Print this pageAbstract: Probability and the Petroleum System
Peter D. Carragher
A key element in the management of exploration programs is the accurate pre-drill assessment of the Probability of Success for wildcat wells. This is often quite a difficult task, given the inherent complexity of many petroleum systems. A comparison of pre- and post-drill probability estimates reveals
that while explorers are reasonably well calibrated in low to moderate risk wildcats, they are often poorly calibrated in high risk wildcats.
Notwithstanding poor data quality or coverage, and the tendency for explorers to be optimistic, it is proposed that there is also a basic problem with the commonly used methodology for estimating the probability of success. The purpose of this talk is to illustrate this problem and propose a solution for one of the root causes of poor calibration in high risk wildcats.
The probability of success for wildcats is usually based upon the proposition that the elements of the petroleum system are independent. However independence is only valid in closely defined conditions.
In certain petroleum systems the elements are plainly independent. For example, in the Gulf of Suez the deposition of Nubia sandstone occurred well before the trap forming tectonic events. In contrast the notion of complete independence is not valid for reservoir deposition and trap formation in growth faulted fields.
A new Probability measure, The Uncertainty Index, addresses the problems associated with independence. This measure has a range of +1 to -1. The Uncertainty Index (UI) can be negative even when the Technical Chance of Success, (based on the assumption of Independence), is in the range of 1 to 25%. Prospects characterized by multiple high risks have negative UIs. When drilled, they very rarely find significant accumulations.
The Uncertainty Index is an additional tool for both data gathering and careful decision making in high risk exploration projects.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994