--> --> Abstract: Intrinsic Fault Seal Uncertainties in Hydrocarbon Migration Analysis, by Øyvind Sylta and Are Tømmerås; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Intrinsic Fault Seal Uncertainties in Hydrocarbon Migration Analysis

Øyvind Sylta1; Are Tømmerås1

(1) Migris AS, Trondheim, Norway.

Fault seal dependent oil and gas prospects are explored for in many sedimentary basins. Changes in the sealing properties of the faults may be critical for the prospectivity of such exploration targets. The outcomes of migration analyses may for many fault seal prospects be quite uncertain. Detailed hydrocarbon migration modeling can often help to describe and thereafter reduce the uncertainties. However, the uncertainties cannot be reduced below the intrinsic uncertainties of the system. The intrinsic uncertainty is an inherent property of the prospect, and helps us focus our analysis towards the more critical uncertainties.

The a-priori uncertainty distribution of the input parameters for the hydrocarbon generation and migration system is described in detail. This includes uncertainties in the thermal field, source rock properties, rock fluid flow properties and fault seal properties. The intrinsic uncertainties of these probability distributions are described as fractions of the standard deviation of each distribution. Hydrocarbon generation and migration software is then used to simulate the transfer of oil and gas within the sedimentary basin from sources to traps through geological time. These simulations are done using a Quantum approach within Monte Carlo simulations.

In the Quantum approach the simulator draws input values from the a-priori statistical distributions with fixed distances from the mean value of each parameter, and uses only these exact values. The exact value used in each simulation is controlled by a Quantum distance parameter, which is defined as a percentage of the standard deviation. In a typical case several hundred faults would be modeled with multiple carriers and seals and using between 10 and 20 input variables for each fault. A random sampling of the input variable spaces in a small but significant number of simulation runs (> 1000) produces trap volumetric results. These results can be used to compile the intrinsic uncertainties of each trap and may relate the intrinsic uncertainties of the traps to the fault seal properties.

The study of uncertainties is a key issue in basin modeling and analysis. When volumetric estimates of uncertainties for prospects can be made using hydrocarbon systems modeling techniques, the next step is to try to reduce these uncertainties. Important questions to address are how small the uncertainties can become, and which of the fault seal input properties can reduce the uncertainties.