Question

When evaluating a fault using an Allan diagram, what specific spatial relationship between the reservoir sand and the juxtaposed rock layer determines if the fault acts as a seal?

Accepted Answer

An Allan diagram is a 2D map that shows the juxtaposition of rock layers across a fault plane, helping geologists visualize whether a permeable reservoir sand is placed directly against a non-permeable layer. A fault acts as a seal when the permeable reservoir sand is moved by fault displacement to be directly opposite a non-permeable rock layer, such as shale or clay. This specific spatial relationship is called juxtaposition sealing. The non-permeable layer, often called the top seal or side seal, acts as a barrier that prevents hydrocarbons from migrating further along the fault plane. If the reservoir sand is juxtaposed against another permeable sand layer, the fault does not form a seal because hydrocarbons can flow through the fault zone into the adjacent sand. The effectiveness of this seal depends entirely on the sand being perfectly positioned against a rock layer with low permeability that blocks fluid movement.

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When evaluating a fault using an Allan diagram, what specific spatial relationship between the reservoir sand and the juxtaposed rock layer determines if the fault acts as a seal?

An Allan diagram is a 2D map that shows the juxtaposition of rock layers across a fault plane, helping geologists visualize whether a permeable reservoir sand is placed directly against a non-permeable layer. A fault acts as a seal when the permeable reservoir sand is moved by fa....