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2018 AAPG International Conference and Exhibition

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Global Insight Into the Controls on Top Seal Based on Analog Studies

Abstract

Seals are the least comprehended physical elements of the petroleum system and they are often the most neglected in pre-drill play and prospect risking even though they are commonly listed as the main reason for prospect failure. We combine the results from several industry post well studies to confirm this point. 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 (top seal) before attempting to evaluate combined mechanisms. An assessment of circa. 1500 proven global reservoir analogs highlights that the majority of large oil fields only require a top seal. Thisi study is also used to evaluate industry paradigms, such as "seal thickness is an indicator of sealing capacity". This paradigm and others will be evaluated. Capillary pressure measurements confirm that the displacement or the threshold pressure of shales is significantly higher than in other lithologies. Only anhydrite has a higher threshold pressure. Global reservoirs have been studied to see how this 1D pressure measurement translates into the ability of each lithology to form a 3D surface that can retain hydrocarbons. We show that over 60% of the reservoirs studied are capped by simple top seals consisting of shales, and an additional ~22% involve shales in combination with other lithologies. Although evaporites seal only 15% of the reservoirs, they trap ~26% of the oil-equivalent ultimately recoverable reserves in these reservoirs.