At the risk of getting into the minutiae there are a variety of tops seal (concrete like, gummy, interlaminated and on and on). The normal seal in this area would be harder than the sand above but not as hard as the carbonate below.
In the case of Miani we have 'unique unknown' since the seal is laid down over the unaltered carbonate - the hydro-thermal fluid enters below - creates the porosity - the rock fails under overburden pressure and collapses. How does this affect the seal above which lowers in response to the collapse? In the famous words of the unknown geo - 'we will find out when we drill' - engineering love it when we talk like that
Another factor that makes drilling through a new play different, is the driller will slow down the bit - feel for the top of porosity and because the pressure of the reservoir is unknown may even increase the mud weight before going into zone. Add to that the damn geo can't give him an accurate depth estimate from seismic and the last bit of a well just takes longer.
