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Encyclopedia > Petroleum geology

Petroleum geology is a term used to refer to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration). It is principally concerned with the evaluation of seven key elements in sedimentary basins: In chemistry, a hydrocarbon is a cleaning solution consisting only of carbon (C) and hydrogen (H). ... Oil Exploration is the search by petroleum geologists for hydrocarbon deposits beneath the Earths surface. ... The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. ...

  • Source
  • Reservoir
  • Seal
  • Trap
  • Timing
  • Maturation
  • Migration
Schematic of some of the seven key elements
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. Schematic of an oil reservoir in a structural trap Created by Charlie Ferrero 2003. ... Old fault exposed by roadcut near Harrisburg, Pennsylvania. ... An oil well is a laymans term for any perforation through the Earths surface designed to find and release both petroleum oil and gas hydrocarbons. ... Reflection seismology is a branch of seismology that uses reflected seismic waves to produce images of the Earths subsurface. ...

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 field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport the Earths chemical components in time and space. ...

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. In geology, the porosity of a rock or sediment is the proportion of the non-solid volume to the total volume of material, and is defined by the ratio: where Vp is the non-solid volume (pores and liquid) and Vm is the total volume of material, including the solid... In geology, permeability is a measure of the ability of a material (typically, a rock) to transmit fluids through it. ... Stratigraphy, a branch of geology, is basically the study of rock layers and layering (stratification). ... Sedimentology is the branch of geology concerned with understanding the characteristics of sedimentary rocks originally deposited in sedimentary basins. ...

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. Evaporites are water-soluble, mineral sediments that result from the evaporation of saline water. ... The Needles, part of the extensive Southern England Chalk Formation Chalk is a soft, white, porous form of limestone composed of the mineral calcium carbonate. ... Shale Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. ...

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. In physics, buoyancy is an upward force on an object immersed in a fluid (i. ...

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.


Major subdisciplines in petroleum geology

Several major subdisciplines exist in petroleum geology specifically to study the seven key elements discussed above.

Analysis of source rocks

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, palaeogeography and sedimentology to determine the likelihood of organic-rich sediments having been deposited in the past. Stratigraphy, a branch of geology, is basically the study of rock layers and layering (stratification). ... Palaeogeography is the study of the ancient geography of the Earths surface. ... Sedimentology is the branch of geology concerned with understanding the characteristics of sedimentary rocks originally deposited in sedimentary basins. ...

If the likelihood of there being a source rock is thought to be high, then next matter to address is the state of thermal maturity of the source, and the timing of maturation. Maturation of source rocks (see diagenesis and fossil fuels) 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 kerogens present and their maturation characteristics) and basin modelling methods, such as back-stripping, to model the thermal gradient in the sedimentary column. In petroleum geology, the maturity of a rock is a measure of its state in terms of hydrocarbon generation. ... In 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). ... Coal rail cars in Ashtabula, Ohio Fossil fuels, also known as mineral fuels, are hydrocarbon-containing natural resources such as coal, petroleum and natural gas. ... Kerogens are chemical compounds that comprise the segment of organic matter in sedimentary rocks insoluble in the normal organic solvents because of its huge molecular weight (upwards of 1,000). ... Basin 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. ... This article may be too technical for most readers to understand. ... Temperature 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. ...

Analysis of reservoir

The existence of a reservoir rock (typically, sandstones and fractured limestones) is determined through a combination of regional studies (i.e. analysis of other wells in the area), stratigraphy and sedimentology (to quantify the pattern and extent of sedimentation) and seismic interpretation. Once a possible hydrocarbon reservoir is identified, the key physical characteristics of a reservoir that are of interest to a hydrocarbon explorationist are its porosity and permeability. Traditionally, these were determined through the study of hand specimens, contiguous parts of the reservoir that outcrop at the surface and by the technique of formation evaluation using wireline tools passed down the well itself. Modern advances in seismic data acquisition and processing have meant that seismic attributes of subsurface rocks are readily available and can be used to infer physical/sedimentary properties of the rocks themselves. In petroleum exploration and development, formation evaluation is used to determine whether a potential oil or gas field is commercially viable. ... Seismology (from the Greek seismos = earthquake and logos = word) is the scientific study of earthquakes and the movement of waves through the Earth. ... Data acquisition is the sampling of the real world to generate data that can be manipulated by a computer. ...

Petroleum exploration

Petroleum is found over all throughout great structures in the earth's crust, being that with such structures deep faults are associated and communicates mantle to crust, i.e. zones of subduction, continental collision (limits between convergent plates) and riftes (limits of divergent plates). In the application of geologic studies for prospection and research of oil geophysical methods are used (seismic, gravimentry, magnetometry, satellite images). Oil is found in such a way in the sea as in land, over all in the sedimentary basins, but also in rocks of the crystalline basement. Reservoirs and seal rocks are studied through sedimentology and stratigraphy, and in the characterization of the traps the studies of the structures that allow economic accumulations. During drilling of an oil well, crossed rocks are described, also searching indications of hydrocarbons. Later, tools are used that investigate radioactive, electric, magnetic and elastic properties of the rocks of the wall of the well (logging) which allow to identify and to evaluate the presence of hydrocarbons.

See also

  Results from FactBites:
Petroleum (889 words)
Crude petroleum (oil) is a naturally occurring hydrocarbon-based liquid which is sometimes present in porous rocks beneath the earth's surface.
Petroleum is usually classified according to the predominance of paraffin or asphaltic compounds and, accordingly, is said to have a paraffin base, an intermediate base, or an asphalt base.
Petroleum is used principally as a source of fuel and lubricating oils.
  More results at FactBites »



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