Geochemical Characterization and 1D Basin Modelling of Afowo Shale: Implications for its Potential Hydrocarbon Source Rock in the Dahomey Basin, Southwestern Nigeria

Deep beneath the earth, ancient layers of rock hold the history of our planet's climate and the potential for the energy that powers our modern world. In the study of petroleum geology, scientists look for specific conditions where organic matter—remains of ancient plants and microscopic life—has been buried, heated, and squeezed over millions of years to transform into oil or natural gas. This transformation is not a simple chemical switch; it is a slow process driven by time and temperature. If the rock is too cool, the organic matter remains immature and cannot generate fuel. If it is too hot, the fuel breaks down or escapes. Finding the "sweet spot" where the heat is just right is the central challenge of exploration, especially in regions where the surface geology tells one story, but the deep underground reality tells another.

In the Dahomey Basin of southwestern Nigeria, a region known for its vast onshore bitumen deposits and recent offshore oil finds, a team of researchers from the University of Lagos has turned their attention to a specific layer of rock called the Afowo Formation. This layer, deposited during the Cretaceous period, is a mixture of sand, clay, and shale. While the presence of massive tar sands on the surface suggests that this area was once a factory for hydrocarbons, recent assessments of the shallow rocks have been confusing. Some studies suggested the rock was rich in organic material but chemically inert, while others pointed to low potential. To resolve this puzzle, the researchers set out to determine not just what the rock is made of, but how hot it has been and whether it has ever reached the temperature required to cook organic matter into usable gas.

The team began by examining samples taken from eight different wells, digging through the geological record to find the Afowo Formation. They analyzed thirteen specific samples of mudstone and shale, looking closely at the organic carbon trapped within them. Their chemical tests revealed that these rocks are indeed rich in organic matter, with some samples containing up to 2.50 percent carbon by weight. This level of richness is significant, indicating that the rock has the raw ingredients necessary to become a source of fuel. However, the type of organic matter matters just as much as the amount. The analysis showed that the organic material is primarily derived from land plants, a composition that tends to produce natural gas rather than liquid oil. This aligns with the region's reputation as a gas-prone province, though the researchers also found traces of marine algae, hinting at brief periods when the sea advanced over the land and deposited different types of organic material.

Knowing what the rock is made of is only half the story; the researchers needed to know if it had ever been hot enough to turn that organic matter into gas. To answer this, they measured the thermal maturity of the samples, essentially checking the "cooking temperature" the rocks have experienced. They looked at the maximum temperature recorded during heating tests and calculated the reflectance of microscopic glass-like particles called vitrinite, which act as a natural thermometer for the rock's history. The results were clear: in the shallow depths where they sampled, the rocks are still immature. The temperatures recorded were too low, and the chemical changes required to generate commercial amounts of gas had not yet occurred. The organic matter is essentially sitting in a state of suspended animation, preserved but not yet transformed.

This finding led the team to build a computer model of the basin's history, a simulation that tracks how the rock layers sank deeper over millions of years and how the heat from the Earth's interior changed over time. They fed their laboratory data into this model to see if it could accurately reproduce the maturity levels they measured. When they tried using a standard, default heat-flow model often applied to similar basins, the simulation failed to match the real-world data. The standard model predicted a different thermal history than what the rocks actually showed. This mismatch was a critical discovery, revealing that the heat flow in this part of the basin is highly variable and cannot be predicted by general rules. The researchers had to create a custom, high-resolution history of heat flow to make the model work, confirming that the shallow rocks have simply not been buried deep enough or heated long enough to generate gas.

The implications of this work are significant for the future of energy exploration in the region. The study confirms that the Afowo Formation is a viable source rock with the potential to generate vast amounts of natural gas, but that potential is currently locked away in the shallow layers because the rocks are too cool. The "immaturity" of these shallow sequences means the gas has not been cooked yet, leaving the resource fully preserved and untapped. The researchers conclude that the key to unlocking this energy lies deeper underground. By shifting exploration focus toward deeper structural targets where the heat is sufficient to mature the rock, the industry may find the commercial gas reserves that the shallow wells have missed. The study provides a calibrated roadmap for the future, suggesting that the answer to the region's energy potential is not in the surface tar sands, but in the deep, hot pockets of the basin where the geological conditions have finally reached the right temperature to turn ancient plant matter into modern fuel.