Overpressure and hydrocarbon distribution, Gulf Coast
| 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 |
Studies of the hydrocarbon distribution in the Gulf Coast relative to the top of overpressure suggest hydraulic fracturing and seal integrity influence the vertical distribution of hydrocarbons, success ratios, and seal risk.[1][2][3]
Success rate vs. depth
The chance of finding an economically successful accumulation in the Gulf Coast decreases with depth. This decrease is a result of several changes, including reservoir quality and migration pathways. Most importantly, however, the distribution of hydrocarbons is closely related to the top of the overpressure zone. Figure 1 shows the distribution of oil and gas production from more than 20,000 wells. Most hydrocarbons are found near or slightly above the top of the overpressured zone. The chance of success is reduced by 90–95% at depths of 2,000–5,000 ft below the top of overpressure.[3]
Hydrocarbon distribution vs. overpressure
The distribution of hydrocarbons relative to the top of overpressure changes as reservoir depth changes is shown in Figure 2. This pattern is consistent with loss of top seal integrity because of hydrofracturing rather than a simple loss of reservoir quality or other variable. Deeper reservoirs have the peak gas occurrence below the top of overpressure. This is consistent with the changing amount of pressure required to fracture the top seal (Pf) with depth. As depth increases, confining pressure increases and thus the Pf increases. As fracture pressure increases, the depth of the first intact top seal increases.
The relationship between depth of reservoir and depth of maximum cumulative production below the top of overpressure is fundamental to seal risk. Further work is required to relate the distribution of hydrocarbons to actual fracture pressure rather than simply depth below top of overpressure.
See also
- Natural hydraulic fracturing of top seals
- Fracture threshold in the real world
- Natural hydraulic fracturing example, North Sea
References
- ↑ Fertl, W. H., and W. G. Leach, 1988, Economics of hydrocarbon reserves in overpressured reservoirs below 18,000 feet in south Louisiana: SPE paper 18146, 16 p.
- ↑ Leach, W. G., 1993a, Fluid migration, HC concentration in south Louisiana Tertiary sands: Oil & Gas Journal, vol. 91, no. 11, p. 71–74.
- ↑ 3.0 3.1 3.2 3.3 Leach, W. G., 1993b, Maximum hydrocarbon window determination in south Louisiana: Oil & Gas Journal, vol. 91, no. 13, p. 81–84.
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