Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Evaluating top and fault seal

  | chapter = Evaluating top and fault seal

  | frompg  = 10-1

  | frompg  = 10-13

  | topg    = 10-94

  | topg    = 10-15

  | author  = Grant M. Skerlec

  | author  = Grant M. Skerlec

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

A fault is dip leaking if hydrocarbons have migrated along the fault plane. Dip-leaking faults can be identified by the presence of a fault plane leak point (FPLP).

A fault is [[dip]] leaking if hydrocarbons have migrated along the fault plane. Dip-leaking faults can be identified by the presence of a fault plane leak point (FPLP).

==Fault plane leak point (FPLP)==

==Fault plane leak point (FPLP)==

[[file:evaluating-top-and-fault-seal_fig10-10.png|300px|thumb|{{figure number|1}}An FPLP limits the hydrocarbon to the structurally independent closure.]]

[[file:evaluating-top-and-fault-seal_fig10-10.png|300px|thumb|{{figure number|1}}An FPLP limits the hydrocarbon to the structurally independent closure.]]

An FPLP is a type of fault-dependent leak point in which the hydrocarbon contact coincides with the intersection of the fault plane and the top of the reservoir. As shown in [[:file:evaluating-top-and-fault-seal_fig10-10.png|Figure 1]], an FPLP limits the hydrocarbon to the structurally independent closure. The lack of hydrocarbons in contact with the fault plane implies leakage has occurred vertically along the fault.<ref name=ch10r76>Smith, D., A., 1966, [http://archives.datapages.com/data/bulletns/1965-67/data/pg/0050/0002/0350/0363.htm Theoretical considerations of sealing and non-sealing faults]: AAPG Bulletin, vol. 50, no. 2, p. 363–374.</ref><ref name=ch10r3>Allard, D., M., 1993, Fault leak controlled trap fill, rift basin examples (abs.), in Ebanks, J., Kaldi, J., Vavra, C., eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg conference, Crested Butte, Colorado, June 21–23.</ref><ref name=ch10r34>Harding, T., P., Tuminas, A., C., 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0006/0700/0738.htm Interpretation of footwall (lowside) fault traps sealed by reverse faults and convergent wrench faults]: AAPG Bulletin, vol. 72, no. 7, p. 738–757.</ref>

An FPLP is a type of fault-dependent leak point in which the hydrocarbon contact coincides with the intersection of the fault plane and the top of the reservoir. As shown in [[:file:evaluating-top-and-fault-seal_fig10-10.png|Figure 1]], an FPLP limits the hydrocarbon to the structurally independent closure. The lack of hydrocarbons in contact with the fault plane implies leakage has occurred vertically along the fault.<ref name=ch10r76>Smith, D. A., 1966, [http://archives.datapages.com/data/bulletns/1965-67/data/pg/0050/0002/0350/0363.htm Theoretical considerations of sealing and non-sealing faults]: AAPG Bulletin, vol. 50, no. 2, p. 363–374.</ref><ref name=ch10r3>Allard, D. M., 1993, Fault leak controlled trap fill, rift basin examples (abs.), in J. Ebanks, J. Kaldi, and C. Vavra, eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg conference, Crested Butte, Colorado, June 21–23.</ref><ref name=ch10r34>Harding, T. P., and A. C. Tuminas, 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0006/0700/0738.htm Interpretation of footwall (lowside) fault traps sealed by reverse faults and convergent wrench faults]: AAPG Bulletin, vol. 72, no. 7, p. 738–757.</ref>

The ability to predict leakage is important in prospect assessment. An FPLP limits the hydrocarbon volume. Where no independent closure exists, prospects may be completely emptied by dip leakage.

The ability to predict leakage is important in prospect assessment. An FPLP limits the hydrocarbon volume. Where no independent closure exists, prospects may be completely emptied by dip leakage.

==Asymmetric dip leakage==

==Asymmetric dip leakage==

Afault may dip seal on one side of a fault and dip leak on the other. This asymmetric dip leakage is caused primarily by variations in the sand-shale ratio of the fault gouge. Other possible controls in some basins include asymmetric fracture density and/or preferential hydraulic fracturing in the hanging wall.<ref name=ch10r93>Weber, K., J., Mandl, G., Pilaar, W., F., Lehner, F., Precious, R., G., 1978, The role of faults in hydrocarbon [[migration]] and trapping Nigerian growth fault structures: Proceedings, Offshore Technology conference, Houston, p. 2643–2652.</ref><ref name=ch10r71>Skerlec, G., M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.</ref>

Afault may dip seal on one side of a fault and dip leak on the other. This asymmetric dip leakage is caused primarily by variations in the sand-shale ratio of the fault gouge. Other possible controls in some basins include asymmetric [[fracture]] density and/or preferential hydraulic fracturing in the hanging wall.<ref name=ch10r93>Weber, K. J., G. Mandl, W. F. Pilaar, F. Lehner, and R. G. Precious, 1978, The role of faults in hydrocarbon [[migration]] and trapping Nigerian growth fault structures: Proceedings, Offshore Technology conference, Houston, p. 2643–2652.</ref><ref name=ch10r71>Skerlec, G. M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.</ref>

==Example of asymmetric dip leakage==

==Example of asymmetric dip leakage==

[[file:evaluating-top-and-fault-seal_fig10-11.png|300px|thumb|{{figure number|2}}Chocolate Bayou field, U.S. Gulf Coast, an asymmetric dip leakage.<ref name=ch10r57>Myers, J., D., 1968, Differential pressures: a trapping mechanism in Gulf Coast oil and gas fields: Gulf Coast Assoc. of Geologists Transactions, vol. 18, p. 56–80.</ref> Copyright: Gulf Coast Assoc. of Geologists.]]

