Minerals and rocks

Take a piece of granite. In it you can see transparent grey grains, pink or white crystals with flat shiny faces, and black flakes. These are three minerals — quartz, feldspar and mica — and granite is a rock, their aggregate. Sandstone is also a rock: fragments of quartz and other minerals cemented by calcite, quartz or clay material. Marble consists of almost pure calcite, but it too is a rock — an aggregate of many crystals.

Two things define a mineral: its chemical composition, written as a formula (for quartz, SiO₂), and its crystal lattice — a regular, repeating arrangement of atoms. The lattice decides the shape of the crystals, the hardness, the density and the way the mineral breaks. The same composition can give different minerals. Calcium carbonate CaCO₃ forms calcite and the denser aragonite, from which many molluscs build their shells. Carbon forms soft graphite and diamond. Substances without a lattice, such as volcanic glass or opal, are strictly speaking not minerals but mineraloids.

The properties of a reservoir are built up at every level: from the lattice (fractions of a nanometre) to the grain (from micrometres to centimetres), from the grain to the rock in a core sample (a cylinder of rock cut from the well), and from the rock to the reservoir. Permeability, for example, depends on which mineral lines the pore walls, on how the grains are packed and cemented, and on how the layers alternate (Rock permeability).

Almost half the mass of the crust is oxygen and more than a quarter is silicon (composition of the crust). So the building block of most minerals is the silica tetrahedron: a silicon atom in the centre with four oxygen atoms at the corners (a tetrahedron is a small pyramid with four faces). The tetrahedron carries a negative charge, so a mineral is “assembled” in two ways. Tetrahedra link to one another through shared corner oxygens. Cations (positively charged ions) of magnesium, iron, calcium, sodium and potassium balance the charge that remains. Aluminium can replace part of the silicon. Such minerals are called aluminosilicates, and an extra cation enters the lattice — for example, potassium in potassium feldspar and in micas.

The more corners a tetrahedron shares with its neighbours, the stronger the framework. You can see this in the ratio of oxygen atoms to silicon atoms. A shared corner belongs to two tetrahedra at once, so it counts as a half.

The main properties follow from this. In the quartz framework, strong bonds run in all directions and there are no weak planes. So quartz is hard, has no cleavage (it does not split along flat planes), and hardly dissolves in water or in acids, except hydrofluoric acid. In sheet silicates, strong bonds act only inside a sheet, and the sheets hold to one another more weakly, through cations or hydrogen bonds. That is why mica splits into thin, flexible flakes. Clay minerals form microscopic plates, and in smectite water can enter between the plates (structure of clay minerals).

How well a mineral resists weathering (breakdown at the surface by air, water and temperature change) also depends on its structure. Minerals that crystallise from a melt first, at high temperature (olivine, pyroxenes, calcium-rich plagioclases), break down fastest at the surface. Quartz, the last mineral to crystallise from a granite melt, breaks down slowest. When feldspars and micas weather, they turn into clay minerals: kaolinite, illite and smectite. So the longer a sand has been transported and reworked, the more quartz it contains. Mature sands are almost entirely quartz (mineralogical maturity).

About six thousand minerals are known, but the sedimentary rocks that a petroleum engineer works with contain only a few dozen. About fifteen minerals make up almost any rock. They are worth knowing by sight, together with their formula, hardness and density.

Besides silicates, sections contain carbonates (calcite, dolomite, siderite), sulfates (anhydrite, gypsum, barite), halides (chloride salts: halite and the potash salt sylvite), sulfides (pyrite) and oxides (hematite, magnetite). Even in small amounts, minerals heavier than 2.9 g/cm³ (pyrite, siderite, barite, iron oxides) change the density log reading noticeably (matrix density).

The geologist holds a core sample, fragments of cuttings (rock chips that the drilling fluid brings up) and a hand lens. You recognise a mineral by a set of properties. Each property alone can mislead, but together they usually give a clear answer.

Hardness

Resistance to scratching. Geologists compare it with the Mohs scale: ten reference minerals from talc (1) to diamond (10), each of which scratches the ones before it. In the field they use everyday tools instead: a fingernail is about 2.5, a copper coin about 3, and window glass or a knife blade about 5.5. Quartz (7) scratches glass, a knife scratches calcite (3), and a fingernail scratches gypsum (2).

Cleavage

The ability to split along flat planes of the lattice, leaving smooth shiny surfaces. Mica has one such plane, feldspars have two, and calcite, halite and anhydrite have three. Quartz has no cleavage. It breaks with a conchoidal fracture (a curved, shell-like surface), like glass.

Lustre

How the surface shines, that is, reflects light: glassy (quartz, calcite), pearly (mica, gypsum on cleavage planes), metallic (pyrite), dull or earthy (clay minerals).

Colour and streak

Colour can mislead, because impurities often cause it. The streak is more reliable: the colour of the mineral's powder when you rub it on an unglazed porcelain plate. For hematite it is cherry red, for pyrite greenish black, and for most light-coloured minerals white.

Density

Heavy minerals feel heavy in the hand. You find the density precisely by weighing (example below).

Reaction with acid

Put a drop of cold 10 per cent hydrochloric acid on the mineral. Calcite fizzes vigorously and releases carbon dioxide. Dolomite as a lump hardly reacts, but it fizzes as a powder or with warmed acid.

