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| Schematic of some of the seven key elements |
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| This figure illustrates a structural trap, where a fault has juxtaposed a porous and permeable reservoir against an impermeable seal. Oil (shown in red) accumulates against the seal, to the depth of the base of the seal. Any further oil migrating in from the source will escape to the surface and seep. |
In general, all these elements must be assessed via a limited 'window' into the subsurface world, provided by one (or possibly more) exploration wells. These wells present only a 1-dimensional segment through the Earth and the skill of inferring 3-dimensional characteristics from them is one of the most fundamental in petroleum geology. Recently, the availability of cheap and high quality 3D seismic data (from reflection seismology) has greatly aided the accuracy of such interpretation. The following section discusses these elements in brief. For a more in-depth treatise, see the second half of this article below.
Evaluation of the source uses the methods of geochemistry to quantify the nature of organic-rich rocks which contain the precursors to hydrocarbons, such that the type and quality of expelled hydrocarbon can be assessed.
The reservoir is a porous and permeable lithological unit or set of units that holds the hydrocarbon reserves. Analysis of reservoirs at the simplest level requires an assessment of their porosity (to calculate the volume of in situ hydrocarbons) and their permeability (to calculate how easily hydrocarbons will flow out of them). Some of the key disciplines used in reservoir analysis are the fields of stratigraphy, sedimentology, and reservoir engineering .
The seal, or cap rock, is a unit with low permeability that impedes the escape of hydrocarbons from the reservoir rock. Common seals include evaporites, chalks and shales. Analysis of seals involves assessment of their thickness and extent, such that their effectiveness can be quantified.
The trap is the stratigraphic or structural feature that ensures the juxtaposition of reservoir and seal such that hydrocarbons remain trapped in the subsurface, rather than escaping (due to their natural buoyancy) and being lost.
Analysis of maturation involves assessing the thermal history of the source rock in order to make predictions of the amount and timing of hydrocarbon generation and expulsion.
Finally, careful studies of migration reveal information on how hydrocarbons move from source to reservoir and help quantify the source (or kitchen) of hydrocarbons in a particular area.
Several major subdisciplines exist in petroleum geology specifically to study the seven key elements discussed above.
In terms of source rock analysis, several facts need to be established. Firstly, the question of whether there actually is any source rock in the area must be answered. Delineation and identification of potential source rocks depends on studies of the local stratigraphy, palaeogeographyPalaeogeography is the study of the ancient geography of the Earth's surface. Related topics plate tectonics Behavior of plates covering the surface of the Earth. tectonophysics Deformation of rocks. paleontology Study of ancient life, often involving fos and sedimentology to determine the likelihood of organic-rich sediments having been deposited in the past.
If the likelihood of there being a source rock is thought to be high, then next matter to address is the state of thermal maturityIn petroleum geology, the maturity of a rock is a measure of its state in terms of hydrocarbon generation. Maturity is established using a combination of geochemical and basin modelling techniques. Organic-rich rocks (termed source rocks) will alter under of the source, and the timing of maturation. Maturation of source rocks (see diagenesisIn geology, diagenesis refers to all the chemical, physical, and biological changes undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering). These changes happen at relative and fossil fuelFossil fuels are coal and hydrocarbon fuels or hydrocarbon containing fuels such as petroleum (including natural gas). The utilization of fossil fuels has fueled industrial development and largely supplanted water driven mills and wood or peat burning fors) depends strongly on temperature, such that the majority of oil generation occurs in the 60° to 120°C range. Gas generation starts at similar temperatures, but may continue up beyond this range, perhaps as high as 200°C. In order to determine the likelihood of oil/gas generation, therefore, the thermal history of the source rock must be calculated. This is performed with a combination of geochemical analysis of the source rock (to determine the type of kerogenKerogens are chemical compounds formed by the low-grade metamorphism (i. diagenesis) of organic molecules derived from decaying plant and animal matter. Kerogens are the precursors to hydrocarbons ( fossil fuels). Labile kerogen breaks down to form heavys present and their maturation characteristics) and basin modellingBasin modelling is the term broadly applied to a group of geological disciplines that can be used to analyse the formation and evolution of sedimentary basins, often but not exclusively to aid evaluation of potential hydrocarbon reserves. At its most basi methods, such as backstripping , to model the thermal gradientTemperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. General description The formal properties of temperature are studied in thermodynamics. in the sedimentary column.