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	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

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	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
						Skeletal
				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. 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.
					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.
					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

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

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