Stochastic Trap Analysis: Relevence for Carbonate Fault Seal Analysis in Exploration and Appraisal

Abstract

Understanding exploration risk involves considering a range of factors, including seal/trap efficiency, charge, and reservoir presence and quality. In many cases this risking considers sealing hydrocarbons against faults as part of the trap element. These seals are required to support large volumes of oil and or gas over near geologic timescales. For risking three-way prospects, juxtaposition analysis is a conservative method for predicting hydrocarbon columns. Whilst conservative in size, these traps have shorter columns and thus are less prone to top seal failure and smaller volumes require less charge. The application of fault rock sealing phenomena will either predict the base juxtaposition case or increases predicted column heights. Fault rock seal prospects with large columns are typically generated and then discounted through other geologic risk factors. If wells do not find the predicted columns, this is often “explained” by lack of charge or trap breach. In clastic sediments considerable work has been done to calibrate fault rock sealing algorithms to predict columns and hydrocarbon water contacts. These calibrations involve back-calculating the seal potential expressed as, across fault pressure difference or capillary entry pressure. Importantly, this back-fitting of seal capacity is generally conducted on single “best” technical models with no direct modeling of uncertainty. It is known that the fault and stratigraphic uncertainties are significant. Stochastic modeling of juxtapositions and fault seal to estimate column heights is vital in our understanding of exploration risk. Results of validations from a range of fields around the world will be presented in which observed hydrocarbon water contacts can be better predicted by probabilistic juxtaposition analysis over the combination of juxtaposition and fault rock models. Clear learnings can be made from fields in clastic sediments for the analysis of carbonate and fractured basement prospects. Incorporating stochastic juxtaposition analysis typically generates smaller lower risk traps, rather than large high-risk traps. Applying these models and methods to fault seal analysis will allow explorers to better define risks and rewards on prospects.

AAPG Datapages/Search and Discovery Article #90334©2018 AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop, Abu Dhabi, UAE, April 16-19, 2018