August 3. Volga-Ural Basin

Oil fields of the Volga-Ural Basin (light green)

For much of Paleozoic time, from Cambrian into Carboniferous, the eastern margin of Baltica, where European Russia is located today, was a passive margin much like western North America was. Shallow seas, as well as deeper slopes, received both clastic and carbonate deposits. By Late Carboniferous time, collision was beginning along this margin with continental blocks that now make up Kazakhstan and Siberia.

The collision began to raise mountains – the Ural Mountains – and along the flank of Baltica, thick sediment deposition together with thrusted slices of older rocks began to press the crust down, forming a typical foreland basin. Permian sediments buried the older limestones that predominated in Devonian time. That burial contributed to the maturation of hydrocarbon source rocks, and the oil and natural gas generated migrated into some excellent reservoirs in broad structures probably related to the collisions.

Romashkino oil field is the largest of some 800 oil fields in the Volga-Ural Basin, and for many years was the largest oil field in the Soviet Union. It was discovered in 1948 and has produced about 17 billion barrels of oil, more than Prudhoe Bay, Alaska, the largest oil field in North America. Romashkino covers about 4,200 square kilometers (1,600 square miles), somewhat more than the area of Rhode Island. It reached its peak of production in the early 1970s when it yielded about 1,600,000 barrels per day. It is largely depleted now, but still produces a few hundred thousand barrels per day. The reservoirs are Devonian carbonates in a broad arch, buried beneath Permian foreland basin sediments.

Even though it is a very mature basin in terms of oil production, the Volga-Ural has been estimated by the USGS to still contain perhaps 1.5 billion barrels of oil and significant reserves of natural gas as well. 

—Richard I. Gibson

Links and references:
Geology 
Oil and Gas Region 

Oil Production 

Map above from USSR Energy Atlas, by Central Intelligence Agency (public domain), 1984