Fracture 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

Fracture traps are divided into three classes: extension, shear, and complex, based on the internal characteristics and geometry of the fractures that make up the reservoir. The outline below shows the classes, some subclasses, and possible styles into which these traps may be subdivided.

Outline

Regime	Class	Subclass	Style
Fracture trap; Lateral boundaries of the trap are provided by change from fractured reservoir to unfractured or less fractured rock or by change from open, permeable fractures to cement-filled or narrow-aperture, low-permeability fractures.	Extension fracture trap; Dominant reservoir fractures are extension fractures.	Parallel fractures; Open fractures in a fractured reservoir, predominantly unidirectional in both strike and dip.	Mineralized fracture; Partially or totally mineralized by postfracture cements, typically calcite, gypsum, or silica.
			Nonmineralized fracture; Contains no postfracture cements or minerals.
		Intersecting fractures; Open fractures in fractured reservoir of several intersecting sets, either along fracture strike or fracture dip.	Mineralized fracture
			Nonmineralized fracture
	Shear fracture trap; The dominant reservoir fractures are shear fractures.	Parallel fractures	Mineralized fracture
			Nonmineralized fracture
		Intersecting fractures	Mineralized fracture
			Nonmineralized fracture
	Complex fracture trap; Dominant reservoir fractures are complex fractures.	Parallel fractures	Mineralized fracture
			Nonmineralized fracture
		Intersecting fractures	Mineralized fracture
			Nonmineralized fracture

Based on the interpretation of the genetic causes of fracture traps, two fracture trap families are recognized: tectonic and nontectonic.

Tectonic fracture trap families

The outline below shows the order of the families of the tectonic fracture trap superfamily and defines some of them.

Regime	Superclass	Class	Superfamily	Family	Subfamily
Fracture	Extension
	Shear
	Complex	Parallel
		Intersecting	Tectonic fracture trap; Fractures were generated by crustal tectonic stresses, whether compressional, extensional, or transpressional.	Fold-related fracture trap; Fractures are intimately associated with and controlled by tectonic folds.	Related to zone of maximum curvature.
					Hydrofractures
				Fault-related fracture traps; Fractures are intimately associated with or controlled by tectonic faults.	Fractures related to normal faults
					Fractures related to wrench faults
				Regional fracture trap; Fractures occur over a broad area unrelated to specific folds or faults and in which fractures are thought to have been created by regional tectonic stresses.

Fractures related to folding

Several mechanisms have been proposed to explain the common occurrence of highly fractured zones in various positions along tectonic folds, including bending-induced fractures along zones of maximum curvature, typically along the flanks of monoclines, anticlines, or synclines, as well as along the plunge axis of anticlines. Another cause of fracturing along compressive anticlines could be due to hydrofracturing during compression and squeezing of the folded rocks. Where appropriate, subfamilies can be established, such as maximum-curvature fracture traps and hydrofracture traps.

Fractures related to faulting

Numerous examples exist in which fracture intensity increases with proximity to faults. A number of fracture traps have been attributed to fault-induced or associated fracturing. Subfamilies can be established based on the type of fault with which the fractures are associated, e.g., normal-fault fracture trap or wrench-fault fracture trap.

Fractures related to regional stresses

Within regionally fractured areas, local fracture swarms with enhanced permeability and fracture frequency often occur. The transition from these high-permeability fracture swarms to areas of lower fracture frequency or fracture permeability often provides local lateral trap boundaries within the regional system. These fracture swarms may or may not be related to or associated with local secondary folds or faults.

Nontectonic fracture families

A wide variety of nontectonic elements have been interpreted to cause fractures and fracture traps. The more common ones include salt solution, piercement by mobile cores, meteorite impact, compaction drape, shrinkage due to cooling or diagenesis, pore-fluid overpressuring, erosional uplift and unloading, and hydrothermal fracturing. Each of these can be used as nontectonic fracture trap families. Where necessary, subfamilies and varieties can be created for any of these families. The outline below shows the order and some definitions for these traps.

Regime	Superclass	Class	Superfamily	Family	Subfamily
Fracture	Extension fractures	Parallel
		Intersecting	Nontectonic fracture trap; Fractures generated by nontectonic stresses, e.g., salt solution, piercement, shrinkage, and over pressuring.	Solution collapse
				Piercement
				Impact
				Compaction drape
				Shrinkage	Chert diagenesis
					Cooling joints
				Overpressuring	Source rock maturation
					Geothermal pressuring
					Clay dewatering
				Unloading
				Hydrothermal

See also

• Structural trap system
• Fold trap regime
• Fault trap regime
• Fracture traps

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