Natural hydraulic fracturing example, North Sea
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| 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 |
| Link | Web page |
| Store | AAPG Store |
Trap description
A trap in the Central graben, North Sea, appears to have been bled dry by 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 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.
- Hydrocarbon 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, Pf. In adjacent fields, the pore pressure is significantly less than Pf.
- 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.
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 repeat formation tester (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.
See also
- Natural hydraulic fracturing of top seals
- Fracture threshold in the real world
- Overpressure and hydrocarbon distribution, gulf coast
External links
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