Depositional trap regime

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
Store	AAPG Store

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.

Classes

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: Stratigraphic

Regime: 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.

Class: 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.

Subclass: Sandstone reservoirs

Partially or completely isolated by seal.

Subclass: Carbonate reservoirs

Partially or completely isolated by seal.

Subclass: Igneous reservoirs

Partially isolated by seal.

Class: 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.

Subclass: Regional sandstone pinch-outs

Regional updip pinch-outs of sandstone into an impermeable facies such as shale or anhydrite.

Subclass: Local sandstone pinch-outs

Local updip pinch-outs of sandstone into an impermeable facies such as shale or anhydrite.

Subclass: Regional carbonate pinch-outs

Regional updip pinch-outs of carbonate into an impermeable facies such as shale or anhydrite.

Subclass: Local carbonate pinch-outs

Local updip pinch-outs of carbonate into an impermeable facies such as shale or anhydrite.

Class: Depositional relief traps

Process forms positive relief on top of the reservoir; this topographic relief between top seal and reservoir creates the trap closure.

Subclass: Sandstone depositional relief traps

Subclass: Carbonate depositional relief traps

Style: Carbonate reservoirs sealed by shale

Style: Carbonate reservoirs sealed by tight carbonate

Style: Carbonate reservoirs sealed by evaporites

Families

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: Depositional reservoirs

Class: Isolated depositional reservoirs

Subclass: Isolated carbonate reservoirs

Superfamily: Marine carbonate reservoirs

Family: Open-shelf (high-energy) carbonates

Subfamily: Shoal

Variety: Oolite

Variety: Skeletal

Family: Tidal-zone carbonates

Subfamily: Tidal channel

Subclass: Isolated sandstone reservoirs

Superfamily: Marine sandstone traps

Family: Shallow-water sandstone reservoirs

Subfamily: Beach

Subfamily: Barrier island

Subfamily: Offshore bar

Family: Deepwater sandstone reservoirs

Subfamily: Turbidites

Subfamily: Turbidite channel

Subfamily: Submarine fans

Superfamily: Alluvial sandstone reservoirs

Family: Fluvial

Subfamily: Channel

Variety: Deltaic

Family: Deltaic

Subfamily: Distributary channel

Subclass: Isolated igneous reservoirs

Superfamily: Intrusive igneous bodies

Family: Intrusive sills

Class: Depositional pinch-outs

Subclass: Sandstone pinch-outs

Superfamily: Marine sandstone pinch-outs

Family: Shallow marine

Updip pinch-out of shallow marine sands into lagoonal or basinal shales and silts.

Subfamily: Barrier bar

Variety: Pinch-out into lagoonal shale

Variety: Pinch-out into marine shale

Family: Deep marine

Subfamily: Turbidite

Updip pinch-out of marine turbidite sandstone into marine shale.

Superfamily: Lacustrine sandstone pinch-outs

Family: Lacustrine delta pinch-out

Subclass: Carbonate pinch-outs

Superfamily: Marine carbonate pinch-outs

Family: Tidal zone

Subfamily: Tidal-flat carbonate pinch-out

Variety: Pinch-out into silts and shales

Variety: Pinch-out into tight dolomites and anhydrite

Family: Open shelf (high energy)

Subfamily: Carbonate bank pinch-out

Reservoir variety: Rudistid limestone bank

Seal variety:

Top: Marine shale

Side: Tight shelf limestone

Class: Depositional relief

Subclass: Sandstone

Superfamily: Eolian sandstone reservoirs

Family: Dune

Superfamily: Marine sandstone reservoirs

Family: Deep water

Subfamily: Turbidite fan

Subclass: Carbonate

Superfamily: Marine carbonate reservoirs

Family: 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 (James and Gelsetzer, 1989). 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.

Subfamily: 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 (James and Geldsetzer, 1989).

Subfamily: 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 (James and Geldsetzer, 1989).

Subfamily: Patch reefs

Small, low-relief carbonate mounds developed by frame-building organisms on top of a shelf.

Subfamily: Mud mounds

Depositional carbonate mounds consisting largely of clean lime mudstone with relatively little macro-fossil debris.

Class: Supraunconformity traps

Subclass: Onlap pinch-out

Superfamily: Nonmarine erosion surface

Family: Sequence-boundary unconformity onlap

Subfamily: Depositional re-entrant

Reservoir variety: Deltaic sandstone pinch-out

Seal variety:

Top: Deltaic shale

Bottom: Subunconformity units

Subclass: Buttress pinch-outs

Superfamily: Nonmarine erosional surface

Family: Sequence-boundary unconformity buttress

Subfamily: Pinch-out against erosional ridge

Reservoir variety: Alluvial sandstone

Seal variety:

Top: Nonmarine shale

Side: Marine shale of subcrop ridge

Bottom: Marine shale beneath unconformity

See also

• Stratigraphic trap system
• Erosional trap regime
• Diagenetic trap regime

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