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  = 10-61

  | topg    = 10-94

  | topg    = 10-61

  | 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

==Trap description==

==Trap description==

A [[trap]] in the [[Central graben]], North Sea, appears to have been bled dry by [[Natural hydraulic fracturing of top seals|natural hydraulic fracturing]]. This trap is a low-relief [[salt structure]] with a [[dry hole]] and has less-than-sufficient [[strain]] to fracture the top [[seal]].

A [[trap]] in the [[Central graben]], [[North Sea]], appears to have been bled dry by [[Natural hydraulic fracturing of top seals|natural hydraulic fracturing]]. This trap is a low-relief [[salt structure]] with a [[dry hole]] and has less-than-sufficient [[strain]] to [[fracture]] the top [[seal]].

==Evidence==

==Evidence==

[[file:evaluating-top-and-fault-seal_fig10-44.png|thumb|{{figure number|1}}. Copyright: Esso Exploration and Production, U.K.]]

[[file:evaluating-top-and-fault-seal_fig10-44.png|300px|thumb|{{figure number|1}}Pore pressure is close to the fracture pressure at the base of the top seal. Copyright: Esso Exploration and Production, U.K.]]

Evidence for hydraulic fracturing and seal rupture include the following:

Evidence for hydraulic fracturing and seal rupture include the following:

* A fossil oil—water contact exists at the synclinal spill point of the trap. The trap was once filled with hydrocarbon, but something happened after trap fill to bleed off hydrocarbons.

* A fossil [[oil—water contact]] exists at the [[Syncline|synclinal]] [[spill point]] of the trap. The trap was once filled with [[hydrocarbon]], but something happened after trap fill to bleed off hydrocarbons.

* Hydrocarbon shows exist throughout the sediment column above the reservoir. These hydrocarbons are direct evidence of a breached top seal.

* Hydrocarbon [[Show evaluation|shows]] exist throughout the sediment column above the [[reservoir]]. These hydrocarbons are direct evidence of a breached top seal.

* The pore pressure is close to the theoretical fracture pressure. In adjacent fields, the pore pressure is significantly less than P_f.

* The [http://www.glossary.oilfield.slb.com/en/Terms/p/pore_pressure.aspx pore pressure] is close to the theoretical [[fracture pressure]], P_f. In adjacent fields, the pore pressure is significantly less than P_f.

* Cores show vertical, open extension fractures rather than the more common shear fractures found in adjacent fields. These fractures are identical to fractures produced by hydraulic fracturing.

* Cores show vertical, open [[extension fracture]]s rather than the more common [[shear fracture]]s found in adjacent fields. These fractures are identical to fractures produced by hydraulic fracturing.

[[:file:evaluating-top-and-fault-seal_fig10-44.png|Figure 1]] shows that the pore pressure is close to the fracture pressure at the base of the top seal (approximately [[depth::1300 ft]]). The pore pressure is inferred from mud weight and RFT measurements (solid dots). Leak-off tests (LOT) help constrain the fracture pressure. The fracture pressure is close to the lithostatic pressure or overburden pressure.

[[:file:evaluating-top-and-fault-seal_fig10-44.png|Figure 1]] shows that the pore pressure is close to the fracture pressure at the base of the top seal (approximately [[depth::1300 ft]]). The pore pressure is inferred from mud weight and [[Wireline formation testers|repeat formation tester]] (RFT) measurements (solid dots). [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=leakoff%20test Leak-off tests] (LOT) help constrain the fracture pressure. The fracture pressure is close to the [[Geostatic and lithostatic pressure|lithostatic pressure]] or overburden pressure.

==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]]