Depositional trap regime

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Traps, trap types, and the petroleum system
Chapter Classification of exploration traps
Author Richard R. Vincelette, Edward A. Beaumont, Norman H. Foster
Link Web page
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Traps in the depositional regime formed primarily by processes that created facies changes between reservoir and seal-quality rocks. Besides deposition by sedimentary processes, this regime also includes deposition by igneous processes.


The basis for the three classes of the depositional regime is the geometric arrangement of the facies resulting from depositional processes. The subclasses describe reservoir composition or lithology. Where described, trap styles can be listed based on the lithology or composition of the sealing rocks. The outline below shows the classes and subclasses of the depositional regime.

System Regime Class Subclass Style
Stratigraphic Depositional; Trap boundaries are created primarily by depositional processes and can involve igneous rocks as well as sedimentary rocks. Three classes are recognized, based on whether the trap involves an isolated reservoir, an updip pinch-out, or depositional relief on top of the reservoir. Isolated (local) depositional reservoirs; Reservoir rock is partially or completely isolated by sealing rocks, which provide top, side, and often bottom seals. These traps are often of limited areal extent, with trap closure defined largely by reservoir distribution. Sandstone reservoirs; Partially or completely isolated by seal.
Carbonate reservoirs; Partially or completely isolated by seal.
Igneous reservoirs; Partially isolated by seal.
Depositional pinch-outs; Depositional processes form an updip pinch-out of permeable rock into impermeable rock. Trap closure is usually created by an updip re-entrant of the pinch-out boundary or by a combination of the pinch-out with other trap elements, such as tectonic nosing or hydrodynamics. Pinch-out boundaries typically involve top, side, and bottom seals. Regional sandstone pinch-outs; Regional updip pinch-outs of sandstone into an impermeable facies such as shale or anhydrite.
Local sandstone pinch-outs; Local updip pinch-outs of sandstone into an impermeable facies such as shale or anhydrite.
Regional carbonate pinch-outs; Regional updip pinch-outs of carbonate into an impermeable facies such as shale or anhydrite.
Local carbonate pinch-outs; Local updip pinch-outs of carbonate into an impermeable facies such as shale or anhydrite.
Depositional relief traps; Process forms positive relief on top of the reservoir; this topographic relief between top seal and reservoir creates the trap closure. Sandstone depositional relief traps
Carbonate depositional relief traps Carbonate reservoirs sealed by shale
Carbonate reservoirs sealed by tight carbonate
Carbonate reservoirs sealed by evaporites


Genetic families for the various depositional trap classes and subclasses have been established based primarily on the genesis of the reservoir. Where desired, trap varieties can be added based on the genesis or origin of the sealing units. Larger superfamilies have been created based on the general depositional environment of the reservoir, i.e., marine, continental, or lacustrine.

Note that the same genetic families and subfamilies can be used for different depositional classes (geometry). This lets cross-correlations be made between different geometric trap classes within similar genetic settings, e.g., isolated reservoirs or pinch-outs within the shallow marine environment. Thus, if desired, trap classes can be combined under similar trap families. Examples of the more common depositional trap superfamilies, families, and subfamilies are given below.

Regime Class Subclass Superfamily Family Subfamily Variety
Reservoir Variety Seal Variety
Top Side Bottom
Depositional reservoirs Isolated depositional reservoirs Isolated carbonate reservoirs Marine carbonate reservoirs Open-shelf (high-energy) carbonates Shoal Oolite
Tidal-zone carbonates Tidal channel
Isolated sandstone reservoirs Marine sandstone traps Shallow-water sandstone reservoirs Beach
Barrier island
Offshore bar
Deepwater sandstone reservoirs Turbidites
Turbidite channel
Submarine fans
Alluvial sandstone reservoirs Fluvial Channel Deltaic
Deltaic Distributary channel
Isolated igneous reservoirs Intrusive igneous bodies Intrusive sills
Depositional pinch-outs Sandstone pinch-outs Marine sandstone pinch-outs Shallow marine; Updip pinch-out of shallow marine sands into lagoonal or basinal shales and silts. Barrier bar Pinch-out into lagoonal shale
Pinch out into marine shale
Deep marine Turbidite; Updip pinch-out of marine turbidite sandstone into marine shale.
Lacustrine sandstone pinch-outs Lacustrine delta pinch-out
Carbonate pinch-outs Marine carbonate pinch-outs Tidal zone Tidal-flat carbonate pinch-out Pinch-out into silts and shales
Pinch-out into tight dolomites and anhydrite
Open shelf (high energy) Carbonate bank pinch-out Rudistid limestone bank Marine shale Tight shelf limestone
Depositional relief Sandstone Eolian sandstone reservoirs Dune
Marine sandstone reservoirs Deep water Turbidite fan
Carbonate Marine carbonate reservoirs Bioherms; Trap results from depositional relief created by porous organic carbonate buildup sealed by overlying and adjacent tight lithologies. These buildups are commonly referred to as reefs. A wide variety of reef traps have been described and classified based upon both the environment of deposition and geometry of the carbonate reservoir. Oil and gas have been trapped in barrier reefs, fringing reefs, platform reefs, atoll reefs, patch reefs, pinnacle reefs, reef mounds (or mud mounts), and carbonate banks.[1] These terms can be used as subfamilies, as noted below. If a more detailed classification is needed, varieties can be established based upon the facies and genesis of the sealing units surrounding the buildups. Pinnacle reefs; High-relief, circular or ovoid mounds created by upward grown of carbonate frame-building organisms in basinal setting. Reef typically contains a significant amount of high-energy carbonate detritus (grain-stones, wackestones) as well as boundstones and framestones. Reef width is less than 10% of height.[1]
Platform reefs; Larger reefal carbonate buildup in which lateral dimensions are measured in kilometers and in which reef width is more than 10 times reef height.[1]
Patch reefs; Small, low-relief carbonate mounds developed by frame-building organisms on top of a shelf.
Mud mounds; Depositional carbonate mounds consisting largely of clean lime mudstone with relatively little macro-fossil debris.
Supraunconformity traps Onlap pinch-out Nonmarine erosion surface Sequence-boundary unconformity onlap Depositional re-entrant Deltaic sandstone pinch-out Deltaic shale Subunconformity units
Subclass: Buttress pinch-outs Nonmarine erosional surface Sequence-boundary unconformity buttress Pinch-out against erosional ridge Alluvial sandstone Nonmarine shale Marine shale of sub crop ridge Marine shale beneath unconformity

See also


  1. 1.0 1.1 1.2 James, N. P., and H. H. J. Gelsetzer, 1984, Introduction, in H. H. J. Geldsetzer, N. P. James, and G . E. Tebbutt,eds.,Reefs—Canada and Adjacent Areas: Canadian Society of Petroleum Geologists Memoir 13, p. 1–8.

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Depositional trap regime
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