Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration). In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum Geologists for Hydrocarbon deposits beneath the Earth's surface

Sedimentary basin analysis

Petroleum geology is principally concerned with the evaluation of seven key elements in sedimentary basins:

A structural trap, where a fault has juxtaposed a porous and permeable reservoir against an impermeable seal. The term sedimentary basin is used to refer to any geographical feature exhibiting Subsidence and consequent infilling by sedimentation 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.

• Source
• Reservoir
• Seal
• Trap
• Timing
• Maturation
• Migration

In general, all these elements must be assessed via a limited 'window' into the subsurface world, provided by one (or possibly more) exploration wells. In Petroleum geology Source rock refers to rocks from which Hydrocarbons have been generated or are capable of being generated West Texas PumpjackJPG|thumb|right|300px|This Pumpjack located south of Midland TX is a common sight in West Texas. 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. Reflection seismology (or seismic reflection) is a method of Exploration geophysics that uses the principles of Seismology to estimate the properties of 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 field of geochemistry involves study of the chemical composition of the Earth and other Planets chemical processes and reactions that govern the composition

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). Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% Permeability in the Earth sciences (commonly symbolized as κ, or k) is a measure of the ability of a material (typically a rock or unconsolidated Some of the key disciplines used in reservoir analysis are the fields of stratigraphy, sedimentology, and reservoir engineering. Stratigraphy, a branch of Geology, studies rock layers and layering ( stratification) Sedimentology encompasses the study of modern Sediments such as sand, mud (silt and clay and understanding the processes that deposit them Reservoir engineering is a branch of Petroleum engineering, typically concerned with maximizing the economic recovery of Hydrocarbons from the subsurface

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. Evaporites (iˈvæpəraɪt are water-soluble Mineral sediments that result from the Evaporation of bodies of surficial Water. Chalk (ʧɔːk is a soft white porous Sedimentary rock, a form of Limestone composed of the Mineral Calcite. Shale (also called mudstone) is a fine-grained Sedimentary rock whose original constituents were Clay minerals or Muds It is characterized by 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. In Physics, buoyancy ( BrE IPA: /ˈbɔɪənsi/ is the upward Force on an object produced by the surrounding liquid or gas in which it is

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, studies rock layers and layering ( stratification) Palaeogeography (sometimes spelled paleogeography) is the study of what the Geography was in times past Sedimentology encompasses the study of modern Sediments such as sand, mud (silt and clay and understanding the processes that deposit them

If the likelihood of there being a source rock is thought to be high, the 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. In Geology and Oceanography, diagenesis is any chemical physical or biological change undergone by a Sediment after its initial deposition and during Fossil fuels or mineral fuels are fossil source Fuels that is Hydrocarbons found within the top layer of the Earth’s crust. 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. Kerogen is a mixture of organic Chemical compounds that make up a portion of the organic matter in Sedimentary rocks It is insoluble in normal organic 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 Back-stripping is a geophysical analysis technique used on Sedimentary rock sequences - it is used to isolate factors which contribute to basin formation/filling other Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature

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. In Petroleum exploration and development formation evaluation is used to determine the ability of a borehole to produce Petroleum. 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. Seismology (from Greek grc σεισμός seismos, "earthquake" and grc -λογία -logia) is the scientific study of Earthquakes Data acquisition is the sampling of the real world to generate data that can be manipulated by a computer

See also

• Important publications in petroleum geology
• Bituminous rocks
• abiogenic petroleum origin
• Fred Meissner, World Renowned Petroleum Geologist, Author, Professor at Colorado School of Mines

External links

• Oil On My Shoes -- Web site devoted to the science and practical application of Petroleum Geology