|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Predicting the occurrence of oil and gas traps|
|Chapter||Evaluating top and fault seal|
|Author||Grant M. Skerlec|
The most important mechanical property for evaluating seal integrity is ductility. Ductile rocks make good top seals; brittle rocks make poor top seals. Shales and salt are two of the most ductile rock types and, not surprisingly, two of the most common top seals.
What is ductility?
Ductility is the amount of strain a seal can withstand before brittle failure and the loss of top seal integrity. Rocks with an extremely high ductility can deform without brittle failure. On the other hand, rocks with low ductility can accommodate only a small amount of strain before fracturing. A seal can be brittle but unfractured; a seal can be ductile but fractured. Fracture depends upon whether the strain exceeds the seal ductility.
Variables that control ductility
Seal ductility is controlled by at least nine different variables. The table below lists these variables and briefly notes how they control ductility.
|Variable||Control on ductility|
|Lithology||Grain mineralogy and cement type control ductility. Brittle seals include dolostone, quartzite, anhydrite, and some shales. Ductile seals include halite, some shales, and some limestones.|
|Composition||Not all limestones or shales have the same ductility. Compositional variations such as total organic carbon and clay mineralogy change ductility.|
|Confining pressure||Increasing confining pressure increases ductility.|
|Pore pressure||Increasing pore pressure decreases ductility.|
|Fluid composition||The presence or absence of fluids and their composition affects ductility.|
|Temperature||Increasing temperature increases ductility.|
|Strain rate||High strain rates decrease ductility.|
|Time||Ductility changes with time as seals undergo burial and diagenesis.|
|Compaction state||Ductility decreases with progressive compaction and diagenesis.|
- Strain analysis of top seals
- Estimating ductility of top seals
- Estimating strain in top seals
- Example: evaluating top seal integrity
- Grunau, H., R., 1987, A world-wide look at the cap-rock problem: Journal of Petroleum Geology, vol. 10, no. 3, p. 245–266., 10., 1111/jpg., 1987., 10., issue-3
- Chong, K., P., Hoyt, P., M., Smith, J., W., Paulsen, B., Y., 1980, Effects of strain rate on oil shale fracturing: International Journal of Rock Mechanics, vol. 17, no. 1, p. 35–43., 10., 1016/0148-9062(80)90004-2
- Corbett, K., Friedman, M., Spang, J., 1987, Fracture development and mechanical stratigraphy of Austin Chalk, Texas: AAPG Bulletin, vol. 71, no. 1, p. 17–28.