Types of Well

Hydrocarbon Trap TypesHydrocarbon Trap Types

American Petroleum Institute, American Petroleum Institute, 19861986

Salt Dome

Fault

Unconformity

Pinchout

Anticline

Source Rock Migration Route* Reservoir Rock Trap* Seal Rock

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Types of Well

Traps

• A Trap - a set of conditions to hold the petroleum in a reservoir and prevent its escape by migration.

• Some definitions concerning the trap nomenclature:• Crest (or culmination) is the highest point of the trap • Spill point is the lowest point at which hydrocarbons

may be contained in the trap. • This lies on a horizontal contour, the spill plane Closure

of the trap is a vertical distance crest to spill plane

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• A trap may contain oil, gas, or both. The oil water contact (commonly referred to as OWC) is the deepest level of producible oil. Similarly, the gas oil contact (GOC), is the lower limit of producible gas.

• Also it is important to gas overlies the oil as the gas has a lower density and water will take the lowest position (the highest density of them).

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• Structural traps are traps whose geometry was formed by post depositional tectonic modification (folding, faulting, and mobility processes).

•

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• Stratigraphic traps accumulate oil due to changes in lithology character rather than faulting or folding of the rock. Such changes may be caused by the original deposition of the rock, as with a reef or channel.

• Alternatively, the change in lithology may be postdepositional, as with truncation or diagenetic traps.

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• Hydrodynamic traps occur as part of other trap types.

• They show the accumulations of oil and gas without any apparent seal.

• In this case the hydrodynamic movement of water prevent the upward movement of the fluids.

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• Combination traps are where two or more trapping mechanisms come together to create trap. In real life many successful oil traps are combination traps.

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Structural traps

• Structural traps are traps whose geometry was formed by tectonic processes after deposition of beds involved.

• They are divided into three main groups:– 1. Anticline traps are produced by compressional

folding, by uplift, and by drape over older tectonically created features

– 2.Fault traps (traps produced by faulting)– 3.Traps formed by deformation of the overburden by

mobile material, such as salt (salt dome trap) or shale.

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• Sometimes it is possible to find a trap where a single tectonic process took place, but frequently two or more of the named processes (folding, faulting, and mobility processes) are involved with equal importance in the creation of a trap.

• Structural traps do not occur at random. The types and distribution of them are closely related to the regional tectonic conditions and history of the region in which they are found.

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• Sometimes it is possible to find a trap where a single tectonic process took place, but frequently two or more of the named processes (folding, faulting, and mobility processes) are involved with equal importance in the creation of a trap.

• Structural traps do not occur at random. • The types and distribution of them are closely

related to the regional tectonic conditions and history of the region in which they are found.

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Anticline traps

• The anticline trap is a classical trap type in petroleum geology.

• An anticline is an example of rocks which were previously flat, but have been bent into an arch.

• Oil that finds its way into a reservoir rock that has been bent into an arch will flow to the crest of the arch, where it may be trapped.

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• The anticline, or fold, traps may be subdivided into two classes:

1. Compactional anticlines and 2. Compressional anticlines.

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• Compactional anticlines are formed by crustal tension.

• Where crustal tension causes a sedimentary basin to form, the floor is commonly split into a mosaic of basement Horsts and Grabens.

• The initial phase of deposition infills this irregular topography.

• Throughout the history of the basin, the initial structural architecture usually persists, controlling subsequent sedimentary.

• Thus anticlines may occur in the sediment cover above deep seated horsts.

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Fault traps

• Fault traps are formed by movement of rock along a fault line. In some cases, the reservoir rock has moved opposite a layer of impermeable rock.

• The impermeable rock thus prevents the oil from escaping. In other cases, the fault itself can be a very effective trap.

• Although many fields are trapped by a combination of faulting, pure fault traps are rare.

• That is why fault plays an indirect but essential role in the entrapment of many fields.

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• Bailey and Stoneley (1981) have shown that there are eight theoretical configurations of petroleum traps associated with faulting.

• These configurations are drawn on the assumption that oil can move across, but not up, the fault plane when permeable sands are juxtaposed.

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Fault traps

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Diapiric traps

• 1-Simple anticlinal • 2-Fault traps • 3-Truncation traps • 4-Unconformity traps • 5-Pinchout traps• Cap rock traps, usually due to leaching and the presence of secondary

porosity

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• Diapiric traps are produced by the upward movement of sediments that are less dense than those overlying them (Selley, 1998).

• In this situation the sediments tend to move upward diapirically, and, may form diverse hydrocarbon traps.

• Diapiric traps are generally caused by the upward movement of salt or, less frequently, overpressured clay.

