Md Qasim:
This link would provide you with essentials of Structural Traps for Hydrocarbon Exploration:
http://course.xsyu.edu.cn/sydz/ydwx/05/traps.pdf
Best
Syed
Traps (of any type) have no role in oil and gas formation. They only host the hydrocarbons after they migrated from a source rock. The source rocks (e.g., organic/bituminous shales etc.) are reponsible for hydrocarbons generation.
All the best.
In certain geologic provinces (e.g. rift basins) you may find that source rocks are deposited in lakes that are later filled with sediments. As regional tension occurs, the fault blocks will pull apart and tilt, resulting in a structural trap of the later sediments. Hopefully that gets covered by seal rock to secure the trap. Once the trap is in place, as the previously deposited source rocks are buried deeper, they heat up and generate hydrocarbons that then migrate into the trap. Fluid densities of hydrocarbons make them want to go up under most circumstances, relative to the naturally water wet rocks surrounding the source rock. Carrier beds/faults will allow the hydrocarbons to flow until they reach either the surface or a trap with a sufficient seal to hold them back.
Structural Traps come in many flavors, and nearly all traps characterized as stratigraphic traps have a structural element.
Structural Traps can be "4-way" features, a structural culmination for which closing contours form a closed trap
As indicated by other answers structural traps can be "3-Ways" Closing contours are in part bounded by a fault. In the case of rift basins and commonly in Deepwater and Deltaic systems these traps are down-thrown to a regional high. So the trap occurs in the footwall with closing contours terminating updip to a master fault.
Fault Seal traps which as also structural will have closing contours terminating against a sealing fault. This fault may occur anywhere along the trap, in dip or strike direction.
As noted stratigraphic traps will have a structural component. Usage of the term Stratigraphic Traps is not always consistent. In Deepwater, many Channels are trapped by a combination of structure in the dip direction, and stratigraphy in the strike direction. The channel margin forms the stratigraphic element of the trap, typically in the strike direction.
Updip pinch-out stratigraphic traps have closing contours which begin downdip then terminate updip to the reservoir termination or pinchout.
I think other answers cover the rest of structural trap concepts.
Cheers ..... Frank
Structural traps are mainly created by tectonic activity. Usually they don't need the presence of a base seal, only top and lateral seal.
Regards
JS
Very good point made by Jamie, Structural Traps only require a Top Seal for Four Way Closures. Additional seals only are required in complex traps, 3-Ways, or simple fault seal. In the more complex cases sealing against a fault adds another dimension.
In contrast, Stratigraphic Traps require both a Top Seal and a Base Seal.
Cheers ..... Frank
STRUCTURAL TRAPS
Structural traps are primarily the result of folding and (or) faulting, or both.
1. Anticlinal (fold) and dome traps
Necessary conditions: An impervious cap rock and a porous reservoir rock; closure occurs in all
directions to prevent leakage (i.e. four-way closure necessary for a dome).
a) Simple fold traps (anticlinal) with axial culmination (fold axis dipping in two or more
directions). The simplest type of trap is formed when a sandstone bed that is overlain by
tight (i.e. low permeability) shale is folded into an anticline. A simple anticline,
however, may not necessarily be a trap. The crest of the anticline must have an apical
culmination (i.e. a peak) somewhere along the fold axis so that hydrocarbons can be
trapped. Anticlinal traps are commonly detected by seismic reflection. In mature
oilfields, most of these simple traps have probably been found, but many anticlinal traps
remain to be discovered offshore and in new prospective areas.
b) Salt domes: Strata around the salt dome curve upward creating traps against the sealing
salt layers (see below for more details).
c) Growth domes. Domes or anticlines that form during sedimentation when one area
subsides more slowly than the surrounding areas. Their formation is concurrent with
sedimentation (i.e. they form during deposition), and not due to later (tectonic) folding.
Growth anticlines may form due to differential compaction over salt domes or other
upward-projecting features in the substrate (topographic highs on the buried landscape).
2. Fault traps
The fault plane must have a sealing effect so that it functions as a fluid migration barrier for
reservoir rocks. There are several common types of fault trap:
a) Normal faults — commonly associated with graben (rift) structures.
