate towards potential reservoir rocks. Expulsion occurs when the pressure within the source rock exceeds the overlying pressure, causing hydrocarbons to move towards areas of lower pressure. This expulsion process can be aided by the presence of natural fractures or faults, which provide pathways for hydrocarbons to escape.

Migration Pathways

Migration pathways play a crucial role in the movement of hydrocarbons from the source rock to potential reservoir rocks. These pathways can include fractures, faults, permeable layers, or even stratigraphic traps. The buoyancy of hydrocarbons, combined with the presence of these pathways, allows them to migrate vertically and laterally through the subsurface. The direction and extent of migration depend on the geological structure and properties of the rocks. Migration pathways need to be present and interconnected for hydrocarbons to accumulate and form a reservoir.

During migration, hydrocarbons can undergo secondary cracking, alteration, and fractionation. This can lead to changes in the composition and properties of the hydrocarbons, influencing the quality and quantity of the accumulated resources. The efficiency of migration depends on factors such as the rock’s permeability, the presence of cap rocks to prevent upward migration, and the timing of hydrocarbon generation and expulsion relative to the formation of potential reservoir rocks.