Holes in Faultrock and the Importance of Trap Characterisation

Abstract

It is important to adequately characterise trap geometries and account for uncertainty in spill and fault leak point elevations. We suggest that in many cases, it can be shown that structural traps are either 1) not filled at all (no hydrocarbons), or 2) completely filled (filled to spill or filled down to the shallowest leak point). This suggests that the typical capacity of source rock regions and kitchen processes to produce hydrocarbons usually exceeds the size of common traps. This, in turn, suggests that much of the effort spent assessing charge efficiency may be better spent on other issues. Recently the AAPG nominated U.S. Allan’s 1989 paper on fault juxtaposition analysis as one of the ten most important papers in oil and gas structural geology. The paper proposed that across fault juxtaposition is the key control on fill level in faulted traps. Since this work has been published a significant number of studies have asserted that fault membrane sealing occurs. In the majority of these works, there are either no case studies, or the case studies that are presented do not include the evaluation of fault juxtaposition or other alternatives which could explain the observations. Much of the work on fault seals assumes that there is contiguous/continuous fault rock or clay smear which enables a capillary entry pressure membrane seal. However, when outcrop exposures of fault zones are examined, the along strike or vertical spatial variability of fault rock properties are usually found to be extreme. Furthermore, these data strongly suggest that the assumption of contiguous and continuous fault rock is commonly erroneous. When assessing exploration risks associated with undrilled prospects it is vital to conduct juxtaposition based trap analysis. In many cases, the key uncertainty is the thickness and nature of the top and base seals, whilst in other cases, the key uncertainty is the fault geometry, particularly the throw or vertical separation profiles of the faults. This analysis is also a fundamental part of any stratigraphic trap risking. Results of a significant number of case studies will be presented in which observed hydrocarbon water contacts are compared with probabilistic predictions of hydrocarbon column heights and trap capacity. Models which consider juxtaposition alone are more accurate, and as such, these should be used in future analyses of charge efficiency.

AAPG Datapages/Search and Discovery Article #90364 © 2019 AAPG Middle East Geoscience Technology Workshop, Integrated Emerging Exploration Concepts: Challenges, Future Trends and Opportunities, Dhahran, Saudi Arabia, December 2-4, 2019