Thermal maturity and hydrocarbons potential generation in the Tumaco and San Juan-Atrato Basins as revealed by TOC, RE-Py and petrographic analysis

Deep beneath the surface of the Earth, in the narrow, turbulent zones where massive tectonic plates collide and one slides beneath another, a complex geological drama unfolds. These subduction zones are not just boundaries of rock; they are recycling centers for the planet, dragging crustal material, sediments, and organic matter down into the hot, high-pressure depths of the mantle. For decades, scientists have wondered what happens to the ancient remains of plants and animals trapped in these sediments. Do they cook into oil and gas, or do they remain locked in a dormant state? This question is particularly pressing in the Colombian Pacific, a region where the ocean floor dives beneath the South American continent. Here, the geology is young and active, characterized by steep slopes and rapid sedimentation. While such areas are known to hold vast amounts of methane hydrates, the potential for conventional oil and gas has remained a mystery, largely because the organic matter buried there has been difficult to study. Understanding whether these sediments have reached the right temperature and pressure to generate fuel is essential for knowing if these frontier basins hold energy resources or if they are simply geological archives of the past.

To solve this puzzle, a team of researchers turned their attention to two specific basins along this coast: the Tumaco Basin and the San Juan-Atrato Basin. They gathered a massive collection of 763 rock samples, digging deep into the geological record from various wells and surface outcrops. The goal was to read the chemical and physical history written inside these rocks. The scientists looked for three main things: how much organic carbon was present, what kind of organic material it was, and how much heat it had endured over millions of years. They used a method called Rock-Eval pyrolysis, which essentially heats the rock in a controlled environment to see what kind of hydrocarbons are released, and they measured the reflectance of tiny bits of fossilized plant material, a technique that acts like a thermometer for the rock's thermal history. By combining these measurements with microscopic observations of the organic matter itself, they could determine if the rocks were "immature" (too cool to generate fuel), in the "oil window" (the sweet spot for generating oil), or "overcooked" (where oil breaks down into gas).

The results from the Colombian Pacific basins tell a story of potential that has not yet been fully realized. The rocks analyzed contained organic matter, but the amount was generally modest, falling into a range the researchers describe as "acceptable" rather than abundant. When they heated the samples to see what would come out, the data showed that the organic material was not yet ready to produce significant amounts of oil or gas. Most of the samples fell into a category known as Type III kerogen, which is derived largely from land-based plant matter and is typically associated with gas generation rather than oil. However, the critical finding was the temperature. The rocks had not been heated enough to trigger the chemical reactions needed to turn that plant matter into fuel. The measurements of thermal maturity, which act as a gauge for how much cooking the rocks have undergone, indicated that the organic matter is still in its early stages, or what geologists call the "immature" phase. It is as if the ingredients for a meal are present in the kitchen, but the stove has not yet been turned on high enough to start the cooking process.

This conclusion was reinforced by looking at the microscopic details of the rock. Under powerful microscopes, the researchers saw the organic particles, including bits of fossilized spores and plant tissue, and measured how much light they reflected. These measurements confirmed the heating data: the rocks had not reached the threshold required for significant hydrocarbon generation. While some deeper samples showed signs of slightly higher maturity, suggesting they are just beginning to enter the zone where oil could form, the vast majority of the basin remains too cool. The study also noted that the organic matter in these basins is a mixture of marine and terrestrial sources, creating a transitional type of material that could eventually produce a mix of oil and gas, but only if it were buried deeper or subjected to more heat over time.

The researchers did find that some specific areas, particularly in the deeper sections of the Tumaco Basin and certain outcrops in the San Juan-Atrato region, showed slightly higher temperatures, hinting that these spots might be on the very edge of becoming productive. However, even these promising areas are not yet fully mature. The study explicitly rules out the idea that these basins are currently rich, conventional oil reservoirs waiting to be tapped. Instead, the evidence suggests that the hydrocarbon potential is limited by the current thermal state of the rocks. The organic matter is there, and it is of a type that could eventually generate fuel, but the geological conditions have not yet provided the necessary heat to unlock it.

In the end, the study provides a clear, data-driven picture of the Colombian Pacific margin. It confirms that while these basins contain organic-rich sediments, they are largely immature. The rocks have not been subjected to enough heat to transform their organic content into the oil and gas that power modern society. This does not mean the basins are devoid of value; they hold important geological records and may contain biogenic gas or methane hydrates. But for conventional oil and gas exploration, the message is one of caution. The ingredients are present, but the recipe has not yet been completed by the forces of the Earth. Further exploration would need to focus on finding deeper, hotter pockets where the cooking process might have finally begun, or to look for different types of energy resources that do not require such intense thermal maturity.