The practice of utilizing geological principles and applying geological concepts to the discovery and recovery of petroleum. Related fields in petroleum discovery include geochemistry and geophysics. The related areas in petroleum recovery are petroleum and chemical engineering. See also: Chemical engineering; Geochemistry; Geophysics
Occurrence of petroleum
Petroleum occurs in a liquid phase as crude oil and condensate, and in a gaseous phase as natural gas. The phase is dependent on the kind of source rock from which the petroleum was formed and the physical and thermal environment in which it exists. As a liquid, oil may be readily transported from producing fields to points of consumption, thus moving in a global market. Natural gas is moved chiefly by pipelines and is therefore tied to land-locked markets. See also: Natural gas; Petroleum
Most petroleum occurs at varying depths below the ground surface, but generally petroleum existing as a liquid (crude oil) is found at depths of less than 20,000 ft (6100 m) while natural gas is found both at shallow depths and at depths exceeding 30,000 ft (9200 m). In some cases, oil may seep to the surface, forming massive deposits of oil or tar sands, such as the Athabasca oil sands of Alberta, Canada, and the tar sands of the Faja de Orinoco in Venezuela. Natural gas also seeps to the surface but escapes into the atmosphere, leaving little or no surface trace. See also: Oil sand
Most petroleum is found in sedimentary basins in sedimentary rocks, although many of the 700 or so sedimentary basins of the world contain no known significant accumulations. Although oil or gas accumulations are commonly known as pools, oil and gas occur in relatively small voids in rocks, such as pores and fractures, and not as underground pools or streams. See also: Basin; Sedimentary rocks
Several conditions must exist for the accumulation of petroleum: (1) There must be a source rock, usually high in organic matter, from which petroleum can be generated. (2) There must be a mechanism for the petroleum to move, or migrate. (3) A reservoir rock with voids to hold petroleum fluids must exist. (4) The reservoir must be in a configuration to constitute a trap and be covered by a seal—any kind of low-permeability or dense rock formation that prevents further migration. If any of these conditions do not exist, petroleum either will not form or will not accumulate in commercially extractable form.
Source rocks for most petroleum generation are those containing high concentrations of animal and plant organic matter. To yield high concentrations of total organic carbon, accumulation of the animal or plant remains should be in a chemical reducing environment so that the carbon is not oxidized. Accumulated organic matter eventually must be buried at depths sufficient to have temperatures of at least 140°F (60°C). On average, temperature increases with depth at a rate of about 18°F/1000 ft (10°C/300 m), but such a geothermal gradient exhibits wide variations. Organic matter exposed to sufficiently high temperatures, over time, changes or matures through stages to a liquid or gaseous hydrocarbon. The process of converting organic matter to petroleum is essentially a cooking process. See also: Organic geochemistry
Rocks that form source beds are commonly fine-grained and low in permeability, and thus make poor reservoirs. In some cases, however, the generated petroleum hydrocarbons do not leave the source rock or place of formation. Excellent examples are the so-called oil shales of the western United States, where vast quantities of petroleum hydrocarbons occur but extraction and recovery are expensive. Generally, migration from the source rock to a porous and permeable reservoir is necessary for commercial extraction. Such migration is initiated as buried source rocks are compacted and fluids are expelled and moved laterally and vertically to reservoirs, but the precise mechanisms for primary migration from source rocks to reservoirs are subjects of continuing debate. See also: Oil shale
Petroleum trapping and reservoir
Migration ends once petroleum liquids and gases are trapped in a reservoir, or if they are not trapped, when they escape to the surface. The reservoir must have sufficient porosity and permeability, either with pores or fractures, to accumulate fluids. Those voids are generally filled initially with water, so that emplacement of petroleum must involve the displacement of some or all of the water. Most reservoir rocks are either sandstone or carbonate; these rocks show wide ranges of porosity and permeability, and they vary significantly in reservoir quality, as demonstrated by the efficiency in extraction. In fact, actual recovery of oil from a reservoir varies from as little as 5% of total original volume to as much as 95%, depending largely on the quality of the reservoir.
A trap is any arrangement of strata that allows the accumulation of oil and gas and precludes further migration. A variety of geologic mechanisms exist. Strata may be arranged by folding to give an anticline or a convex-upward trap. Areal changes in permeability of a reservoir from porous to impermeable will stop migration and cause a trapping of oil and gas. A reservoir may terminate in any direction due to subsequent erosion or to the original process by which it formed, such as a coral reef; such terminations are pinch-out traps. A fault or a vertical displacement of strata may result in permeable strata abutting impermeable strata, creating a fault trap. Or traps may be formed by vertical movement of salt or shale to form diapir traps.
