Urengoy
A huge natural gas field
For what it’s worth, I shall continue to present science as well as I can, in the hope that logic, reason, and knowledge will prevail. Operating “as normal” in these posts does not mean I acquiesce to or support things happening in the USA now. Beyond that statement, I’ll try to stick to science here.
Urengoy in the West Siberian Basin of Russia is the second largest natural gas field in the world after South Pars in Iran and Qatar. Urengoy originally contained a gas volume estimated variously from 8.1 trillion cubic meters (tcm) to 11.0 tcm or more, compared to the largest historic conventional gas field in the US (and #8 in the world) Hugoton in Kansas, Oklahoma, and Texas at 2.3 trillion cubic meters. Russia also has the #3, #5, #7, #10, and #11 ranking natural gas fields. For a sense of scale, Urengoy field is about 200 km (125 miles) long.
The fields are associated with fault uplifts in the subsurface (the surface is essentially flat and shows no evidence of the buried structure). Those uplifts, technically called horsts, can be identified using aeromagnetic data because the adjacent grabens (down-faulted basins) are filled with magnetic basalt, part of the vast Siberian basalt flows that erupted about the end of the Permian Period 251 million years ago. The uplifts do not have the magnetic rocks so they produce magnetic lows even though they are physical highs in the subsurface. The natural gas is trapped in multiple fluvial sandstone packages of mostly Cretaceous age that are draped over the uplifts in the subsurface.
The long narrow magnetic high (red and purple, west of Urengoy in the map above) is a graben that’s part of an extensive rift system that underlies the West Siberian Basin. The rift system may represent a time of extension following compression that produced the Ural Mountains during Permian time, perhaps analogous to the U.S. Basin and Range in Nevada, Utah, and adjacent areas, following extensive collisional compression that formed much of the Rocky Mountains.
The Soviet Union used magnetic data to explore successfully in the basin in the 1960s, finding dozens of natural gas and oil fields that continue to produce prolifically today. Many trillions of cubic feet of natural gas from the West Siberian Basin have heated homes in Russia and much of Europe for decades.
Today, the basalt flows are exposed over extensive areas of Siberia, but west of those exposures, in the relative lowlands occupied by two huge river systems, the Yenisey and the Ob, the basalt flows are found in the subsurface of the West Siberian Basin as described above.
A few million years after the start of the Triassic and probably continuing for millions of years into the Triassic, a rift began to form in this West Siberian area. It was perhaps something like the rifts, the grabens, that were forming in eastern United States, but it was unlike them in that the whole system ultimately failed – no ocean was created by the rifting here, whereas the Atlantic formed from the rifting between North America and Africa. The West Siberian rifts were pretty big, though – one extends continuously for at least 1,800 kilometers.
Now visualize something like Nevada’s basin and range – long linear rift valleys adjacent to long linear mountain ranges. As soon as topographic relief exists, erosion begins to fill those valleys – but this time, let’s focus on the mountains that are being eroded. The Permian and early Triassic flood basalts covered this region in vast, continuous sheets. When the rifting began, the sheets were broken – some segments were dropped down into the grabens, while other segments of the sheets were left on top of the mountain ranges, the horsts, a German word for heap or pile, something standing higher than the surrounding land.
The basalt on the tops of the mountain ranges got eroded away, leaving the basalt in the down-dropped basins. An alternative interpretation is that the basalts were erupting as the basins and ranges were forming, so the lavas flowed into the low-lying basins and were never deposited on the mountain tops, but I think the preferred interpretation is that the basalt flows were eroded off the mountains. In any case, what’s left, deep in the subsurface, is linear basins full of basalt and linear uplifts barren of basalt. The whole thing subsided even more, millions of years later, so all that complex structure, basins and ranges, is completely buried by later sediments, and the surface of the West Siberian Basin today is fundamentally a flat marshy plain.
So how do we know the basalt flows are down there? One way of course is to drill into the subsurface and get samples. But that’s expensive and random, at least initially. Another way is to recognize that basalt is a dark, iron-rich rock, and a good bit of that iron is in the form of magnetite – an iron oxide mineral that is highly magnetic. We can measure the earth’s magnetic field at a distance from the magnetic rocks, from an airborne magnetometer, so that we can infer the distribution of magnetite-rich rocks in the subsurface. When Soviet geophysicists did that in the 1950s and 1960s, they revealed the long linear magnetic highs – representing zones where there was a lot of magnetite – alternating with long linear magnetic lows, where magnetite was absent or less abundant. You’ve probably guessed that what they were defining were the grabens filled with basalt and the buried uplifts where the basalt had been eroded away.
So what? Well, those uplifts are buried beneath Jurassic and Cretaceous rocks that include some rich hydrocarbon source rocks and excellent reservoirs. The sedimentary rocks draped over the deep Triassic uplifts contain some of the largest natural gas fields in the world. And the gas fields coincide almost perfectly with the low values in the magnetic data, the uplifts where basalt is absent. Back in 1990 I did an extensive analysis of the magnetic map of the Soviet Union for oil exploration, and in the process discovered this correlation between magnetic lows and gas fields. The Soviets had known it for decades, of course, but it was pretty satisfying to unravel the details of the relationships myself.
The work I did in 1989-1990 identified an excellent analog to Urengoy in the magnetic data, shown in the cross section and model above. The problem was that the feature to the east (right) was near the Yenisey River, and permafrost was patchy or absent there. Permafrost constitutes an important aspect of the trapping mechanism for the gas at Urengoy; it its absence, gas reached the surface and was not trapped in the analog structure. Russian geologists told me that it is indeed an excellent structural and lithologic analogy to Urengoy, but it had been drilled and found barren.
Changing climate is allowing more and more permafrost to melt in Siberia and elsewhere, allowing more and more subsurface natural gas to escape, sometimes violently, resulting in explosive craters across the Siberian landscape.
The origin of the word Urengoy is uncertain. It may come from indigenous Khanty and Nenets words meaning “an island in a former river bed,” or less likely something about a hill with larches or yellow grass, but there’s really not much in the way of hills in the West Siberian Basin. It’s also been suggested that the word means “rotten place” in reference to a Stalinist gulag camp there.
We care from afar Richard. Keep up your good work.
Thank you for your acknowledgement of our situation. I fear for the global community as well as our own. I've been ignoring news and focusing on geology.