Global Insight into Seals to Help Overcome the Most Likely Reason for Exploration Failure
Seals are often given as the most frequent reason for exploration failure and are the least comprehended physical elements of the petroleum system and of hydrocarbon traps. Despite this they are often the most neglected in pre-drill play and prospect risking. Seals are defined as rocks that have capillary entry pressures that are large enough to match or exceed the buoyancy forces of migrating or trapped hydrocarbons. They can occur as single surfaces capping a reservoir or as complex combinations of surfaces on the top, laterally, and on the bottom of reservoirs. A trapped hydrocarbon column will exert buoyancy pressure on all sealing surfaces unequally. Therefore, it is necessary to understand first how this pressure operates on the simplest sealing surface before attempting to evaluate combined mechanisms. We have evaluated this relationship by correlating the attributes of successful seals with the height of hydrocarbon columns and with the degree to which traps are filled to their maximum capacity. Variations in these relationships are noted based on seal lithology and depositional environment. Capillary pressure measurements confirm that the displacement or the threshold pressure of shales is significantly higher than in other lithologies, with the exception of anhydrites. The majority of the reservoirs studied are capped by simple top seals consisting of shales or involve shales in combination with other lithologies. Although evaporites seal a small percentage of the reservoirs studied, they trap a disproportionately high percentage of resources. Other sealing lithologies are argillaceous limestone, chalk, marl, undifferentiated tight carbonates, and tight sandstones/conglomerates. No correlation is seen between seal thickness and column height. This is important for those who use seal thickness as a proxy for sealing capacity in regional studies to remove play areas where seal capacity is deemed to be high risk. We will share a number of best practices for explorers that will help teams to better define the uncertainty surrounding the column height distribution used for the calculation of probabilistic volumes of exploration prospects.
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