Sedimentary processes and depositional environments

Ethan's first task is to describe a core from a Tamarack Lake field well. Laid out in boxes on the long tables of the core laboratory, nine metres of rock show a story he cannot yet read: fine grey mudstone at the base, then thin sandy layers that thicken upward, then clean, well-sorted sandstone with gently tilted laminations, capped by a pebbly layer and dark shale. Nadia walks along the boxes. 'This is a shoreline,' she says. 'The sea was getting shallower, the beach was building outward, and then something changed and the sea flooded back in.' Reading rocks this way, as the record of the place where they formed, is the core skill of petroleum geology, because the place decides the shape, size and quality of the reservoir.

From weathering to deposition

Sedimentary rocks begin as weathering breaks older rocks down. Physical weathering, such as frost wedging and abrasion, breaks rock into fragments. Chemical weathering dissolves some minerals and alters others: feldspar turns to clay, while quartz, which resists chemical attack, survives as sand grains. Rivers, wind, glaciers and ocean currents then carry the products away. The transported pieces, from clay through silt, sand and gravel, are called clastic or siliciclastic sediment. Dissolved ions travel to lakes and the sea, where organisms and chemical reactions turn them into carbonate rocks and evaporites such as salt and anhydrite.

Grain size is the first thing a geologist records. The Udden-Wentworth scale divides sediment into clay (finer than 1/256 mm), silt (1/256 to 1/16 mm), sand (1/16 to 2 mm) and gravel (coarser than 2 mm), with sand further split into very fine to very coarse. The size of grains a current can move depends on its speed: a fast river carries gravel, a slow one only mud. When the current slows, the largest grains drop first. This is why grain size changes in predictable ways across a delta or down a beach, and why it records the energy of the environment.

Stokes's law gives the settling speed of a small sphere of diameter d and density ρs falling through a fluid of density ρf and viscosity μ. It holds for silt and clay, where flow around the grain stays smooth; larger sand grains fall more slowly than the law predicts because turbulence adds drag. The law explains why clay settles only in very quiet water, such as lakes, lagoons and the deep sea, while sand drops out wherever currents slacken.

# Settling speed of grains in still water from Stokes's law
RHO_S, RHO_F = 2650.0, 1000.0     # quartz grain and fresh water, kg/m3
MU, G = 1.0e-3, 9.81              # water viscosity Pa.s at about 20 C, gravity m/s2

def stokes(d_mm):
    d = d_mm / 1000.0
    return (RHO_S - RHO_F) * G * d * d / (18 * MU)

def nice(seconds):
    for unit, s in (("days", 86400), ("hours", 3600), ("minutes", 60)):
        if seconds >= s: return f"{seconds / s:.1f} {unit}"
    return f"{seconds:.0f} seconds"

print("grain              diameter (mm)  speed (mm/s)   time to fall 5 m")
for name, d in (("fine clay", 0.0005), ("coarse clay", 0.002), ("silt", 0.02), ("very fine sand", 0.08)):
    v = stokes(d)
    print(f"{name:18s} {d:10.4f}   {1000 * v:12.4f}   {nice(5 / v):>16s}")

grain              diameter (mm)  speed (mm/s)   time to fall 5 m
fine clay              0.0005         0.0002         257.4 days
coarse clay            0.0020         0.0036          16.1 days
silt                   0.0200         0.3597          3.9 hours
very fine sand         0.0800         5.7552       14.5 minutes

The output makes the point vividly. A clay particle takes weeks or even months to fall five metres, so any current at all keeps it moving; a very fine sand grain falls the same distance in about a quarter of an hour, and coarser sand much faster still. Clay therefore accumulates only where water is still for long periods, and the presence of a thick mudstone tells a geologist that the setting was calm.

Sorting, rounding and sedimentary structures

Besides size, geologists describe how uniform the grains are and how they are arranged. A well-sorted sand has grains of nearly the same size, typical of beaches and windblown dunes where waves or wind have winnowed the finer material away. A poorly sorted sediment, such as a glacial deposit or a debris flow, mixes clay to boulders. Sorting matters to engineers because well-sorted sands have the highest porosity and permeability: no small grains fill the spaces between the large ones. Rounding records distance of travel: grains carried far are smoothed; those deposited near their source stay angular.

