Top and fault seal evaluation
Sufficiently intense deformation in excess of top seal ductility can fracture a top seal. In addition, sufficiently high pore pressures in excess of the fracture pressure can induce natural hydraulic fracturing. Both top seal and fault seal are fundamental to prospect and play assessment as well as to production and field development. Despite our understanding of the variables that control seals[1] practical techniques are few and seal is commonly risked in an intuitive, qualitative manner. However, quantitative seal analysis, using those few techniques available, improves success ratios and reduces costly errors in field development.
Importance of seal
Top seal and fault seal are important because they control the following:
- Presence or absence of hydrocarbons
- Percent fill
- Vertical and lateral distribution of hydrocarbons
- Migration pathways and charge volumes
- Distribution and movement of hydrocarbons during field development
Seals are fundamental; no seal, no trap. Seals, or their absence, also define leak points that control the percent fill for hydrocarbon accumulations. Assessment of percent fill without the ability to risk seal (top or fault) is reduced to a statistical guessing game. They control the vertical and lateral distribution of hydrocarbons, both within individual fields and within basins.
Seals control migration pathways into traps. A trap may be empty not because a fault leaked once-trapped hydrocarbons but because a fault sealed and prevented hydrocarbons from migrating into a trap and filling it in the first place. Similarly, top seals can restrict vertical migration into shallow traps and control the vertical and lateral distribution of hydrocarbons within a basin.
Hydrocarbons migrate until they encounter the first intact seal. Because of variations in seal integrity and capacity, drainage areas are four-dimensional. Prospect analysis using drainage areas defined by simple structure-depth maps on the top reservoir can be very misleading. Plays appear and disappear in response to seal behavior.
Seals also control the movement of hydrocarbons during production. Efficient field development, well placement, ultimate recovery, and economic success or failure depend on risking seal.
See also
References
- ↑ Downey, M., W., 1984, Evaluating seals for hydrocarbon accumulations: AAPG Bulletin, vol. 68, no. 11, p. 1752–1763.
External links
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