--> Resource Assessment of a Prospect With Two Separate Structural Closures Linked by a Saddle - Challanges, Solutions, and Pitfalls in Creating a Geologically Correct Probabilistic Model

2018 AAPG International Conference and Exhibition

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Resource Assessment of a Prospect With Two Separate Structural Closures Linked by a Saddle - Challanges, Solutions, and Pitfalls in Creating a Geologically Correct Probabilistic Model

Abstract

This paper will focus on how to model the predicted success scenarios of a prospect with two or more structural closures, sharing a saddle above the maximum HWC. To model this statistically correct, we need to honour Risk and Volume dependencies as well as possible fluid communications between the reservoirs over the saddle. The objective is to build a stochastic model that generates realistic combination of success and failures for the separate closures based on the geological model. 3D seismic and improved subsurface resolution has led to multi-compartment prospects beeing the new norm, challanging the old assessment methods. The objective of prospect assessment today, is not only to model the overall Chance of Success (COS) and the total volume resource uncertainty range. It also needs to model the COS for each possible success scenarios and the volume distributions of these scenarios, given that all reservoirs/compartments are tested. To capture a prospects success scenario in a probabilistic model, it is imperative to handle parameters like correlations, risk dependencies and potential fluid communications between the different reservoir units/compartments in a prospect. For the geoscientist, the objective of modern prospect assessment is to model his/her understanding of the possible success scenarios risk and volume uncertainty in one stochastic model. To do this, a new industry best practise for prospect assessment is developing based on a three-step process: A) break the prospect down into individual reservoirs/compartments that can succeed/fail independently, these are assessed individually with respect to risk and volume. B) Enrol the assessed reservoirs/compartments into a prospect where risk dependency, correlations and possible fluid communication is modelled. C) Verify that the output fits the predictions in the geological model. This assessment method will be illustrated using a prospect containing two separate structures linked by a saddle, where the HWC range can exceed the saddle. The key assessment challenge are linked to modelling the communication between the two closures over the saddle and the effect trap/seal failure on either closure will have on the HWC. This paper will demonstrate the practical application of this three-step process as well as illustrate the potential errors in COS and volumes of the prospect if the communication over the saddle is not modelled correctly.