Geol 463.3 Supplementary Notes 463-RWR-5
3
b) Strike-slip faults — these may not be sealed due to incremental movements, but
basement-controlled strike-slip faults commonly produce good anticlinal structures in
overlying softer sediments.
c) Thrust faults — commonly associated with compressional tectonics (e.g., the Front
Ranges in Alberta).
d) Growth faults — Growth faults typically form in sediments that are deposited rapidly,
especially at deltas. Faulting occurs during sedimentation (i.e. syndepositionally), such
that the equivalent strata on the downthrow side will be thicker than on the upthrow side.
The throw between corresponding strata declines upwards along the fault plane. Minor
fault planes with an opposite throw (antithetic faults) may also form in the strata that
curve inward towards the main fault plane. The fault plane is commonly sealed,
preventing further upward migration of oil and gas. Fault traps may also form when
sandstone beds are offset against the fault plane. Some petroleum traps, however, form
in "roll-over" anticlines on the down-faulted block. Growth faults may reduce porewater
circulation in sedimentary basins; consequently, undercompacted clays, which may
develop into clay diapirs, are often associated with growth faults.
Some general points about fault traps…
The geometry and timing determine whether faults will be effective in forming fault traps:
1. Dead faults that predate basinal sediments only affect the underlying basement – they
play no direct role in hydrocarbon trapping in the younger sedimentary pile.
2. Continuously developing faults (growth faults) — these are active during sedimentation
and are major petroleum traps (e.g., Niger Delta).
3. Young (late) faults —these form late during sedimentation; depending on their initiation
and growth, they may or may not be effective as traps.
4. Late regenerated faults —these are new movements on old faults — they are more likely
to destroy than form traps, but may be effective.
Many petroleum fields are closely linked to faulting, but traps that result from faulting alone are
less common. There are three common fault – petroleum pool associations:
1. The fault itself makes the trap without an ancillary trapping mechanism such as a fold —
normal faults are the most common examples.
2. The fault creates another structure (e.g., a fold or horst) that in turn forms the main trap.
3. The fault may be a consequence of another structure that forms the main trap — e.g., the
extensional crestal faults that form above some anticlines.
Geol 463.3 Supplementary Notes 463-RWR-5
4
Important point: Faults are highly ambiguous features. They may leak, acting as permeable
conduits for fluid flow (including oil and gas migration), but more commonly act as seals unless
they are rejuvenated after petroleum has pooled.
They form seals because of fault gouges or differential pressures either side of the fault plane:
Gouge
(mudcake)
along fault
plane
shale
Overpressured
Overpressured
Normal P
Normal P
SEAL
SALT DOME RESERVOIRS
Salt domes form when salt is less dense than the overlying rock, and the salt moves slowly
upwards due to its buoyancy. For this to happen, there must be a minimum overburden and the
thickness of the salt deposits must be more than ~100 m. Upward movement of salt through the
sedimentary strata, and associated deformation is termed halokinetics or salt tectonics.
Movements may continue for several hundred million years.
Traps may form (1) in the strata overlying the salt dome, (2) in the top of the salt domes (the cap
rock - caused by brecciation and dissolution), (3) in the strata that curve upward against the salt
intrusion, (4) due to stratigraphic pinch-out of strata around the salt dome:
Salt dome reservoirs produce major oilfields where basinal sediments contain thick salt deposits.
Salt deposits are common in Permian-Jurassic sediments around the Atlantic Ocean. Examples
include the Gulf of Mexico, where there is Permian and Jurassic salt, the Permian Zechstein salt
in NW Europe and the North Sea.
Dear Md Osim,
This link would provide you information about all type of trap including Structural Traps for Hydrocarbon Exploration:
http://www.kau.edu.sa/Files/0054337/Subjects/types%20of%20oil%20traps.pdf
Best regards,
Waseem
In petroleum geology, a structural trap is a type of geological trap that forms as a result of changes in the structure of the subsurface, due to tectonic, diapiric, gravitational and compactional processes. These changes block the upward migration of hydrocarbons and can lead to the formation of a petroleum reservoir.
Structural traps are the most important type of trap as they represent the majority of the world's discovered petroleum resources. The three basic forms of structural traps are the anticline trap, the fault trap and the salt dome trap.
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