Located in a remote part of SE Siberia, the giant Yurubchen-Tokhomo Field, which has been on production since 1987, hosts oil and gas in the oldest sedimentary reservoir in the world – in dolomites and dolomitic limestones of Riphean (early Proterozoic) age. This ancient reservoir is composed of stromatolitic bindstones, which display a remarkably complex internal configuration.

At the Yurubchen-Tokhomo, productivity is highest where the reservoir is cut by extensive subvertical fractures and on the flanks of palaeovalleys in its heavily leached upper parts (Fig. 1). To unlock the potential of these elusive areas, horizontal wells were strategically drilled perpendicular to the fractures to maximise connectivity between wells and porous zones in the oil leg. However, well performance was heavily influenced by other factors, including:

• Connection to porous zones in the gas cap, which led to gas breakthrough (Fig. 2)
• Fracture width – narrow fractures are less likely to be productive than wide fractures.

By 2021, the Yurubchen-Tokhomo field had transitioned entirely to horizontal wells, with all 139 active producers operating this way. Cumulative production reached 93 MMBO by the end of the year – however, much of the field’s potential remains untapped, leaving exciting opportunities for further development.