Geologic modelling is the applied science of creating computerized representations

Geologic modelling (or modeling) is the applied science of creating computerized representations of portions of the Earth's crust, especially oil and gas fields and groundwater aquifers.

In the oil and gas industry, realistic geologic models are required as input to reservoir simulator programs, which predict the behavior of the rocks under various hydrocarbon recovery scenarios. An actual reservoir can only be developed and produced once, and mistakes can be tragic and wasteful. Using reservoir simulation allows reservoir engineers to identify which recovery options offer the safest and most economic, efficient, and effective development plan for a particular reservoir. Geologic modelling is a relatively recent subdiscipline of geology which integrates structural geology, sedimentology, stratigraphy, paleoclimatology, and diagenesis. In 2 dimensions a geologic formation or unit is represented by a polygon, which can be bounded by faults, unconformities or by its lateral extent, or crop. In geological models a geological unit is bounded by 3-dimensional triangulated or gridded surfaces. The equivalent to the mapped polygon is the fully enclosed geological unit, using a triangulated mesh. For the purpose of property or fluid modelling these volumes can be separated further into an array of cells, often referred to as voxels combining the word volumetric and pixel. These 3D grids are the equivalent to 2D grids used to express properties of single surfaces.

1 Geologic modelling components
1.1 Structural framework
1.2 Rock type
1.3 Reservoir quality
1.4 Fluid saturation
2 Geologic modelling software
Structural framework
Incorporating the spatial positions of the major boundaries of the formations, including the effects of faulting, folding, and erosion (unconformities). The major stratigraphic divisions are further subdivided into layers of cells with differing geometries with relation to the bounding surfaces (parallel to top, parallel to base, proportional). Maximum cell dimensions are dictated by the minimum sizes of the features to be resolved (everyday example: On a digital map of a city, the location of a city park might be adequately resolved by one big green pixel, but to define the locations of the basketball court, the baseball field, and the pool, much smaller pixels - higher resolution - need to be used).
Rock type
Each cell in the model is assigned a rock type. In a coastal clastic environment, these might be beach sand, high water energy marine upper shoreface sand, intermediate water energy marine lower shoreface sand, and deeper low energy marine silt and shale. The distribution of these rock types within the model is controlled by several methods, including map boundary polygons, rock type probability maps, or statistically emplaced based on sufficiently closely spaced well data.
Reservoir quality
Reservoir quality parameters almost always include porosity and permeability, but may include measures of clay content, cementation factors, and other factors that affect the storage and deliverability of fluids contained in the pores of those rocks. Geostatistical techniques are most often used to populate the cells with porosity and permeability values that are appropriate for the rock type of each cell.
Fluid saturation
A 3D finite difference grid used in MODFLOW for simulating groundwater flow in an aquifer.
Most rock is completely saturated with groundwater. Sometimes, under the right conditions, some of the pore space in the rock is occupied by other liquids or gases. In the energy industry, oil and natural gas are the fluids most commonly being modelled. The preferred methods for calculating hydrocarbon saturations in a geologic model incorporate an estimate of pore throat size, the densities of the fluids, and the height of the cell above the water contact, since these factors exert the strongest influence on capillary action, which ultimately controls fluid saturations.
Geologic modelling software
Software packages have been designed for geologic modelling purposes to display, edit, digitise and automatically calculate parameters required by engineers, geologists and surveyors.
Three-dimensional stratigraphic modelling

Geomodeller3D
GSI3D
Gocad
Earth Vision
Irap RMS
Landmark
Petrel
PetroMod
Igeoss (Dynel2D/3D and Poly3D)
MineSight
Three-dimensional stratigraphic, faulting & bedrock modelling
Geomodeller3D
PetroMod
Modeling magma crystallizing processes
COMAGMAT-NET 3.3
COMAGMAT-Windows 3.58
MELTs
Groundwater modelling
FEFLOW
MODFLOW
GMS
Visual MODFLOW
ZOOMQ3D

BANDED IRON FORMATION

Banded iron formations (also known as banded ironstone formations or BIFs) are a distinctive type of rock often found in primordial sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite, alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed around three thousand million years before present (3 Ga), include banded iron layers, and the banded layers are a common feature in sediments for much of the Earth's early history.
Banded iron beds are less common after 1.8 Ga, although some are known that are much younger. The conventional concept is that the banded iron layers were formed in sea water as the result of oxygen released by photosynthetic cyanobacteria, combining with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the substrate, which may have been anoxic mud (forming shale and chert). Each band is similar to a varve. The banding is assumed to result from cyclic variations in available oxygen. It is unclear whether these banded ironstone formations were seasonal or followed some other cycle. It is assumed that initially the Earth started out with vast amounts of iron dissolved in the world's acidic seas. Eventually, as photosynthetic organisms generated oxygen, the available iron in the Earth's oceans was precipitated out as iron oxides. At the tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced pulses of free oxygen in the surface waters, alternating with pulses of iron oxide deposition

Water flowing over iron-rich beds
Later banded iron formations
Until fairly recently, it was assumed that the rare later banded iron deposits represent unusual conditions where oxygen was depleted locally and iron-rich waters could form then come into contact with oxygenated water. An alternate explanation of these later rare deposits is undergoing much research as part of the Snowball Earth hypothesis — wherein it is believed that an early equatorial supercontinent (Rodinia) was totally covered in an ice age (implying the whole planet was frozen at the surface to a depth of several kilometers) which corresponds to evidence that the earth's free oxygen may have been nearly or totally depleted during a severe ice age circa 750 to 580 million years ago (mya) (See Cryogenian period, from 800 million years ago (mya, boundary defined chronometrically) to approximately 635 mya) prior to the Ediacaran wherein the earliest multicellular lifeforms appear. Alternatively, some geochemists suggest that BIFs could form by direct oxidation of iron by autotrophic (non-photosynthetic) microbes. The total amount of oxygen locked up in the banded iron beds is estimated to be perhaps twenty times the volume of oxygen present in the modern atmosphere.

Banded iron beds are an important commercial source of iron ore.
Banded iron formation
Archean eon -- time of formation of earth and first rocks
Chlorophyta -- a secondary source of oxygenation
Green algae -- see above
Iron (metallic element)
Ironstone (metallic mineral)
Metals -- group of similar atomic elements
Micropaleontology -- study of ancient microbes and microfossils
Mineralology -- study of minerals and their chemistry
Oxygen catastrophe -- the ancient oxygenation of the atmosphere
Prokaryota -- primitive, unicellular organisms without a nucleus
Archaea -- a newly-described domain of above microbes; includes extremophiles
Bacteria -- the more-familiar domain of above microbes
Cyanobacteria -- a bacterial kingdom previously called "blue-green algae"
Proterozoic eon -- second eon of geologic time (the time of oxygenation)
Paleoproterozoic era -- earliest/lower rock layer
Mesoproterozoic era -- middle geologic layer
Neoproterozoic era -- latest/upper rock layer
Taconite (mineral)
References
Jelte P. Harnmeijer, 2003, Banded Iron-Formation: A Continuing Enigma of Geology, University of Washington Doc format
Klein, Cornelis, 2005, Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins, American Mineralogist; October 2005; v. 90; no. 10; p. 1473-1499; DOI: 10.2138/am.2005.1871 http://ammin.geoscienceworld.org/cgi/content/short/90/10/1473 abstract.
Andreas Kappler, et al., 2005, Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria, Geology; November 2005; v. 33; no. 11; p. 865–868; doi: 10.1130/G21658.1 http://www.gps.caltech.edu/~claudia/papers/kappleretal_GEO2005.pdf