Diagenetic trap regime
|Exploring for Oil and Gas Traps
|Treatise in Petroleum Geology
|Traps, trap types, and the petroleum system
|Classification of exploration traps
|Richard R. Vincelette, Edward A. Beaumont, Norman H. Foster
Diagenetic traps are those in which the trap boundaries are due to postdepositional diagenetic processes, which can create new reservoirs or new seals.
The hierarchical structure and definitions for diagenetic traps are shown below.
|Diagenetic reservoirs; Form where porosity is created as a result of local diagenetic processes such as dissolution or dolomitization.
|Secondary dolomite reservoirs; Form as a result of localized secondary dolomitization.
|Leached (secondary) porosity: Form as a result of localized leaching of nonreservoir-quality rock to create enhanced porosity and permeability. Common beneath unconformity surfaces.
|Diagenetic seals: Trap boundaries created by plugging original reservoir porosity and permeability by diagenetic cements or minerals.
|Secondary anhydrite barriers; Form as a result of secondary anhydrite precipitating in original pore spaces.
|Secondary clay barriers; Form as a result of secondary clay, commonly kaolinite, precipitating in original pore space.
|Secondary pore-throat trap; Caused by a reduction in the size of pore-throat radii of reservoir rocks by depositional or diagenetic processes. These trap types typically have relatively porous units in the sealing facies that are impermeable to oil migration due to capillary restraints but contain producible water.
Discussion of diagenetic trap families
Families of diagenetic traps based on genesis of the diagenetic processes that created or modified either the reservoir or the seal are somewhat difficult to establish because not all diagenetic processes are well understood and disagreements often exist about the cause of specific diagenetic processes that trapped hydrocarbons. Furthermore, almost all reservoirs and seals have undergone some degree of diagenesis since deposition, making diagenesis at least a secondary trapping element in many fields.
The common occurrence of dolomitized reservoirs is a classic case in point. Dolomitized reefs and dolomitized tidal-flat carbonates are common oil and gas reservoirs. Although dolomitization was a critical element in creating commercial porosity and permeability in these reservoirs, exploration efforts usually focus on defining the original depositional fairway of these reservoirs, i.e., reefs or tidal flats. As a consequence, the primary trap classification of these reservoirs occurs under the depositional trap regime. For our purposes, diagenetic traps are those in which trap boundaries are created by diagenetic processes largely independent of the original depositional environment of reservoir or seal.
Below is a suggested classification outline for some of the superfamilies, families, and subfamilies of the Diagenetic Reservoirs and Diagenetic Seals classes.
|Secondary dolomite reservoirs
|Ascending hot brines
|Leached (secondary) porosity
|Karst and cavern formation
|Secondary anhydrite barrier
- Stratigraphic trap system
- Depositional trap regime
- Erosional trap regime
- Postaccumulation cementation
- Reservoir quality