[[file:evaluating-top-and-fault-seal_fig10-11.png|300px|thumb|{{figure number|2}}Chocolate Bayou field, U.S. Gulf Coast, an asymmetric dip leakage.<ref name=ch10r57>Myers, J. D., 1968, Differential pressures: a trapping mechanism in Gulf Coast oil and gas fields: Gulf Coast Assoc. of Geologists Transactions, vol. 18, p. 56–80.</ref> Copyright: Gulf Coast Assoc. of Geologists.]]

An example of asymmetric dip leakage is the Chocolate Bayou field, U.S. Gulf Coast, shown in [[:file:evaluating-top-and-fault-seal_fig10-11.png|Figure 2]]. All of the hanging wall gas [[accumulation]]s are limited by FPLPs and dip leak. The three gas accumulations in the footwall, however, are all dip sealing; all have gas columns in contact with the fault plane. This pattern of behavior is common in both the U.S. Gulf Coast and the Niger Delta.<ref name=ch10r93 />

An example of asymmetric dip leakage is the Chocolate Bayou field, U.S. Gulf Coast, shown in [[:file:evaluating-top-and-fault-seal_fig10-11.png|Figure 2]]. All of the hanging wall gas [[accumulation]]s are limited by FPLPs and dip leak. The three gas accumulations in the footwall, however, are all dip sealing; all have gas columns in contact with the fault plane. This pattern of behavior is common in both the U.S. Gulf Coast and the Niger Delta.<ref name=ch10r93 />

==Direction of dip leakage==

==Direction of dip leakage==

Although dip leakage is generally understood as the migration ''up'' the fault zone due to buoyant forces, leakage ''down'' the fault zone can also occur. Refraction of flow lines can occur when fluids cross a fault zone of differing [[permeability]] from the wall rock. Fluid flow across a fault of lower permeability than the adjacent wall rock could dip leak down the fault zone.<ref name=ch10r38>Hubbert, M., K., 1953, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0037/0008/1950/1954.htm Entrapment of petroleum under hydrodynamic conditions]: AAPG Bulletin, vol. 37, no. 8, p. 1954–2026.</ref> Similarly, a fault zone with higher permeability than the wall rock will refract flow lines up the fault zone. Pressure differences along and across faults can also cause down-dip flow in the area of lowest pressure.<ref name=ch10r43>Knutson, C., A., Erga, R., 1991, Effect of horizontal and vertical permeability restrictions in the Beryl reservoir: Journal of Petroleum Technology, vol. 43, p. 1502–1509., 10., 2118/19299-PA</ref><ref name=ch10r60>Niemann, J., Krowlow, M., 1992, Delineation of a pressure fault seal from shale resistivities, in Ebanks, J., Kaldi, J., Vavra, C., eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg Research conference, unpublished abstract.</ref>

Although dip leakage is generally understood as the migration ''up'' the fault zone due to buoyant forces, leakage ''down'' the fault zone can also occur. Refraction of flow lines can occur when fluids cross a fault zone of differing [[permeability]] from the wall rock. Fluid flow across a fault of lower permeability than the adjacent wall rock could dip leak down the fault zone.<ref name=ch10r38>Hubbert, M. K., 1953, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0037/0008/1950/1954.htm Entrapment of petroleum under hydrodynamic conditions]: AAPG Bulletin, vol. 37, no. 8, p. 1954–2026.</ref> Similarly, a fault zone with higher permeability than the wall rock will refract flow lines up the fault zone. Pressure differences along and across faults can also cause down-dip flow in the area of lowest pressure.<ref name=ch10r43>Knutson, C. A., and R. Erga, 1991, Effect of horizontal and vertical permeability restrictions in the Beryl reservoir: Journal of Petroleum Technology, vol. 43, p. 1502–1509., 10., 2118/19299-PA</ref><ref name=ch10r60>Niemann, J., and M. Krowlow, 1992, Delineation of a pressure fault seal from shale resistivities, in J. Ebanks, J. Kaldi, and C. Vavra, eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg Research conference, unpublished abstract.</ref>

==Caveat==

==Caveat==

Faults in most basins are at equilibrium. Those faults that were going to leak have leaked. The present structure and juxtapositions control fault seal behavior and spill points. However, disequilibrium may exist in basins now undergoing rapid migration and fill. The Los Angeles basin (California) is one example. Faults may be dip leaking, but the rate of charge is greater than the rate of leakage. These faults appear to dip seal—despite seeps along the surface trace of the faults, which indicate dip leakage.

Faults in most basins are at equilibrium. Those faults that were going to leak have leaked. The present structure and juxtapositions control [[fault seal behavior]] and spill points. However, disequilibrium may exist in basins now undergoing rapid migration and fill. The Los Angeles basin (California) is one example. Faults may be dip leaking, but the rate of [[Calculating charge volume|charge]] is greater than the rate of leakage. These faults appear to dip seal—despite seeps along the surface trace of the faults, which indicate dip leakage.

==See also==

==See also==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Evaluating top and fault seal]]

[[Category:Evaluating top and fault seal]]