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THRONDSEN,TORBJ√ėRN and MAGNUS WANGEN, Institute for Energy Technology, Kjeller, Norway

Abstract: Prospect Evaluation in the Framework of Petroleum Systems and Basin Modelling

The ultimate goal of petroleum system analysis in exploration is to contribute to find more petroleum and reduce risk in the exploration process. Basin modelling is in this context just an aid, but as such indispensable in performing a proper analysis.

Petroleum system logic may be regarded as having two main aspects in relation to exploration. The first is to understand or predict on a regional to semiregional scale the occurrence or lack of hydrocarbon accumulations in an area, i.e. to delineate the extent of proven or hypothetical petroleum systems. This is in accordance with the original definition of Magoon & Dow (1994). The second aspect which is highlighted in this presentation is to focus on and contribute to the evaluation of potential prospects with respect to hydrocarbon content, phase behaviour, preservation, etc. The key objective here is to contribute to prospect definition, risk assessment and ranking before drilling.

The first approach is useful at an early stage of exploration when decisions are to be taken to go into an area and do more detailed investigations, as a structure for larger regional overviews, or to provide general background evaluations. However, this comprehensive approach may be impractical when it comes evaluation of single prospect candidates. It is likely to suffer from the lack of detailed regional maps, be time-consuming with much effort in non-relevant areas, and out of phase with the seismic mapping related to the upcoming prospect.

The normal procedure in prospect definition is that potential candidates are defined from seismic interpretation and some idea of the presence of a reservoir, trapping mechanism and mature source rock. More proper evaluations regarding the presence and type of hydrocarbons in the trap, degree of filling and state of preservation are performed after the initial prospect identification. There is no reason to believe that this sequential order of the work process will change. The role and importance of seismic interpretation to detect and map potential prospects is undisputable. However, petroleum system logic has a clearly defined contribution to prospect evaluation, and is essential when it comes to final launching of prospects and risk assessment.

Risk evaluation is performed after the volume and type of petroleum (oil, gas, two-phase) in the prospect is assumed. In fact, it is the presence of a given volume of a given petroleum type (oil, gas, two-phase) that is subjected for risk evaluation. The type of petroleum is inferred from comparison with nearby analogs, evolution along a migration route (fill/spill, phase evolution), from basin modelling or simply assumed as the only type of interest to consider. Petroleum system analysis may have an important contribution in this initial step to estimate the most likely type of hydrocarbons to be expected in the prospect. Standard risk factors are the presence of trap, reservoir, seal, hydrocarbon generation, migration and preservation of trapped hydrocarbons. These factors are more or less identical to the essential elements and processes in a petroleum system, although the use of petroleum system logic has a more pronounced weighting on the dynamics (timing). It is obvious that the concepts of petroleum system analysis can be very useful in the risk assessment of potential prospects.

The focus of petroleum system analysis to an actual prospect candidate will effectively.limit the study area of interest to the catchment area for the prospect and actual analogs to be used in the evaluation. This delimination of the study area is often important when it comes to access to relevant maps and turnover time for the work process. Both factors are essential when it comes to creative participation and contribution of petroleum system analysis in project definition.

Basin modelling is indispensable in petroleum system analysis for description and quantification of hydrocarbon generation, migration, phase behaviour, preservation and timing in relation to other essential factors. True 3-D or pseudo 3-D (2-D) basin modelling is a necessity as volumetric calculation of hydrocarbon generation and migration is essential to estimate the degree of filling and type of trapped hydrocarbons. The type of trapped hydrocarbons is not only dependent on the type of supplied hydrocarbons, but also by the phase evolution in the trap which is governed by the pressure, temperature and the volume of supplied hydrocarbons relative to trap volume. It is here essential that the basin modelling can be performed within a time schedule and on a format that is relevant for the actual prospect generation.

The role of petroleum system analysis and volumetric basin modelling with respect to prospect evaluation and risk assessment will be examplified by several examples from the Norwegian North Sea.

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