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• Salt has a density of about 2.03 g/cm3. • Recently deposided clay and sand have densities less than

salt. • As the clay and sand are buried, however, they compact,

losing porosity and gaining density. • Ultimately, a burial depth is reached when sediments are

denser than salt. • Depending on a number of variables, this occur between

about 800 and 1200 m. • When this point is reached, the salt will tend to flow up

through the denser overburden.

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• In some salt structures the overlying strata are only up domed, whereas in others the salt actually intrudes its way upward, the latter are referred to as piercement structures.

• That's why salt creates numerous traps of oil and gas. • Such as:

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Stratigraphic traps

• Stratigraphic traps has a geometry that is formed by changes in lithology.

• The lithological variations may be depositional (channels, reefs, and bars) or postdepositional (truncations and diagenetic changes).

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• Levorsen (1934) defined a stratigraphic trap as "one in which the chief trapmaking element is some variation in the stratigraphy, or lithology, or both, of the reservoir rock, such as a facies change, variable local porosity and permeability, or an up structure termination of the reservoir rock, irrespective of the cause" (Richard C. Selley, 1998).

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• Stratigraphic traps are divided into these main groups:

• Pinchout traps oil and gas is trapped where a layer of reservoir rock ends in a wedge surrounded by impermeable rocks.

• Unconformity traps have been formed due to break in the depositional sequence of sediments.

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• Isolated, lenticular bodies, commonly of sandstone, form closed traps.

• Some of them are prolifically produced because they are filled with oil without free water; most of them are small.

• Some biostromal bodies of limestone or dolomite also provide tabular or lenticular traps, commonly passing laterally into denser limestones.

• Also this trap type includes chanels and bars.

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• Massive traps that are consist of reef traps and massive erosion traps.

• Reef traps have been formed by carbonate buildups.

• Erosion massive trap is a case when eroded metamorfic, sedimentary or igneous rocks are overlapped by impermeable rock.

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Pinchout traps

• A pinchout trap, or a wedged layer of reservoir rock, traps fluids and gases if surrounded by impermeable rocks.

• There are two types of pinchout traps:

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• Permeability pinchouts within otherwise continuous formations provide traps in both clastic and carbonate succession (F.K. North, 1985).

• In sandstone reservoirs, the loss of permeability in the updip direction is due to increasing shaliness of the sand or passage into a band of secondary cementation

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• More striking are the carbonate representatives of this trap category, about equally divided between updip transition from dolomitized to undolomitized limestone and from limestone (occasionally dolomite) into evaporate time equivalents.

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• Depositional pinchout traps within otherwise a continuous succession are characteristic for in sandstones, which have much higher horizontal permeabilities than limestones.

• There are two cases: • In the first case, the sand body wedges out up

the depositional dip, as a beach and bar sand do. Such sands are in trap position from the outset.

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• They may also possess optimum source reservoir seal relations if they merge down dip into basinal shales and up dip into delta plain or lagoonal sediments of low permeability.

• They may also instead merge down dip into deep water turbidite sands associated with the basinal shales.

• Under regression, their updip wedge outs may remain exposed too long, or be covered by continental sands which permit escape of hydrocarbons directly to the surface.

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• In the second case, the depositional pinchout is down dip, resulting in a sandstone body wedged between two shales.

• If such a pinchout is to acquire trap position, its original dip must somehow be preserved.

• This is easily achieved by uplift of the source area of the sand, and the consequent loading of the depositional basin by molasse sediments.

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Unconformity trap

• An unconformity is a break in the depositional sequence of rocks.

• If the underlying beds were tilted, eroded, and then covered with flat lying impermeable rocks, then oil and gas may be trapped at the unconformity.

• A large percentage of the known global petroleum reserves are trapped adjacent to the worldwide unconformities and south rocks of Late Jurassic to mid Cretaceous age.

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• The reservoir may be capped by almost any sedimentary rock type, but commonly by shale, or evaporites (F.K. North, 1985).

• A special case is when asphalt is the seal; the oil accumulates under the surface products of its own inspissation, especially if these cover dipping beds.

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• Most traps have had their reservoir quality enhanced by epidiagenesis (Selley, 1998).

• This secondary solution, porosity induced by weathering, is particularly well known in limestones, but also occurs in sands and basement.

• Weathering of limestones ranges from minor moldic and vuggy porosity formation to the generation of karstic and collapse breccia zones of great reservoir potential. (Rafaelsen, in prep)

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Reef traps

• Reefs, or carbonate buildups, possess all the ancillary trapping mechanisms of buried hills: flanking clastic sediments of high porosity, facies changes over their crests, and drape anticlines in post reef strata (F.K. North, 1985).

