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Sealing or Breaching? Predicting Seal Failure Mechanisms Across a Sedimentary Basin: An Exercise in the Caswell Sub-Basin


Capillary leakage and mechanical breach are main causes underlying a seal failure of a hydrocarbon trap. Both are determined by a combination of several geological elements (e.g., rock type, hydrocarbon type, mechanical compaction, and diagenesis); this brings a challenge in evaluating a sealing mechanism in a quantitative way. Here, we present a simple approach to illustrate a vertical change in the sealing mechanism based on arithmetic models of the capillary and the rock strength (mechanical seals). Our method allows a quick characterization of a primary sealing mechanism in a sedimentary basin, without a sophisticated basin modeling. Our study comes from the Caswell sub-basin, the Northwest shelf of Australia. We predict a regional depth profile of the capillary seal based on grain size, compaction trend, interfacial tension, the wettability of rocks, and capillary threshold pressures measured by mercury injection experiments. A vertical profile of the fracture pressure is also derived from leak off test values. The result shows that the capillary seal plays an important role in the sealing capacity in the shallow section irrespective of its grain size, while the mechanical seal becomes more important in the deeper section. No matter which component is the primary control, potential hydrocarbon column height increases with the burial depth. This approach enables a characterization of hydrocarbon prospectivity throughout a sedimentary basin. In the shallower section, small hydrocarbon column height is expected with the limited amount of hydrocarbon leakage that is chiefly controlled by the capillary seal. In the deeper section, a larger potential hydrocarbon column can be expected with an increased chance of the mechanical seal failure that could lead to total loss of the accumulation. We assume that this relationship appears to be more complicated in a basin with more complex geological history, because historical maximum effective stress controls the capillary pressure of seal, whereas fracture pressure reflects present-day minimum stress. This study suggests the importance of the integrated evaluation approach of both capillary and mechanical seal capacities. It provides us a better understanding of the sealing phenomena and hydrocarbon prospectivity in a sedimentary basin.