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1SINTEF Petroleum Research

Recent advances in petroleum assessment,Trondheim 19 20 October 2005

Risk assessment.

Principles and experiencesKjell ygard

SINTEF Petroleum Research


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Risk assessment

The purpose of risk assessment in petroleum explorationis to estimate the probability of discovery prior to drilling

of a mapped prospect.

Play level evaluationCalculation of the economic value of prospectsAssessment of the undiscovered resources in a given area duringplay evaluation.

Ranking of prospects


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Risk assessment

High degree of uncertainty in exploration for oil and gas

Half of drilled exploration wells are dry

About 50% of the discoveries are non profitable

Geologists perform risk assessment in a subjective

manner


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Geological risk assessment

The Geological risk assessment requires an evaluation ofthose geological factors that are critical to the discovery of

recoverable quantities of hydrocarbons.The probability of discovery is defined as the product of thefollowing major probability factors, each of which must be

evaluated with respect to presence and effectiveness:Probability of:

Reservoir

Trap Hydrocarbon charge

Retention of hydrocarbons after accumulation


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The Probability Consept

1. Pprob. = 1 Prisk

2. The probability of the simultaneousoccurrence of several independent events isequal to the product of their individualprobabilities. P = Presx Ptrapx PHCx Pret

3. Given the occurrence of several mutuallyexclusive events, P = Pa+ Pb

4. The probability of either one or both of twoindependent events can be estimated bycalculating the risk that neither of the eventswill occur. (1-P) = (1 - Pa) x (1 Pb)



Oil Prospect Reserve

Monte Carlo Calculation

1 ITERATION

Transform

ation

Access

Sou

rceArea

Conver

tibility

SealEfficiency

Gas/O

ilRa

tio

RecoveryF

ac

tor

Volum

eGasC

ap

OilSaturation

Porosity

NetTrapVolum

e

Minimum

Most Likely

Maximum

Phase

Form

atio

n

Volum

e

Factor



Risk vs. Uncertainty

Risk is the probability that the parameter of interest

fails to work at the minimum level expected

Uncertainty is the variation in or the range of

possible outcomes



Prospect Probabilities

The probability of discovery is a value that is based partlyon objective knowledge and historical data, partly on

extrapolations and partly on our subjective judgements oflocal geological parameters.

Post-drill evaluation

Reliability of the database

Relevant geological models



Reservoir

Probability of existence of reservoir facies with minimumnet thickness and net/gross-ratio

Probability of effectiveness of the reservoir, with respect tominimum porosity, permeability and hydrocarbonsaturation



Reservoir probability guidelines

Presence of effective reservoir facies

Porosity evaluation



Trap mechanism

Probability of presence of the mapped structure with aminimum rock volume as prognosed in the volume

calculation. Geometrical body.

Probability of effective seal mechanism for the mappedstructure.

T m h i m b bilit g id li



Trap mechanism probability guidelines

Structural/stratigraphic complexityregarding rock volume Effective seal mechnism



Petroleum Charge

Probability of effective source rock in terms of theexistence of sufficient volume of mature source rock of

adequate quality located in the drainage area of themapped structure.

Probability of effective migration of hydrocarbons from the

source rock to the mapped structure at the right time.

Petroleum Charge= Effective drainage area X Thickness

map X TOC and HI X Transformation X Expulsion XSecondary migration


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Retention after accumulation

The probability of effective retention of petroleum in the prospect afteraccumulation


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Hydrocarbons indicators

Oil seepage, pockmarks at sea-bottom, gas anomalies inseismic data etc., are all indicators of hydrocarbons being

present.

A direct hydrocarbon indicator (DHI) is defined as a

change in seismic reflection character (seismic anomaly)which is the direct result of the reservoirs fluid contentchanging from water to hydrocarbons.

Real or false:- Opal A/CT and Opal CT/Quartz


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Direct Hydrocarbon Indicator

Rock PropertiesShould See Should not See

Do See

Do not See

DHI- Attribute

Poor rockproperties model

Seismic artifact

No prospect

Poor seismicquality

Relay on

geologicmodells andPetroleum

System

Seismic


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Risk assessment

Predicted

Not predicted

Not predicted Predicted

Ge

ophysica

ldriven

Geological driven

Reservoir presence

Reservoir deliverability

Seal presence

Seal capasity

Source presence

Access to charge

Retention afteraccumlation


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Petroleum Risk Assessment litterature

http://www.ccop.or.th/projects/RiskAssess.pdf

Rose, P R, 1987. Dealing with risk and uncertainty in exploration: how can weimprove? AAPG Bulletin, v. 77, no 3, p 319-326

Rose, P R, 1992. Chance of success and its use in petroleum exploration, in RSteinmetz, ed., The business of petroleum exploration: AAPG Treatise ofPetroleum Geology, Handbook of Petroleum Geology, p 71- 86

White, D A, 1993. Geologic risking guide for prospects and plays: AAPG Bulletin, v

77, p 2048-2061Otis, R and Schneidermann N, 1997. A process for evaluating exploration prospects.AAPG Bulletin, v 81, no 7, p 1087-1109.

Ulmishek, Gregory F., 1986: Stratigraphic aspects of petroleum resourceassessment, in Rice, D.D., ed., Oil and gas assessments Methods andapplications: AAPG Studies in Geology #21, p. 59-68.

Milton, N. J., and Bertram, G. T., 1992. Trap Styles-A New Classification Based onSealing Surfaces. AAPG Bulletin, v 76, no. 7, p 983-999.

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