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Greetings from 37,000 feet. This is a very interesting question and as with most aspets of assessment of chance there are not any hard rules, just some guidelines.
First, playing by the Exxon ‘rules’, if you have a reservoir penetration within your play outline, Play Chance for reservoir is 100%. Having said that, Play Chance is actually specific to each play SEGMENT. Most plays are subdivided into a manageable number of pieces (we advocate 5-25) that we call Common Risk Segments (horrible term) or more simply, play segments, where the boundaries reflect both significant and abrupt changes in geology. Often these reflect changes in GDE or structural fabric. If you are in an unpenetrated segment, you cannot assign a Play Chance value of 100% (I have been known to use a lot of 95% values, depending upon the situation).
The Play Chancevalue you land upon in your assessment is often best derived using Delphi methods, making sure that the perspective is cleary stated – this is not the reservoir chance for the first well to be drilled in the area but rather of finding reservoir at any location in the play segment. Rarely do I have values of less than 0.3 for any individual chance element at play scale.
One final tip. You will likely be familiar with the nine-box chance matrix espoused by R&A – this provides reasonable values based upon confidence and good/bad news for Pg values for individual chance factors in one prospect. As such it is not applicable at play level. Having said that, I find that the values for OVERALL Play Chance often reconcile nicely with the values in the matrix. This can provide an element of consistency to your assessment process.
Hope this helps.
Rose & Associates