The Elephant in the Room: Seal Rock

The Seal Under Scrutiny

Well logs and mud log painted a concerning picture. The main top seal lacked the typical characteristics of an effective seal (not clay-rich or even shale-rich). Instead, it comprised roughly 300 meters of loose sand, friable sandstone with some shale beds, indicating a low shale gouge ratio and poorly compacted beds with larger pore throats. The overburden pressure was simply not enough to adequately compact the seal rocks. William C. Dawson’s work on the influence of depositional conditions on shale properties and seal character resonated here. It made me question how established regional high (Figure 2) could be explored twice without proper assessment of the top seal’s ability to hold hydrocarbons. Does that mean that shallow reservoir targets are riskier regarding SEAL integrity? Not necessarily.

Why Seal Effectiveness is Often Ignored

Seal effectiveness is often the most challenging element to predict in exploration, hence its frequently neglected. In his LinkedIn blog, Alan Foum mentioned that a significant amount of technical work is done on reservoir prediction using sequence stratigraphy, sedimentology, and reservoir geology. Basin modeling or petroleum systems modeling (and geochemistry) examines the source and migration, reducing the risk on hydrocarbon charge and helping predict the hydrocarbon phase. Seismic interpretation defines the trap, and advanced seismic attributes further aid in reservoir characterization. Yet, seal prediction remains largely ignored. Let’s peg deeper into this issue and find out why it has entrenched in the exploration.

What makes the Seal Conundrum

A study titled “Central North Sea Post Well Analysis” revealed a concerning distribution of exploration failures: Seal (38%), Trap (28%), Reservoir (17%), and Charge (14%). Despite this significantly high number, seal prediction is often ignored due to the industry-wide dogma that seal effectiveness depends solely on facies, thickness, and lithology, ignoring that in all cases, sealing capability depends upon the radius of the pore throat, densities, and interfacial tension. As Zhiyong He explains, the inability to measure seal capacity from seismic data forces explorationists to rely on depositional environment-based ranges and probabilistic migration models. This explains why many geologists, particularly when dealing with thick shale-rich systems, take seals for granted, assuming even a few meters can hold a significant hydrocarbon column. However, thickness alone doesn’t determine seal capacity and column height.

Seal Failure: Friend or Foe? Rethinking a Contentious Concept

Alan Foum references J. Karlo’s video outlining various causes of seal failure (micro-porosity, brittle fractures, hydraulic fracturing, fault displacement, and fault rock composition). While these mechanisms are well-established, recent research has challenged the blanket application of some aspects of the ‘contentious seal failure concept.’ In light of this, let’s scrutinize more closely why seal integrity remains a neglected factor in the oil and gas industry.