Hydrocarbon traps

Hydrocarbon traps shape where permeable reservoir rocks (carbonates, sandstones) are covered by rocks alongside depression permeability (caprocks) that are capable of preventing the hydrocarbons from further upward migration. Typical caprocks are compacted shales, evaporites, in addition to tightly cemented sandstones in addition to carbonate rocks.

The caprock ask non travel 100% impermeable to water, oil or gas. If the upward loss of hydrocarbons is less than the furnish of hydrocarbons from the source rocks to the trap, hydrocarbons may soundless accumulate…

Basic Trap-Fluid Nomenclature

Traps are ordinarily classified according to the machinery that produces the hydrocarbon accumulation. The 2 primary groups of traps are those that are formed past times structural deformation of rocks (structural traps), in addition to those that are related to depositional or diagenetic features inwards the sedimentary sequence (stratigraphic traps).

Many traps termination from both of these factors (strati-structural or combination traps). H5N1 mutual illustration is stratigraphic pinch-out (e.g., a sandstone lens wedging into mudstone) that is combined alongside tectonic tilting (which allows hydrocarbons to pond inwards the updip component subdivision of the sandstone wedge). Other traps termination mainly from fracturing (which creates the reservoir

porosity) or hydrodynamic processes.

There are many classifications of hydrocarbon traps inwards use, but most bring 90% inwards common…

1. Anticlinal (fold) in addition to dome traps

a) Simple crimp traps (anticlinal) alongside axial culmination (fold axis dipping inwards 2 or more
directions). The simplest type of trap is formed when a sandstone bed that is overlain by
tight (i.e. depression permeability) shale is folded into an anticline. H5N1 unproblematic anticline,
however, may non necessarily travel a trap. The crest of the anticline must bring an apical
culmination (i.e. a peak) somewhere along the crimp axis therefore that hydrocarbons tin be
trapped. Anticlinal traps are commonly detected past times seismic reflection. In mature oilfields, most of these unproblematic traps bring in all likelihood been found, but many anticlinal traps rest to travel discovered offshore in addition to inwards novel prospective areas.

b) Salt domes: Strata to a greater extent than or less the tabular array salt dome bend upward creating traps against the sealing tabular array salt layers (see below for to a greater extent than details).

c) Growth domes. Domes or anticlines that shape during sedimentation when i expanse subsides to a greater extent than piece of cake than the surrounding areas. Their formation is concurrent alongside sedimentation (i.e. they shape during deposition), in addition to non due to afterward (tectonic) folding.
Growth anticlines may shape due to differential squash over tabular array salt domes or other upward-projecting features inwards the substrate (topographic highs on the buried landscape).

2. Fault traps

The mistake plane must bring a sealing effect therefore that it functions every bit a fluid migration barrier for

reservoir rocks. There are several mutual types of mistake trap:

a) Normal faults — commonly associated alongside graben (rift) structures.
b) Strike-slip faults — these may non travel sealed due to incremental movements, but basement-controlled strike-slip faults commonly create proficient anticlinal structures inwards overlying softer sediments.
c) Thrust faults — commonly associated alongside compressional tectonics (e.g., the Front Ranges inwards Alberta).
d) Growth faults — Growth faults typically shape inwards sediments that are deposited rapidly, specially at deltas. Faulting occurs during sedimentation (i.e. syndepositionally), such that the equivalent strata on the downthrow side volition travel thicker than on the upthrow side.

The throw betwixt corresponding strata declines upwards along the mistake plane. Minor mistake planes alongside an contrary throw (antithetic faults) may likewise shape inwards the strata that bend inward towards the primary mistake plane. The mistake plane is commonly sealed, preventing farther upward migration of oil in addition to gas. Fault traps may likewise shape when sandstone beds are root against the mistake plane. Some crude oil traps, however, shape inwards “roll-over” anticlines on the down-faulted block. Growth faults may trim back porewater circulation inwards sedimentary basins; consequently, undercompacted clays, which may educate into clay diapirs, are oft associated alongside growth faults.

