Evaluating the hydrocarbon production potential of Shemshak's formation Shale in Shahrud's Dehmolla area, using Rock-Eval Device

Deep beneath the Earth's surface, layers of rock act as ancient archives, preserving the remains of plants and animals that lived millions of years ago. When these organic remains are buried deep enough and subjected to intense heat and pressure over vast stretches of time, they transform into the energy sources we rely on today: oil and natural gas. Geologists study these rocks to understand if they contain enough organic material to generate fuel, what kind of fuel they might produce, and how far along the transformation process they have progressed. This field of study, known as organic geochemistry, allows scientists to read the chemical signatures trapped within stone to reconstruct the history of energy formation. In regions where coal and shale are abundant, such as the mountainous Alborz range in Iran, understanding these underground layers is crucial for determining whether they hold significant reserves of hydrocarbons or if the organic matter has simply turned into inert rock.

In the Dehmolla area, located southwest of the city of Shahrud in Iran, researchers focused their attention on a specific geological layer called the Shemshak Formation. This formation is famous for containing coal-bearing sediments from the Jurassic period, a time when vast forests covered the land. To investigate the potential of these rocks to generate hydrocarbons, a team of scientists from the University of Shahrood and Amirkabir University of Technology collected samples from the ground. They gathered twenty distinct pieces of rock, including black shale, green shale, coal, and sandstone, representing the different types of material found in this formation. The goal was to determine not just how much organic carbon was present, but also the quality of that carbon and whether the rocks had been heated enough to release oil or gas.

To analyze these samples, the team used a specialized instrument called a Rock-Eval pyrolysis device. This machine works by heating a small rock sample in a controlled environment, essentially cooking it to release the gases and liquids trapped inside. As the temperature rises, the machine measures the amount of free hydrocarbons already present in the rock, the amount of hydrocarbons that can be released by heating, and the amount of carbon dioxide produced from the burning of organic matter. By measuring these specific outputs, the scientists could calculate the total amount of organic carbon in the rock and determine the type of organic matter it contained. This process is a standard method used by oil companies worldwide to evaluate whether a rock formation is a viable source of energy.

The results revealed a wide spectrum of potential within the Shemshak Formation. The amount of organic carbon in the samples varied dramatically, ranging from a very low 0.08 percent to an exceptionally high 23.24 percent. This variation meant that the rocks fell into different categories of quality. The green shale and sandstone samples contained very little organic material, classifying them as poor sources for energy production. In contrast, the black shale and coal samples were rich in organic carbon, with some reaching levels that indicate excellent potential for generating hydrocarbons. The type of organic matter found in the rocks was primarily derived from land-based plants, which is typical for coal-rich environments. This organic matter is classified as Type III and Type IV kerogen, which are generally known for producing natural gas rather than liquid oil.

The study also examined the thermal maturity of the rocks, which refers to how much heat and pressure they have endured over geological time. The data showed that the rocks in this area have reached a stage of maturity ranging from mature to super-mature. The darker samples, particularly the black shales and coals, had undergone the most intense heating. This high level of maturity suggests that these rocks have passed the stage where they would generate liquid oil and have moved into the zone where they produce dry gas. In fact, most of the samples indicated that the organic matter had been converted into gas, with some samples showing signs of entering the metagenesis stage, where even gas begins to break down. The presence of high oxygen levels and low hydrogen levels in the organic matter further confirmed that these rocks formed in environments that were highly oxidized, likely on land rather than in deep, oxygen-poor oceans.

Ultimately, the investigation confirmed that the Shemshak Formation in the Dehmolla area holds significant promise for natural gas, specifically within its black shale and coal layers. While the green shales and sandstones in the same formation offer little value as energy sources, the coal-rich sections possess the necessary organic richness and thermal history to generate substantial amounts of dry gas. The findings suggest that these rocks have already completed the process of turning ancient plant matter into gas, making them a viable target for future energy exploration in this region of the Alborz mountains.