A Paleogene Pressure Cooker
The natural gas of the Fergana Basin comes from an ancient, tropical past. Around 50 million years ago, during the Eocene epoch, this intermountain depression was covered by the shallow, brackish waters of the proto-Paratethys Sea. The warm climate fueled immense blooms of algae, which died and sank to the bottom, forming thick layers of organic-rich mud. Over millions of years, these sediments were buried under thousands of meters of additional rock, with the Paleozoic basement now lying as deep as 8 to 10 kilometers in the basin's center.
This deep burial subjected the Eocene source rocks, specifically the Suzak beds, to immense heat and pressure. The total organic carbon content in some of these layers reaches as high as 19 percent by weight. As temperatures increased with depth, the complex organic matter (kerogen) derived from the algae was thermally cracked in a process called catagenesis. This process broke down the large organic molecules into smaller, simpler ones, creating vast quantities of methane, the primary component of natural gas. The gas then migrated into porous reservoir rocks, primarily sandstones and carbonates from the Cretaceous and Paleogene periods, where it was trapped by overlying impermeable layers, forming the gas fields known today.