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Diagenetic seals effectively block the movement of reservoir hydrocarbons in many sandstone and carbonate rock units. Diagenetic processes that create these trapping seals include (1) chemical compaction through pressure-solution of silicate and carbonate minerals, (2) concentration of insoluble clay minerals and organic matter during chemical compaction, (3) cementation by authigenic minerals, (4) volume increase of rock constituents resulting from hydration or replacement, (5) coalescive recrystallization, (6) mechanical deformation of ductile constituents, and (7) implacement of immobile organic residue derived from crude oil and natural gas. Sealing cements include silica minerals, clays, zeolites, carbonates, sulfates, chlorides and, subordinately, several other mine al groups. The sealing capacity of the rocks is related to (1) the amount of residual porosity, (2) the pore geometry, (3) the modulus of elasticity, (4) the resealing capacity, and (5) the phases and physical properties of pore-filling media.
Diagenetic seals develop in a wide array of diagenetic environments during eodiagenesis (before burial), mesodiagenesis (during burial), and telodiagenesis (after burial). Diagenesis can convert any sandstone or carbonate lithology into a sealing rock. The direct factors that control the formation of diagenetic seals include (1) textural and mineralogical composition, (2) degree of lithification, (3) burial history, (4) fluid regime and history of chemical composition, pressure, and migration of pore-filling media, (5) thermal history, and (6) tectonic stress. These direct factors are, in turn, controlled by other parameters such as the lithology of intercalated sediments, tectonic history, structural position, and spatial relationship to unconformities or faults.
Diagenetic seals in sandstones and carbonate rocks encase reservoir
rocks with either depositional or diagenetic porosity. Diagenetic reservoir porosity may originate before or after the establishment of an effective diagenetic seal. Hydrocarbon traps with diagenetic seals may conform in their geometry as well to structure or stratigraphy as to diagenetic facies. Therefore, some structural and stratigraphic traps may, in part or entirely, depend on diagenetic seals.
An example of such sealing is the Recinus Cardium "A" pool. In this field, lateral, top, and bottom sands have formed as a result of mesogenetic cementation at the margins of the sand body. The limiting injection pressure in these seals is approximately 550 psi (3,790 kPa) (mercury against air), which translates to a pore throat radius of 0.195 microns. This seal could effectively withstand a hydrocarbon-water injection pressure of 51 psi (352 kPa), which represents a trapped hydrocarbon column of between 200 and 500 ft (61 and 92 m). This clearly indicates that diagenetic seals can be tremendously effective.
Detailed analysis of diagenetic seals in sandstones and carbonate rocks can considerably improve our ability to predict their occurrence and to recognize their spatial and temporal relationship to reservoir rocks and hydrocarbon migration.
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