Sunday sand: green grains of Groningen (and a lot of gas)

Having celebrated the sand enthusiasts of the Netherlands in my previous post, it’s an appropriate time for a Dutch sand (again, thanks to Carla) – one that displays, with subtle understatement, some seasonal colours (although, given the current weather conditions in Northern Europe, this should perhaps be white). Near the city of Groningen, in Northern Holland, is a lake, the Sellingerbeetse. It’s not a natural lake, but rather one that formed from waters flooding a very large hole in the ground, a hole created by sand mining.

 
These sands were deposited during the ice ages, but are probably marine, glacial debris being reworked and winnowed in shallow seas of an interglacial period, and their subtle colours reflect this. The beautiful blue-green hue results from grains being coated with a thin veneer of the mineral glauconite, so named from the Greek glaucos, meaning, oddly enough, blue-green. Glauconite is an iron-potassium member of the phyllosilicate group of minerals, the micas, whose crystalline form appears as thin sheets  - if you insist, glauconite’s empirical formula is K_0.6Na_0.05Fe³⁺_1.3Mg_0.4Fe²⁺_0.2Al_0.3Si_3.8O₁₀(OH)₂

Glauconite is a common mineral in sedimentary rocks, particularly sands and sandstones, and was first recognised as being widespread during the nineteenth century research voyage of HMS Challenger. It forms by the chemical alteration of unstable mineral grains, for example, the mica, biotite, contained in sands gently moving around and accumulating in shallow marine conditions. Many of the green sands of the world result from their glauconite content, exceptions being where the grains themselves are green minerals, such as the olivine sands of Hawaii. The mineral has been used as a pigment and, because of its potassium content, a fertiliser.

But while we’re talking about Groningen, it should be mentioned that, deep below the fields of crops, is another kind of field, and one that owes its fame to sand. Europe’s largest gas field, and the tenth biggest in the world, lies below Groningen and takes its name from the city and the province. Discovered in 1959, the Groningen gas field covers an area of 900 square kilometres and contained a staggering 99 trillion cubic feet, 2,800 billion cubic metres of recoverable natural gas, a large portion of which still remains. Its discovery revealed the foolishness of the conventional wisdom of the time that there was no oil or gas in Northern Europe or the North Sea; enough said about conventional wisdom. The rocks that contain the gas, the reservoir, are sandstones, 250 million years old, formed during the dire conditions of the Permian period when desert environments dominated much of the planet and the greatest extinction of life was imminent (see a couple of posts from last year). Blowing dunes bordered densely saline seas and flash floods delivered their cargoes of sand to the salt flats and the shorelines. Great thicknesses of sand and salt accumulated and were buried, but the spaces between the sand grains ultimately provided these rocks with sufficient porosity to store the huge volumes of gas that eventually migrated into them. These rocks, known in Northern Europe as the Zechstein formation, form the reservoirs for much of the region’s gas reserves, and were responsible for kick-starting the successful search for indigenous energy sources, along with the technology and infrastructure to effectively manage extraction and use. Furthermore, the scale and value of the Groningen gas field was eventually recognised as worthy of more than simply mass production: it is now managed as a “swing producer,” its remaining reserves conserved except during times of demand that exceeds the capacity of smaller fields – and, judging by the weather conditions at the moment, Groningen may well be fulfilling exactly this function as I write. A detailed description and history of the Groningen gas field can be found here.

Sands modern and ancient, green and red (the dire conditions continued - the Zechstein is overlain by the Triassic sandstones that I wrote about earlier this month); we should leave the arid world of the Permo-Triassic return to the relatively benign surface of Groningen with another image of Carla’s glauconitic Sellingerbeetse grains.