• They add two other vital characteristics of their own: they are themselves excellent reservoirs and they are commonly very well sealed.

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• Organic reefs range is size from gigantic barrier forming structure, through large atolls, discreate bioherms, to tiny isolated bodies only a few square meters in area. (Rafaelsen et al, 2003)

• Most reefs, large or small, are carbonate constructions of colonies of algae, corals, molluscs, and some groups of now extinct organisms such as stromatoporoids, with symbiotic crinoids, brahiopods, and foraminifera.

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• Reefs are inherently both reservoir rocks and traps. • They owe their reservoir capacities to their

lithologis. • With high primary, skeletal porosity, enhanced by

the solution of aragonitic fossils, reefs readily acquire a unique internal texture;

• reticulating, breccia like networks of permeable channelways, fissures, and vugs, independent of bedding or of any other original structure.

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• The importance of reefs as traps, however, stems from their tendency to acquire natural convexity (North, 1985).

• Colonial marine organisms grow both outward and upward, in order to maintain an essential constant relation to water depth.

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• As important to a reef trap as its inherent convexity and its porosity is its seal.

• During a transgressive depositional cycle reefs become buried by muds or marls which make excellent seals.

• In regressive cycles barries reefs or atolls become overlapped by evaporites or marls from the back reef of lagoonal regime.

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Combination traps

• Many oil and gas fields around the world are not due solely to structure or stratigraphy or hydrodynamic flow, but to a combination of two or more of these forces (Selley, 1998).

• Such fields may properly be termed as combination traps.

• Most of these traps are caused by a combination of structural and stratigraphical processes.

• Structural hydrodynamic and stratigraphic hydrodynamic traps are rare.

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Hydrodynamic traps

• The fourth group of traps is the hydrodynamic trap.

• This type of traps occur as part of other previous considered trap types.

• In these traps hydrodynamic movement of water is essential to prevent the upward movement of oil or gas.

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• Where the water is moving hydrodynamically down permeable beds, it may encounter upward moving oil (Selley, 1998).

• When the hydrodynamic force of the water is greater than the force due to the buoyancy of the oil droplets, the oil will be restrained from upward movement and will be trapped within the bed without any permeability barrier.

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• Hydrodynamic traps often have tilted oil water contacts.

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Summary

• Anticline traps occur in all basin types and commonly occur as inverted swells and uplifted blocks in structurally controlled basins.

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• Fault traps are common for compressional and continental rift basins.

• Fault trapping mechanisms are typical for thrusting belts and uplifted blocks within acreages being under active tectonic movements during their geological history.

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• Salt basins formed over vast areas during Devonian and Early Permian and provide excellent sealing traps, particularly close to and beneath salt domes.

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• Stratigraphic traps occur within pinching out zones, which have been developed on the flanks of the uplifted anticline plays.

• Stratigraphic traps are quite often associated with unconformity traps.

• Isolated lenticular bodies depend on paleofacies environments.

• The reservoirs have formed mainly in offshore and onshore environments of tidal and delta plains.

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• Reef reservoirs are common for carbonate successions and they reflect the paleoenvironments in which they were formed.

• They form good porosity and permeability reservoirs and contain many oil and gas fields.

• Basins with thick sedimentary cover and long geological history often contain many types of traps.

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4. Traps• If nothing stops oil from rising, it will

reach surface – Ex: The La Brea tar pits

• Traps can be rocks that do not allow fluids to pass through them, or folds and faults in the rock can trap petroleum

• Anticlinal Theory• Petroleum Accumulates in

Structural Closure58 12:33

Migration of Petroleum (Traps)

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What do we get from oil?• 1 barrel = 42 gallons of

crude oil• 83% becomes fuel

– Gasoline, diesel, jet fuel, heating oil, and liquefied petroleum gas (propane and butane)

• 17% other– Solvents, fertilizers,

pesticides, plastics * These add up to 44.6 gallons because volume is increased during the refining process.

US Energy Information Administration60

How much oil do we use?

• US consumes 20,680,000 barrels of oil each day (2007)

• US motor gasoline consumption 9,286,000 b/d (390 million gallons/day) (2007)

• World consumes 83,607,000 b/d (2005)

US Energy Information Administration

US oil consumption 1980-2006

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Where do we get our oil from?

US Energy Information Administration62

Oil exports by countryBarrels per day

US Energy Information Administration63

Oil imports by countryBarrels per day

US Energy Information Administration64

Are We Running Out of Oil?Marion King Hubbert (1903-1989)• Shell geophysicist• Hubbert’s Peak and Curve

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Global Oil Flows

BP Statistical Review, 200866

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