In
sedimentology, compaction is the process by which a
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
progressively loses its
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...
due to the effects of pressure from loading. This forms part of the process of
lithification
Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...
. When a layer of sediment is originally deposited, it contains an open framework of particles with the pore space being usually filled with water. As more sediment is deposited above the layer, the effect of the increased loading is to increase the particle-to-particle stresses resulting in porosity reduction primarily through a more efficient packing of the particles and to a lesser extent through elastic compression and
pressure solution. The initial porosity of a sediment depends on its
lithology.
Mudstones start with porosities of >60%,
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
s typically ~40% and
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
s sometimes as high as 70%. Results from
hydrocarbon exploration
Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology.
Exploration methods
Visi ...
wells show clear porosity reduction trends with depth. Compaction trend estimation and decompaction process are useful for analyzing numerical basin evolution (e.g., subsidence) and evaluating hydrocarbon reservoirs and geological storages.
In sediments compacted under self-weight, especially in sedimentary basins, the porosity profiles often show an exponential decrease, called Athy's law as first shown by Athy in 1930. A mathematical analytical solution was obtained by Fowler and Yang to show the theoretical basis for Athy's law. This process can be easily observed in experiments and used as a good approximation to many real data.
Differential compaction
If there is a variation in thickness and compactability of a sequence, loading by later deposits will give rise to spatially varying amounts of compaction. This form of compaction is a function of the lithology of the base sediment. Both the thickness and structure of the later sequence will be controlled by the underlying geology in the absence of any active tectonics. Buried tilted
fault blocks in a
rift basin often produce large
anticlinal Anticlinal may refer to:
*Anticline, in structural geology, an anticline is a fold that is convex up and has its oldest beds at its core.
*Anticlinal, in stereochemistry, a torsion angle between 90° to 150°, and –90° to –150°; see Alkane_st ...
closures in the post-rift section that may form traps for
hydrocarbons
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
e.g. the
Daqing Field, the largest
oil field in the
People's Republic of China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
, in the Songliao Basin.
[http://energy.cr.usgs.gov/WEcont/regions/reg3/P3/tps/AU/au314412.pdf, USGS report on the Songliao Basin]
References
Geological processes
Sedimentary rocks
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