Reservoirs must be covered by a seal of impervious strata, such as salt or shale, to prevent further migration and to effect petroleum accumulation. With changing geologic conditions over time, the reservoir may be breached or modified, causing the trapped oil and gas to escape. This movement, known as secondary migration, may be to the ground surface or to another reservoir and trap. See also: Diapir; Limestone; Salt dome; Sandstone; Shale
The aim of petroleum geologists is to find traps or accumulations of petroleum. The trap not only must be defined but must exist where other conditions such as source and reservoir rocks occur. Most structures or rock configurations that could contain and hold petroleum in fact do not. Thus, even after applying the best scientific information, a well that is drilled to an accurately defined trap may not encounter hydrocarbons.
A variety of techniques are used by the petroleum geologist to reconstruct geologic events and define an area or a prospect to be drilled. Geologic structures shown at the surface, such as folds or unconformities, can be projected into the subsurface. Accumulation of hydrocarbons at the ground surface may occur as seeps. In the early days of oil and gas exploration, reliance was placed on such surface manifestations; and in remote, unexplored areas of the world, surface conditions are yet a clue to the existence of petroleum at depth.
Most of the world’s oil and gas accumulations show no surface manifestation. To locate these traps, the geologist must rely on subsurface information and data gathered by drilling exploratory wells and data obtained by geophysical surveying. These data, once interpreted, are used to construct maps, cross sections, and models that are used to infer or to actually depict subsurface configurations that might contain petroleum. Such depictions are prospects for drilling. If, on drilling, the trap is found to be as reconstructed by the petroleum geologist and if all other conditions for oil and gas accumulation exist, a discovery is made. If the prospect does not exist as envisaged, the well encounters no oil and is recorded as a dry hole. On average, less than one exploratory well in ten contains commercial quantities of oil or gas. Exploration efficiency has improved remarkably in recent years with advances in the technology of seismic reflection acquisition and processing. Running seismic surveys in a closely spaced grid allows three-dimensional imaging of the Earth’s subsurface. In certain areas, especially offshore where seismic is shot through a uniform medium of seawater, character of reflections commonly allows direct detection of hydrocarbon accumulations. Accordingly, in areas amenable to advanced seismic technology, success rates for oil and gas discovery are 35% or higher. See also: Geophysical exploration; Oil and gas well drilling
Oil and gas must be trapped in an individual reservoir in sufficient quantities to be commercially producible. That quantity is determined by the price or value of oil or gas and the cost to find it. In the United States, extensive drilling since around 1890 has led to the discovery of nearly 30,000 oil and gas fields. These fields range in size from some with less than 100,000 barrels (16,000 m3) of oil to the largest field so far discovered in the United States, Prudhoe Bay in Alaska, which contained more than 1010 bbl (1.6 × 108 m3) of recoverable oil. Worldwide, 25% of all oil discovered so far is contained in only ten fields, seven of which are in the Middle East. Fifty percent of all oil discovered to date is found in only 50 fields.
Worldwide, about 4 × 106 oil and gas wells have been drilled. About 85% percent of the wells have been drilled in the United States. As a result, most of the large and fairly obvious fields in the United States have been discovered, except those possibly existing in frontier or lightly explored areas such as Alaska and the deep waters offshore. Few areas of the world remain entirely untested, but many areas outside the United States are only partly explored, and advanced techniques have yet to be deployed in the recovery of oil and gas found so far. See also: Petroleum reserves
Although in the United States the most obvious and most readily detectable prospective traps have been tested, many traps remain that are subtle and not easily definable. It is known that petroleum is accessible in the subtle traps because many have been found, commonly by accident. This occurs as a particular prospect is being tested and drilling encounters an unexpected accumulation. The challenge to the petroleum geologist is to develop models and to use ever-advancing technology to improve the ability to detect these subtle structures.
In addition to exploration for difficult-to-detect traps, greater efforts in petroleum geology along with petroleum engineering are being made to increase recovery from existing fields. Of all oil discovered so far, it is estimated that there will be recovery of only 35% on the average. In the United States alone, the amount of oil known in existing reservoirs and classed as unrecoverable is more than 3.25 × 1011 bbl (5.2 × 1010 m3), twice the volume produced to date and 50 times the amount of oil the United States uses every year. Recovering some part of this huge oil resource will require geological reconstruction of reservoirs, a kind of very detailed and small-scale exploration. These reconstructions and models have allowed additional recovery of oil that is naturally movable in the reservoir. If the remaining oil is immobile because it is too viscous or because it is locked in very small pores or is held by capillary forces, techniques must be used by the petroleum geologist and the petroleum engineer to render the oil movable. If oil is too viscous or heavy to flow, steam can be injected into the reservoir to raise the temperature and thus lower the oil viscosity. If oil is locked in small pores, gas can be injected to expand the fluid and cause it to escape and move.
J. Gluyas and R. Swarbrick, Petroleum Geoscience, 2003
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R. C. Laudon, Principles of Petroleum Development Geology, 1995
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A. I. Levorsen, Geology of Petroleum, 2d ed., 1967, reprint 2001
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L. B. Magoon and W. G. Dow (eds.), The Petroleum System—from Source to Trap, 1994
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