As you may have guessed by now, I find sands and sandstones fascinating - fascinating and vital in so many different ways to our daily lives. But while, more often than not, they co-operate and fulfil the roles we need them too, sands can misbehave and frustrate our expectations and best-laid plans; today’s sand is one of those pesky ones, and a current source of some personal frustration.
All over the world, ancient sandstones deep below the earth’s surface act as reservoirs – vast storage containers for supplies of water, oil, and gas. In the case of water, such a reservoir is referred to as an aquifer, from the Greek for “water carrier.”As I wrote in Sand, in the infamous Chapter 9:
How many people in the world rely on water supplies flowing or pumped from underground? Controversial though the matter may be, how many of us rely on oil and gas flowing or pumped from underground? These critical resources do not occur in subsurface lakes, rivers, or caverns; they inhabit the microscopic holes in buried rock, very often the spaces between sand grains. Any rock that, like a sponge, is sufficiently porous to hold significant amounts of water or hydrocarbons is called a reservoir…. Ninety-five percent of America’s freshwater is underground, and a large proportion of it—30 percent of the water used for agriculture in the United States—is contained in one of the world’s great aquifers, the Ogallala. The Ogallala, the main part of the High Plains aquifer, is formed from essentially unlithified sands, silts, and gravels that were carried off the eroding Rocky Mountains during the last twenty million years. Ancient sand dunes form parts of the aquifer, and the Nebraska Sand Hills are built on the Ogallala sediments. The reservoir is huge: it underlies an area of about 450,000 square kilometers (174,000 sq mi), covering parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. It is close to the surface, easily accessible, and 300 meters (1,000 ft) thick in parts, with an average thickness of 60 meters (200 ft).
The Ogallala contains a staggering volume of water – enough to fill Lake Huron – but it’s not inexhaustible, and the resource is being used up faster than it is being naturally replenished.
So our resources of underground water and hydrocarbons don’t reside in great caverns, pools or lakes, but rather, very commonly, in the minute spaces, or pores, between the grains of an old sandstone. How well it behaves as a useful reservoir depends on both the volume of those spaces between the grains – the porosity – and how easily the fluids can flow out of those spaces – the permeability. The trouble is that, once a sand is buried below the surface, subjected to pressures of the overlying sediments, heated up, and saturated with waters laden with various concoctions of dissolved minerals, all kinds of things can happen to clog up the porosity and bung up the permeability. Today’s sand is a lesson in exactly what can happen – as revealed by the stunning details of the technology of imaging by a scanning electron microscope (best to click on the image to enlarge).
The sandstone in question is around ten million years old, and was deposited as sand banks in the shallow seas of ancient Sumatra. Just as it does today, Sumatra then lay on a major plate boundary and was routinely shaken by earthquakes and blasted by volcanoes. A variety of rock and volcanic mineral fragments mixed with the quartz sands carried to the seas, and warm waters with sometimes exotic chemical recipes coursed through the sands as they were buried – hence my problem. Ever since the early part of the last century when the Dutch discovered oil in Sumatra, many of these old sandstones have proved to be effective reservoirs – but they can be fickle and unpredictable.
The image on the left at the head of this post shows the grains of one such sandstone – a very fine-grained one, each grain less than a tenth of a millimeter across (the scale shows 10 micrometers, a micrometer being one thousandth of a millimeter – a human hair is typically between 20 and 80 micrometers in thickness). The grains are mostly quartz, but they are far from being smooth, clean and sparkling; instead, they are coated in what looks like crushed cornflakes. Look at those coatings in more detail in the image on the right (ten times more detail), and they resolve into masses of individual flakes and fragments – mineral grains. Beautiful clusters of flat, flaky crystals of mainly clay minerals. The sand originally contained mud and clay, but chemical cooking once it was buried has changed much of that clay, experimented with the volcanic material, and grown new clay minerals – which have effectively clogged up the pores of this sandstone. All that remains of initial porosity between the grains is the occasional pore space labeled as “IP”; in a very few places, original minerals have been dissolved away to form new, secondary, pore spaces – “SP.” The quality of this sandstone as a reservoir has been severely – if not totally – degraded.
Clay minerals come in an astonishing variety of forms, but they are dominantly aluminium phyllosilicates – mixtures of Al and silica, together with a wide variety of other elements such as magnesium and iron, water molecules bound into their structure. The “phyllo” part of “phyllosilicate” comes from the original Greek for “leaf” (as in today’s “phyllo dough”) – these minerals form the thin, flat, leaf-like plates that are clearly visible in the right-hand image. They are very much like the platy mica minerals, and the spaces between the molecular sheets can absorb all kinds of other molecules. Here’s a typical formula for a clay mineral:
Some clays, such as kaolin, so important for ceramics and other uses, are relatively well-behaved, but others, such as the evil-sounding smectite, absorb water and expand, bunging up the porosity further; as far as reservoir quality is concerned, smectite is, indeed, evil.
So our sandstone contains all kinds of clays, including kaolin (K) and smectite, plus “Sid” – siderite, iron carbonate that completely changes the electrical conductivity of the rock; since one of the ways in which such a possible reservoir is evaluated is through measurement of its electrical conductivity (water being more conductive than oil or gas), Sid plays vicious games with such measurements and makes them almost impossible to interpret.
So, sand misbehaving: clogged-up porosity, plugged-up permeability, and screwed-up electrical properties. One of my colleagues has a three-year-old granddaughter who classifies her world into things that are “good” and those that are “not good.” This is definitely “not good."