Special properties

Halite tastes salty and sylvite tastes bitter-salty (do not taste everything: some minerals are poisonous). Magnetite is attracted to a magnet. Mica flakes are flexible. Transparent calcite shows double refraction (you see a double image through it).

You find the density of a small fragment with Archimedes' principle. Weigh it in air, then weigh it hanging in a liquid of known density. The difference between the two scale readings is the weight of the liquid that the fragment pushes aside, and from it you find the volume of the fragment.

The laboratory finds the mineral composition more precisely. Thin sections are slices of rock 30 µm thick, glued to glass. Under a microscope with light shining through, you recognise minerals by colour, shape and optical properties. Pores filled with dyed resin show as blue patches. X-ray diffraction (XRD) gives the amount of each mineral, including the very fine clay fractions. Scanning electron microscopy (SEM) shows the shape of clay minerals on pore walls. At the rig, during mud logging, geologists describe the cuttings under a binocular microscope. They find the share of calcite and dolomite by calcimetry: they measure the volume of carbon dioxide released when crushed cuttings react with hydrochloric acid.

By origin, rocks fall into three classes. The features that tell them apart in a sample are directly related to how the rock formed.

Igneous rocks crystallise from a melt, called magma. Intrusive (plutonic) rocks solidified at depth. They cooled slowly, so the crystals had time to grow and you can see them with the naked eye. Granite and gabbro look like this. Extrusive (volcanic) rocks poured out onto the surface and cooled quickly. Their crystals are small, part of the material is glass, and there are often voids left by gas bubbles. Rhyolite and basalt are examples. Bodies of magma that solidified in cracks across the layers are called dykes. Those that pushed in between the layers are called sills.

By composition, igneous rocks fall into two main groups. Felsic (acid) rocks are light-coloured and rich in silica (silicon dioxide, SiO₂), with quartz and feldspars (granite, rhyolite; about 70% SiO₂). Mafic (basic) rocks are dark, heavy and rich in iron and magnesium, with plagioclase and pyroxene (gabbro, basalt; about 50% SiO₂). The ultramafic rocks of the mantle (peridotites) are still poorer in silica. Felsic rocks and rocks close to them in composition dominate the upper part of the continental crust, and mafic rocks make up the oceanic crust. That is why the two types of crust differ in density (more detail).

Sedimentary rocks form at the Earth's surface from the broken-down products of other rocks and from substances dissolved in water. Clastic (terrigenous) rocks are made of fragments carried by water, wind or ice and cemented after burial. They include conglomerates, sandstones, siltstones, clays and mudstones. Chemical and biochemical rocks either precipitate from water chemically or build up from the skeletons of organisms. They include limestones, dolomites, siliceous rocks and evaporites (rocks left when water evaporates: rock salt, anhydrite, gypsum). Organic rocks — coals — form from plant remains. The main features of sedimentary rocks are bedding (layers), a texture of fragments (clasts) and the remains of organisms. How to name and classify them by composition and grain size is covered in the topic Sedimentary rocks and their classification.

Metamorphic rocks form when an existing rock recrystallises in the solid state, without melting, under high temperature, high pressure and hot solutions. Contact metamorphism happens around igneous bodies, where heating plays the main role. Regional metamorphism happens over large areas during deep burial and mountain building, where pressure and shearing add to the heating. Under pressure, flat (platy) minerals line up in parallel, and the rock gets a layered, banded texture called foliation. As temperature and pressure rise, clay turns into slate, then into schist and gneiss. Rocks made almost entirely of one mineral do not become foliated: quartz sandstone turns into quartzite, and limestone into marble.

No rock lasts for ever. All rocks take part in the rock cycle. Weathering breaks down igneous rocks that are brought up to the surface, and the broken-down material is carried away and deposited in basins. As the sediments are buried, they are compacted and cemented, and they turn into sedimentary rocks. With further burial and heating, sedimentary rocks become metamorphic. At still higher temperature they melt and make new magma. The path can be cut short at any stage. A metamorphic or sedimentary rock brought up to the surface is broken down again, and an igneous rock can become metamorphic without ever having been a sediment.

Two things drive the cycle. One is the Earth's internal heat together with plate tectonics, which buries and raises huge blocks of lithosphere (Plate tectonics and sedimentary basins). The other is solar energy, which moves the water and air that break down rocks at the surface. The cycle runs over tens and hundreds of millions of years. So in the section of a single well, rocks formed in completely different conditions may lie one above another.

For a reservoir, the class of the rock is not the only thing that matters. It also matters which minerals the rock contains and where they are: in the grains, in the cement or on the pore walls.

An engineer gets information on minerals and rocks from several sources. Cuttings are small fragments of rock that the drilling fluid carries out of the well. They are described at the rig as soon as the interval has been drilled. Core and sidewall core (core cut from the borehole wall) give samples for thin sections, X-ray analysis and measurements of rock properties (Core analysis). Logging gives a continuous but indirect picture of the mineral composition (Lithology from the photoelectric factor and spectral gamma ray, Lithology and porosity from crossplots). The description of a rock includes colour, texture, size and shape of grains, minerals, cement, visible porosity and reaction with acid. It is called the rock's lithological description, or lithology.