Risk, Uncertainty and Priorities - Quantitative Models
5
Summary
Several major points result from the study presented here:
451
1. Perhaps the dominant point of the numerical experiments is that they indicate the development of significant overpressure under both salt sheets in the
last 0.8 MaB.P. through to present day. The exact degree of overpressure and
its precise buildup rate are controlled by variations in salt speed and salt
thickness, but the buildup of overpressure always occurs for a laterally extensive salt sheet.
2. An overpressure increase (relative to regional) of about 100-300 atmospheres
occurs under and, in the vicinity of the salt, corresponding to about 200 atm
per km thickness of salt.
In the present situation it is clear that the ±(200-300)m uncertainty on a salt
thickness of around 2000 m, is causing over half of the uncertainty in excess
pressure values, with salt speed and sub-salt lithology uncertainties providing
roughly equal contributions of about 10-25% each to the total uncertainty in excess pressure depending on the individual case. The variations in excess pressure estimates at break-out are about ± 30 atm on a mean value of about 170 atm.
Thus, if one can accept a high break-out value of 200 atm while drilling, then
there is no need to attempt to improve the uncertainty on salt thickness prior to
drilling, which is responsible for about 58-65% of the ± 30 atm. On the other
hand, if one cannot accept that level of uncertainty on excess pressure, then a
concerted effort should be made first to improve salt thickness estimates rather
than concentrating on salt speed or sub-salt lithology
Basin modeling has become a useful tool in petroleum exploration, especially
in those areas where not many wells are available. However, when data availability decreases, the uncertainty involved will increase and so will the risk. Therefore the uncertainty and associated risk have to be assessed when applying basin
modeling techniques to such areas. Risk analysis in basin modeling should include the resolution, sensitivity, uniqueness and precision of modeling results.
The Monte Carlo approach is not appropriate in practice because of the complexity of the system being modeled; too much computer time is taken to perform risk analysis for any basin analysis model due to the multiplicity of processes related to the tectonic and strata development, sediment deposition and
diagenesis, petroleum generation, migration and accumulation in a sedimentary basin.
The probability method for risk analysis in basin modeling is numerically
quick without loss of accuracy in risk assessment. The problem does not require
massive Monte Carlo computer runs and can be performed on a personal computer or workstation in a timely manner, which is of importance in risky and
competitive petroleum exploration.
5
Summary
Several major points result from the study presented here:
451
1. Perhaps the dominant point of the numerical experiments is that they indicate the development of significant overpressure under both salt sheets in the
last 0.8 MaB.P. through to present day. The exact degree of overpressure and
its precise buildup rate are controlled by variations in salt speed and salt
thickness, but the buildup of overpressure always occurs for a laterally extensive salt sheet.
2. An overpressure increase (relative to regional) of about 100-300 atmospheres
occurs under and, in the vicinity of the salt, corresponding to about 200 atm
per km thickness of salt.
In the present situation it is clear that the ±(200-300)m uncertainty on a salt
thickness of around 2000 m, is causing over half of the uncertainty in excess
pressure values, with salt speed and sub-salt lithology uncertainties providing
roughly equal contributions of about 10-25% each to the total uncertainty in excess pressure depending on the individual case. The variations in excess pressure estimates at break-out are about ± 30 atm on a mean value of about 170 atm.
Thus, if one can accept a high break-out value of 200 atm while drilling, then
there is no need to attempt to improve the uncertainty on salt thickness prior to
drilling, which is responsible for about 58-65% of the ± 30 atm. On the other
hand, if one cannot accept that level of uncertainty on excess pressure, then a
concerted effort should be made first to improve salt thickness estimates rather
than concentrating on salt speed or sub-salt lithology
Basin modeling has become a useful tool in petroleum exploration, especially
in those areas where not many wells are available. However, when data availability decreases, the uncertainty involved will increase and so will the risk. Therefore the uncertainty and associated risk have to be assessed when applying basin
modeling techniques to such areas. Risk analysis in basin modeling should include the resolution, sensitivity, uniqueness and precision of modeling results.
The Monte Carlo approach is not appropriate in practice because of the complexity of the system being modeled; too much computer time is taken to perform risk analysis for any basin analysis model due to the multiplicity of processes related to the tectonic and strata development, sediment deposition and
diagenesis, petroleum generation, migration and accumulation in a sedimentary basin.
The probability method for risk analysis in basin modeling is numerically
quick without loss of accuracy in risk assessment. The problem does not require
massive Monte Carlo computer runs and can be performed on a personal computer or workstation in a timely manner, which is of importance in risky and
competitive petroleum exploration.