Sedimentary structures are features formed during or just after deposition, and they are the best clues to the environment. Ripples and cross-bedding form as currents push sand along in migrating bedforms; the direction the cross-beds dip shows which way the current flowed. Hummocky cross-stratification forms under storm waves on the shelf. Graded beds, coarse at the base and fining upward, record a single waning flow such as a turbidity current. Mud cracks show the sediment dried out at the surface. Burrows and tracks, called trace fossils, show where animals lived and how much oxygen the water held.

Geologists also watch how grain size changes upward through a section. A coarsening-upward package, mud passing up into sand, typically records a shoreline or delta building out into deeper water, so that sandier, shallower deposits settle on top of muddier, deeper ones. A fining-upward package, sand passing up into mud, often records a river channel filling and being abandoned. On a gamma-ray log, which reads high in shale and low in clean sand, coarsening-upward packages look like a funnel and fining-upward packages like a bell. Lesson 9 uses these shapes to correlate wells.

Depositional environments

A depositional environment is a place with a particular set of physical, chemical and biological conditions, such as a river channel, a beach or the deep-sea floor. Each produces a distinctive combination of rock types, structures, fossils and geometries, called a facies. Walther's law, one of the most useful ideas in the field, states that facies found stacked vertically in a continuous sequence were once side by side on the surface. As a shoreline moves, the environments move with it and lay their deposits on top of each other. Reading a vertical core therefore reveals a map.

The Cardium illustrates why environment matters so much. Its best sandstones and conglomerates formed at the shoreface, where waves winnowed sand into long, narrow bodies parallel to an ancient coastline that ran roughly northwest to southeast across Alberta. A field such as Pembina, discovered in 1953, follows that trend for tens of kilometres. Knowing the trend lets geologists predict where the next sand body will be, and it tells engineers that wells placed along the trend will stay in good rock longer than wells drilled across it.

Deep water: turbidity currents and fans

Not all sand stays near the shore. On steep continental slopes, sediment piles up until an earthquake, a storm or simple overloading triggers a slide. The slide mixes with water into a turbidity current, a dense, fast-moving cloud of sediment that races down submarine canyons at speeds of several metres per second and can travel hundreds of kilometres across the basin floor. As it slows, it drops its load: coarse sand first, then finer sand, silt and finally mud, building the graded bed called a turbidite. Thousands of these events, stacked over millions of years, build submarine fans of channels, levees and sheet-like lobes.

Turbidite fans hold some of the largest oil fields discovered in the last fifty years, in the deep Gulf of Mexico, offshore Brazil and West Africa. Their sands can be clean and thick, but they are threaded with mud layers that may divide a reservoir into compartments, and channels can be cut and filled many times. Predicting how well the sand bodies connect is one of the hardest problems in deepwater development, and it relies heavily on seismic images, discussed in Lesson 10.

Describing core in practice

A core description follows a routine. The geologist works from the base upward, recording rock type, grain size, sorting, sedimentary structures, fossils, burrowing intensity, colour, oil staining and fractures, and plotting them on a graphic log whose width increases with grain size. Photographs under white and ultraviolet light record the core before it is sampled, because oil fluoresces under ultraviolet and shows which layers hold it. Plugs are then drilled for porosity and permeability tests at regular intervals, often every 30 centimetres, and the results are placed beside the description so that rock quality can be tied to facies. That link between facies and measured properties is what lets a geological model predict reservoir quality between wells.

From sediment to rock: diagenesis

Once buried, sediment becomes rock through diagenesis, the physical and chemical changes that happen at the temperatures and pressures of the upper few kilometres of the crust. Compaction squeezes grains together and drives out water. Cementation precipitates new minerals, such as quartz, calcite or clay, in the pore spaces and binds the grains. Dissolution can remove unstable grains or cement and create new pores. Together, these processes usually reduce porosity and permeability with depth, but dissolution and early cements that prop the grains apart can preserve unusually good porosity deep in a basin.

Diagenesis is one reason two sandstones deposited in the same environment can perform very differently. A beach sand buried quickly beneath a thick shale may keep much of its porosity; the same sand exposed to circulating groundwater may be cemented solid with calcite. Petrographers study thin slices of rock under the microscope to work out which cements formed when, and that sequence, called the paragenesis, helps predict reservoir quality in places no well has yet reached. Lessons 3 and 4 return to diagenesis for sandstones and carbonates.