Geologic modelling 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.
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).
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 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.
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.
An important part of geologic modelling is related to geostatistics. In order to represent the observed data, often not on regular grids, we have to use certain interpolation techniques. The most widely used technique is kriging which uses the spatial correlation among data and intends to construct the intepolation via semi-variograms.
Mining geologists use modelling to determine the geometry and placement of mineral deposits in the subsurface of the earth. They then determine the concentration and volumes of the minerals investigated. Economic constraints are applied to the model determining the value of mineralization. Plans for mineral extraction are made determined by the ability of the miner to make an economic extraction of the defined ore.
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.
* Paradigm Gocad and SKUA
* Seismic to Simulation
* Turner, A. K. & Gable, C. (2007). "A review of geological modelling. In: Three-dimensional geologic mapping for groundwater applications, Workshop extended abstracts," (PDF). Denver, Colorado. http://www.isgs.uiuc.edu/research/3DWorkshop/2007/pdf-files/turner.pdf.
* Kevin B. Sprague & Eric A. de Kemp. (2005) Interpretive Tools for 3-D Structural Geological Modelling Part II: Surface Design from Sparse Spatial Data http://portal.acm.org/citation.cfm?id=1046957.1046969&coll=&dl=ACM
* de Kemp, E.A. (2007). 3-D geological modelling supporting mineral exploration. In: Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication 5, p. 1051-1061. http://gsc.nrcan.gc.ca/mindep/method/3d/pdf/dekemp_3dgis.pdf