Some full general points well-nigh mistake traps…

The geometry in addition to timing create upwards one’s hear whether faults volition travel effective inwards forming mistake traps:

  1. Dead faults that predate basinal sediments alone comport on the underlying basement – they play no forthwith role inwards hydrocarbon trapping inwards the younger sedimentary pile.
  2. Continuously developing faults (growth faults) — these are active during sedimentatioand are major crude oil traps (e.g., Niger Delta).
  3. Young (late) faults —these shape belatedly during sedimentation; depending on their initiation in addition to growth, they may or may non travel effective every bit traps.
  4. Late regenerated faults —these are novel movements on sometime faults — they are to a greater extent than probable to destroy than shape traps, but may travel effective.
Many crude oil fields are closely linked to faulting, but traps that termination from faulting lone are less common. There are 3 mutual mistake – crude oil puddle associations:
  1.  The mistake itself makes the trap without an ancillary trapping machinery such every bit a crimp —normal faults are the most mutual examples.
  2. The mistake creates some other construction (e.g., a crimp or horst) that inwards plough forms the primary trap. 
  3. The mistake may travel a termination of some other construction that forms the primary trap — e.g., the extensional crestal faults that shape higher upwards some anticlines.
Important point: Faults are highly ambiguous features. They may leak, acting every bit permeable conduits for fluid menstruum (including oil in addition to gas migration), but to a greater extent than commonly human activity every bit seals unless they are rejuvenated after crude oil has pooled.

Salt domes shape when tabular array salt is less dense than the overlying rock, in addition to the tabular array salt moves piece of cake upwards due to its buoyancy. For this to happen, at that topographic point must travel a minimum overburden in addition to the thickness of the tabular array salt deposits must travel to a greater extent than than 100 m. Upward displace of tabular array salt through the sedimentary strata, in addition to associated deformation is termed halokinetics or tabular array salt tectonics. Movements may transcend away on for several hundred 1000000 years.

Traps may shape (1) inwards the strata overlying the tabular array salt dome, (2) inwards the top of the tabular array salt domes (the cap stone – caused past times brecciation in addition to dissolution), (3) inwards the strata that bend upward against the tabular array salt intrusion (4) due to stratigraphic pinch-out of strata to a greater extent than or less the tabular array salt dome:
Salt dome reservoirs create major oilfields where basinal sediments incorporate thick tabular array salt deposits. Salt deposits are mutual inwards Permian-Jurassic sediments to a greater extent than or less the Atlantic Ocean. Examples include the Gulf of Mexico, where at that topographic point is Permian in addition to Jurassic salt, the Permian Zechstein tabular array salt inwards NW Europe in addition to the North Sea.

Stratigraphic traps are created past times whatever variation inwards the stratigraphy that is independent of structural deformation, although many stratigraphic traps involve a tectonic constituent such every bit tilting of strata.
Two primary groups tin travel recognized —

Primary stratigraphic traps termination from variations inwards facies that developed during sedimentation.
These include features such every bit lenses, pinch-outs, in addition to appropriate facies changes.

Examples include:

  • Primary pinch out of strata, e.g., strata that pinch out updip inwards less permeable rocks such every bit shale;
  • Fluvial channels of sandstone that are isolated in addition to surrounded past times impermeable clay-rich sediments;
  • Submarine channels in addition to sandstone turbidites inwards strata rich inwards shale;
  • Porous reefs that are surrounded past times shale, etc.
Secondary stratigraphic traps termination from variations that developed after sedimentation, mainly because of diagenesis. These include variations due to porosity enhancement past times dissolution or loss past times cementation.
Paleogeomorphic traps are controlled past times buried landscape. Some are associated with
prominences (hills); others alongside depressions (valleys). Many are likewise partly controlled by
unconformities therefore are likewise termed unconformity traps.
Hydrodynamic Traps

If porewater menstruum inwards a sedimentary basin is rigid enough, the oil-water contact may deviate from the horizontal because of the hydrodynamic shear stress that is laid up. In some cases, oil may accumulate without closure. Flow of fresh (meteoric) H2O downwards through oil-bearing rocks commonly results inwards biodegradation of the oil in addition to formation of asphalt, which may in addition to therefore shape a cap stone for oil.

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